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Microelectrochemical approach towards the analysis of ... · 1 Introduction 1.1 Background and...
Transcript of Microelectrochemical approach towards the analysis of ... · 1 Introduction 1.1 Background and...
Microelectrochemical approach towards the analysis of electrochemical noise signals related to
intergranular stress corrosion cracking of austenitic stainless steel
Mikroelektrochemischer Ansatz zur Analyse
elektrochemischer Rauschsignale der interkristallinen Spannungsrisskorrosion von
austenitischem rostfreiem Stahl
Der Technischen Fakultät der Universität
Erlangen-Nürnberg zur Erlangung des Grades
D O K T O R – I N G E N I E U R
vorgelegt von
Mathias Stefan Breimesser M.Sc.
Erlangen – 2012
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Als Dissertation genehmigt von der Technischen
Fakultät der Universität Erlangen-Nürnberg
Tag der Einreichung: 12.06.2012
Tag der Promotion: 24.09.2012
Dekanin: Prof. Dr.-Ing. M. Merklein
Berichterstatter: Prof. Dr. sc. techn. S. Virtanen, WW IV
Jun.-Prof. Dr.-Ing. A. Heyn, Uni Magdeburg
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Zusammenfassung
Die elektrochemische Rauschmesstechnik ist eine vielversprechende Methode zur Untersuchung
und Früherkennung von Korrosionsprozessen. Zum besseren Verständnis der Rauschsignale, die
während der Initiierung und des Wachstums von interkristalliner Spannungsrisskorrosion in
austenitischem rostfreiem Stahl auftreten, wurde eine Kombination von zwei experimentellen
Techniken angewandt: Einerseits wurden makroskopische elektrochemische Rauschmessungen
durchgeführt. Andererseits wurden mittels der elektrochemischen Mikrokapillartechnik die
Initiierung und das Wachstum einzelner Mikrorisse untersucht, mit dem Ziel, ein besseres
Verständnis für die makroskopischen Rauschsignale zu erreichen und diese zu modellieren.
Um vergleichbare Resultate zu erhalten, muss ein experimentelles System gewählt werden, dass
sowohl in mikro- wie in makroskopischen Experimenten anwendbar ist. Eine 0.01 M Lösung von
Kaliumtetrathionat mit einem pH-Wert von 2.2 erfüllt diese Anforderung. In Kombination mit
der Mikrokapillartechnik ist es möglich, Probenoberflächen systematisch abzusuchen und aktive
Oberflächenpositionen anhand des Korrosionspotentials zu identifizieren, und das Stromsignal
eines einzelnen wachsenden Mikrorisses aufzuzeichnen.
Mikrokapillarmessungen auf geschliffenen Proben unter konstanter Verformung zeigen, dass sich
Probenoberflächen grösstenteils passiv verhalten. Wenige spezifische Punkte sind aktiv und
zeigen Rissinitiierung. Aktive Punkte treten überwiegend an den Grenzflächen zwischen
Mangansulfid-Einschlüssen und dem Metall auf. Rissinitiierung hängt jedoch in hohem Masse
von der Oberflächenbehandlung ab. Ist die Oberfläche oxidiert, so tritt Rissinitiierung verzögert
auf. Werden geschliffenen Oberflächen zusätzlich vibrationspoliert, so zeigen alle Korngrenzen
aktives Verhalten und interkristalline Korrosion, auch auf unbelasteten Proben.
Stromsignale einzelner Mikrorisse bestehen aus Überlagerungen von Transienten mit abruptem
Anstieg und langsamerem Abfall. Die gemessenen Ladungsmengen korrelieren dabei gut mit den
Mikrorisslängen und ermöglichen ein Abschätzen der Dicke der aufgelösten Metallschicht
entlang einer Korngrenze. Die gefundenen Stromsignale können als Serie von Passivfilmrissen,
gefolgt von anodischer Metallauflösung und Repassivierung gedeutet werden.
In makroskopischen Versuchen kann aufgrund der Aggressivität des Elektrolyts die Initiierung
von Spannungsrisskorrosion unter konstanter Last unterhalb der Streckgrenze des Materials
innert Stunden oder weniger Tage beobachtet werden. Elektrochemische Rauschmessungen
können dabei klar den Übergang von passivem zu aktivem Verhalten detektieren. Risswachstum
ist gekennzeichnet durch charakteristische Strom- und Spannungstransienten. Drei Haupttypen
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von Transienten können identifiziert werden, welche sich metastabiler Lochkorrosionsbildung,
Initiierung und Wachstum von Mikrorissen, sowie beschleunigtem Risswachstum zuordnen
lassen. Die Stromrauschsignale, welche Initiierung und Wachstum von Mikrorissen anzeigen,
gleichen dabei in hohem Masse den potentiostatischen Stromsignalen, die mit der Mikrokapillare
auf einzelnen Mikrorissen gemessen werden können.
Ein makroskopisches Rauschsignal lässt sich somit nicht alleine durch Aufsummierung von
mikroskopischen Risswachstumssignalen simulieren, einzelne Komponenten des Rauschsignals
können jedoch anhand der Mikrokapillarexperimente erklärt werden. Es ist nicht abschliessend
möglich, aufgrund der vorliegenden Ergebnisse einen bestimmten Mechanismus der
Spannungsrisskorrosion auszuschliessen oder definitiv zu bestätigen. Ein Mechanismus basierend
auf Passivfilmbruch, anodischer Metallauflösung und Repassivierung kann die beobachteten
Ergebnisse jedoch schlüssig erklären.
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Abstract The electrochemical noise technique is a promising method for the study and early detection of
corrosion processes. To gain a better understanding of the electrochemical noise signals related to
the initiation and growth of intergranular stress corrosion cracks in austenitic stainless steel, a
combination of two experimental techniques was applied: First, macroscopic electrochemical
noise measurements were performed. Second, the electrochemical microcapillary technique was
applied to study the initiation and growth of single micro cracks, with the aim to gain a better
understanding of macroscopic noise signals and to model them.
To obtain comparable results, an experimental system has to be chosen, which is applicable for
both micro- and macroscopic experiments. A 0.01 M solution of potassium tetrathionate with a
pH value of 2.2 fulfils this requirement. In combination with the microcapillary technique it
allows a systematic scanning of a sample surface, identification of active surface spots by
measurement of the open circuit potential, and the subsequent measurement of current signals
caused by single growing micro cracks.
Microcapillary measurements on ground samples under constant deformation conditions show
that most part of a sample surface exhibits passive behaviour. Crack initiation occurs at few
specific active spots. These active spots are mostly found on the boundary between manganese
sulphide inclusions and metal. However, crack initiation depends to a high degree on surface
preparation. On oxidised surfaces, crack initiation can occur delayed. Vibration polishing of
ground surfaces on the other hand renders all grain boundaries susceptible to intergranular
corrosion, independent of applied stress.
The current signal of a single micro crack can be explained as a superposition of transients with
fast rise and slower decay. The detected charge values correlate well with micro crack lengths
and allow estimating the width of the dissolved metal layer along a grain boundary. These current
signals can be interpreted as a series of passive film rupture events, followed by anodic metal
dissolution and repassivation.
In macroscopic noise measurements, the aggressive electrolyte allows the monitoring of the
initiation of stress corrosion cracks under constant loading conditions in a matter of hours or few
days. Electrochemical noise measurements can clearly detect the transition from passive to active
behaviour. Crack propagation is indicated by characteristic current and potential transients: Three
main transient types can be identified and assigned to metastable pitting, micro crack initiation
and growth, and accelerated growth steps. Current noise signals indicating micro crack initiation
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and growth highly resemble the potentiostatic current signals measured with the microcapillary
on single micro cracks.
A macroscopic noise signal can therefore not be modelled by simply adding up microcapillary
signals, but microcapillary measurements can explain certain components of a macroscopic
signal. It is not possible to finally confirm or dismiss a specific mechanism for stress corrosion
cracking based on the presented results. However, a mechanism based on passive film rupture,
anodic metal dissolution and repassivation coherently explains the found results.
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Contents Zusammenfassung............................................................................................................... 3 Abstract ............................................................................................................................... 5 Contents............................................................................................................................... 7 List of abbreviations and symbols....................................................................................... 9 1 Introduction............................................................................................................... 11
1.1 Background and motivation .............................................................................. 11 1.2 Objectives and experimental approach ............................................................. 12 1.3 Relevance of the work....................................................................................... 14 1.4 Outline of the thesis .......................................................................................... 15
2 Theoretical background............................................................................................. 16 2.1 Stress corrosion cracking .................................................................................. 16
2.1.1 Definition and phenomenology................................................................. 16 2.1.2 Mechanism of IG SCC of sensitised stainless steel .................................. 18 2.1.3 Loading methods....................................................................................... 25
2.2 The electrochemical microcapillary technique ................................................. 26 2.2.1 Overview of microelectrochemical measuring techniques ....................... 26 2.2.2 Features of the electrochemical microcapillary ........................................ 28 2.2.3 Applications of the electrochemical microcapillary technique................. 29
2.3 The EN technique.............................................................................................. 31 2.3.1 Definition .................................................................................................. 31 2.3.2 Measuring methods ................................................................................... 32 2.3.4 Application of the EN technique to monitor SCC .................................... 36
2.4 Conclusion......................................................................................................... 39 3 Experimental procedures and development of an experimental system ................... 40
3.1 Material ............................................................................................................. 40 3.2 Macroscopic EN measurements........................................................................ 42
3.2.1 Sample preparation.................................................................................... 42 3.2.2 Measuring setup ........................................................................................ 42
3.3 Microcell measurements ................................................................................... 44 3.3.1 Sample preparation.................................................................................... 44 3.3.2 Measuring setup (analogous to [23])......................................................... 45
3.4 Initial experiments with neutral sodium thiosulphate electrolyte ..................... 47 3.4.1 Macro scale EN measurements ................................................................. 47 3.4.2 Microcell tests ........................................................................................... 48
3.5 Evaluation of new electrolytes .......................................................................... 49 3.5.1 Selection criteria........................................................................................ 49 3.5.2 Acidic thiosulphate solutions .................................................................... 50 3.5.3 Chloride solutions ..................................................................................... 51 3.5.4 Potassium tetrathionate solutions.............................................................. 54
3.6 Final measuring procedures .............................................................................. 58 3.6.1 Macroscopic CERT tests (Figure 27a)...................................................... 58 3.6.2 Macroscopic CL tests (Figure 27b)........................................................... 59 3.6.3 Microcapillary measurements ................................................................... 59 3.6.4 Post test inspection.................................................................................... 60
3.7 Summary ........................................................................................................... 61 4 Investigation of initiation and propagation of IG cracks on the micro scale ............ 62
4.1 Signal characteristics......................................................................................... 62 4.1.1 Passive behaviour...................................................................................... 62
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4.1.2 Active surface spots .................................................................................. 63 4.1.3 Discontinuous crack growth behaviour and repassivation........................ 65
4.2 Location of crack initiation sites ....................................................................... 66 4.3 Influence of applied stress and DOS on the crack growth behaviour ............... 70
4.3.1 Influence on crack sizes ............................................................................ 70 4.3.2 Influence on current signal........................................................................ 71 4.3.3 Crack growth direction.............................................................................. 71
4.4 Quantitative comparison of current signals and micro crack dimensions......... 72 4.5 Systematic array measurement.......................................................................... 76
4.5.1 Distribution of active sites ........................................................................ 76 4.5.2 Variation of OCP values ........................................................................... 77
4.6 Influence of the surface state............................................................................. 79 4.6.1 Behaviour of polished surfaces ................................................................. 79 4.6.2 Influence of oxide film formation over time............................................. 81
4.7 Discussion ......................................................................................................... 83 4.7.1 General validity of the results ................................................................... 83 4.7.2 Variation of OCP values on passive surface spots.................................... 83 4.7.3 Micro crack initiation................................................................................ 84 4.7.4 Micro crack propagation ........................................................................... 85
4.8 Summary ........................................................................................................... 88 5 Macro scale EN measurements ................................................................................. 90
5.1 Results of CERT tests ....................................................................................... 91 5.2 Results of CL tests ............................................................................................ 93
5.2.1 General behaviour of solution annealed samples...................................... 93 5.2.2 General behaviour of sensitised samples .................................................. 94
5.3 Interpretation of current transient shapes........................................................ 102 5.3.1 Classification of transient types .............................................................. 102 5.3.2 Transition to macroscopic cracking ........................................................ 109 5.3.3 Integration of characteristic current signals ............................................ 109
5.4 Further analysis of EN signals ........................................................................ 111 5.4.1 Comparison of statistical parameters ...................................................... 111 5.4.2 Analysis in the frequency domain........................................................... 112
5.5 Discussion ....................................................................................................... 113 5.5.1 General results......................................................................................... 113 5.5.2 Crack initiation under CERT and CL conditions.................................... 114 5.5.3 Crack initiation and propagation: mechanism and influence of stress.... 114 5.5.4 Identification of IG SCC and use of EN for monitoring applications..... 116
6 Comparison of macro and micro scale measurements............................................ 118 6.1 Direct upscaling: Macrocapillary measurements ............................................ 118
6.1.1 Passive surface behaviour ....................................................................... 118 6.1.2 Current signals of active surfaces ........................................................... 119 6.1.3 Comparison with microcapillary measurements..................................... 121
6.2 Discussion of microcapillary and macro scale EN measurements.................. 123 6.2.1 Comparability of macro and micro scale results..................................... 123 6.2.2 Comparison of current transients during crack growth........................... 124 6.2.3 Mechanistic considerations ..................................................................... 127
Summary and conclusions............................................................................................... 130 Outlook............................................................................................................................ 132 Acknowledgement........................................................................................................... 134 References ....................................................................................................................... 135 Appendix ......................................................................................................................... 144
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List of abbreviations and symbols a) Abbreviations AC Alternating current
AISI American Iron and Steel Institute
CE Counter electrode
CERT Constant extension rate tensile
CL Constant load(ing)
DOS Degree of sensitisation
EBSD Electron backscatter diffraction
ECN Electrochemical current noise
EMPA Eidgenössische Materialprüfanstalt
EN Electrochemical noise
EPN Electrochemical potential noise
FFT Fast Fourier transformation
IG Intergranular
OCP Open circuit potential
PSD Power spectral density
PSD I Current power spectral density
PSI Paul Scherrer Institut
RE Reference electrode
SCC Stress corrosion cracking
SCE Saturated calomel electrode
SEM Scanning electron microscope/microscopy
TG Transgranular
WE Working electrode
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b) Symbols a Surface crack length
Acrack Crack wall area
b Micro crack depth
c Width of dissolved metal along a micro crack
F Faraday constant: 96485 J·K-1·mol-1
M Molar mass
I Current
i Current density
ia Activation current density
ir Reactivation current density
Rn Noise resistance
U Potential
Zn Noise impedance
ρ Density
σ Strain
σy Yield stress
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1 Introduction 1.1 Background and motivation Stress corrosion cracking (SCC) is a phenomenon, which is observed on susceptible metals and
alloys, when subjected to stress in the presence of an aggressive medium. The combination of
stress and corrosion causes quasi-brittle fractures in normally ductile alloys, and cracking well
below yield stress σy. One of the first documented examples of SCC was the so called “season
cracking”, observed by British troops in India in the 19th century: brass cartridge cases that were
stored for extended times in horse stables during the monsoon months suffered cracking,
especially where the cartridges were crimped to the bullets. The combination of the ammonia
rich, humid atmosphere and significant residual stress in the cartridge cases could be identified as
the cause of this phenomenon in 1921 [1].
Today, SCC is still a common phenomenon and a dangerous material degradation process in
many fields of engineering. The use of better materials and the avoidance of potentially harmful
operating conditions reduced the number of SCC failures, but they could not be totally avoided so
far. Stress corrosion field failures are a major problem in the petroleum, chemical, and nuclear
sectors [2-10], where austenitic steels and nickel-based alloys find widespread application as
construction materials, often under aggressive environmental conditions. While resistant against
uniform corrosion, these materials can suffer SCC under certain specific conditions. In the
nuclear industry, intergranular (IG) SCC has been reported to occur under boiling water and
pressurised water reactor conditions [6, 9, 10], affecting the integrity of various reactor
components, e.g. pressure vessel penetrations, coolant piping or heat exchanger tubing. SCC in
these components often results in significant economic losses, and in few cases can also affect
system integrity.
The early detection of SCC could significantly reduce potential corrosion damage and would
increase plant operation safety. Therefore, there is a high demand for a reliable, continuous and
non-destructive monitoring system for the early detection of localised corrosion phenomena in
industrial applications. A measuring technique that might fulfil these requirements is the
electrochemical noise (EN) technique. EN is defined as fluctuations of potential (electrochemical
potential noise, EPN) or current (electrochemical current noise, ECN), typically in a low
frequency region (< 10 Hz), which are produced from a corroding electrode on its own, without
need of external current or potential sources. The EN technique is a promising tool for
standardised testing of the corrosion resistance of metals and protective coatings, or for
monitoring and diagnostic applications [11]. The fundamentals of corrosion monitoring are found
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in [12] and [13]. Extended field studies were done at Hanford Site [14]: EN measurements were
performed inside storage tanks for high-level nuclear waste. The aim of these measurements was
to detect SCC and control the rate of uniform corrosion in the highly aggressive nitrate/hydroxide
system. Various difficulties arose during these tests [15, 16], illustrating the possible challenges
that might occur in a monitoring application: Probe design had a significant influence on the
outcome of a noise measurement. Only certain sample designs that were tested, exhibited SCC,
while others only showed uniform corrosion. Furthermore, certain samples showed SCC, but
crack propagation could not be detected during the EN measurement. This was attributed to the
cathodic reactions taking place mostly on the working electrode, due to an unfavourable design. It
is concluded that a series of problems can occur during the implementation of the EN method in a
real setup, but in the experience of the authors, none of these problems are insurmountable, if
enough time and money is invested in development. Further examples of the application of EN
probes in industrial environments are presented in [17-21]. The use of EN as a diagnosis tool to
identify critical chemical conditions is described in [22].
The basic understanding of EN signals is still limited, and the identification of specific corrosion
processes by analysing these signals is challenging. While a differentiation between uniform
corrosion, localised corrosion and passive states has been possible in certain cases, particular
localised corrosion processes, such as pitting and SCC, are still difficult or even impossible to
discern.
1.2 Objectives and experimental approach The main objectives of this thesis are to improve the understanding of EN signals produced
during IG SCC of stainless steel, and to gain new insight in the underlying mechanisms of crack
initiation and propagation. Characteristic EN patterns should be found, which allow the
reproducible and unambiguous identification of the early stages of IG SCC. A novel twofold
approach to study the initiation and propagation of IG SCC should be applied. In a “top-down”
approach, EN measuring technique should be used to study IG SCC on macroscopic samples. A
“bottom-up” approach aimed at the study of single EN sources with the electrochemical microcell
technique. Combination and analysis of the two approaches is expected to lead to new insights
into the phenomenon of IG SCC itself, especially initiation and early propagation of cracks, as
well as a better understanding of the resulting EN signals.
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Top-down approach
On the macroscopic scale, classic EN measurements were performed on dynamically and
statically stressed tensile specimens. In this “top-down” approach, the EN signals of a multitude
of active sites on the working electrode were measured simultaneously. Both current and
potential EN signals were recorded and the corrosion behaviour of the sample could be
monitored. Macroscopic EN measurements are well established for some corroding systems, but
the production of meaningful data is far from being trivial. The macro scale experiments in this
thesis were based on preliminary work performed at the Paul Scherrer Institute (PSI). In the
course of this thesis, the existing test setup for EN measurements at room temperature was
utilised and optimised.
EN data was analysed in time and frequency domain, to derive characteristic signal parameters
indicating SCC (initiation).
Bottom-up approach
On the micro scale, single cracking events were investigated using the electrochemical
microcapillary technique. This part of the thesis was done at the Swiss Federal Institute of
Materials Science and Technology (EMPA) in Dübendorf. The idea behind this “bottom-up”
approach is to decrease the surface area exposed to the electrolyte and to measure the current and
potential signals of single active corrosion sites on a surface. Due to the small area exposed to the
electrolyte (1 - 1000 µm2), it is often possible to directly correlate surface damage to a measured
current signal. Signals of single active sites might then be extrapolated to model a macroscopic
signal. The electrochemical microcapillary technique has been used before to study the influence
of stress on pitting corrosion [23-25]. However, it has not been used so far to monitor the
initiation and early propagation of single micro cracks. This was a new and untested application
of the technique, which could open up new applications for the electrochemical microcapillary
technique.
Experimental system
The basis for this study is a well-understood system. American Iron and Steel Institute (AISI) 304
austenitic stainless steel was chosen, due to its widespread use in many industrial applications,
and its known susceptibility to SCC in chloride or sulphur containing environment. Additional
thermal treatment of the solution annealed base material further increased its tendency to suffer
from IG SCC.
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Although known for a long time, a variety of effects on IG SCC of austenitic stainless steels are
still subject of research: The influence of applied [26] and local stress values (including the effect
of crack coalescence) [27], surface preparation [28], inhibiting and accelerating effects of
different metal cations [26, 29, 30] and anions [26, 30-32], cold work [33], or thermal
sensitisation [26, 34], to name a few.
To produce meaningful and comparable results with both approaches, experiments on both scales
had to be performed under reasonably comparable experimental conditions. This limits the choice
of parameters. The microcell setup is not suitable for measurements under pressure or at
temperatures above the boiling point of the electrolyte. All experiments were therefore performed
under atmospheric pressure and at room temperature. Furthermore, experiments were mainly
performed under constant loading (CL) conditions. Initially, an aqueous solution of sodium
thiosulphate was considered as a suitable electrolyte, which was successfully applied in
preliminary experiments [35]. After a series of tests on both setups, this electrolyte was
dismissed, and other electrolytes had to be considered. An acidic solution of potassium
tetrathionate was finally found to be suitable for both approaches.
1.3 Relevance of the work The combination of electrochemical methods on the macro and micro scale to characterise IG
SCC, presented in this study, is new. The monitoring of single micro crack initiation and
propagation by electrochemical measurements has not been reported yet. The results should help
to clarify if characteristic signals exist, which clearly indicate and identify IG SCC. The ability to
detect smallest current and potential fluctuations also allows an investigation of the very early
signs of single cracking events. Information gained by these experiments might clarify some open
questions concerning the underlying mechanism of IG SCC (initiation) for the investigated
system.
This thesis is part of the KORA I and II1 projects of the Laboratory for Nuclear Materials at the
PSI. The results gained in this fundamental study will complement experiments on IG SCC in
high-purity water at elevated pressure and temperature. Aim of these studies is the application of
the EN technique as a monitoring tool under boiling water reactor conditions.
1 The KORA research project is considered with environmentally-assisted cracking in light water reactor structural materials. It is split up into three sub-projects. In KORA I, corrosion fatigue of austenitic stainless steels is studied. KORA II is focussed on the optimisation of the EN technique for monitoring applications. Sub-project III is focussed on SCC in weld seams between Nickel-base alloys and low-alloy steels.
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1.4 Outline of the thesis Following this introduction, a background chapter will provide some basic information on the
topics covered in this thesis. An introduction into SCC is given, and an overview of the applied
measuring techniques is presented.
The main body of the thesis is split into three parts. First, the applied experimental conditions and
methods are presented and the process of establishing a suitable experimental system for further
experiments on both macro and micro scale is described.
The following chapters present the results of systematic electrochemical microcell measurements
and macroscopic EN measurements. EN signals are analysed and correlated with corrosion
processes. In a final chapter, the results of micro and macro scale measurements are compared,
and general conclusions and suggestions for possible future work are made.
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2 Theoretical background
2.1 Stress corrosion cracking 2.1.1 Definition and phenomenology
The cracking of metals below their elastic limits in an aggressive environment is known as
environment assisted cracking. According to [36], this term includes the phenomena of SCC,
corrosion fatigue and hydrogen embrittlement.
SCC can be defined as the formation (often at stress levels below σy) and sub-critical growth (at
stress intensity factors well below the fracture toughness in air) of cracks in a material under the
simultaneous and synergistic interaction of approximately constant mechanical tensile stress and
a corrosive environment. It is considered to be the most important, as well as the most dangerous
type of corrosion, and causes major economical losses. It is a synergistic process of mechanical
and chemical factors, which causes damages that would not occur under load alone in absence of
an aggressive electrolyte and susceptible material. The stress can be externally applied shear or
tensile stress, or residual stress in the material, induced by mechanical (cold work) or thermal
treatment. SCC often affects metals and alloys that are protected against uniform corrosion by a
passive layer. In fact, SCC occurs in certain well known combinations of materials and chemical
environments. As illustrated by the presented example of SCC on brass cartridges exposed to an
ammonia rich atmosphere, SCC occurs under very specific conditions, when a susceptible
material is subjected to (internal or external) stress and is exposed to specific environmental
conditions (chemical species, temperature, humidity). To sustain crack growth, corrosion must
proceed at the crack tip, while the crack walls remain passive. Important material environment
combinations causing SCC are shown in Table 1. Depending on material and environments, IG or
transgranular (TG) crack propagation is observed.
Corrosion fatigue describes a reduction of fatigue initiation life and an accelerated fatigue crack
growth in a material under cyclic or fluctuating mechanical stress in a corrosive environment.
Contrary to SCC, corrosion fatigue occurs over a broad range of environmental, material and
loading conditions. This phenomenon is not considered further in this study.
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Table 1: SCC susceptible material-environment combinations (taken from [37]).2
Material Environment Concentration3 Temperature3 Mode4 Carbon steel Hydroxides
Nitrates Carbonate/bicarbonate
Liquid ammonia CO/CO2/H2O Aerated water
High Moderate
Low
High Moderate Moderate
Low Low
Very high
IG IG IG TG TG TG
Low-alloy steel (e.g. Cr-Mo, Cr-Mo-V)
Water Moderate TG
High strength steels Water Chloride Sulphide
Low Low Low
Mixed Mixed Mixed
Austenitic stainless steels (incl. sensitised)
Chloride Hydroxide
High High
High very High
TG Mixed
Sensitised austenitic stainless steels
Aerated water Thiosulphate or polythionate
Low Low
Very high Low
IG IG
Duplex stainless steels Chloride High Very high TG Martensitic stainless
steels Chloride + H2S high
Chloride ( usually + H2S high) High
Moderate Moderate
Low TG TG
High-strength aluminium alloys
Water vapour Chlorides
Low Low Low
TG IG
Titanium alloys Chlorides Methanol N2O4 high
High Low Low Low
TG TG TG
Copper alloys (excluding Cu-Ni)
Ammoniacal solutions and other nitrogenous compounds
Nitrate Cupric sulphate
Low
Moderate Moderate
Low
Low Low
IG
TG TG
Nickel-based alloys Water Caustic
Pb2+
Chloride + H2S
High
High
Very high Very high Very high Very high
IG IG IG IG
Hydrogen embrittlement describes quasi-brittle fracture of higher strength materials, which can
occur with relatively low hydrogen concentrations [38]. Atomic hydrogen in metals has a small
solubility and large mobility, a high molar partial volume, and a large electronic interaction,
strongly influencing the surrounding metal atoms. Hydrogen tends to segregate at trap centres
such as impurities, grain boundaries or regions of high hydrostatic stress, and it reacts in a variety
of (sometimes contradictory) ways with metals and alloys. The most important effect of dissolved
2 The table presents the systems for which SCC problems are well established and of practical importance. The absence of a metal-environment combination does not mean that SCC has not been observed. 3 There are rarely well-defined temperature or concentration limits for SCC, and the ratings given here are indicative only. As an approximate guide the terms used equate to the following ranges of values. Low: up to 10-2 M, ambient temperature; moderate: up to 1 M, below 100 °C; high: ~1 M, ~100 °C; very high: near saturation, above 100 °C. Significantly increased local concentrations may be obtained under the influence of local boiling or evaporation, or by accumulation in pits and crevices, and cracking is often obtained for nominal concentrations that are much lower than is indicated here. 4 The fracture mode is classified as IG, where cracks go along the grain boundaries, TG where cracks go across the grains, or mixed where there is a combination of the two modes, or where the mode can vary depending on the conditions. There are often circumstances that can cause the fracture mode to change (e.g. chloride SCC of sensitised austenitic stainless steel may give IG cracking).
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hydrogen on the mechanical properties of steels is the production of brittle fractures where
normally ductile behaviour would be expected [38]. Different theories explain this effect, and not
all predict an embrittlement: The hydrogen-enhanced localised plasticity mechanism explains
crack propagation by highly localised slip due to local softening by hydrogen at the crack tip. The
hydrogen-enhanced decohesion model explains hydrogen embrittlement by a weakening of
interatomic bonds in the metal, allowing brittle fracture. Other possible effects of hydrogen are
the formation of brittle hydrides (e.g. in titanium alloys), the reaction of hydrogen with certain
alloy components (e.g. reaction with iron carbide in steels and formation of methane), or the
recombination of hydrogen in voids and other micro structural defects, resulting in very high
overpressures, which can assist cracking. Hydrogen embrittlement can accompany SCC at the
crack tip, and the discerning of both phenomena is not always possible. In these cases, hydrogen
embrittlement can be considered as one possible mechanism of SCC.
2.1.2 Mechanism of IG SCC of sensitised stainless steel
Several mechanistic models have been developed. No single model can explain all observed
phenomena, and it seems likely that several different mechanisms account for SCC in different
systems. This section introduces mechanistic aspects that are currently discussed for IG SCC of
thermally sensitised austenitic stainless steel. As indicated in Table 1, these materials are
susceptible towards IG SCC at room temperature, when exposed to certain sulphur species. The
susceptibility towards IG attack depends on the properties of a grain boundary, i.e. grain
boundary chemistry, misorientation, and precipitates.
The effect of grain boundary geometry on IG crack growth is e.g. presented in [39], for different
austenitic materials. It is shown that twin boundaries are crack-resistant, and the intersection of a
twin with a random boundary may act as a barrier to crack advance. No statistically valid
conclusion could be made, if any other grain boundary geometry had a beneficial effect on the
resistance towards cracking, and it is emphasised that chemical characteristics must be considered
as well.
Grain boundary chemistry of austenitic stainless steels highly depends on thermal treatment: At
temperatures between 500 – 700 °C the formation of chromium-rich carbides (Cr23C6) along the
grain boundaries is observed [34, 40-42]. This “sensitisation” leads to a chromium depletion of a
narrow seam along the grain boundaries, and renders the grain boundaries vulnerable towards
corrosion attack if the chromium content is below a critical value (~12% Cr). Extended
experiments and theoretical considerations on the chromium depletion theory are described in
19
[42], and it is concluded that this theory is capable of interpreting all effects related to IG
corrosion. Other theories for IG corrosion are rebutted.
A model to predict the extent of grain boundary depletion is described in [41]. Diffusion
processes of chromium at the grain boundary towards growing carbides and the evolution over
time of chromium concentration are analysed. A distinction is made between chromium
concentrations at the interface between bulk metal and carbides, and the concentration at points
between adjacent grain boundary carbides. These concentrations can be different, and it is
concluded that both have to be lower than a critical value for sensitisation to occur.
Crack initiation
SCC is usually split up into an initiation and a propagation stage. This differentiation is not well
defined [43]. For a technical/engineering definition, “crack initiation” can be based on detection
limits, or on a minimum crack length related to the spacing of some metallurgical features, e.g.
grain size. Andresen proposed a more scientific definition [44]: He defines crack initiation as the
“formation of a mechanical distinct geometry that will tend to grow in preference to its
surroundings”. An interpretation of SCC initiation as a sequence of different phases, as proposed
by Staehle [45], is shown in Figure 1.
Figure 1: Possible definition of SCC initiation phases according to Staehle [45].
A number of typical initiation sites are documented, which can lead to the formation of such a
distinct geometry: The origin of a single crack is usually some kind of weakness or irregularity on
a sample surface. Slip steps, pits, machining defects or microstructural heterogeneities, e.g.
(sulphide) precipitates, which are intersected by the surface, are possible initiation sites. If a
material is susceptible to IG corrosion, SCC can be initiated at attacked grain boundaries. Crack
20
initiation is therefore strongly dependant on the surface state. Surface roughness and structure, as
well as residual stress or cold work, which is introduced during fabrication or grinding can cause
crack initiation as well. A mechanically induced rupture of a passive film can also serve as an
initiation point.
Crack initiation at pits can occur, when a high enough stress concentration is reached at the
bottom of the pit. Trethewey mentioned in [46] that stress concentration at the pit base is more
concerned with the shape of the pit than its depth: If dissolution at the pit walls continues at the
same speed as at the pit base, the pit will grow and maintain a hemispherical shape, which will
not increase the stress at the base much. For crack initiation to occur, the corrosion at the pit
bottom has to accelerate, while pit walls repassivate.
In general, SCC initiation mechanisms are more difficult to investigate than crack propagation:
Crack initiation often exhibits long initiation times and strong variation between samples. A high
number of time consuming experiments are necessary to produce statistically valid results. Crack
propagation on the other hand can be studied on pre-cracked samples, hence bypassing the
initiation time, and the variation of crack growth rates typically shows less scatter than crack
initiation.
Crack propagation
Two main mechanisms are considered for crack growth: cracks either proceed by preferential
anodic dissolution at the crack tip, or by a sequence of brittle fracture events. Both mechanisms
are still discussed for the case of IG SCC of austenitic stainless steels. Different variations of the
film rupture or slip-dissolution model [47] exist, which explain crack growth by anodic metal
dissolution. Cracking by brittle fracture is mostly explained by hydrogen embrittlement [48].
Both mechanisms will be discussed further below, regarding IG SCC of sensitised stainless
steels. Other mechanisms, such as the surface mobility model [49] or the film induced cleavage
model [50] have been developed, but they find use for systems other than sensitised stainless
steel: The film induced cleavage model is typically applied to noble alloy systems, such as Au-Ag
or Au-Cu, where SCC proceeds through brittle fracture of de-alloyed surfaces. The surface
mobility model explains SCC as a surface diffusion process where atoms diffuse away from the
crack tip. It was applied to a wide range of systems. It is stated in [51], that the surface mobility
model is inadequate for the description of systems with pre-existing crack paths, such as
sensitised austenitic stainless steels. SCC in such cases is often better explained by an anodic
dissolution mechanism.
21
Crack propagation by film rupture
The film rupture model explains crack growth as a stepwise process: the local rupture of a passive
surface film is followed by anodic dissolution of the freshly exposed metal at the crack tip, and
finally repassivation of the active crack tip occurs. Metal dissolution proceeds along the
sensitised grain boundaries, while the crack walls remain passive, due to their higher chromium
concentration. The film rupture model is often used to explain results of slow strain rate tensile
(SSRT) tests. Crack propagation rates can then be modelled as a function of strain rate and the
fracture strain of the passive oxide layer [52]. The film rupture mechanism requires dynamic
plasticity at the crack tip. For crack propagation under CL conditions, this can be due to low-
temperature creep. Another explanation is that dynamic strain is caused by crack growth itself
[53]: A plastic strain distribution exists ahead of a crack tips, with stress inversely proportional to
the distance from the crack tip. Each crack growth step shifts this stress distribution, causing a
strain transient in front of the crack.
A special case of the film rupture mechanism is the slip-dissolution model, incorporating the
effect of slip planes: The periodic breaking of the passive film is explained by local plastic strains
at the crack tip, which lead to the formation of slip steps (Figure 2). The shear stress in such a
case depends on the orientation of the slip plane and the applied stress.
Figure 2: The slip-dissolution mechanism explains crack propagation by film rupture due to material dissolution
along slip planes [36].
As stated by Gutman in [54], the term “slip-dissolution model” is often incorrectly used as a
synonym for “film rupture mechanism”. The basic film rupture mechanism does not require the
22
presence of a slip plane, and brittle passive films can already rupture at loads below σy of the base
material. Gutman further critically discusses the film rupture model for SCC and points out
inconsistencies in certain models for crack growth speed and the rates of initiation and
repassivation events. Nevertheless, the film rupture model is widely accepted and is used in many
studies on austenitic stainless steels:
Roychowdhury et al. studied IG SCC of AISI 304 steel in thiosulphate solutions [55] and
explained crack growth by a film rupture mechanism. They stated that a balance between film
rupture events and film formation could be established by changing the thiosulphate
concentration and the application of potentials. IG and TG SCC of stainless steel under boiling
water reactor conditions was attributed to a film rupture mechanism by Arganis-Juarez et al. in
[56] (Figure 3). A film rupture mechanism is also proposed to explain the EN signals of IG and
TG SCC of stainless steel in thiocyanate solution in [57] (Figure 4): The sudden exposure of bare
metal surface leads to immediate metal dissolution, which is indicated by a sudden rise in current,
accompanied by a potential drop of the corroding metal. With proceeding repassivation the
current signal drops and the potential rises back to the initial value. This results in characteristic
current and potential transients. Further details on the EN signals related to SCC are described in
Section 2.3.4.
Figure 3: Stress-time curve and EN time series for solution annealed AISI 304 stainless steel at 288 °C, 8 MPa, air
saturated water (approximately 4000 ppb oxygen and +200 mV/standard hydrogen electrode, outlet conductivity
0.59 μS/cm. Transients were explained by slip-dissolution, due to their shape and polarity [56].
23
Figure 4: EPN, ECN and displacement rate of a tensile specimen during a slow-rate-load-tensile test in aqueous
thiocyanate solution, measured by Leban et al. [57]. Transients are explained by a slip-dissolution mechanism.
Crack propagation by hydrogen embrittlement
As explained in 2.1.1, hydrogen embrittlement can be a phenomenon on its own, or it can act as a
mechanism of SCC. It explains crack propagation as a series of brittle fractures, induced by
absorbed hydrogen in front of the crack tip. The cathodic corrosion half reaction provides the
hydrogen by reduction of protons. The locally produced hydrogen then diffuses into the metal. It
accumulates in the plastic zone in front of the crack tip, in the region of highest hydrostatic stress.
The absorbed hydrogen induces slip steps, finally leading to quasi-brittle fracture events. This
mechanism explains highly discontinuous crack propagation. Gomez-Duran and Macdonald
postulate a hydrogen embrittlement mechanism for SCC of AISI 304 stainless steel in aqueous
thiosulphate solution [58, 59]. They state that the discrete crack propagation events they found
during square wave loading experiments are of the size of grain boundaries (35 – 108 µm), and
are too large to be explained by slip-dissolution events. Therefore, the detected crack growth
steps are better explained by hydrogen embrittlement than by a slip-dissolution mechanism.
24
Effect of aggressive sulphur compounds
Most experimental work presented in this thesis was done using acidic or neutral solutions of
thiosulphate or tetrathionate. Cragnolino und Macdonald review the effects of various sulphur
species on austenitic stainless steel in [60]. A whole class of sulphur species exist, which can
induce IG SCC in sensitised steels, e.g. hydrogen sulphide, sulphur dioxide, thiosulphate,
polythionates or thiocyanate. These substances can form complex systems, as many of them are
not stable in aqueous solution and can react with each other. A variety of disproportionation
reactions are described for tetrathionate and thiosulphate:
−⎯⎯←
⎯→⎯−+ ++ 3
232 HSOSOSH (1) [60, 61]
OHOSSOSH 22
642
32 32256 +++ −⎯⎯←⎯→⎯
−+ (2) [61]
OHSOSOSH 22
42
32 2432 +++ −⎯⎯←⎯→⎯
−+ (3) [61]
−⎯⎯←⎯→⎯
−− + 232
264 22 OSeOS (4) [62]
32322 SOSHOSH +⎯⎯←⎯→⎯ (5) [62]
SSOOHSOSH +++ ⎯⎯←⎯→⎯ 2232 (6) [62]
+−⎯⎯←⎯→⎯
− +++ HSOOSOOH 222 24
23222 (7) [62]
It is still under debate, how these sulphur species accelerate SCC. It is proposed, that the
metastable sulphur oxy-anions disproportionate in the acidic pH at the crack tip, and atomic
sulphur is released (reactions 1 – 3, 6). Sulphur then adsorbs on the metal, where it can have
multiple effects [63]: it can catalyse the anodic dissolution of the metal, and it blocks the surface
sites required for the adsorption of oxygen and thereby inhibits repassivation. Passive films
formed in the presence of adsorbed sulphur additionally exhibit more defects than those formed
under sulphur-free conditions. High concentrations of sulphur species may also directly cause the
breakdown of a passive film, when the formation of metal sulphides is thermodynamically more
stable than the oxide. Considering hydrogen embrittlement, it is known that sulphide species,
which are adsorbed to the metal surface, can accelerate the absorption of hydrogen [2, 64].
Furthermore, the inhibition of repassivation can further facilitate hydrogen uptake. The observed
acceleration of IG SCC by sulphur compounds therefore does not dismiss or favour one of the
two mechanisms.
25
2.1.3 Loading methods
The type of applied stress and the specimen shape highly influence the outcome of any SCC test.
An overview of different specimen designs and testing procedures is given in [65]. The applied
stress can either be static or dynamic, where “dynamic” usually means the application of slow
strain rates.
Static loading
Historically, most SCC testing was done under static loading. Static loads can be applied easily
by bending a suitable specimen, either applying a constant displacement or CL to a sample.
Constant displacement tests can suffer from relaxation and exhibit a decrease in stress with
propagating cracks. In CL tests, on the other hand, stress increases with growing cracks, due to a
reduction of the sample cross section. Typical specimens are bent beams, U-bend specimens or
C-rings [65]. Depending on the cell design, the stress values may vary significantly over the
surface of a bent sample, and for all bent specimens the stress varies through the sample profile.
Statically loaded specimens can be tested in the elastic or plastic region: The advantage of elastic
straining is that the magnitude of the applied stress can be calculated from the measured strain
and modulus of elasticity. Plastic strain tests are generally faster due to the more aggressive
loading conditions, but the stress conditions are not known precisely.
For experiments dealing with the monitoring of SCC initiation and propagation, systems under
static loading can be difficult to use, as time to crack initiation may be extremely long, and vary
between samples. Kovac et al. describe the monitoring of IG SCC of austenitic stainless steels
with a combination of EN, acoustic emission, digital imaging and specimen elongation under CL
in [66]. In these experiments, it was necessary to increase the load several times during a
measurement, which caused transients in the EN signals (marked regions at 15 h and 25 h in the
blue and red curves in Figure 5). Crack initiation occurred directly after such a loading step. The
following crack growth could then be monitored by all applied techniques, although increased
acoustic emission activity was detected only after a crack reached a certain length (Figure 5,
purple curve).
26
Figure 5: Monitoring of IG SCC under CL, using elongation of specimen, EN and acoustic emission. EN transients
occurred, when the load was increased stepwise at t = 15 and 25 h. Crack growth set in after the second increase,
clearly detectable with all applied measuring methods [66].
Dynamic loading
Dynamic loading is generally a more severe testing method than static loading, and it can produce
SCC in a reasonable time frame in systems where static loading cannot. This allows the use of
less aggressive environments. Constant extension rate tensile (CERT) tests are commonly
performed in the study of SCC on the laboratory scale. In these tests, slow strain rates are applied.
A too high strain rate may lead to ductile fracture without any contribution of SCC. Typical rates
range from 10-7 to 10-5 s-1. A drawback of CERT tests is that the application of slow strain rates
needs more sophisticated testing equipment. These setups allow the application of uniaxial tensile
stress, which leads to a uniform stress over the cross section of a sample.
2.2 The electrochemical microcapillary technique 2.2.1 Overview of microelectrochemical measuring techniques
Corrosion processes are mainly studied on a macroscopic scale. Typical laboratory tests are
performed on samples in the region of mm2 – cm2. But corrosion can be a highly localised
phenomenon. The electrochemical behaviour of two surface spots at a distance of a few µm can
be totally different, depending on the microstructure of the metal. Furthermore, the
miniaturisation of electronic and mechanical devices creates microstructures, which cannot be
27
studied with macroscopic methods. To perform electrochemical measurements on this scale, the
application of microelectrochemical methods becomes necessary. One group of micro scale
methods are scanning techniques [67]: Macroscopic surfaces are immersed in an electrolyte, and
micro- or ultramicroelectrodes are used to scan the surface and measure inhomogeneities in
potential or current density. A range of methods, based on this concept, have been developed, e.g.
scanning vibrating electrode technique [68], or the local electrochemical impedance spectroscopy
[69]. The lateral resolution of these techniques depends on the conductivity of the electrolyte. The
main drawback of these methods is that they do not allow the selective polarisation of a small
surface region.
A combination of a scanning technique and a small area technique (discussed below) is the free
droplet method [70]: An electrolyte droplet, hanging freely on the tip of a capillary is brought
into contact with the sample surface. This technique can be applied on non-wetting surfaces, e.g.
most electropolished metal surfaces. Capillary diameters typically range from a few µm to some
100 µm. The dimension of the wetted area is in the same order of magnitude. The electrolyte
droplet can be moved over the surface during a measurement, therefore surface scanning is
possible. The technique is suited for very sensitive surfaces, as no mechanical stress is applied by
the capillary. The scanning droplet cell was used to study the different passivation potentials of
various polycrystalline materials, depending on grain orientation (Zn: [71]. Zr, Ta: [72]. Au:
[73]). A drawback of this technique is the constant evaporation of electrolyte at the droplet and
resulting concentration build up.
The second group of microelectrochemical methods are the small area techniques: In these
methods the surface area exposed to the electrolyte is reduced to the region of µm2 or even nm2 in
the case of nanoelectrochemical techniques. This allows the polarisation of a limited surface area
and the investigation of local corrosion behaviour. The localisation of corrosion processes on a
small surface area allows a direct correlation of measured corrosion currents and the corrosion
attack observed after an experiment. Different small area techniques are applied: A common
method is to reduce the size of the working electrodes, e.g. by using embedded metal wires as
electrodes [74-76], or by masking techniques [77-80]. Early techniques used micro hardness
imprints to damage isolating surface films and thereby create micro spots of exposed metals [77].
More sophisticated coating techniques have been developed, and advanced electrochemical cell
designs allow the combination of electrochemical and optical monitoring of micro spots, as
demonstrated by Chiba et al. [81].
The electrochemical microcapillary technique, developed at ETH Zürich and further improved at
EMPA Dübendorf [82-84], uses silicone coated glass microcapillaries as electrochemical cells.
28
The capillaries containing the electrolyte are positioned on a working electrode, exposing only an
area of the size of the capillary opening to the electrolyte. Reference and counter electrode are
connected to the wide end of the capillary. This cell design allows a reduction of the exposed
surface to a few µm2. Installation of the microcell in a modified socket of a light microscope
allows precise positioning on the surface. It is possible to precisely perform measurements on
single surface features (e.g. grain boundaries, inclusions, precipitates) and study the effects of
these surface features on the local anodic behaviour of the bulk metal. When a three electrode
setup is used, all common electrochemical measurements can be performed, e.g. potentiodynamic
and potentiostatic current measurements, open circuit potential (OCP) measurements or cyclic
voltammetry.
The capillary tips are coated with a soft silicone rubber to create a flexible seal, which prevents
leaking even on rough surfaces. (see Figure 6) The silicone is hydrophobic in nature, which
reduces the tendency of crevice corrosion to occur. The silicone coatings also protect sample
surfaces from damage by the glass capillary5. Many of the specific features of the microcapillary
technique rely on the properties of the silicone coating at the tip, which makes it a key point for
reproducible and meaningful measurements.
Figure 6: A 100 µm microcapillary with silicone sealant. a) Surface quality of the silicone sealant; b) Deformability
of the silicone sealant [84].
2.2.2 Features of the electrochemical microcapillary
Microcapillary measurements show significantly less background noise than macroscopic
measurements, due to the reduced number of EN sources present on a studied surface spot. The
5 Soft metal surfaces, e.g. aluminium alloys can be damaged, when uncoated glass capillaries are pressed on the surface. This can even happen with coated capillaries, if the coating is thin and if the capillary is pressed on the surface too hard, or if a very sensitive surface is studied.
29
reduction of the surface area, together with the use of a specially developed high resolution low
noise potentiostat, allows the detection of single events in the fA-range. Low pass filtering of the
signals further reduces noise. A Faraday cage and screened cables protects the signal from
interferences.
The microcapillary is a stationary measurement, as opposed to scanning techniques like the
scanning droplet method. However, by performing consecutive measurement in an array pattern
on a surface, a discontinuous step-by-step scanning of a surface is possible [82, 85]. The
incorporation of microsensors into the microcapillary cell allows monitoring of additional
experimental parameters. The possibility of simultaneous potentiodynamic current measurements
and pH measurements was demonstrated in [84].
A main feature of the microcell technique compared to other small area techniques is its
applicability on a wide variety of surfaces. Small area techniques usually either require
complicated sample preparation (surface masking) or they are limited to very specific samples
(e.g. embedded wires). The microcapillary technique uses a different approach and reduces the
size of the electrochemical cell. Sample preparation is the same as for a macroscopic
measurement or even simpler. Measurements can be performed on a variety of polished or rough
surfaces, as long as the specimen fits in the setup. The silicone seal of the capillary serves as the
outer boundary of the investigated working electrode. It is not necessary to apply any covering
lacquer on the sample to prevent crevice corrosion.
2.2.3 Applications of the electrochemical microcapillary technique
Local OCP and pitting potentials
The microcapillary has been used to study the initiation of pitting corrosion at MnS-inclusions in
AISI 304 stainless steel in [86]. Potentiodynamic measurements were performed on single
inclusions. It was shown that pitting behaviour depended on the geometry of each inclusion:
Shallow inclusions showed dissolution, accompanied by metastable current transients. On
narrow, deep MnS-inclusions, the metastable events were followed by stable corrosion. SEM
images of attacked inclusions revealed that corrosion proceeded mainly along the edges of the
inclusions. This selective dissolution led to the formation of micro crevices.
The microelectrochemical investigation of pit initiation on Al2024 is described in [87]. The
pitting potential of various intermetallic particles was identified, and micro cracks in the bulk
material were found to be initiating points for pitting. Further studies on aluminium alloys and
high purity aluminium are described in [88-95], illustrating the potential of the microcapillary
technique to study localised corrosion phenomena and identify the influence of different
intermetallic particles on corrosion behaviour.
30
Influence of stress
The microcapillary technique has also been applied to study the influence of surface stress on the
initiation of pitting [23-25]. Similar to the work described in [86], pitting of 304 stainless steel in
1 M NaCl was studied in [23] as a potential initiation step for SCC. Microcapillary measurements
were performed on stressed and unstressed stainless steel samples to evaluate the influence of
applied stress on the pitting behaviour. A three point bending cell was used to apply constant
deformation to the sample. It was found that certain shallow MnS-inclusions would lead to
metastable pitting on unstressed surfaces, but could lead to stable pitting on stressed surfaces.
With increasing stress, the dissolution rate of MnS-increased and the dissolution potential was
shifted to more negative values.
The influence of residual stress on pitting sensitivity was studied by Oltra and Vignal in [24].
Microelectrochemical measurements were combined with stress mapping on a resulphurised AISI
316L stainless steel. Localised electrochemical measurements allowed a correlation of pit
initiation and stress gradients.
Krawiec et al. investigated the influence of stress on the local electrochemical behaviour of an
AlCu4Mg1 aluminium alloy [25]. After the application of plastic strain, surfaces showed high
densities of micro cracks, especially in precipitates, along matrix/precipitate interfaces and along
the grain boundaries. It was found that pitting potentials of stressed surfaces were systematically
lowered compared to unstressed surfaces. Stressed surfaces appeared to be highly heterogeneous:
The most active surface spots corresponded well with the location of extended micro cracks and
surface damages.
Microelectronics
Local electrochemical surface conditions on micro engineered parts are important quality factors:
The quality of insulating layers or the purity of conducting lines can be studied with the
electrochemical microcapillary technique. Such an application in microelectronics is shown in
[84]: Microcapillary measurements on Au lines showed lower overpotentials on the edges of a
line, compared to the centre, indicating a higher concentration of impurities.
In summary, the electrochemical microcapillary has been successfully used to study differences
in local electrochemical behaviour on several systems. The technique was especially useful for
measurements of OCP and pitting potentials on different intermetallic particles and surface
defects, on both stressed and unstressed samples. It was also possible to study the initiation
31
mechanism of stable pitting corrosion. So far, the technique has not been used to track the
electrochemical signals of single growing stress corrosion cracks.
2.3 The EN technique The first application of the EN technique for monitoring of corrosion was published by Iversion
in 1968 [96]. In an earlier study, he observed the formation of “hollow whisker” on metals
exposed to acidic solutions of potassium hexacyanoferrate (II) and (III), which indicated that
bursts of ions were formed on point anodes on the metal surfaces. As these bursts appeared
sporadically, he concluded that it must be possible to measure the potential or charge fluctuations
created during formation of these whiskers. Using a high impedance voltmeter, connected to a
chart recorder, he measured the potential fluctuations between a platinum foil and various metal
sheets, immersed in different electrolyte solutions. Since this early work, EN measurements have
been widely used to study the corrosion behaviour of a variety of metal/electrolyte combinations.
Better measuring equipment with smaller instrument noise has allowed measuring EN of passive
systems and even under extreme experimental conditions, e.g. high purity water at temperatures
and pressures common for nuclear reactors. New data analysis methods have been developed, and
methods for automated online monitoring are being developed. In this section, different
measuring and data analysis methods of EN measurements are introduced, and examples of the
successful application of the EN technique for corrosion monitoring in different environments are
summarised.
2.3.1 Definition
EN describes electrochemical current and potential fluctuations, which are produced on an
electrode undergoing an electrochemical reaction. EN occurs naturally, and does not need any
external stimulation. The nature of these fluctuations depends on the ongoing anodic and cathodic
processes on the electrode surface. EN measurements record these fluctuations, which the
corroding system constantly produces by itself. EN signals can then be analysed to gain
information on the ongoing corrosion processes.
EN sources in corroding systems are small, stochastic events that cause sudden charge bursts and
change the electrode potential [97]. Typical examples are local oxide film breakage, subsequent
metal dissolution and repassivation during pitting corrosion, or the slow formation and sudden
detachment of a hydrogen bubble from a metal surface.
A general theoretical description of EN has not yet been established. However, there exists an
analysis for the special case of shot noise [97-99]. Shot noise analysis can be applied when a
32
corrosion signal consists of discrete events, such as the current transients of metastable pitting.
Each transient provides a certain charge to the power of the signal. This charge value and the
number of transients over time define the total power of the signal and can be calculated from
EPN and ECN signals [99]. For shot noise to be applicable, it must be assumed that single
corrosion event are uncorrelated, and the system under investigation is stationary. The shot noise
theory therefore does not fully explain typical corroding systems, in which these assumptions
usually are not valid.
Depending on the system, EN signals are usually in the range of µV to mV, and nA to µA. An
example of a single transient, which was detected during an experiment in the course of this
thesis, is shown in Figure 7. The current signal shows a rapid rise of 55 nA, lasting approximately
3 min before the signal drops to the initial level. The potential shows a rapid drop of 150 mV,
until the current stops flowing. The slow potential rise back to its initial value indicates the
recharging of the double layer capacitance. This transient illustrates that the current is the primary
parameter, and the potential just responds to the local corrosion currents [99].
Figure 7: Typical EPN (green) and ECN (red) transient, detected on sensitised stainless steel in a 0.01 M potassium
tetrathionate solution at pH 2.2, at room temperature. The fast current peak indicates a growth step of a micro crack
or formation of a metastable pit.
2.3.2 Measuring methods
ECN and EPN can be measured separately or simultaneously. If only EPN is measured, it can
either be measured between two equivalent electrodes or between one working electrode (WE)
and a low-noise reference electrode (RE). Similarly, ECN is either measured between a working
electrode and counter electrode (CE) under potentiostatic control, or it is measured between two
equivalent electrodes. In this case, both electrodes may corrode, and the source of a single event
33
can be deduced from the transient direction/polarity. When EPN and ECN are measured
simultaneously on a freely corroding system, a three electrode setup is applied. ECN is measured
between working and counter electrode, which are at the same potential. Their potential is
measured against the reference electrode. Possible configurations are shown in Figure 8 [100].
Figure 8: EN measurement principles: a) EPN measurement under galvanostatic conditions; b) ECN measurement
under potentiostatic conditions; c) ECN measurement (I) without external polarisation and possibility to measure
EPN (U) simultaneously [100].
The measurement of both ECN and EPN signals allows the calculation of noise resistance Rn or
noise impedance Zn (see Section 2.3.3). Eden stated in [101], that many problems in simultaneous
ECN and EPN measurement can be solved if a setup with three identical electrodes is used. The
advantages of this setup are that no Haber-Luggin capillary has to be used, which eliminates
possible sources of contamination and the impedances of the electrodes are (nearly) identical.
With identical impedances it is actually possible to calculate Zn from the data. For certain
applications, it is not possible to use two identical electrodes for ECN measurements. This is the
case when corrosion is limited to one electrode, e.g. during typical SCC experiments, where only
one electrode is stressed. Under these conditions, it is not possible to obtain the properties of one
single electrode, because a system with asymmetric electrodes contains too many unknowns [102,
103].
Another important point in cell design is the surface area of the electrodes, because the ECN
power is proportional to the area, while the EPN power is inversely proportional to the area. The
electrode area also determines the number of corrosion sites that can be monitored. The influence
U
I
amplifier
WE 1 WE 2
RE
computer
(c) (a)
U
amplifier
¨
WE 1
WE 2
RE
computer
WE 3
galvanostat
ampl
ifier
I
WE 2
RE
computer
potentiostat
R
(b)
WE 1
34
of the electrode area, as well as other parameters, such as measuring frequency and experiment
duration, are discussed in [104].
A critical factor in EN measurements are external interferences. A typical interference is the
50 Hz power line hum. Low-pass filtering of the measured data is therefore always necessary.
The maximum frequency that can be analysed, called Nyquist frequency, is determined as one
half of the scanning frequency. Any frequencies above this value cannot be detected and might
cause aliasing effects, if not filtered out. Other interferences may be caused by vibrations or
electromagnetic field fluctuation in the proximity of the measuring setup. To avoid the pick up of
external interferences, EN measurements should be performed in a Faraday cage, and cables
between the data processing unit and the electrodes should be properly shielded and kept as short
as possible. Further interferences may arise from the formation of ground loops: Ground loops
are created when elements of an electrical circuit (or electrochemical cell) are grounded through
multiple connections. Potential differences between the different ground connections can then
induce current flow through the circuit. To avoid ground loops, the whole experimental setup
should be connected to ground at a single point.
2.3.3 Data analysis
The main aim of the analysis of EN data is the identification of rate and type of corrosion
processes. While the measurement of EN signals is basically straightforward, the interpretation
and analysis of EN data is a rather complex task. No standards exist, and a multitude of different
methods for EN analysis are available: Visual examination and peak counting, statistical methods
of the time domain data as well as transformation to the frequency domain have all been used and
proved their usefulness for certain systems. It is not yet decided which parameters are suited best
to analyse EN data, and often a method that works well in one study gives only poor results in a
different case. An overview of the different possible methods is given in [99]. Some of these
methods shall be introduced:
Visual analysis: Direct interpretation of transients
The most straightforward method and at the same time a very powerful approach to analyse EN
data is the direct examination of signals in the time domain. EN signals of localised corrosion
phenomena often exhibit characteristic transients. The analysis of transient shapes, amplitudes
and distribution over the time record can help discerning between different corrosion processes.
Transient shapes are intensely investigated to understand the underlying mechanisms of different
corrosion reactions [105, 106]. The visual analysis also directly reveals trends or long term
35
fluctuations, which especially occur in EPN measurements, and it often reveals signal
interferences. The downside of this analysis technique is that it is usually difficult to automate.
Statistical methods
A further possibility to analyse EN data are basic statistical methods. In these calculations, EN
measurements are treated as sequences of unrelated points. Detailed description of statistical
parameters can be found in [99]. Among the most often used parameters are:
Mean: The mean is easily understood as the average signal value. For the current I, the mean is
calculated as follows:
∫==max
0max)(1 t
dttIt
Imean (8)
Standard deviation, variance and root mean square: These parameters are related and basically
give information on the extent of signal fluctuations. For statistical signal analysis in this thesis
(Section 5.4.1) the standard deviation was used. The standard deviation of a current signal is
calculated as follows:
∫ −=max
2
max))((1..
t
o
dtItIt
devstd (9)
By dividing the standard deviation of the EPN by the standard deviation of the current, Rn can be
calculated. Rn can be used to estimate the corrosion rate of a system, similar to the polarisation
resistance.
Skew and kurtosis: These are nondimensional higher-order statistical parameters. The skew is a
measure for the shape of a distribution of values. The kurtosis is a measure of the shape of a
distribution relative to the normal distribution.
A weakness of the statistical methods is the indifference towards baseline trends and changes of
the signal characteristics over time. Leban et al. stated in [107], that the non-stationarity of most
corrosion processes renders analysis with the mentioned statistical tools unreliable. While passive
state, localised corrosion and uniform corrosion could show significant differences in the
estimated parameters, differentiating between different kinds of localised corrosion seemed not
possible. It is further proposed that mathematical techniques to improve a signal stationarity can
lead to incorrect interpretations.
36
Analysis in the frequency domain
EN data is commonly analysed in the frequency domain. The most prominent frequency-based
technique is the estimation of power spectral densities (PSD). PSD plots show the distribution of
the signal power as a function of frequency. The units of the PSD are V2/Hz or A2/Hz. The two
methods commonly used for the estimation of power spectra are the fast Fourier transformation
(FFT) and the maximum entropy method. For the calculation of the FFT, the following equations
are applied [108]:
∫∞
∞−
−= dtetxfX tfiπ2)()( (10)
The corresponding PSD is:
2)(2)( fXT
f Tx =Ψ (11)
The equations for the transformation in the maximum entropy method can be found in [99]. The
frequency range that can be analysed with these methods is limited by two factors. The upper
limit is defined by the Nyquist frequency. The low frequency limit of PSD spectra is determined
by the duration of the measurement. However, in real corroding systems, the non-stationarity of
many corrosion processes limits the usefulness of the frequency domain analysis of very long
measurements. Typical EN power spectra consist of a plateau at low frequency values, followed
by a power drop in proportion to 1/fn. The roll-off slope was expected to be characteristic of the
type of corrosion occurring, but so far, analysis of the roll-off slope has often been found to be an
unreliable parameter [97].
When ECN and EPN are simultaneously recorded, the potential PSD can be divided by the
current PSD for each frequency, to derive a value with the unit of Ω2. The square root of this
value is known as the noise impedance Zn. Just like Rn, it can be used to estimate the corrosion
rate. Zn is related to conventional electrochemical impedance, and it can be used to estimate the
AC impedance spectrum of a system. This application is limited by the low signal power above
1 Hz, which is typically encountered in EN measurements, but noise measurements can provide
very low frequencies, which might offer an advantage over conventional AC impedance
measurement.
2.3.4 Application of the EN technique to monitor SCC
The use of EN measurements to monitor SCC processes on the laboratory scale has been
demonstrated in several cases [56-59, 66, 107, 109-113], many of them using stainless steels as a
37
model system. SSRT tests were predominantly performed to ensure a reproducible initiation of
SCC. Fewer studies were done on experimental systems using CL.
Various electrolytes and electrode materials were used by several groups of scientists to measure
EN signals of passive phases, pitting corrosion and SCC. Only a few examples are given in the
following paragraphs, and some examples have already been presented in earlier sections: EN
measurements under simulated boiling water reactor conditions are presented in [56] (see also
Figure 3). Transients, appearing above the σy, were attributed to slip-dissolution events. A
transition from IG to TG SCC to ductile fracture was monitored. Further studies in boiling and
pressurised water reactor conditions are presented in [112] and [113]. IG SCC of sensitised 304
stainless steel, exposed to thiocyanate solutions was studied in [57] (see also Figure 4). Different
transient shapes were detected during SSRT tests: Fast, sharp transients were interpreted as
fracture events of metal ligaments. Slow current fluctuations were interpreted as crack
propagation along the grain boundaries, according to a slip-dissolution mechanism. Cyclic
loading/unloading experiments on pre-cracked specimens of the same material are described in
[58, 59], and a hydrogen embrittlement mechanism was proposed for IG SCC of austenitic steel
in thiosulphate solutions.
Different corrosion processes are directly compared in [107]. Statistical and spectral analysis
techniques showed significant differences between passive state, localised and uniform corrosion,
but it was not possible to discern the different localised corrosion processes. The reason for this
shortcoming was attributed to the non-stationarity of the system. SSRT tests on sensitised 304
stainless steel in aqueous thiosulphate solutions are described in [35] and [109]. Single current
transients were attributed to the rapid crack propagation along single grain boundaries. Crack
growth is assumed to arrest at grain junctions, where the growth direction changes. Crack tip
dissolution widths and current densities were estimated from transient analysis, calculating the
amount of metal that was dissolved during crack growth. Furthermore, a jumping probability is
introduced in [109], and a model for crack propagation rate based on this jumping probability is
proposed. The differentiation of localised and uniform corrosion processes is also presented in
[110], for Alloy 690 that was subjected to SSRT tests at 90 °C in a thiosulphate electrolyte: EN
analysis showed current peaks with high intensity and low frequency during SCC, while passive
or uniformly corroding samples showed signals of lower intensity, but higher frequency. It is
concluded that EN is a powerful method to detect initiation and propagation of SCC, but it is
difficult to identify a specific mechanism based on the signal shape. Interrupted SSRT tests are
described in [111]: Using 304H stainless steel and a thiosulphate electrolyte, repeated SSRT
cycles were applied and interrupted shortly after reaching σy. Fast transients were interpreted as
38
single rupture events, and a rise in baseline current indicated continuous metal dissolution of
grain boundaries. It was found that crack propagation stopped during phases of static strain,
unless stable macro crack growth had already started.
A comparison of these studies shows that EN signals often exhibit characteristic singular
transients, indicating stepwise cracking events. These transients typically occurred when certain
loading conditions were applied, e.g. right at a stepwise change of the applied load, or during
dynamic straining phases above a certain load level.
Current and potential transients usually lay in the region of nA-µA and mV, respectively.
Frequency and amplitude as well as baseline signal noise varied considerably between the studied
systems. This variation is easily understandable, as different sample materials, surface
preparations and environmental as well as loading conditions were studied. Hence, single
transients are generally not directly comparable between different studies.
Combination of EN with other monitoring techniques
EN signals are often difficult to analyse, and it is normally not possible to unambiguously
correlate a specific EN signal pattern or transient to a certain corrosion process or a particular
stage of the underlying mechanism. An important step towards the understanding of EN signals
and the application of reliable monitoring systems is the combination of EN measurements with
other monitoring techniques:
The combination of EN measurements with in situ atomic force microscopy on corroding
samples of coated stainless steels samples is described in [114]. The growth of metastable pits
could be observed with in situ atomic force microscope scans and compared to characteristic EN
signals. Depending on the coating metastable pit generation occurred with different rates.
However, the two measuring techniques were not simultaneously performed on the same sample.
In recent years, the combination of EN with acoustic emission was applied for the monitoring of
IG SCC. Several studies describe this approach [66, 115-117]. SCC initiation and its early stage
propagation could be detected by EN measurements, whereas the acoustic emission technique
detected rapid crack propagation events, which caused relatively large plastic deformation of a
specimen. In [66], EN and acoustic emission measurements are further combined with
simultaneous elongation measurements (see also Figure 5) and digital image correlation, creating
a comprehensive detection and monitoring system for SCC.
39
2.4 Conclusion The phenomenon of IG SCC of thermally sensitised stainless steel is basically well understood.
Cracks are known to propagate along the sensitised grain boundaries, and crack initiation occurs
on well known weak points on a surface. However, it is still very difficult to predict, where and
when crack initiation will occur. Even for a well studied system as AISI 304 stainless steel in
aqueous thiosulfate solutions at room temperature, numerous studies, by means of EN
measurements or other methods, could not yet definitely determine the mechanism responsible
for crack propagation. Film rupture mechanisms are widely accepted, especially for experiments
where dynamic loads are applied, but the theory is incomplete, and the cause of the film rupture
and repassivation events is still unclear. Some researchers also suggest hydrogen embrittlement
as the dominating mechanism. The present study may not definitely answer all of these open
questions, but the combined use of EN measurements and microcell experiments might clarify
some points for the studied system.
The existing data from microcapillary studies on AISI 304 stainless steels so far mostly covered
the initiation of pitting corrosion. Experiments were predominantly performed in chloride
containing electrolytes. The study of single micro cracking events is a new application of the
technique and, if successful, might trigger further studies on similar systems. However, it has to
be demonstrated first, that the initiation of a crack under the microcapillary is possible and
reproducible.
EN measurements on stressed samples are well established on the laboratory scale, and various
methods for data analysis are available. In general, EN measurements allow differentiating
between passive behaviour, uniform corrosion and localised corrosion. The identification of
specific types of localised corrosion has been reported in several studies, but results seem to be
very specific for the applied experimental conditions. No universally valid criteria for signal
interpretation exist.
The monitoring of IG SCC subjected to SSRT tests in thiosulfate solutions is documented in
several studies. This system is well suited for EN measurements, because IG SCC can be initiated
in a controlled time span and little additional corrosion processes occur. Less data is available for
EN measurements under CL conditions, as crack initiation can exhibit very long incubation
times, making laboratory experiments tedious and unpredictable. In the planned twofold
approach, the need to perform experiments on both macro and micro scale under comparable
conditions, require either CL or constant deformation. This raises the question, if crack initiation
can be observed on sufficient samples, and how the EN signals of crack propagation under CL
conditions will differ from the transients observed under dynamic loading.
40
3 Experimental procedures and development of an
experimental system
3.1 Material All samples were machined from a rod of austenitic stainless steel (Deutsche Industrie Norm DIN
1.4301, UNS S30400). The material chosen for this study was selected to be suitable for the study
of IG SCC initiation: The steel had a high carbon content (0.06 wt.%) to ensure good thermal
sensitisation properties. The selected steel showed a sulphur content of 0.029 wt.% and a high
number of MnS-inclusions. The chemical composition is shown in Table 2. The material was
solution annealed at 1050 °C and water quenched. The material exhibited a mean grain size of
30 - 100 µm and a hardness of 150 HV15. The yield stress σy and the ultimate tensile strength of
the material were 291 MPa and 686 MPa respectively (at 25 °C). For further heat treatment the
material was split up in different batches. One batch was kept as solution annealed reference
material. The other batches were thermally sensitised at 620 °C for 1 to 24 h. The degree of
sensitisation (DOS) for each material was measured by the double loop electrochemical
potentiokinetic reactivation test, according to JIS G 05801986:
Samples of each batch were immersed in an aqueous solution of 0.5 M H2SO4 and 0.01 M KSCN
at a constant temperature of 30 °C. An Ag/AgCl electrode and a platinum coupon were used as
reference and counter electrodes for the following measurement. A potential sweep from -
500 mV/Ag/AgCl to +330 mV/Ag/AgCl with a rate of 100 mV/min was applied (activation),
followed by a second sweep in the opposite direction (reactivation). DOS values were calculated
from the resulting polarisation curves according to equation 12:
100% ⋅=a
r
iiDOS (12)
where ir and ia indicate the maximum current densities during reactivation (sweep in cathodic
direction) and activation (sweep in anodic direction), respectively. The resulting polarisation
curves and DOS values are shown in Figures 9 and 10. The heat treatments and DOS values of all
sample batches are listed in Table 3.
Table 2: Chemical composition of the investigated material (in wt.%).
C Si Mn P S Cr Mo Ni Co Cu Nb Ti Fe AISI 304 0.062 0.15 0.53 0.023 0.029 18.3 0.273 8.59 0.169 0.26 0.019 0.001 Balance
41
Table 3: DOS values and heat treatments of the material batches.
Material designation Heat treatment DOS
C Solution annealed: 1050 °C / 30 min / water quench 0.25 %
F Sol. ann. + sensitised: 620 °C / 1 h / water quench 1.2 %
IRK, IRO Sol. ann. + sensitised: 620 °C / 5 h / water quench 11 %
G, GI Sol. ann. + sensitised: 620 °C / 9 h / water quench 15.2 %
D, HRK, HI Sol. ann. + sensitised: 620 °C / 24 h / water quench 22.4 %
Figure 9: Double loop electrochemical potentiokinetic reactivation tests of all material treatments (0.5 M H2SO4,
0.01 M KSCN, T = 30 °C sweep rate: 100 mV/min.) The sweep direction is indicated by arrows.
Figure 10: DOS values against sensitisation time for all material treatments.
42
3.2 Macroscopic EN measurements All macroscopic tensile tests with EN measurements were performed at PSI, using an
electromechanical tensile machine with an electrolyte recirculation loop.
3.2.1 Sample preparation
Round bar specimens (gauge length = 36 mm, diameter = 6 mm) with a V-shaped or U-shaped
notch in the centre were machined from the different heat treated material batches (Figure 11). A
connecting wire was spot welded on each specimen for connection to the zero resistance ammeter
and voltmeter. Samples were degreased with ethanol in an ultrasonic bath and then dried using
hot air. The clean samples were covered with shrinking tape and/or covering lacquer
approximately 1 mm above and below the notch, to leave only the centre area of approximately
65 mm2 around the notch exposed to the electrolyte. Care had to be taken to avoid crevices at the
protective covering. The protected specimens were then fixed in an electromechanical tensile
machine (CORMET 10kN SSRT Unit). A cell with the counter electrode and the reference
electrode was assembled around the sample. The setup was shielded by a Faraday cage against
electromagnetic interference.
Figure 11: Schematic of the used samples with U- or V-notch. Linear dimensions are given in millimetres. Samples
were protected with shrinking tube and covering lacquer (grey area); only a small area around the notch was exposed
to the electrolyte.
3.2.2 Measuring setup
ECN and EPN were recorded simultaneously using a zero resistance ammeter and high resistance
voltmeter, specifically designed for EN measurements (EcmNoise, IPS Germany) and verified
according to a guideline for EN measuring devices [108]. EN data was recorded with sampling
frequencies of either 2 or 10 Hz. Accordingly, low-pass filtering at 0.8 or 4 Hz was applied to
avoid aliasing effects. Besides the raw data, a high-pass filtered (0.01 Hz) signal was recorded as
well. High-pass filtering removed any slow current and potential trends or “direct current”
43
components from the signal. The remaining “alternate current” (AC) signals only showed fast
signal changes, such as the transients typically detected during localised corrosion processes.
ECN was measured between the notched sample and a C-ring made of the same steel (solution
annealed) as the samples. The polarity of the ECN measurement was chosen in a way, that
positive current indicate anodic processes on the working electrode. In the initial setup a Pt-wire
was used as reference electrode. For later experiments, EPN was record with the help of a second
steel coupon or a saturated calomel electrode (SCE), which was connected to the experimental
cell by a Haber-Luggin capillary and an additional glass frit to inhibit electrolyte contamination.
A schematic of the test setup (with a saturated calomel electrode in place) is shown in Figure 12.
The cell configuration with a Pt-wire as reference electrode is shown in Figure 13.
Figure 12: Schematic of the experimental setup for simultaneous ECN and EPN measurement. The whole cell was
protected by a Faraday cage. The electrolyte recirculation loop is not shown.
Figure 13: Electrode setup of the EN measuring cell, with tensile specimen as working electrode (centre), solution
annealed stainless steel C-ring as counter electrode (connection to the right, surrounding the working electrode) and a
Pt-wire as reference electrode (connected to the left, in between the other two electrodes). The electrolyte outlet can
be seen in the upper right corner.
44
Connecting cables between the cell and the measuring instruments were shielded and kept as
short as possible. To avoid ground loops, the Faraday cage, all cables and the voltmeter and
ammeter were connected to the same mains ground. Some specific interference signals that were
encountered during measurements are presented in Appendix I.
For a measurement, all electrodes were connected to the ammeter and voltmeter. The electrolyte
recirculation loop was connected to the cell and the Faraday cage was closed. The electrolyte
recirculation loop consisted of a 5 L storage container, the electrolyte pump, a cooling loop and
the cell. The cooling loop cooled the electrolyte to 21 °C before entering the sample cell. The cell
volume was 0.2 L. The electrolyte flow rate was adjusted to 0.9 L/h, establishing quasi-stagnant
electrolyte flow around the sample.
Nominal stress values for macroscopic experiments were calculated from the force applied by the
electromechanical tensile machine and the area cross-section of the round notched samples at the
notch root. These values are stated in the experiments presented later. Local stress values varied
greatly from these nominal values, due to the geometry of the notch. Finite element calculation of
stress distribution for nominal loads typically applied during CL experiments in this study (230 –
275 MPa) showed stress values above 600 MPa at the notch root and values below 200 MPa on
the notch walls.
3.3 Microcell measurements All experiments on the micro scale were performed at EMPA Dübendorf. The development of the
measuring and capillary making procedures introduced in this section is described in the literature
[82-84].
3.3.1 Sample preparation
0.9 mm thick coupons of 2.5×1 mm size were cut from the heat treated bulk material. For the
majority of the experiments, the samples were ground with 600 – 4000 grit SiC emery paper and
tap water, until a mirror finish was reached. Surfaces prepared this way showed few scratches,
which did not interfere with the measurements. Samples were then rinsed with ethanol and dried
with hot air. Measurements were performed directly after grinding. The surface of a sample after
preparation is shown in Figure 14. The influence of a change of the surface treatment on sample
behaviour was studied in a limited number of experiments. These measurements are discussed in
section 4.6.1.
45
Figure 14: Specimen after surface preparation with 600 - 4000 grit SiC emery paper. Surfaces prepared this way
showed scratches from the grinding procedure. MnS-inclusions were clearly visible as dark grey particles (shown in
the centre of the picture).
3.3.2 Measuring setup (analogous to [23])
Microcapillaries were manufactured from borosilicate glass capillaries (GB150F-8P, Science
Products GmbH) with a ratio of outer to inner diameter of 1.5 mm/0.84 mm. The glass capillaries
were pulled to a tip with less than 1 µm diameter, using a pulling machine (WPI PUL-1).
Capillary shapes could be influenced by regulating the temperature applied to the middle of the
glass tube. Pulled capillaries were ground to the desired end diameter by hand, using a 4000 grit
SiC emery paper and water as lubricant. When the desired diameter was reached, the capillary
was flushed thoroughly to remove any residual glass particles. The ground tips were coated with
a transparent silicone rubber (DOW CORNING ® 732 multi-purpose sealant) to form a rim
around the capillary edge. To do that, capillaries were repeatedly dipped into fresh droplets of
silicone sealant and flushed with ethanol. This way, the rubber seals could be built up over time.
For most experiments capillary diameters between 50 and 100 µm were used, exposing surface
areas of 2000 - 8000 μm2 to the electrolyte. These sizes were in the region of single grains, so
most measurements hit at least one grain boundary. Smaller capillaries made it increasingly
difficult to find grain boundaries or active surface spots. Larger capillaries often hit more than
one active spot and made signal interpretation difficult. Good capillaries showed very little
dripping and allowed for several measurements on the same sample without accidentally
contaminating the surface with electrolyte.
The microcell setup consists of an acrylic glass cell, which can be fixed in the objective revolver
of a light microscope, and has an opening for a microcapillary at the bottom and for the counter
electrode and the reference electrode at the top. To prepare the setup for a measurement, a
capillary was inserted into the microcell, and the counter and reference electrodes were
connected. A saturated calomel electrode, connected through a Haber-Luggin capillary was used
as reference and a Pt wire was used as counter electrode (see Figure 15). The cell was filled with
46
the electrolyte and was then mounted in the modified socket of the light microscope objective
carousel. A sample was fixed on the microscope stage. By rotating the objective revolver, the
microcell could be switched to an ordinary microscope objective, and the surface could be
searched for suitable measuring locations. Once a desired position was found, the capillary could
be switched back in for the measurement. A high resolution potentiostat/galvanostat (Jaissle
IMP 83 PC T-BC) with a current detection limit of 1 fA was used. A scanning frequency of 9 Hz
was applied. A Faraday cage shielded the experimental setup from external electromagnetic
interference. Measurements were performed under atmospheric pressure and at room temperature
(23 °C). Figure 12 shows the measuring setup used in this study.
Figure 15: Experimental setup of the electrochemical microcapillary and bending cell [23].
The application of stress was limited by the experimental setup, as the whole loading unit had to
fit on the light microscope table. A three point bending cell was used to bend the samples and
create the specified surface stress (also shown in Figure 12). The same setup has already been
used for microcapillary measurements to study the effect of stress on pit initiation on stainless
steels [23]. Deformation was applied by turning a screw underneath the sample coupon. The
nominal stress was calculated from the deformation, assuming only elastic deformation. For some
experiments, nominal stress values of up to 720 MPa are stated, although σy of the material was
much lower (291 MPa). Obviously, significant plastic deformation was induced in these
experiments.
Before each measurement, the capillary was flushed with fresh electrolyte. Using the light
microscope, a suitable position for a measurement was selected, and the capillary was positioned.
The Faraday cage was then closed and the power cables to the potentiostat and the equipment
inside the cage (lighting, microscope) were unplugged for minimising external noise.
47
3.4 Initial experiments with neutral sodium thiosulphate electrolyte 3.4.1 Macro scale EN measurements
The initiation of IG SCC under dynamic loading conditions in thiosulphate solutions has been
proven in a series of studies [58, 59, 109, 111] (see also Section 2.3.4). Thiosulfate
concentrations as low as 6·10-7 M [118] have been reported to induce IG SCC. Early CERT
measurements showed stable current and potential signals during unloaded phases. Increased
activity occurred when high stress levels were reached. An exemplary CERT measurement on a
highly sensitised specimen in 0.1 M Na2S2O3 is shown in Figure 16. Figure 17a and b highlight
the first rapid current and potential transients. A slow current rise and potential drop, which set in
at approximately 250 MPa, indicates increasing activity of the strained surface. Subsequent
cracking events, indicated by fast current transients and simultaneous potential drops, are likely
due to mechanical initiation of film rupture events. The sample was unloaded when 300 MPa
were passed and after a current transient reached 1 µA.
Figure 16: EPN (green) and ECN (red) measurement during a CERT test on a fully sensitised sample (DOS: 22%) in
0.1 M Na2S2O3. Applied strain is indicated in blue. Current and potential transients and a rise in base line current and
potential drop occurred shortly before 300 MPa were reached.
48
Figure 17: a) Detail view of the transient signals shown in Figure 16. Current transients (red) show a fast rise and
exponential decay. The potential (green) shows a less pronounced response b) The high-pass filtered (0.01 Hz) AC
signal clearly shows the two events as potential drops and current oscillations.
3.4.2 Microcell tests
For the first microcell experiments, the same electrolyte was used as for the preliminary
macroscopic experiments. Potentiodynamic current measurements were performed on random
surface spots with 40 µm capillaries in aqueous 0.01 – 0.5 M sodium thiosulphate solution.
Exemplary current curves, measured on highly sensitised material (DOS 22%) at various degrees
of deformation in 0.01 M Na2S2O3, are shown in Figure 18. Current curves indicated passive
behaviour. OCP values varied between -350 and -250 mV/SCE, and anodic current densities
around 10-4 mA/mm2 were measured. Surface spots exhibiting MnS-inclusions showed typical
dissolution peaks around 400 mV/SCE. Comparison of current signals on bulk metal and on grain
boundaries showed no change in behaviour. While neutral Na2S2O3 solutions of concentrations
well below 0.01 M have been successfully used in macroscopic experiments, concentrations up to
49
0.5 M did not show any sign of crack initiation with the microcapillary technique, even at high
applied deformations and anodic polarisation of the sample. As mentioned before, the small
surface areas exposed during microcapillary measurements exhibit a more noble behaviour than a
macroscopic surface, because fewer surface weaknesses are present. More important, the more
stable behaviour of samples in microcapillary experiments is attributed to the applied types of
loading. Crack initiation and propagation in these electrolytes only occurs under dynamic loading
conditions by mechanical film rupture, which could not be done during microcapillary
measurements.
Figure 18: Potentiodynamic current measurements in 10 mM thiosulphate solution on differently stressed samples of
the highly sensitised steel (DOS: 22%) wit a potential sweep rate of 1 mV/s. All samples show a passive behaviour.
Dissolution of MnS-inclusions (if present on the micro spot) sets in at 400 mV/SCE. The applied load does not show
any influence.
3.5 Evaluation of new electrolytes 3.5.1 Selection criteria
Downscaling of the exposed sample surface and switching from a macroscopic electrochemical
cell design to the microcell had two major effects:
• The application of load to the sample was restricted to a three point bending cell. Samples
could only be strained to a fixed value of deformation at the beginning of an experiment, and
no adjustment of stress was possible during a measurement.
50
• With decreasing surface area the number of structurally “weak” points present on the
investigated surface is reduced, and the chance to hit an “active” surface spot decreases. A
sufficiently aggressive system has to be chosen for micro scale measurements, which may
therefore be overly aggressive on the macro scale.
For experiments with the microcapillary technique, a chemical environment had to be found that
fulfilled the following requirements:
• A suitable electrolyte has to rapidly induce IG SCC under the predefined working
conditions (room temperature, atmospheric pressure and constant deformation applied to the
sample) to make a detection of active spots with the microcapillary possible. And while IG
SCC is a highly localised type of corrosion, where even a small number of initiation points
can be very dangerous in practice, a high number of initiation sites considerably facilitates
their study.
• To unambiguously correlate certain signals with IG SCC, other localised corrosion
processes occurring simultaneously have to be minimised. In particular, crevice and pitting
corrosion have to be avoided.
• Ideally, the electrolyte should be non-toxic and should not exhibit an extreme pH value.
The stability of the electrolyte and a possible decomposition during long term experiments
must be considered.
3.5.2 Acidic thiosulphate solutions
To increase the aggressiveness of the initially used electrolyte based on sodium thiosulphate,
sulphuric acid was added to lower the pH value. Macroscopic as well as microscopic
measurements at pH 3 showed similar results as measurements in neutral electrolytes. A further
lowering of the pH was limited by the stability of the thiosulphate ion under acidic conditions: At
room temperature and a pH around 7, aqueous solution of thiosulphate were reported to be stable
over days to weeks [119]. Air oxidation occurred only under elevated temperature and high
oxygen pressure. Under acidic conditions, thiosulphate easily decomposes and forms elemental
sulphur (see equations 1-3 in Section 2.1.2). A rate law for the disproportionation reaction 1 of
thiosulphate to sulphite and sulphur is deduced in [120]. The found reaction rate is first order
with respect to H+, and second order with respect to S2O3-:
22
3200 ][][ −+ ⋅⋅= OSHkR 120 66.0 −−= smolk (13)
51
Thiosulphate decomposition in the presented exposure tests was indicated by an opaque
precipitate of colloidal sulphur in the electrolyte and the formation of hydrogen sulphide, readily
detectable by its characteristic smell. The applicability of such unstable, continuously
decomposing electrolytes was deemed unsuitable for further study.
3.5.3 Chloride solutions
Pitting corrosion induced by chloride ions has already been studied extensively with the microcell
[23, 83]. Chloride solutions were therefore considered for measurements in the present system.
Macroscopic exposition tests on stressed samples in concentrated acidic MgCl2 and NaCl
solutions showed severe corrosion damage on the exposed surfaces (Figure 19a and b). Grinding
of the damaged surface after an experiment and additional bending revealed IG SCC in both
environments (Figure 19c and d). These aggressive electrolytes had the potential to initiate IG
SCC under the applied conditions, but the simultaneous occurrence of pitting made them
unsuitable for further experiments.
The result of an exposure test in a less concentrated electrolyte is shown in Figure 20. A highly
sensitised sample was exposed to 1 M NaCl at pH 7 and stressed to nominal 360 MPa. No signs
of cracking were observed after 72 h. The deformation was increased to nominal 540 MPa. After
additional 72 h, the surface around the area of highest deformation showed IG cracking. The
surface also exhibited additional corrosion attack in some regions.
52
Figure 19: a and b) Light microscope images of two sample surfaces (DOS 22%) after 24 h of exposition to
5 M MgCl2 (a) and 5 M NaCl (b), both at pH 1.5. c) and d) Light microscope images of the same sample surfaces
shown in a) and b), after polishing. Stress corrosion cracks are revealed on both samples.
Figure 20: a) SEM image of IG cracking on a macroscopic exposure specimen at nominal 600 MPa, after 6 d
exposure to 1 M NaCl. Cracks appeared around the area of highest deformation on the sample. b) Additional
corrosion occurred on some parts of the surface.
Microcell measurements were performed using 0.001 – 1 M sodium chloride solutions at pH
values 2 – 7. Potentiodynamic current measurements on unstressed samples often showed pitting
53
or crevice corrosion in the region of 200 – 800 mV/SCE, both on sensitised as well as solution
annealed samples (Figure 21).
Figure 21: Potentiodynamic current measurement on unstressed material, using the electrochemical microcapillary
and a 1 M NaCl solution at pH 2. A potential sweep rate of 1 mV/s was applied. Both solution annealed (a) and
sensitised (b, DOS: 22%) material suffered stable pitting corrosion in the range of 100 – 800 mV/SCE.
While macroscopic immersion tests in chloride solutions showed the occurrence of IG SCC, it
was not possible to isolate this phenomenon from pitting corrosion and to study it exclusively
with the microcell technique. Macroscopic EN measurements that were performed in chloride
solutions, showed micro crack formation on sensitised samples, but also extended crevice
corrosion. The initiation of IG SCC did not occur faster than with thiosulphate. These electrolytes
therefore did not provide any advantages for macroscopic measurements, and crack propagation
was not observed on the micro scale, the use of chloride for further experiments was dismissed.
54
3.5.4 Potassium tetrathionate solutions
The tetrathionate ion is closely related to thiosulfate in its potential to initiate IG SCC in
sensitised steels, but it exhibits a higher stability at low pH. Therefore, stable electrolytes with
low pH values can be prepared. The initiation of IG SCC on stainless steel under CL conditions
in acidic tetrathionate solutions is described in [121]: a 0.1 M solution of potassium tetrathionate
with pH 2 initiated IG SCC on passively deformed stainless steel samples. These results could be
reproduced in the current study. Stressed samples exposed to this electrolyte showed clear IG
SCC in the region of highest load after hours of exposure. Additionally, exposure to the
electrolyte caused a dark blue/black deposition of sulphides on the sample surfaces, mostly
around the freshly formed IG cracks.
With the macroscopic EN measurements in mind, tests with more diluted electrolytes were
performed. Applied nominal stress in the elastic (180 MPa) region, just beyond the transition
from elastic to plastic deformation (360 MPa), and in a highly plastic deformation region
(> 360 MPa) were chosen. Table 4 summarises the immersion experiments performed with
various tetrathionate solutions and stress values.
Table 4: Results of macroscopic immersion tests of different tetrathionate electrolytes.
c(K2S4O6) c(H2SO4) Nominal stress Surface inspection 0.1 M 0.01 M 180 MPa IG SCC, IG corrosion, dark precipitates 0.01 M 0.01 M 180 – 900 MPa IG SCC, IG corrosion, dark precipitates 0.01 M 0.005 M 180 – 360 MPa IG SCC, precipitates 0.01 M 0.001 M 360 MPa No corrosion on surface 0.01 M 0 360 MPa No corrosion on surface
0.001 M 0.01 M 360 MPa Black precipitate layer, extended IG corrosion, more prominent in region of
highest load. 0.001 M 0.005 M 180 – 360 MPa IG SCC, dark precipitates
Control test without tetrathionate: 0 0.01 M 720 MPa Grain boundaries attacked at centre of
highest loading 0 0.05 M 720 MPa Grain boundaries attacked at centre of
highest loading Control test without load:
0.1 M 0.01 M 0 IG corrosion
High concentrations of tetrathionate and acid caused IG corrosion and IG SCC at stress values
below σy. Sample surfaces showed dark blue precipitate films, which can be attributed to the
formation of sulphides on the surface [122]. For constant tetrathionate concentrations, raising the
pH generally reduced IG corrosion. At pH 2.2 (0.005 M H2SO4), IG cracks could still be
observed at 180 MPa nominal stress (Figure 22b). At higher pH values, crack initiation below σy
was not reproducible any more.
55
Tetrathionate concentrations below 0.01 M gave contradictory results: A 0.001 M solution at pH
1.8 caused severe IG corrosion on the whole specimen (Figure 22c). This increased corrosion was
accompanied by formation of a very dense precipitate layer, compared to higher tetrathionate
concentrations. At pH 2.2, IG SCC occurred with moderate precipitate deposition on the surface.
It is unclear why the 0.001 M solution at pH 1.8 showed an increase in aggressiveness compared
to 0.01 M solutions. But the tendency to attack all grain boundaries of a sample and the dense
precipitate layer made it difficult to identify micro cracks.
A tetrathionate concentration of 0.01 M and pH values of 1.8 and 2.2 caused IG SCC below σy of
the material, with little IG corrosion on the unstressed surface and with moderate sulphide
precipitation on the surface. These electrolytes were considered for further experiments.
Figure 22: Light microscope images of samples exposed to various tetrathionate electrolytes over 24 h. a) 0.1 M
K2S4O6, pH 1.8, 180 MPa: IG cracks formed over the whole width of the sample. The surface shows dark blue
sulphide depositions. b) 0.01 M K2S4O6, pH 2.2, 180 MPa: single cracks appeared near the centre of highest
deformation. The surface shows colour changes around cracks and MnS-inclusions. c) 0.001 M K2S4O6, pH 1.8,
360 MPa (after removing of sulphide deposits): extended IG corrosion. Cracks cannot be identified.
56
Macro scale EN measurements
A CERT test (labelled RTL 30) was performed on a highly sensitised specimen exposed to
10 mM K2S4O6 at pH 1.8. The EN signals are shown in Figure 23: Already during the initial
stabilising phase at 10 MPa, the current exceeded 1 µA. The ECN signal jumped to 10 µA at
160 MPa, causing a signal overload. After reaching 200 MPa, complete fracture of the sample set
in. The potential shows a corresponding fast drop at 160 MPa stress, followed by a continuous
decrease until the sample broke. Inspection of the fracture surface after the experiment revealed
almost complete IG fracture of the sample cross section (Figure 24). In some regions of the
sample, small pits were found, following the grain boundaries (Figure 24b).
Figure 23: EPN (green) and ECN (red) measurement during a CERT test on highly sensitised material (DOS: 22%)
in 0.01 M K2S4O6 at pH 1.8 (RTL 30). Current signal overload occurs at 15 h. The sample breaks apart after 18 h.
Figure 24: SEM image of the completely fractured round notched sample after the CERT test shown in Figure 23. a)
The fracture surface shows almost complete IG fracture. b) Some regions showed additional pitting and IG
corrosion.
In a series of follow-up experiments it was tried to optimise experimental procedures and sample
preparation. All further experiments were conducted in 0.01 M K2S4O6 at pH 2.2. Results are
presented in detail in Section 5. In the process of optimising experimental conditions, current
57
signal fluctuations during the stabilising phase could usually be limited to a region of 0 – 5 nA. In
some cases, better values were achieved (stable current signals of 850 ±50 pA over several
hours).
Potentiodynamic current measurements with tetrathionate electrolytes on the micro scale initially
gave results very similar to those obtained in thiosulphate solutions. After several measurements
on highly deformed samples, a different behaviour emerged. Distinct surface spots showed an
OCP shifted to more negative values and immediate active dissolution in the anodic region, while
the largest part of a surface showed a passive behaviour, identical to unstressed samples (Figure
25). Inspection of surface spots that exhibited active signals revealed micro cracks along a few
grain boundaries. Cracks propagated predominantly in the direction perpendicular to the applied
load (Figure 26). Based on these results, 10 mM K2S4O6 at pH 2.2 was chosen for the subsequent
systematic studies.
Figure 25: Potentiodynamic current measurement (potential sweep rate = 1 mV/s) on a passive surface spot (black),
and on an active spot (red), which resulted in IG SCC under the microcapillary. Active spots generally showed a
lower OCP and immediate active behaviour in the anodic region. The current rise in the passive curve at
500 mV/SCE can be attributed to dissolution of a MnS-inclusion.
58
Figure 26: SEM image of the first micro crack that was initiated under the microcapillary, measured on highly
sensitised material (DOS 22%) in 0.01 M K2S4O6 at pH 1.8 and a nominal stress of 180 MPa. The direction of
applied load is predominantly horizontally, resulting in a crack growth in vertical direction on the image. The surface
area exposed to the electrolyte underneath the capillary is indicated by the dotted circle.
3.6 Final measuring procedures 3.6.1 Macroscopic CERT tests
The experimental procedures for CERT experiments is shown in Figure 27a. The electrolyte
recirculation loop was filled with the appropriate test solution from the beginning. An initial load
of 1000 N (~50MPa) was applied to the sample. The system was allowed to stabilise under these
constant conditions for 48 – 72 h 6. The conditioning phase was also used to identify interferences
in the signal and adjust the system. After the system had stabilised, and ECN and EPN
measurements showed stable signals, a pull rod extension rate of 3.5·10-9m (nominal strain rate:
2·10-7 s-1) was initiated. At the end of an experiment, either due to complete failure of a sample,
or when a certain maximum stress was reached, the specimen was unloaded. The measurement
was continued for some hours, to monitor the change of the noise signal after unloading: A
sudden decrease in noise activity after unloading confirmed that the detected noise signals were
caused by the applied stress. The specimen was finally removed from the setup and prepared for
surface inspection.
6 In exceptional cases, the conditioning phases could be shortened or had to be extended.
59
3.6.2 Macroscopic CL tests
The experimental procedures for CL experiments is shown in Figure 27b. CL experiments were
performed in aqueous solutions of potassium tetrathionate at pH 2.2, adjusted with sulphuric acid.
Any dynamic loading of the specimen exposed to the tetrathionate was avoided. For these
experiments the electrolyte recirculation loop was initially filled with distilled water and the
specimen was stressed to the desired nominal load. The system was allowed to stabilise for 48 –
72 h. When potential and current signal were stable, the missing components of the electrolyte
were added to the loop. Measurement durations then ranged between a few hours and up to
10 days, before the experiments were stopped.
Figure 27: Illustration of the experimental procedure for macroscopic EN measurements.
a) CERT test: The sample is exposed to the complete electrolyte from the beginning. During a stabilising phase a
constant pre-load is applied. At t = 0 a constant extension rate is initiated.
b) CL test: The sample is stressed to the desired level from the beginning. During a stabilisation phase, the sample is
exposed to distilled water. At t = 0, tetrathionate and sulphuric acid are added.
3.6.3 Microcapillary measurements
For potentiodynamic scans, samples were prepolarised at -500 mV/SCE for 1 min. The scan rate
was 1mV/s and scans ranged from -500 to 1000 mV/SCE. For potentiostatic measurements,
initial OCP measurement was performed for 1 min before the current measurement at a defined
constant potential was started. Potentiostatic measurements lasted between a few seconds and
several hours. For selected experiments, extended OCP measurements were performed,
monitoring the fluctuations of the OCP over up to 2 h.
To find the few potential crack initiation sites, which were spread over a mostly passive sample
surface, the surfaces had to be systematically scanned. To quickly discern active from passive
areas, the distinct difference in OCP values, which was found in the initial potentiodynamic
current measurements, could be used: Locations exhibiting OCP values in the range of
-200 mV/SCE often showed crack initiation, while surface spots with OCP values near
60
0 mV/SCE usually remained passive. The use of the microcapillary for systematic scanning has
already been explained in detail [82, 85].
To perform measurements selectively on active sites, the capillary was placed spot by spot in an
array of rectangular shape (see Figure 28). Opposite to electrochemical droplet cells, which can
change position while still in contact with the surface, the microcapillary has to be lifted off the
surface after each positioning and put in place again at the next one. After positioning, the OCP
drift was observed on the potentiostat for a few seconds to minutes. If the potential rose to
potentials around 0 mV/SCE no measurement was performed and the capillary was moved to the
next stop. If the OCP on a spot stayed around -200 mV/SCE for a few seconds, a measurement
was started. This procedure allowed a fast identification of active spots over a selected area.
Measurements could be selectively performed on active surface spots.
Figure 28: Illustration of the point by point scanning technique using the microcapillary technique. The capillary has
to be lifted from the surface and repositioned after each measurement.
3.6.4 Post test inspection
After a completed experiment, the exposed surfaces of the tensile specimens were investigated
using SEM and stereo light microscopy. SEM measurements were performed using a LEO 440 or
a LEO 1455 model. Additional stressing of the samples under air after an experiment above σy
could be applied to improve the visibility of cracks. Selected samples were mechanically
overloaded and fractured in air. The resulting fracture planes often revealed the crack walls,
which allowed their direct inspection and an estimation of the crack wall areas.
For microcapillary experiments, the first assessment of corrosion damage was done by light
microscope immediately after lifting the capillary from the surface. The highly localised
measurements allowed the direct identification of any corrosion damages on the exposed surface.
Intergranular cracks were often directly visible, using an objective with 20x magnification.
The localisation of investigated surface spots on cleaned specimens in the SEM was challenging,
as sample surfaces showed little features that would facilitate orientation. Microcapillary
61
specimens were usually removed from the bending cell for SEM inspection. This caused cracks
to partially close again. To increase their visibility in the SEM, microcapillary samples were often
plastically deformed after a measurement, to open the micro cracks. The extensions of selected
micro cracks were measured by preparing cross sections of the cracks and grinding them layer by
layer. Depth measurements allowed an estimation of the crack wall area.
3.7 Summary The first aim of this thesis was the development of a system that allowed the monitoring of IG
SCC of austenitic stainless steel on the micro and macro scale, using the electrochemical
microcell technique and EN measurements during macroscopic tensile tests. The combination of
these two measuring techniques required the definition of experimental conditions for both
scales, which were reasonably similar to be able to compare the result. The initially proposed
system of neutral thiosulphate solutions was well suited for CERT tests on the macro scale, but
this system was not aggressive enough to initiate IG SCC on passively stressed samples in a
reasonable time frame. Microcapillary experiments under potential control showed no difference
in behaviour between the sensitised grain boundaries and the bulk material, and no IG cracking
was observed.
A more aggressive chemical environment had to be found for the micro scale experiments, but
not overly aggressive on the macro scale. Lowering the pH of the thiosulphate was limited by the
decomposition of the electrolyte. Chloride solutions showed crack initiation under constant
deformation in a matter of days, but pitting corrosion predominantly occurred during
potentiodynamic current measurements. Current signals related to micro crack propagation could
therefore not be isolated from pitting corrosion processes
Potassium tetrathionate solutions at pH values below 3 were finally found to be suitable
electrolytes, as they fulfilled the initially stated requirements very well: They were easy and safe
to handle, showed IG SCC initiation under constant deformation, and it showed only little other
corrosion processes besides IG SCC. Besides these requirements, this experimental system
showed crack initiation only at few, well defined surface spots. Active spots could be easily
identified by a shift in OCP compared to passive surface spots, and they showed almost
immediate crack initiation and growth when in contact with the electrolyte. This allowed making
measurements selectively on active spots.
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4 Investigation of IG crack initiation and
propagation on the micro scale
The growth of single micro cracks was studied by potentiostatic current measurements. A data set
was established by scanning sample surfaces and performing measurements selectively on active
spots. All experiments were performed in 0.01 M K2S4O6 at pH 2.2 as described in Section 3.5.4.
Samples were polarised at potentials in between the OCP of typical active and passive surface
spots (Figure 25). Measurements were performed on material with DOS of 22% and 15% and
lasted between a few seconds up to several hours. The resulting current – time curves are
analysed and compared to SEM images of micro cracks that had formed on the surface during the
measurement. Finally, potential and limitations of the microcapillary technique for the study of
IG SCC are discussed.
4.1 Signal characteristics This section illustrates the typical signal characteristics that were observed. A statistical signal
analysis will be presented in sections 4.3 - 4.5. Immediately after a freshly ground surface spot
came in contact with the electrolyte, OCP values around -400 mV/SCE were measured. This
potential was stable for a few seconds. A rapid potential rise occurred over the following minutes.
The same reaction of AISI 304 stainless steel exposed to tetrathionate solutions was reported for
macroscopic samples by Kowaka and Kudo in [122]. After this initial potential drop, surface
spots either exhibited active or passive behaviour, indicated by the OCP:
4.1.1 Passive behaviour
OCP values measured on the majority of surface spots for both solution annealed and sensitised
material showed a continuous potential rise towards a constant value in between 0 and
200 mV/SCE (Figure 29a). Exact values varied between samples, and between surface spots on
the same sample. No corrosion attack was observed on these “passive” surface spots after a
measurement. The same behaviour was found on bulk material, grain boundaries and inclusions,
for all tested materials. Potentiostatic current measurements on passive surface spots showed
negative current signals, due to the applied potential below the OCP of the passive surface.
Signals often showed a slight trend towards less negative values over several hours. Cathodic
63
current densities after 10 min, measured with a 60 µm capillary on passive surfaces, ranged
between -0.4 - 0 mA/cm2 (Figure 29b).
Figure 29: a) Typical OCP curve of a surface spot exhibiting passive behaviour, measured on a highly sensitised
sample (DOS: 22%) with a 60 µm capillary over 2 h in 0.01 M K2S4O6 (pH 2.2). b) Typical potentiostatic current
measurement on a passive surface spot at -150 mV/SCE, measured on a highly sensitised sample (DOS: 22%) with a
60 µm capillary over 2 h in 0.01 M K2S4O6 (pH 2.2).
4.1.2 Active surface spots
Surface spots which showed an OCP anywhere between -400 and -100 mV/SCE for longer than a
few seconds are designated “active” (Figure 30a). These surface spots were found on sensitised
material exclusively. Active surface spots were mostly found on stressed surfaces. Very few spots
showed initial active behaviour without stress.
Potentiostatic current measurements on these spots showed fluctuating current signals
immediately after polarisation. Single current transients showed fast rise and often irregularly
shaped decay (Figure 30b and c). Initial currents could reach amplitudes above 250 nA directly
after polarisation for very short time spans. Current peak amplitude then declined and showed
values up to 100 nA.
64
Active surface spots on stressed samples showed intergranular cracks after an experiment. Crack
growth was not stopped at the capillary rim, but continued underneath the silicone seal of the
capillary, and occasionally even beyond it. Experiments at low stress (below 150 MPa) showed
very short cracks, which were difficult to find.
Figure 30: a) OCP measurement on an initially active surface spot on highly sensitised material (DOS: 22%),
measured with a 60 µm capillary over 2 h in 0.01 M K2S4O6 (pH 2.2). b) Potentiostatic current measurement on an
active surface spot on highly sensitised material (DOS 22%) at -150 mV/SCE, measured with a 60 µm capillary over
2 h. c) Potentiostatic measurement on an active spot on partially sensitised material (DOS 15%), measured under the
same conditions.
65
4.1.3 Discontinuous crack growth behaviour and repassivation
The classification of active and passive surface spots is inaccurate, as repassivation commonly
occurred during measurements. Current signals showed a slow decay towards a constant or
slightly drifting passive value in the cathodic current region, equivalent to the current signals
measured on surface spots that were passive from the beginning. Samples could passivate after
significant crack growth, or just seconds after a measurement on an initially active surface spot
was started (Figure 31). SEM images of such surfaces after the measurements showed only
limited surface damage (Figure 32).
Figure 31: Potentiostatic current measurement (-150 mV/SCE, DOS: 22%, 0.01 M K2S4O6 (pH 2.2) on a surface
spot that showed initial activity, indicated by an OCP value of -290 mV/SCE after 1 min. The measurement shows
mostly passive behaviour, only interrupted by two dissolution events.
Figure 32: SEM image of the surface spot after potentiostatic measurement over 1 h, shown in Figure 31. The MnS-
inclusion to the right shows dissolution along the inclusion-metal interface. A 10 µm long micro crack has formed.
Furthermore, reinitiation after a passive phase was observed occasionally, directly proving that
micro crack growth is a discontinuous process, even at constant deformation conditions. This
behaviour is shown in the current signals in Figure 30b and c. Reinitiation usually occurred by a
66
fast current rise. It is not clear, if an initially passive surface spot would show the transition to
active behaviour over time. Such behaviour was not observed in any measurements on passive
surface spots, however their number and duration was limited. Therefore it might still be possible
for crack initiation to occur delayed on certain passive surface spots.
4.2 Location of crack initiation sites An important feature of the presented experimental system is the potential to identify crack
initiation sites on a mostly passive metal surface. The nature of these active spots was further
investigated. The presence of a sensitised grain boundary was obviously a necessity for IG SCC
initiation, but apparently it was not sufficient on its own: The majority of the grain boundaries on
a surface did not show active behaviour. The question arises, what conditions have to be fulfilled
to create an active surface spot.
Post-test inspection of micro cracks revealed, that in most cases, crack initiation occurred at
MnS-inclusions. In some of these cases the MnS-inclusions were still intact after the
measurement (see Figure 33). This is in accordance with the results from potentiodynamic scans:
MnS-inclusions showed dissolution at potentials above 400 mV/SCE, while IG SCC could be
initiated below 0 mV/SCE. In other cases the inclusions were attacked (as seen in Figure 32),
which could lead to the partial or complete break out of an inclusion. Cracks could also initiate at
defects, which were created or revealed during surface preparation (Figure 34). These were rare
cases, as the applied surface preparation procedure usually did not create such large defects. For
certain micro cracks, no clear initiation point could be identified. The causes of crack initiation in
these cases could not be determined. It is likely that very small MnS-inclusions, or some kind of
weakness in the oxide layer of the surface, are responsible for these cases of crack initiation.
Typical weak spots for the case of pitting corrosion are e.g. local variations of oxide film
structure and composition, thermally induced film rupture events or electrostriction [123], and the
same surface features probably also account for crack initiation. A final identification of the
cause of crack initiation in the described cases went beyond the scope of this work. It also
remains unclear, if certain grain boundaries were more susceptible to crack initiation, due to an
orientation that leads to very high shear stress.
In an attempt to quantify the occurrence of active surface spots, the results of an array
measurement with 99 investigated surface spots were evaluated (see also Section 4.5), comparing
the number of active spots with the number of MnS-inclusions. The results are listed in Table 5.
Of 99 measurements, 4 surface spots showed crack initiation in the absence of a visible MnS-
67
inclusion or other particular surface feature, while crack initiation at an inclusion was observed in
20 cases. This underlines the importance of MnS-inclusions for the initiation of IG cracks in the
present material.
Figure 33: SEM image of a short crack after 15 s of polarisation at -150 mV/SCE. The crack initiated right at the
intersection of a grain boundary and a MnS-inclusion, indicated by a small pit. The inclusion is still intact.
Figure 34: SEM image of a crack that initiated at a large defect. These cases were rare, as the samples rarely
exhibited such defects.
68
Table 5: Evaluation of 99 consecutive measurements on the same sample (DOS: 22 %, load: 360 MPa), concerning
the influence of MnS-inclusions on crack initiation.
MnS-inclusion present No visible inclusions total
Passive behaviour: 9 57 66
No crack, only MnS-inclusion attacked
Crack initiation at MnS-inclusion
Cracking without MnS-inclusion present total:
Active dissolution: 9 20 4 33
All measurements 99
For most experiments, a detailed investigation of a surface spot before a microcapillary
measurement was not possible: The scarce distribution of active points made it necessary to scan
large surface areas with the microcapillary. These areas were too large to be inspected by SEM in
detail prior to the measurements. However, it was possible in two cases, to obtain SEM images of
surface areas that would exhibit active behaviour in the following measurements: A sample was
stressed to a nominal load of 720 MPa, and SEM images were recorded of random “suspect”
surface spots on the stressed surface. The selected surface spots exhibited MnS-inclusions, which
facilitated the later identification and increased the chance for active behaviour. Microcapillary
measurements were then performed on the exact same positions. Two surface spots were hit
precisely and showed active behaviour: Figure 35a and b show SEM images of these two MnS-
inclusions on a stressed surface. The inclusions show no cracks or defects, which might serve as
initiation sites. Potentiostatic current measurement on the two surface spots (Figure 36) indicated
active metal dissolution. After the measurements, both inclusions showed cracking, but no other
sign of corrosion (Figure 35c and d). This implies that the first step in the initiation of IG SCC is
a chemical attack of the surface.
69
Figure 35: Two examples of MnS-inclusions before and after a potentiostatic current measurement. a) and b) show
MnS-inclusions on an already stressed surface before the measurements. No sign of mechanical damage can be seen
prior to the measurement. c) and d) show the same inclusions after potentiostatic measurement at -150 mV/SCE for
30 min (c) and 15 min (d). Besides being cracked, both inclusions are still intact and do not show any significant
corrosion attack.
Figure 36: a and b) Corresponding potentiostatic current measurements to the micro cracks shown in Figure 35c and
d. Measurements were performed with a 50 µm capillary on highly sensitised material (DOS: 22%) at -150 mV/SCE
in 0.01 M K2S4O6 (pH 2.2). Currents of 40 – 80 nA indicate active metal dissolution.
70
4.3 Influence of applied stress and DOS on the crack growth behaviour To quantify the influence of applied stress and of the sensitisation of the material on the crack
growth behaviour, crack lengths and integrated current signals were compared for partially and
fully sensitised material, and for two stress values. All experiments were performed with a 60 µm
capillary, and samples were polarised to -150 mV/SCE for 2 h. The stress values selected were
720 and 360 MPa. Both values were in the region of plastic deformation and usually produced
large and clearly visible cracks.
4.3.1 Influence on crack size
Mean crack surface lengths for the four possible combinations of the two applied stress values
and the two material treatments are shown in Figure 37. Mean crack lengths were higher for
highly sensitised materials and for higher DOS of the material. The largest cracks clearly
appeared at high stress. Differences between the other material – stress combinations are smaller,
and error bars are large for these calculations, due to the large spreading of crack lengths and the
limited number of experiments. Mean crack growth velocities are 0.04 µm/s for highly sensitised
material at high stress, and in the range of 0.02 µm/s for the other combinations of material and
stress.
Figure 37: Mean surface crack lengths after 2 h potentiostatic current measurements for differently sensitised
materials and applied stress values. Both sensitisation and increased stress leads to longer cracks.
The observed influence of stress on crack growth confirms the earlier macroscopic immersion
tests of sample coupons. Stress is clearly a driving force for crack propagation.
71
4.3.2 Influence on current signal
Integration of current signals gave the dissolution charge values for each investigated crack. A
baseline correction was applied to account for the negative sum currents measured during passive
phases: a positive current value was added to each signal, so that passive phases showed a value
of 0 nA. All active phases then exhibited positive current values, and integration of the corrected
signal resulted in the total anodic charge produced by active metal dissolution.
The same trend as for the crack lengths is expected for the mean charge value of the differently
stressed materials. The values are shown in Figure 38. Again, the highly stressed material shows
clearly the highest charge values. The differences between the other values are even smaller than
for the crack lengths. This can be explained by additional current that originated from dissolving
MnS-inclusions. The number and shape of MnS-inclusions varied greatly between samples, and
crack initiation on the tested partially sensitised material mainly occurred on large MnS-
inclusions, which were often attacked during tests. The resulting dissolution currents were
independent of the applied stress and therefore obscured its influence on crack growth.
Figure 38: Mean charge values after 2 h potentiostatic current measurements for differently sensitised materials and
applied stress values. A combination of high stress and high sensitisation caused the highest charge values. The other
combinations of stress and sensitisation show similar values.
4.3.3 Crack growth direction
Crack growth was limited to grain boundaries, and therefore the orientation of the grains
determined the exact growth direction of a crack. In all SEM images of micro cracks presented,
the applied main load was in horizontal direction. Accordingly, crack growth proceeded
predominantly vertically, i.e. orthogonal to the direction of the highest stress. Due to the
72
geometry of the used three point bending cell, additional stress components in vertical direction
were also present: The screw used to deform the specimens touched the sample in one point in
the centre. Stress is highest directly above the screw and decreases radially around this centre
point. As stress was applied to the sample only in one point, and measurements were spread over
an area around this centre of highest deformation, the sample surface was subjected to biaxial
stress, and both intensity and direction of stress varied between surface positions. This explains
the deviation of crack growth from the expected direction in some cases, such as the small cracks
in Figure 35 d, which proceeded to the right.
4.4 Quantitative comparison of current signals and micro crack dimensions So far, the comparison of current signals with micro cracks was mainly made qualitatively. To
quantify the correlation between crack dimensions and current signals, charge transfer values
from current data were compared to the estimated crack wall areas, using Faraday’s law. To
estimate crack wall dimensions, some assumptions had to be made:
Cracks were found to initiate at distinct weak points, and crack growth had to proceed from these
points, following the grain boundaries. Dissolution of the crack walls was restricted to the
sensitised regions along grain boundaries. The width of these regions, and with it the width of the
dissolution zone, were predetermined by the extent of the chromium-depleted zone. This width
was assumed to be constant. Extended dissolution and crack propagation were therefore limited
to two dimensions in the metal: crack propagation in depth and in length along the grain
boundaries, mainly perpendicular to the applied load. With a constant dissolution rate along the
grain boundaries from the initiation point in every direction, the result would be a semi-circular
disc, with a radius of half the visible surface crack length. Growth speed might be higher at the
surface due to higher stress (stress is declining with crack depth in the case of constantly
deformed samples). Therefore cracks were expected to have approximately a semi-elliptical
shape. The result of these assumptions is a model shape for a micro crack of a semi-elliptical disc
with a constant width. The volume of a crack can then be derived from the width of the metal
dissolution zone (marked c in Figure 39) and the area of a crack wall (marked A in Figure 39). A
similar calculation has already been described by Wells et al. [109]. There the calculation was
based only on the measurement of surface crack lengths, and a semicircular crack shape was
assumed.
73
Figure 39: Illustration of the crack wall area calculation: Cracks were assumed to have a semi-elliptical shape with
crack walls along the grain boundaries. The crack wall area A can be estimated from the visible crack length a and
the maximum crack depth b. The width of the dissolved metal layer c can be calculated from A and the volume of
dissolved metal, derived from the current signal using Faraday’s law.
To confirm the assumption of semi-elliptical crack shape, and to estimate the mean crack depth,
the cross sections of 11 selected cracks were investigated using light microscopy and SEM:
Sample profiles were progressively ground with SiC paper to reveal the depth profile along the
surface crack. After each grinding step the depth of the crack was measured. A combination of
two SEM images, showing the surface crack and the depth profile, is shown in Figure 40. Cracks
showed highest depths around the centre of the surface crack line, and depth decreased
continuously towards the crack tips, confirming a semi-elliptical shape. A mean ratio of
maximum crack depth to surface length of 0.47 ± 0.07 was found. As expected the main depth
was smaller than half of the visible crack length on the surface. This value of 0.47 was used to
estimate the depths of all other investigated cracks and to calculate their wall areas. The measured
values as well as the linear fit are shown in Figure 41.
74
Figure 40: Combination of two SEM images to illustrate the estimation of crack shapes and ratios between the
length on the surface a and the depth in the crack centre b.
Figure 41: Maximum depths (measured optically on ground sample profiles) vs. surface lengths for selected cracks,
measured optically on the sample profiles. A ratio of 0.47±0.07 between surface length and maximum crack depth
was found.
Current values from potentiostatic measurements were used to calculate electrode charge transfer.
Again, a baseline correction was applied to each current measurement to account for the cathodic
currents measured during passive phases. The resulting wall area and charge transfer values are
plotted in Figure 42. A best fit line for the data has a slope of 2.2 mC/mm2, which is the amount
of charge flown per area of the crack wall. For the calculation of the trend line, several data
points (open data points in Figure 42) were omitted: For these particular experiments, surface
investigation after the measurements revealed extended corrosion of MnS-inclusions, leaking of
the capillary or other types of corrosion. The current signal from these corrosion processes
75
perturbed the current signal related to crack propagation. Accordingly, these data points mostly
exhibit a higher charge than would be expected from the crack size.
The final ratio of 2.2 mC/mm2 was compared to passive charge transfer values measured on
freshly ground steel samples. The charge of passivation per area ranged between 0.03 and
0.09 mC/mm2, i.e. values are more than 20 times smaller than the ratio found for crack walls:
Significant additional metal dissolution must occur during active crack growth in the chromium-
depleted regions along the grain boundaries.
Figure 42: Charge vs. estimated crack wall areas of 50 micro cracks (both degrees of sensitisation). A trend line was
calculated for the filled symbols. Open symbols indicate measurements that were omitted from the calculation of the
trend line.
The ratio between charge and crack wall area can directly be used to calculate the width of the
dissolved metal layer along the grain boundary. Using Faraday’s Law the corresponding volume
of dissolved metal was calculated from the charge as follows:
FzQMV⋅⋅⋅
=ρ
(14)
(V: volume; M: molar mass; Q: charge; ρ: density; z: charge number; F: Faraday constant)
The molar mass M and density ρ of iron and a charge number z of 2 were assumed for the
calculation. The width of the dissolution front could then be calculated for each crack, simply by
dividing the obtained volume V by the estimated crack wall areas. A mean width of 80 nm was
found, which is in the expected range for critical chromium depletion (< 12 wt.%) along
sensitised grain boundaries, [124, 125]. Similar values for dissolution widths along grain
76
boundaries were found in macroscopic studies of IG SCC of stainless steel in thiosulphate
solutions [35, 109, 111].
The presented calculations incorporated measurements on both material treatments (DOS 15%
and 22%). As can be seen in Figure 42, similar values were found for both partially and fully
sensitised material. This is surprising, as the degree of sensitisation directly influences the
concentration profile of chromium at the grain boundaries, and therefore should define the
dissolution width. It is possible, that the number of measurements on the partially sensitised
material was too small to statistically detect a variation. Another possibility is that the width of
the area with a critical chromium concentration (e.g. < 12 wt.%) is similar for both degrees of
sensitisation: As shown in [41], the chromium concentration profile of a grain boundary shows a
very steep depletion for short sensitisation times. With increasing time, the width of the depleted
zone increases, while the depth of the zone decreases. The profiles for two different sensitisation
times therefore intersect at one point. If this point is close to the critical chromium concentration,
both degrees of sensitisation may lead to similar metal dissolution.
4.5 Systematic array measurement As introduced earlier, an array measurement was performed to study the distribution of active
surface spots on an area of approximately 1.4 mm2: A series of short measurements (t = 10 min)
were performed in an array of 10×10 spots on a single sample (DOS 22%, nominal stress
360 MPa). Measurements were carried out on each spot, independent of the initial OCP value. On
the first column OCP measurements were performed over 10 min. On the subsequent 9 columns
the OCP was measured for 90 s, followed by a potentiostatic current measurement at
-150 mV/SCE over 10 min. All 99 measurements (one measurement was skipped) were
performed on three consecutive days without any regrinding of the surface between the
measurements.
4.5.1 Distribution of active sites
An image of the array after the measurements is shown in Figure 43a. The array area with a
dimension of 1.1×1.3 mm was marked using a micro hardness imprinter. These imprints served
as orientation points in the following analysis. An overview over the number of active and
passive spots on the array was already shown in Table 5. The scanned surface area exhibited a
high number of large MnS-inclusions (> 10 µm), and many of the inclusions, on which a
measurement was performed, were attacked. Crack initiation mostly occurred at these inclusions.
Only four measurements showed cracking in the absence of MnS-inclusions.
77
Figure 43b shows the array surface after vibration polishing. Most traces of the microcapillary
measurements disappeared. Only few large cracks are still visible. MnS-inclusions can be seen
clearly as longish, dark grey particles that are horizontally oriented. The position of each
microcapillary measurement is indicated by a circle. Surface spots exhibiting active metal
dissolution during a measurement are indicated by red circles. They seem evenly distributed over
the surface, and no pattern emerges.
Figure 43: a) Surface after 99 consecutive microcapillary measurements with a 60 µm capillary on highly sensitised
material (DOS: 22%), at -150 mV/SCE in 0.01 M K2S4O6 (pH 2.2). The scanned array area was marked at the
corners and along the sidelines with micro hardness imprints. b) The same surface area, after vibration polishing for
30 min in silica emulsion. Positions of measurements are indicated by circles. Red circles indicate spots which
showed activity. Black circles indicate passive spots.
4.5.2 Variation of OCP values
The presented array measurement produced a set of 99 OCP values, measured on the same
surface, under the same experimental conditions and with the same capillary. The only factor that
changed was the time span between surface preparation and each measurement. Such a dataset
allows a comparison of OCP values of a large number of both active and passive surface spots.
This gives insight in the variation of OCP values over a sample surface, which is typically
observed with the microcapillary technique. The first 60 s of OCP measurements for all data
points of the array are plotted in Figure 44a. Curves that led to localised corrosion are plotted in
red. Black curves indicate passive surface spots, on which no significant corrosion damage
occurred.
78
Figure 44: a) OCP measurements over 1 minute on 99 positions on the same sample. b) Distribution of OCP values
after 1 min. OCP values of passive surface spots are concentrated around 0 mV/SCE. OCP values of active surface
spots are distributed over region of -450 - -100 mV/SCE. c) Distribution of OCP values relative to the time passed
since the first measurement. No trend appears, indicating that the time that passed after surface preparation did not
significantly influence the behaviour of the surface.
OCP values are spread over a region of 500 mV, but a separation into two potential regions can
be seen for active and passive spots: Figure 44b shows the distribution of OCP values after 1
minute. Passive OCP values are concentrated in the potential region between
-80 and 90 mV/SCE, indicated by the grey band in Figure 44a, with few exceptions. A mean OCP
of 2 mV/SCE and a standard deviation of 27 mV were found. Active spots showed a higher
variation. OCP values are spread between -420 and -150 mV/SCE, with a mean value of
-300 mV/SCE. A concentration of values is observed in the region of -420 to -300 mV/SCE,
indicated by the red band in Figure 44a.
The high variation between active spots is coherent with the described crack propagation
mechanism. Potential values are expected to fluctuate during crack initiation and early growth,
due to the discontinuous nature of the process. The variation of OCP values on passive surfaces
indicates inhomogeneities of the oxide layer that formed on the surface during grinding. With the
applied surface preparation, using SiC grinding paper and water, the OCP of the passive surface
varied by 170 mV between different surface spots on the same sample.
Figure 44c shows the OCP values of all measurements after 1 min, relative to the time that passed
since the first measurement of the array. Similar distribution of OCP values for passive and active
surface spots occurred. No decline in the number of active surface spots appeared over time.
Apparently, time spans of 50 h did not significantly influence the behaviour of the surface.
79
4.6 Influence of the surface state Surface preparation has a large effect on the topography of the surface, residual stresses and
structure of the oxide layer, and therefore on corrosion resistance of a surface [126-128]. Morach
demonstrated in [129], how the formation of metastable pitting corrosion on austenitic stainless
steel varied with the used grinding paper. Corrosion resistance of ground surfaces continuously
increased from 320 to 4000 grit SiC paper, which was explained by a decrease of both surface
roughness and plastic deformation of the top layer. A similar effect of surface roughness to the
formation of pitting corrosion was reported by Burstein and Pistorius [130]. Furthermore, it was
shown in [129] that supposedly identical manual surface preparation, performed by different
operators, can lead to systematically different results, due to different amounts of pressure
applied to a sample during grinding. Hence, an optimisation of grinding parameters, such as the
type of abrasive paper, grinding pressure and speed or the lubricant can significantly increase
corrosion resistance of a surface [131].
Possible effects of surface preparation on SCC initiation are exemplarily described in [28]: The
initiation frequencies of cracks on four different mechanical surface finishes are compared.
Cracks predominantly initiated at pits, which often occurred at local defect sites, such as deep
grooves created during grinding. However, the surface roughness values did not necessarily
correlate with initiation rate. Depending on the grinding method, lower roughness was
accompanied by higher residual tensile stress, which could lead to more pitting.
In this study, care was taken from the beginning to prepare all samples for microcapillary
measurements by the same procedure, explained in detail in Section 3.3.1.: Coupons were ground
up to 4000 grit SiC in water and then degreased in ethanol and dried with hot air. The effect of a
deviation from this procedure is described below. Additionally, the effect of the time between
surface preparation and start of a measurement was studied.
4.6.1 Behaviour of polished surfaces
It was evaluated how fine polishing changed the behaviour of surfaces. These tests should clarify
if it was possible to perform microcapillary measurements on surfaces with very little residual
deformation. This would allow EBSD measurements on specimens after electrochemical
measurements, and it would be possible to analyse grain orientations adjacent to the micro crack
and determine their effect on crack growth direction.
The additional surface preparation consisted of consecutive polishing steps with 3 µm and 1 µm
colloidal diamond suspension and a final step of vibration polishing in silica suspension.
Vibration polished surfaces were scratch-free in the light microscope. Surfaces prepared in this
80
manner were found to be highly vulnerable to IG corrosion: Microcapillary measurements
showed selective dissolution of grain boundaries, both on stressed and unstressed samples (Figure
45). Under these conditions, crack initiation can occur anywhere on the grain boundaries, or at
several locations at once, and the additional current signal from the widespread IG corrosion
obscures the current signs of crack initiation and propagation.
Figure 45: SEM image of an unstressed sensitised sample (DOS: 22%) that was vibration polished and then
subjected to a potentiostatic current measurement for 1 min (60 µm capillary, -150 mV/SCE, 0.01 M K2S4O6, pH
2.2). The surface shows extended IG corrosion.
This behaviour can be explained by chemical and mechanical etching of the grain boundaries
during vibration polishing with the alkaline polishing agent. This dependence of the sample
behaviour on the surface preparation (grinding vs. polishing) illustrates how sensitive the studied
system reacts to changes of parameters, and how an apparently improved surface quality can
increase corrosion
Hence, micro electrochemical measurements on vibration polished surfaces did not produce any
meaningful results. This limited further surface analysis, and it was not possible to
electrochemically monitor the growth of micro cracks and then perform EBSD measurements on
the same surface. For EBSD measurements, samples have to be polished after the electrochemical
measurements. This in turn changes the top surface layers, and a direct comparison of both
measurements is not possible for very short cracks and their initiation points (see also Appendix
II).
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4.6.2 Influence of oxide film formation over time
Series of microcapillary measurements were normally started immediately after grinding. The
number of experiments performed on one sample varied with the time it took to find a suitable
position for a measurement, and with the duration of the measurements. As shown in Figure 44c,
time differences of up to 50 h did not seem to have an effect on the OCP values or the occurrence
of active surface positions.
Further experiments were performed to determine, if longer time periods between surface
preparation and the start of a measurement influenced the sample behaviour. Samples were
ground and then immersed in distilled water for several days. They were then stressed, and
measurements were performed. Figure 46a shows typical OCP signals measured on a sample that
had been immersed in aerated distilled water for 17 d. The sample was stressed directly before
the measurements. All measurements showed markedly higher initial OCP values compared to
freshly ground surfaces. After a period of a few seconds up to several minutes, the potential
dropped, and the curves subsequently showed either active or passive behaviour, just as on
freshly ground surfaces. A second series of experiments were performed, where stress was
applied before the exposure period: Figure 46b shows the corresponding OCP signals for
measurements on a sample that was stressed directly after grinding, and was then immersed in
aerated distilled water for 20 d. The same behaviour as before was found. SEM investigation of
micro cracks for both series of experiments showed no particularities. Initiation occurred at MnS-
inclusions (Figure 46c and d).
Apparently the oxide films, which had formed over several days, did not prevent crack formation,
but initially slowed it down. As mentioned in Section 2.1.2, sulphur-containing electrolytes can
cause the breakdown of a passive film, by replacing metal oxides by sulphides. The observed
delay in crack initiation might therefore be due to this reaction. Variations in initiation time can
then be attributed to variations in oxide film thickness or structure.
Furthermore, it was observed that the outcome of an experiment was not influenced by the timing
of the application of stress: Crack initiation still occurred on samples, which were deformed
before an oxidation period of several days. This indicates that the first step of crack initiation in
the studied system is a chemical process, which does not require a mechanically induced surface
defect right before the measurement.
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Figure 46: a) Exemplary OCP measurements in 0.01 M K2S4O6 (pH 2.2) on a highly sensitised sample (DOS: 22%)
which was additionally oxidised in aerated water for 17 d after grinding and stressed directly before the
measurements. Initial high potential were found, which did not occur on freshly ground surfaces. After the potential
dropped, the distinct behaviour of active and passive surface spots was found. b) Exemplary OCP measurements in
0.01 M K2S4O6 (pH 2.2) on a highly sensitised sample (DOS: 22%) which was stressed and then oxidised in aerated
water for 20 d. The same behaviour was found as before. c) SEM image of the micro crack formed during the active
OCP measurement shown in a). d) SEM image of the micro crack formed during the active OCP measurement
shown in b).
83
4.7 Discussion 4.7.1 General validity of the results
The presented experiments were successful, as they allowed the initiation of single micro cracks
and the monitoring of current signals during micro crack initiation and early propagation. Crack
depth measurements confirmed the assumption of basically semi-circular micro crack shapes, as
it was assumed in earlier studies [109].
The inspection of micro cracks in the SEM revealed exclusively IG SCC on most surface spots.
In some cases, additional corrosion along large inclusions occurred, but these cases could be
easily identified. Together with the correlation of charge transfer values with micro crack
dimensions, this leaves little doubt that the measured current signals were caused by anodic
dissolution of metal along the grain boundaries, and the course of the current signal reflects the
propagation of a micro crack.
Crack propagation was not restricted to the surface area exposed to the electrolyte: The
electrolyte continuously filled the growing crack enclave, and crack growth continued underneath
the silicone seal of the capillary. An influence of the increasing solution resistance with crack
growth was not found: Cracks that grew well beyond the walls of the capillary still showed the
same ratio of charge to estimated crack wall area.
4.7.2 Variation of OCP values on passive surface spots
A factor of uncertainty for the microcapillary technique is the quality of the microcapillary tip,
and the comparability of measurements with different capillaries: Any inconsistency in
experimental results is easily attributed to influences of flawed capillaries. This uncertainty was
mostly avoided in the presented study: Almost all measurements were performed with the same
60 µm capillary. Comparability of measurements was improved further during the array
measurement presented in Section 4.5. These measurements were not only performed with the
same capillary, but also on the exact same sample surface, and any differences between samples
introduced during surface preparation were avoided. The found variation of OCP values on
passive surfaces, ranging from -100 to +100 mV/SCE, can therefore most likely be attributed to
inhomogeneities of the surface oxide layer. Small variations in the exchange current density
could then result in the observed potential variation. This would be intrinsic to the studied surface
and could be influenced by the type of surface preparation. Another possible reason for the
observed OCP fluctuations between measurements is a variation in oxygen concentration at the
capillary tip between measurements. The oxygen concentration was not measured and no data on
its variation is therefore available.
84
Currently, a work in progress with J. DeRose at EMPA studies this variation of OCP and pitting
potentials during microcapillary measurements on stainless steel and aluminium. It could already
be established, that the use of ethanol instead of water during grinding of the surface reduces the
spreading of OCP values on stainless steel. This can be explained by a more homogeneous oxide
layer forming during surface preparation. Further studies are needed to quantitatively assess
these effects.
4.7.3 Micro crack initiation
Crack initiation occurred on sensitised samples above and below σy. The current signals related to
the very first initiation events could not be monitored, as initiation often set in immediately when
the capillary was positioned. The location of crack initiation on the studied material generally
confirmed the known possible weak spots, as described in literature [36, 46]: Initiation was
observed at inclusions, surface defects and, in case of vibration polished surfaces, on any grain
boundary due to widespread IG corrosion. For this special case, the initial statement in Section
4.2 has to be corrected: Apparently the presence of a grain boundary can be sufficient for IG SCC
initiation, depending on the surface preparation. Crack initiation obviously strongly depends on
the surface state. Any observation concerning the surface weaknesses responsible for crack
initiation might only be valid for one specific type of surface preparation. This has to be taken
into account for the comparison of microscopic and macroscopic results, which are discussed in
Section 6.
For samples that were ground with 4000 grit SiC paper, MnS-inclusions were by far the
dominating initiation sites on the surface. The results shown in Section 4.2 imply that MnS-
inclusions were still intact after plastic deformation of the surface. Fracture of inclusions occurred
during measurements and was very likely not involved in the initiation process. This observation,
together with the tendency of MnS-inclusions to be attacked even on unloaded specimens,
heavily implies that crack initiation was caused by localised corrosion at the metal-inclusion
interface.
Besides cracks growing from MnS-inclusions, a small number of cracks initiated on surface
spots, which did not exhibit any visible weakness. The underlying cause for crack initiation on
these sites could not be found. An initiation due to solely mechanical film rupture seems unlikely.
The experiments presented in Section 4.6.2 have shown that crack initiation could still occur on
surfaces that were exposed to aerated water for several weeks after grinding and deformation.
During this time, any ruptured surface oxide film would have had ample time to reform. A more
85
probable explanation for crack initiation would be the formation of metastable pitting at a grain
boundary triple point or a weakness in the surface oxide layer.
4.7.4 Micro crack propagation
The main characteristics of current signals during micro crack propagation are current transients
with a fast rise and slower decay, and the occurrence of passive phases, followed by reactivation,
without any external change of the applied stress. A detail view of a potentiostatic current
measurement, illustrating these features, is shown in Figure 47. A mechanism must account for
these findings. As described in Section 2.1.2, two main theories for crack propagation need to be
considered: a mechanism based on anodic dissolution, and a mechanism based on brittle fracture.
Figure 47: Detail view of a superposition of current peaks. This crack propagation event exemplarily shows the
typical fast current rise, fluctuating current signal consisting of several superimposed events, and repassivation.
Anodic dissolution
In the case of anodic dissolution, each current peak during a measurement represents a
dissolution event at the crack front. The superposition of many single transients indicates several
simultaneous dissolution events along the crack front, which continuously appear and repassivate.
This is in good agreement with the studies from Newman et al. [132]. They concluded that along
a crack front, even within one grain, all possible stages of the film rupture mechanism are present
simultaneously.
While single events are occurring along a crack front, stress distribution along the crack front is
constantly changing, and the crack opens further, which might trigger further film rupture events.
When all single dissolution events repassivate and no new active dissolution events are formed,
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crack growth arrests, and the surface shows fully passive behaviour. This might happen, when a
complete grain boundary is dissolved and the triple points of the next grains are reached, where
metal dissolution becomes less favourable. As soon as the passive film on the crack front is
ruptured, crack growth reinitiates and proceeds. Integration of the transient group shown in
Figure 47 gives a charge of 8 µC, equivalent to a crack with a surface length of approximately
90 µm. This is in the typical size range of grain boundaries. The single transients that form the
whole superposition therefore account only for a fraction of metal dissolution of a single grain
boundary.
A crack growth mechanism based on anodic dissolution depends on the formation of a corrosion
cell at the crack front. In the studied system the formation of a corrosion cell can be explained by
the microstructure of the studied material: The crack front consists of the grain boundary metal,
which is depleted in chromium and is therefore susceptible to dissolution. The crack walls
quickly repassivate, due to their higher chromium concentration. Hence, crack propagation is
limited to a narrow metal layer, and an electrochemical opening of the crack by metal dissolution
is not possible. The occurrence of IG corrosion on highly polished samples (see Section 4.6.1),
independent of applied stress, further supports an anodic dissolution mechanism. Obviously the
electrolyte is capable of attacking the sensitised grain boundaries even in the absence of external
stress. Application of stress might merely promote this process by opening the formed cracks and
accelerating the diffusion of electrolyte and reaction products, and by mechanical rupture of the
passive film. While the used electrolyte is already quite aggressive, the crack geometry further
promotes the formation of a chemical environment which favours crack propagation. The narrow
crack crevice limits diffusion processes. Dissolved oxygen in the electrolyte is quickly consumed,
which hinders repassivation. Additionally, dissolved metal ions are hydrated and release protons,
lowering the pH in the crack and creating a more aggressive local chemistry.
The main question remaining is the nature of the film rupture events after a complete passivation
of the crack front. It is not clear, which effect is responsible for this reactivation. Film rupture
occurred under constant deformation conditions, a purely mechanical cause therefore seems less
likely. The formation of local aggressive chemical environments, i.e. a “chemical” weakening of
the passive film is a probable trigger for film breakdown and re-initiation of a passive crack front.
For such a mechanism, no need arises to invoke the effect of slip planes on crack growth
reactivation.
87
Brittle fracture
A mechanism based on hydrogen embrittlement might explain the observed current signals as
well. Atomic hydrogen is formed inside a crack by reduction of protons. This is promoted by
highly acidic conditions inside the cracks due to metal dissolution and subsequent hydrolysis of
metal ions. Reactivation after a passive phase can be readily explained by a brittle fracture event
that takes place after a sufficient amount of hydrogen is absorbed in front of the crack tip.
Possible mechanisms for embrittlement along the grain boundaries are hydrogen formation and
enrichment along the grain boundaries, or the reaction of hydrogen with segregated alloy
components and subsequent decohesion of the grain boundary. After a brittle fracture event, the
crack opens and crack walls are attacked by the electrolyte. The chromium-depleted metal is
dissolved, leading to the observed ratio of charge and crack dimensions. Repassivation of the
crack walls occurs when the chromium content of the base metal exceeds a minimal value. The
accelerating effect of the tetrathionate electrolyte can be explained by the catalysis of hydrogen
absorption by adsorbed sulphur, as well as a retarded repassivation (see also Section 2.1.2). The
acidity of the electrolyte might further promote hydrogen evolution. According to Gomez-Duran
and Macdonald [58], hydrogen embrittlement would likely cause fracture dimensions that
correspond to the spacing of certain microstructural features, e.g. the grain size. The current
signals found in microcapillary measurements are composed of much smaller single events,
which are superimposed and form the peak groups as shown in Figure 47. This would imply
brittle fracture events of only fractions of grain boundaries.
A final dismissal of any mechanism is not possible. Both mechanisms may explain the observed
IG cracking along boundaries, and the correlation of crack dimensions and detected charge. A
mechanism based on film rupture and anodic dissolution does not explain the nature of the
reactivation events under constant deformation conditions, but this can be attributed to low
temperature creep or chemical reactions with the oxide film. The occurrence of IG corrosion on
certain surfaces proves the susceptibility of the sensitised grain boundaries towards anodic
dissolution, and the application of stress might accelerate the dissolution process along a crack
path. The hydrogen embrittlement mechanism does not explain IG corrosion on unstressed
polished surfaces, and it would suggest larger current transients, which correspond to fracture
dimensions in the range of whole grain boundaries. The film rupture mechanism and crack
propagation by anodic dissolution therefore suggests itself more than a hydrogen embrittlement
mechanism. Further studies are necessary to better understand the chemical reactions of
tetrathionate and its decomposition product at the crack tip.
88
4.7.5 Limitations of the presented experiments
The established measuring technique required a rather time consuming scanning process of the
surface to find active surface spots. This made it difficult to establish a large data set for
statistical evaluation. Furthermore, current signals measured on active cracks showed large
variations due to the local microstructure, e.g. orientation of grain boundaries and presence of
inclusions. Therefore only few parameters were investigated, and statistical evaluation showed
large error bars. On the other hand, the fact that these variations between initiation points can be
studied with this method is remarkable, and no other method so far allows the study of single
micro crack growth.
It was avoided so far to make a statement on the density of active sites on the differently
sensitised materials. It would be an interesting question, how the applied stress or the DOS of the
material influence the number of initiation sites. But the number of active sites on a selected area
depended to a high degree on the abundance and size of MnS-inclusions, two parameters that
varied greatly between samples, independent of stress and DOS. This is not a problem of the
technique, but a property of the studied phenomenon. Multiple array measurements, as described
in Section 4.5, with arbitrary positioning of the capillary, could establish a database large enough
to make a valid conclusion on this point.
Another limitation was the limited control of the applied stress. The simple three point bending
cell used for all microcell measurements only allowed for constant deformation at the beginning
of a measurement. A more advanced experimental setup, with the ability to apply active loading,
might allow CERT tests with less aggressive electrolytes on a microscopic scale.
4.8 Summary It has been demonstrated that the early stages of crack growth of single micro cracks can be
monitored over hours by electrochemical means. The microcapillary technique proved to be well
suited for the study of single micro cracks in stainless steel. The flexible positioning and spot by
spot scanning for active behaviour made it possible to measure selectively on active crack
initiation sites.
The specific current signals of single growing cracks can be explained satisfactorily by a crack
growth mechanism of film rupture and anodic metal dissolution along the sensitised grain
boundaries. However, the results of the presented experiments did not allow proving or
dismissing a specific mechanism for IG crack propagation in the studied system.
Some information was obtained on the location and cause of crack initiation, but experiments
with varying surface preparation illustrated, that this information is specific for the investigated
89
sample and its surface state only. A comparison of initiation sites between microcapillary
experiments and the corresponding macroscopic experiments was therefore limited. The initiation
of cracks in absence of any visible surface weakness, such as a MnS-inclusion, could not be
explained satisfactorily. A detailed surface analysis to assess such potential weak spot would
have to be done before the actual electrochemical measurement.
90
5 Macro scale EN measurements
This section presents the results of EN measurements, performed on macroscopic samples under
CL and slow strain rate, exposed to the tetrathionate electrolyte. Selected measurements are
presented to illustrate the variety of EN signals recorded. Typical signal features that were
repeatedly encountered during these measurements are analysed and assigned to different
corrosion processes.
A series of challenges arose during these tests, partially due to the change from the initially used
thiosulphate electrolyte to the more aggressive tetrathionate solution (Section 3.5.4). Despite the
large number of experiments performed, only a small number were regarded as successful. These
experiments, running between 24 and 250 h, provided a large data set of different EN signals,
which are introduced in this section. For identification, all measurements were labelled “RTL”
(room temperature loop), followed by a two digit number. An overview of all experiments
presented in this section is given in Table 6.
Table 6: Overview of selected macroscopic EN measurements. All measurements were performed in 0.01 M K2S4O6
at pH 2.2, except RTL 30 (pH 1.8).
Designation DOS Load type Time under CL
Remarks on EN signal Surface inspection
Shown in
RTL 30 (pH 1.8)
22% CERT High activity during the whole measurement, I-transient > 10 µA, U < -300 mV/SCE
severe IG SCC, pitting, IG corrosion
Figure 23
RTL 47 22% CERT I-rise and U-drop during loading ramp
severe IG SCC, extended pitting
Figure 48
RTL 42 15% CERT fast transients during loading ramp
micro cracks Figure 49
RTL 53 0.3% CL 250 h passive signal - Figure 50 RTL 44 15% CL 215 h stepwise signal changes
after 170 h. no large transients.
micro cracks Figure 51
RTL 43 22% CL 24 h stepwise signal changes, I-transient > 1 µA
severe IG SCC, pitting
Figure 52
RTL 49 22% CL 72 h high density of micro transients two I-transients > 1 µA
severe IG SCC, extended pitting
Figure 54
RTL 51 22% CL 72 h Stepwise signal change, I-transient > 10 µA
severe IG SCC, pitting
Figure 56
91
5.1 Results of CERT tests Initial experiments were performed under constant extension rate conditions. The first
macroscopic EN measurement in tetrathionate solution was already introduced in Section 3.5.4
(Page 54): A highly sensitised specimen was exposed to 10 mM K2S4O6 at pH 1.8. Large fracture
events occurred during this CERT test after 15 h of dynamic loading, reaching 160 MPa. Failure
of the sample occurred after 18 h. Besides extended IG SCC, the sample exhibited IG corrosion
on parts of the exposed surface (Figure 24). These additional corrosion processes contributed to
the overall EN signal and caused the high currents immediately after the sample was exposed to
the electrolyte. For all following experiments a pH value of 2.2 was chosen to reduce the
aggressiveness of the electrolyte. CERT measurement performed under these milder conditions
exhibited passive phases with few transients before the constant extension rate phase, as can be
seen in Figure 48 (RTL 47). Accelerating crack growth at high loads was indicated by several fast
transients and finally a current rise above 1 µA and current drop by 400 mV. Slow current rise
and potential drop, as well as single current transients of several hundred nA, indicated
approaching macroscopic cracking (Figure 48c). Unloading stopped crack growth, and
accordingly the current and potential signals returned to their initial values. Post-test inspection of
these samples in the SEM still revealed extended pitting corrosion on the whole exposed surface
and IG SCC at the notch root of the sample.
92
Figure 48: EPN (green) and ECN (red) measurement during a CERT test (RTL 47) on highly sensitised material
(DOS: 22%) in 0.01 M K2S4O6 (pH 2.2). Applied strain is indicated blue. a) Overview over 65 h. b) High-pass
filtered (0.01 Hz) AC signal of the measurement. c) Detail view of transients that occurred before current rises above
1 µA and potential drops by 400 mV. Transients indicated with numbers were integrated for further evaluation
(Section 5.3 and Appendix III).
CERT experiments on partially sensitised material (15% DOS) showed markedly less activity
than on the highly sensitised material, as shown in Figure 49 (RTL 42). No continuous crack
growth set in. Current transients with fast rise and exponential decay appeared during the
constant extension rate phase, at stress levels above σy, and no further transients appeared after
unloading of the sample. The raw EPN signals in Figure 49 only shows a very weak potential
response to fast current transients, which cannot be discerned easily from the base line potential
fluctuations. The high-pass filtered signal (Figure 49b) clearly shows corresponding potential
transients. SEM investigation revealed micro cracks and very few pits.
93
Figure 49: a) EPN (green) and ECN (red) measurements of a CERT test (RTL 42) on partially sensitised material
(DOS: 15%) in 0.01 M K2S4O6 (pH 2.2). Applied strain is indicated blue. b) The high-pass filtered (0.01 Hz) AC
signal clearly shows both potential and current transients.
5.2 Results of CL tests CL tests were performed according to the schematic shown in Figure 27 (page 59). Samples were
stressed to the desired level from the beginning of an experiment. During the stabilising phase,
the experiments were exposed to distilled water only. In ideal cases, EN signals during these
stabilising phases reached constant values after a few hours. At the end of the stabilising period, a
concentrated solution of tetrathionate and sulphuric acid was added to the electrolyte loop to
establish the final concentration of 0.01 M K2S4O6 and pH 2.2. The manipulations on the setup
during addition caused vibrations of the system and induced potential and current fluctuations for
a limited period of time.
5.2.1 General behaviour of solution annealed samples
The solution annealed material showed very little activity when exposed to tetrathionate. It did
not suffer from IG SCC in the investigated time frame, and metastable pitting corrosion only
occurred scarcely. A typical measurement on solution annealed material is shown in Figure 50
(RTL 53). The sample was stressed to 275 MPa, close to σy (290 MPa). Current fluctuations over
250 h are below 10 nA, and the signal exhibited stable phases below 1 nA over hours.
94
Figure 50: EPN (green) and ECN (red) measurement of a CL test (RTL 53) at 275 MPa on solution annealed
material (DOS: 0.3%) in 0.01 M K2S4O6 (pH 2.2). The signal exhibits very few fluctuations over a time span of
250 h (11 d).
5.2.2 General behaviour of sensitised samples
EN measurements on sensitised samples generally showed increased activity compared to
solution annealed material, with highly sensitised material being more active than the partially
sensitised one. Little activity was detected during the stabilising phase in distilled water.
Transients often appeared directly after the addition of tetrathionate to the electrolyte
recirculation loop. ECN measurements showed distinct current transients in positive directions
and EPN measurement showed corresponding potential drops as seen in CERT tests. Under CL
conditions, samples exhibited highly non-stationary signal behaviour: In many cases, activity
increased with time, indicating consecutive corrosion attack and increasing crack growth.
Transient shapes, amplitudes and occurrence frequencies varied not only between the different
materials, but between each measurement, even under identical experimental conditions.
CL experiments on partially sensitised material (DOS 15%)
Two CL experiments on partially sensitised material are shown in Figure 51a and b. The first
experiment, designated RTL 44, was performed at 230 MPa. EN signals showed fluctuations over
150 h of exposure to tetrathionate, after which rising current and a potential drop were observed.
During a mostly passive period, the reference electrode leaked and ran dry, leading to data loss of
the EPN signal. The following experiment, designated RTL 48, was performed at 275 MPa, but
showed less activity than the previously presented experiment. This contradictive result indicates
that variations in surface micro structure have a large influence on the corrosion resistance of a
specific sample. Few single transients occurred over 200 h of passive behaviour. SEM
investigation revealed micro cracks and few pits on both samples, spread over the whole exposed
surfaces.
95
Figure 51: EN EPN (green) and ECN (red) measurement during CL tests on partially sensitised material (DOS:
15%) in 0.01 M K2S4O6 (pH 2.2). a) RTL 44, performed at 230 MPa. b) RTL 48, performed at 275 MPa.
Selected CL experiments on fully sensitised material (DOS 22%)
CL tests on fully sensitised material at nominal loads of 230 MPa or higher showed macroscopic
cracking in the studied time range. RTL 43, shown in Figure 52, was performed at a constant
stress of 230 MPa. The disturbance of the signals at 0 h was caused by manipulation on the setup
during the addition of the electrolyte. As in the case of RTL 47 (Figure 48), macroscopic cracking
was preceded by a baseline current increase and fast transients of several hundred nA, and
corresponding potential drop. The sample was unloaded and removed after 24 h. SEM images of
the samples taken after the measurement confirmed IG SCC in the notch exposed to the
electrolyte (Figure 53). Only little pitting corrosion appeared on the sample surface. The sample
was mechanically overloaded to complete fracture, to reveal the crack wall areas. Figure 53b
shows a fracture plane. The region of IG cracking can clearly be discerned from ductile fracture.
96
Figure 52: EPN (green) and ECN (red) measurement during a CL test (RTL 43) on highly sensitised material (DOS:
22%) at 230 MPa in 0.01 M K2S4O6 (pH 2.2). a) Overview over 3.5 h, until large transients occurred. b) High-pass
filtered (0.01 Hz) AC signal of the measurement. c) Detail view of two transient groups that occurred before a large
current rise and potential drop. Transients indicated with numbers were integrated for further evaluation (Section 5.3
and Appendix III). d) High-pass filtered detail view.
97
Figure 53: SEM images of the sample from RTL 43. No cracks were visible directly after unloading. a) Two IG
cracks appeared in the notch after additional mechanical loading in air after the experiment. b) Mechanical
overloading to complete fracture of the sample revealed the crack wall area.
In a subsequent experiment, labelled RTL 49, the load was increased to 275 MPa (Figure 54). A
series of current transients below 40 nA occurred directly after the addition of tetrathionate and
the current baseline showed a slow increase. These small signals were not accompanied by an
increasing baseline current, and they were followed by a passive phase. As already observed for
experiments on partially sensitised material, a higher load did not necessarily lead to faster
macroscopic cracking.
Initial fast transients < 40 nA are shown in detail in Figure 54c. Such small and fast transients
often did not appear in the unfiltered potential signal. The high-pass filtered signals in Figure 54b
and d clearly show both current and potential transients. These fast signals are likely caused by
metastable pitting, which set in immediately after the surface came in contact with the
tetrathionate solution. SEM images of RTL 49 (Figure 55) are in agreement with this
interpretation. Besides extended IG SCC, the surface showed circular pits over large areas of the
exposed notch (Figure 55b).
98
Figure 54: EPN (green) and ECN (red) measurement during a CL test (RTL 49) on highly sensitised material (DOS:
22%) at 275 MPa in 0.01 M K2S4O6 (pH 2.2). a) Overview of the whole measurement. b) High-pass filtered
(0.01 Hz) AC signal of the measurement. c) Detail view of the initial phase, exhibiting a high number of small
current transients. Transients marked by numbers were integrated for further analysis (Section 5.3 and Appendix III).
d) High-pass filtered signal of fast transients.
99
Figure 55: SEM image of RTL 49 after the measurement. a) The mechanically overloaded sample reveals extended
IG cracking along the notch. b) The surface exhibited a high number of pits. Some of them were scattered over the
whole exposed area and exhibited mostly circular shapes, while others were concentrated along grinding grooves,
forming surface cracks.
RTL 51, presented in Figure 56, was conducted under identical conditions as RTL 49, shown
before, at a nominal stress of 275 MPa. It exhibited a distinctly different behaviour, which was
confirmed by the EN signal and the inspection of the sample in the SEM after the experiment.
After an initial disturbance during addition of tetrathionate, this measurement exhibited very clear
stepwise transitions from passive behaviour to different levels of activity, finally resulting in a
current jump above 11 µA and potential drop below -300 mV. After this large transient, constant
current rise and potential drop indicated the transition to continuous, accelerating crack
propagation. High-pass filtering of the signal clearly shows the large current and potential
transients, as well as the transition from passive to active behaviour at 25 h (Figure 56b and d).
Detail view of the transition from passive behaviour to macroscopic crack growth is shown in
Figure 57. It illustrates the order of magnitude of the stepwise change in current. The initial
change in current by 50 nA is barely visible, and the following current jumps of 150 and 200 nA
are still small compared to the final current jump by 10 µA at 55 h.
100
Figure 56: EPN (green) and ECN (red) measurement during a CL test (RTL 51) on highly sensitised material (DOS:
22%) at 275 MPa in 0.01 M K2S4O6 (pH 2.2).a) Overview of the measurement. b) High-pass filtered (0.01 Hz) AC
signal of the measurement. c) Detail view of the transition from passive to active behaviour. d) Detail view of the
high-pass filtered signal.
101
Figure 57: Detail view from Figure 56. The transition from passive behaviour to micro crack propagation and to
macroscopic cracking illustrates the different scales of signal jumps. After macroscopic cracking sets in, the current
rises continuously until the sample is unloaded.
SEM investigation of the sample revealed IG cracking almost completely around the
circumference of the sample (Figure 58a). Mechanical overloading in air after the measurement
revealed that IG SCC extended over ca. 70% of the sample cross section (Figure 58b).
102
Figure 58: a) SEM images of RTL 51 after the measurement. IG cracking occurred along the notch. Additional pits
can be found in some regions of the notch. b) The sample was mechanically overloaded to complete fracture after the
measurement. The fracture plane reveals extended IG SCC over approximately 70% of the sample cross section.
5.3 Interpretation of current transient shapes The main tool for the interpretation of EN data was the analysis of single transients. Current
transients were characterised by amplitude, duration and shape. Selected signals were integrated
separately to calculate the respective charge transfer values and estimate the extent of the
corrosion damage related to these discrete events.
5.3.1 Classification of transient types
Each macroscopic EN measurement exhibited a unique signal pattern. To facilitate the
interpretation of the various signals, which occurred over the course of these measurements,
classes of similar current transients are introduced. Figure 59 gives an overview of the three main
types of signals that were extracted from ECN signals. These classes are by no means absolute or
complete. However they encompass reasonably well the majority of transients that occurred.
103
Figure 59: Typical current transient shapes encountered in CERT and CL tests.
Type I: Peaks of short duration (1 – 5 s) and low amplitude (< 50 nA). These signals might indicate metastable
pitting or micro crack initiation and growth steps.
Type II: Series of superimposed transients, typically exhibiting fast rise and slow decay, and lasting 10 – 30 min. No
single transients can be isolated. Amplitudes are in the range of 30 – 50 nA.
Type III: Single transients of longer duration (5 s – 5 min) and high amplitude (50 – 1000 nA). These signals were
often detected before larger current transients (> 1µA).
Type I incorporates fast and small current peaks that were encountered during many
measurements on sensitised material (both 15% and 22% DOS). Only few of these transients
appeared on solution annealed material. They often appeared directly after the addition of
tetrathionate to the electrolyte system during CL tests. Transients exhibited durations of a few
seconds. Amplitudes were mostly below 20 nA, occasionally reaching higher values. Transient
shapes showed variations and fell generally in two categories: Either a slowly increasing current
rise was followed by an abrupt decay, or the inverse shape occurred, with a sudden current rise
and a slower (exponential) decay (Figure 60). These transient shapes might be explained by two
different underlying mechanisms: Type Ia (Figure 60a) shows the characteristic shape expected
for a film rupture event, with sudden exposure of bare metal causing fast current rise, followed
by exponential decay due to continuous repassivation [133]. Such transients might be related to
micro crack initiation and growth steps, IG corrosion events or the formation of closely packed
pits along grinding groves. Type Ib (Figure 60b) on the other hand exhibits a transient shape that
is characteristic for metastable pitting. As described in [134], the current rises fast with pit growth
after initiation. Pits grow by undercutting the passive film, until the pit breaks open and the local
composition of electrolyte inside the pit is disrupted. This immediately stops the corrosion
process, leading to a sudden current drop.
104
Figure 60: Type I current peaks, exhibiting similar amplitude and duration, but inverse shapes:
a) Fast rise and exponential decay. b) Slowly accelerating rise and sudden decay.
Large numbers of small pits were found on samples after experiments which exhibited high
densities of these transients. A particularly high number occurred during experiment RTL 49
(Figure 54). This corresponds with a high number of circular pits scattered over the whole
surface, found after the experiment (Figure 55b). Fewer pits occurred on the partially sensitised
material to some degree. No pitting corrosion was found on crack wall areas of IG cracks that had
formed (Figure 61a). Only the machined outer surface was attacked. Occasionally very high
densities of shallow pits occurred, ultimately resulting in uniform corrosion of limited surface
areas (Figure 61b). Pits in the notched area were often lined up in grinding groves or surface
scratches; closely packed, they formed kinds of micro cracks through the top layer of the surface
(Figure 55b). These attacked groves could therefore serve as initiation sites for IG cracking. As
opposed to typical pits, these surface damages often did not exhibit circular shapes. They more
closely resembled very small cracks. It is likely that stress accelerated the formation of these
specific pits via film rupture.
105
Figure 61: a) High density of metastable pits along grinding groves, next to an IG crack. The crack wall area shows
no significant pitting. b) Extended formation of shallow micro pits caused uniform corrosion on some surface areas.
Type II transients prominently appeared in experiment RTL51, shown in Figure 56, where a clear
transition from a passive signal to an active signal occurred. No single transient can be discerned,
as many events are superimposed, but a grouping of events can be observed, exhibiting fast
current rise and slow decay. Between these transient groupings, the decreasing current signal
approaches the passive level, suggesting a discontinuous behaviour of the underlying processes.
Maximum amplitudes of the transient groups are in the range of 30 – 50 nA, and their duration
ranges between 10 – 30 min. The current signal in experiment RTL 51 shows a transition at 41 h
(Figure 56c and detail views in Figure 62a and b). The frequency of events increases and the
grouping becomes less clear. The signal does not reach values close to the passive level any
more, which leads to an overall rise in baseline current (Figure 62b). Similar signals appeared in
several measurements, often at an elevated base current (RTL 44, Figure 62c). Such signals are
likely caused by simultaneous metal dissolution processes at multiple active sites. The fast rise
and slow decay of the observed transient groups might be explained by a film rupture mechanism
exhibiting extended periods of anodic dissolution before repassivation occurs. This corresponds
to the propagation of IG micro cracks along single grain boundaries: Film rupture initiates the
dissolution, which then proceeds along one or several grain boundaries until it stopped, possibly
at a grain boundary triple point. This interpretation is supported by the current signals detected
with the electrochemical microcapillary technique under potentiostatic conditions: They showed
very similar transient patterns, which could be attributed to the propagation of single micro
cracks.
106
Figure 62: Type II transient series over 10’000 s (3 h). a) Excerpt from the current measurement of RTL 51, shown
in Figure 57 (27 – 30 h). Single transients could usually not be discerned, but transient groupings appear. The signal
baseline is close to 0 nA. b) Excerpt from the same experiment (41 - 44 h): An increase in transient density occurred,
leading to a rise in baseline current. c) Excerpt from a measurement on partially sensitised material (RTL 44):
Superposition of an increased baseline current with type II transients.
Type III transients (Figure 63) reached amplitudes of 50 – 1000 nA. Transients lasted from a few
seconds to several minutes. Typically, a fast current rise is followed by differently shaped decays.
Current decays exhibited different shapes, e.g. a sudden drop to the initial base line, an
exponential decay, or a “chair”-like shape, where current slowly approaches an elevated value,
and final repassivation is delayed (Figure 63a-c). Some type III transients did not show a
complete decay back to the initial current level. Instead, baseline current stabilised at an increased
107
level, creating a step shape in the signal (Figure 63d). Step sizes that occurred during CL
experiments before large current transients ranged between 5 and 300 nA.
The distribution of type III signals during measurements was non-uniform. Single transients
could occur occasionally during a measurement, but more often these transients were grouped
together: During some CERT and CL experiments on fully sensitised material, such transient
groups appeared right before large current jumps (as seen in Figure 48 and Figure 52), where they
apparently mark the transition of micro crack propagation to macroscopic cracking. A possible
explanation for these transients under CL conditions is the coalescence of micro cracks, leading
to the accelerated dissolution of metal along a whole grain boundary, or even several grains.
Series of type III transients could then be explained as a cascade of coalescence events, which
increase the real stress on the sample, due to the continuously decreasing surface cross section
during crack growth. These series of transients could then finally culminate in macroscopic
cracking. Wells et al. [109] uses a simple geometrical model to explain the shapes of current
transients, assuming a semicircular crack front progressing along a grain boundary. A transient
shape similar to the one shown in Figure 63c was derived, explaining the “chair-like” shape as
caused by the limitation of a single grain boundary.
Type III transients also prominently occurred during CERT tests on partially sensitised material
(Figure 49). Under these conditions, type III transients were likely triggered by mechanical
rupture of the protective oxide layer on the surface.
108
Figure 63: Illustration of typical type III transients. a) Fast rise and fast decay. b) Fast rise and exponential decay. c)
Fast rise and delayed current drop. d) Series of type III current transients occurring before a current jump above
1 µA. Dotted lines indicate stepwise baseline current increase following some of the transients.
As initially stated, the classification of transient types into three categories is incomplete and to
some extent ambiguous. Transients of type Ia and type III could exhibit the same typical shape of
fast rise and exponential decay: The only discerning criterion may then be the amplitude. Current
transients with amplitudes in the range of 20 – 100 nA could therefore not clearly be attributed to
one or the other type. The transition from type Ia to type III is continuous, which implies that
micro crack growth events range from fractions of single grain sizes to whole grain boundaries or
even several grains.
109
5.3.2 Transition to macroscopic cracking
Macroscopic cracking occurred on highly sensitised samples and was indicated by fast current
and potential fluctuations with amplitudes of several µA and several hundred mV. Repassivation
could still occur after these events during CL experiments, but SEM inspection showed IG cracks
with lengths of several hundred µm.
The detail view of such a transient in Figure 64 shows that the current rise, albeit steep, is not
sudden, but rather smooth and constantly accelerating. The current rises over a period of ca.
240 s. This indicates that this large transient is not caused by a single brittle fracture event of the
sample, which is followed by repassivation of the exposed crack walls. The transient shape is
better explained by a continuously growing crack. With the expanding crack front, the number of
film rupture events rapidly increases, and metal dissolution along grain boundaries is accelerated.
Finally, repassivation becomes faster than crack growth/metal dissolution, and the current drops.
Figure 64: Detail view of the large transient that occurred during experiment RTL 51 (Figure 56).
5.3.3 Integration of characteristic current signals
To verify the interpretation of current transients introduced above, typical current transients of all
types were integrated over time to calculate the amount of charge. The equivalent volume of
dissolved metal was calculated using Faraday’s Law, as described in Section 4.4 (Page 72).
Macroscopic ECN curves generally showed too much activity to assign single surface defects to
certain current peaks. A proper correlation of crack wall area and charge value was only possible
for experiment RTL 51: the sample was overloaded in air after the measurement and the area of
110
the sample cross section that showed IG cracking was estimated using the SEM. To account for
the intergranular surface roughness, an estimated correction factor of 1.41 was applied. A crack
wall area of 58 µm2 was found. Integration of the ECN curve gave total charge of 0.15 C, which
is equivalent to 5.5 · 10-3 mm3, using Faraday’s law. From these values, a mean crack width of
94 nm was calculated. This estimation of the cracked surface was conservative, as only the
visible crack wall area could be taken in account. Therefore the result agrees reasonably well with
the value of 80 nm that was calculated on the base of microcapillary data. For further
calculations, this value is used. Charge values, equivalent metal volumes, theoretical crack sizes
and crack growth rates for selected current transients and transient groupings are listed in
Appendix III. For the calculation of crack lengths and their propagation rates, crack shapes were
approximated to be semi-elliptical, with a ratio of maximum depth to surface length of 0.47 7, as
described in 4.4.
Integration of type I transients gave values equivalent to metal dissolution of 0.026 – 6.8 µm3.
Based on the interpretation presented earlier, such volumes would account for hemispherical pits
with diameters of 0.5 – 3 µm, as proposed for type Ib, or micro crack growth steps of 1 – 15 µm
for type Ia. Both results agree with the expected sizes of these events.
Integration of type II signal sequences such as the one shown in Figure 62a gives dissolution
charge values of 120 – 150 µC. This is equivalent to crack lengths of 380 – 430 µm and would
account for quasi-continuous crack propagation along a few grain boundaries. Crack propagation
rates range from 0.053 to 0.067 µm/s. Signal sequences during the subsequent phase of increased
activity (Figure 62b) show slightly higher values of 0.067 – 0.071 µm/s. For single micro cracks,
monitored with the microcapillary, mean propagation rates of 0.02 – 0.04 µm/s were calculated,
and crack lengths varied between a few µm up to 450 µm. The values are in a similar order of
magnitude, which supports the proposed interpretation of these transients. The higher rates
measured on the macroscopic scale indicate, that either several micro cracks were propagating in
parallel, or several active spots along the crack fronts of micro cracks were active at the same
time.
Integration of type III transients gives theoretical crack lengths of 21 – 106 µm, which
corresponds to the typical grain sizes of the material (30 – 100 µm). These events exhibited
growth rates of 0.4 – 6.2 µm/s, which is considerably faster than type II transients. This implies
the formation of cracks along a few grain boundaries in a matter of seconds or minutes.
7 An alternative would be the assumption of semi-circular crack shapes, as it was done by Wells et. al.[109]. By assuming a semi-elliptical shape with 0.47, calculated surface crack lengths increase by 3.2%, which is insignificant, compared to the large variation between the transients.
111
5.4 Further analysis of EN signals 5.4.1 Comparison of statistical parameters
Mean and standard deviation were calculated for selected measuring periods of 1 h (from
different tests), exhibiting the characteristic transient types introduced in 5.3. Table 7 shows mean
and standard deviations for passive phases and phases exhibiting transients of types I-III. Mean
potential values allow an estimation of the activity of a specimen, with lower values indicating
higher activity. This worked well for the comparison of different sequences of the same
measurement, e.g. the phases of different activity during RTL 51. The comparison of mean
potential values between different experiments was less reliable, as OCP values varied between
samples. Mean current values gave a more reliable comparison of surface activities. While mean
values do not reflect the signal shape, the standard deviations of current and potential signals give
an indication of signal fluctuation: The lowest values for both current and potential were found
for passive states. Increasing values were found from type I to type II and up to type III
transients, which exhibited the highest values.
A remarkable difference can be seen between type III signals measured during CERT tests on
partially sensitised material (RTL 42 and 35), and the CERT and CL measurements on fully
sensitised material (RTL 43 and 47). Signals measured on the fully sensitised material exhibited
the highest mean currents and lowest mean potentials, as well as the highest standard deviations.
The sequences on partially sensitised material, although exhibiting high transients, showed low
mean current values and markedly lower standard deviations, confirming significantly less
corrosion on this material.
The results presented in Table 7 generally depended on the selected time interval. Single
transients were unevenly distributed, and depending on the number of transients in a specific time
interval, the statistical analysis will give differing results. The non-stationarity of the typical EN
signals therefore limits the validity of statistical parameters.
112
Table 7: Mean and standard deviation for selected EPN and ECN signals exhibiting characteristic EN patterns.
Sequence Sensitisation loading Signal type Potential Current
Mean (mV/SCE)
Std.dev. (mV)
Mean (nA)
Std.dev. (nA)
RTL 51, 14 – 15 h Fully sens. CL Passive 205 0.0955 -2.18 0.278 RTL 53, 175 – 176 h Sol. ann. CL Passive 190 0.239 -0.620 0.0228
RTL 49, 1 h – 2 h Fully sens. CL Type I 259 1.55 -0.141 1.24 RTL 42, 40 h – 41 h Part. Sens. CERT Type I 246 0.526 4.66 0.0905 RTL 51, 28 h – 29 h Fully sens. CL Type II 172 4.76 15.5 8.20 RTL 51, 29 h – 30 h Fully sens. CL Type II 165 4.52 17.5 6.99 RTL 51, 46 h – 47 h Fully sens. CL Type II 137 1.77 31.3 4.38 RTL 51, 47 h – 48 h Fully sens. CL Type II 137 2.07 29.5 4.71
RTL 44, 207 h – 208 h Part. Sens. CL Type II 68.8 0.930 159 4.73 RTL 42, 47 h – 48 h Part. Sens. CERT Type III 197 10.5 10.8 10.3 RTL 35, 24 h – 25 h Part. Sens. CERT Type III 201 0.206 9.45 9.41
RTL 43, 2 h – 3 h Fully sens. CL Type III 89.6 46.9 36.2 42.2 RTL 47, 63 h – 64 h Fully sens. CERT Type III 35.9 9.78 75.5 28.6
5.4.2 Analysis in the frequency domain
The macroscopic EN signals were transformed into the frequency domain, using FFT. PSD I
curves were calculated for the measuring periods selected in Section 5.4.1, according to equation
11 (Section 2.3.3):
2)(2)( fXT
f Tx =Ψ (11)
Typical curves are shown in Figure 65.
Figure 65: PSD I of 1 h periods of EN signals, exhibiting different characteristic transients. a) Comparison of PSD
of passive state (measured on solution annealed material) and type I-III transients (measured on highly sensitised
material). b) Comparison of different periods during the same experiment, exhibiting passivity and different phases
of type II transients.
113
The curves in Figure 65a show clearly different PSD levels and roll-off slopes for the different
signals. The highest PSD was calculated for type III signals, exhibiting a characteristic
fluctuation in the roll-off slope, which is due to the single events dominating the current signal
[99]. Clearly the lowest PSD is calculated for a passive phase on solution annealed material.
Figure 65b compares PSD spectra calculated from 1 h periods during phases of experiment RTL
51 (Figure 56). The passive phase (14 – 15 h) exhibits the lowest PSD. The other passive periods
showed similar values. It could not be discerned between the initial phase of type II transients,
which showed clearly grouped transients (28 – 29 h and 29 – 30 h), and the later phase of
increased frequency of type II events (46 – 47 h and 47 – 48 h). Type II signals at higher current
(e.g. shown in Figure 62c) give similar PSD curves.
The transformation of ECN signals to the frequency domain generally allowed distinguishing
phases of different activity, but it did not provide any advantage over the analysis of the signal in
the time domain for the present case. Like the statistical analysis, the results depend on the
selected time interval, due to the non-stationarity of the observed EN signals.
5.5 Discussion 5.5.1 General results
The presented macroscopic EN measurements confirm that the chosen electrolyte can initiate IG
SCC under CL conditions in a matter of hours or days. This allowed the monitoring of the
transition from a passive surface to micro crack formation and growth and finally to macroscopic
cracking without changing the applied load. Additional corrosion besides IG SCC occurred
during these measurements in the form of metastable pitting corrosion. Due to the rather high
corrosion activities of sensitised samples, it was not possible to assign a certain signal transient to
a specific surface damage. Thus, there is some degree of uncertainty in the interpretation of EN
signals.
Signals showed a highly non-stationary behaviour. Therefore the most part of signal analysis was
done by visual inspection of EN signals in the time domain and by integration of current
transients to calculate the equivalent dissolved metal volumes. Signal analysis in the frequency
domain or by statistical parameters highly depended on the selected time frame. Furthermore, the
signal shape and the time until initiation of IG SCC under CL conditions showed high variation.
This limited the possibility to derive an initiation mechanism or to predict the occurrence of IG
SCC for the investigated system (see also Section 5.5.4). Additional macroscopic experiments are
necessary to derive a statistical evaluation of crack initiation times.
114
5.5.2 Crack initiation under CERT and CL conditions
Metastable pitting corrosion was likely part of the initiation mechanism of SCC on the studied
samples. SEM images of cracked samples showed closely packed pits along grinding grooves.
The coalescence of these pits could lead to micro cracks in the upper surface layers, which could
then serve as initiation points for IG SCC. For experiments under CL, crack initiation was
therefore very likely triggered by ongoing corrosion processes on the surface, i.e. by
“(electro)chemical” initiation. This type of initiation was mainly governed by the electrolyte and
the surface micro structure of each sample. Variations in surface roughness and oxide thickness
between samples could then explain the observed variation of EN signals and crack initiation
time between experiments.
While metastable pits are most likely initiation points for crack growth under CL, crack initiation
under CERT tests could also result from mechanical rupture of the surface film at particular weak
points, i.e. by “mechanical” initiation. This occurred reproducibly at nominal stress levels near
σy, which made CERT experiments more predictable and therefore easier to perform. It was also
possible to initiate cracks on partially sensitised samples in CERT tests, while CL tests on this
material showed only little activity during the studied time spans.
Concerning a possible correlation of the shown results with IG SCC in real applications, both
“(electro)chemical” and “mechanical” initiation are not directly comparable to the conditions
present in a real system. Under real conditions, initiation is typically a very slow process and may
be caused by a combination of both mechanisms. But irrespective of the initiation event, the EN
signals measured during corrosion of a real system are expected to be similar to the signals
detected on the presented experimental system, as long as cracks in the real system propagate by
the same mechanism of anodic dissolution along corrosion-sensitive pathways. It cannot be
expected that systems which tend to suffer from other forms of SCC (e.g. TG SCC of 304
stainless steel in environments containing chloride [135]), exhibit comparable signals.
5.5.3 Crack initiation and propagation: mechanism and influence of stress
The EN signals encountered in this study could be satisfactorily explained as caused by anodic
metal dissolution. Small transients with slow increase and sudden decrease were attributed to the
formation and repassivation of metastable pits, as described in the literature [134]. Transients
with fast rise and slower decay were interpreted as events of film rupture and subsequent
repassivation [133]. While pitting corrosion was usually avoided in other studies by choosing less
aggressive electrolytes, the characteristic transient shape of film rupture events was reported in a
multitude of studies on IG SCC in AISI 304 stainless steel (see also Section 2.3.4): Wells et al.
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[109] performed EN measurements on AISI 304 stainless steel under CERT conditions in
thiosulphate solution: their ECN signals featured very low baseline currents, ranging below 2 nA.
This allowed the clear detection of distinct current signals similar to type II and III transients.
Current transients were successfully correlated with micro cracks. As mentioned in Section 5.3.1,
transient shapes for dissolution of single grain boundaries were derived from a simple
geometrical model, with results corresponding to type III transients with “chair-like” shape, that
were found in this study.
Current transients detected on the same material in thiocyanate solution under CERT conditions
again showed the common transient shape of fast rise and slow decay [57]: Transients exhibited
similar amplitudes as the type III transients in the present study. The amplitudes were reported to
correlate with the sample displacement rate, which was measured in parallel. A second signal
type was reported to occur, characterised as smooth current fluctuations, which often occurred
directly after a large transient. These smooth fluctuations were attributed to anodic dissolution
along grain boundaries. A rather high DC current above 7 µA is reported for these EN
measurements, which is attributed to pitting corrosion. Such high base current signals make it
highly unlikely to detect signals similar to type II transients, which might be attributed to the
propagation of a single micro crack.
The explanation of IG SCC proceeding only via film rupture and anodic dissolution is at odds
with the studies of Gomez-Duran and Macdonald, which were already introduced in section
2.1.2: In particular, EN signals similar to the type III transients, were attributed to hydrogen
embrittlement: These transients were found to be equivalent of IG cracks with lengths of 35 –
108 µm, similar to the values found in the present study. The authors claimed that such fracture
dimensions are too large to be consistent with a slip dissolution mechanism and are better
explained by brittle fracture events along single grain boundaries, noting that the dimension of
brittle fracture events should correspond to the spacing of some metallurgical asperity, such as
precipitates or grain boundary triple points [58]. The EN measurements in the present study
showed that during CL tests, these large transients of type III were preceded by the much smaller
type I and II transients and rising baseline currents. For these small transients, the argument of
fracture dimensions is not valid, and an explanation based on anodic dissolutions is more likely.
Type III transients are then explained as anodic dissolution events that are accelerated by high
local stress.
IG SCC in the studied system can therefore be described as a sequence of different phases, with
varying duration and intensity between experiments: Initiation and micro crack growth (type I
and II transients), micro crack coalescence (type III transients, stepwise signal changes), large
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fracture events (current transients beyond 1 µA), and transition to macroscopic cracking and
sample failure. A mechanism based on hydrogen embrittlement cannot be ultimately dismissed
for certain aspects of crack propagation, but it is not necessary for the explanation of the
encountered EN data.
It was expected that an increase in stress leads to faster macroscopic crack growth during CL
experiments, as it was found for microcapillary experiments. Such an influence of stress did not
clearly emerge from the performed experiments: Experiments at nominal loads below 230 MPa
did not show macroscopic cracking in the studied time frame. But increasing the stress from
230 MPa to 275 MPa did not necessarily lead to faster cracking in the experiments performed in
this study. And while most cracks on round notched samples initiated near the notch root, where
the highest local stress values occurred, cracks were also found on the notch walls, in regions of
significantly lower stress. Other influences, such as the specific surface morphology, apparently
had a large influence on crack initiation and propagation as well. A larger data set is required to
confirm a statistically significant effect of stress on the occurrence of macroscopic cracks.
5.5.4 Identification of IG SCC and use of EN for monitoring applications
The transition from passive behaviour to active metal dissolution could be detected very well in
both EPN and ECN signals. EPN signals required the application of high-pass filtering to resolve
fast transients with durations of only few seconds. No further signal treatment was necessary to
detect these fast events in the ECN signal.
For small transients, an unambiguous differentiation between metastable pitting and SCC
initiation was not possible in the studied system. For environments where no metastable pitting
occurs, signal interpretation is expected to be much clearer, allowing a reliable early detection of
film rupture events and subsequent micro crack growth.
Considering CL and CERT experiments on fully sensitised material, type III current transients
indicated the onset of macroscopic cracking and might serve as a warning sign. In RTL 47 (see
Figure 48), these transients appeared over a period of 12 min before a large current transient
exceeded 1 µA. Likewise, a warning time of 30 min passed in RTL 43 (Figure 52) between the
occurrence of type I transient series and a current jump above 1 µA. In RTL 49 and 51, no such
transients announced the upcoming macroscopic cracking events. Instead, stepwise signal
changes, which preceded macroscopic cracking, could serve as warning signs in these cases.
Type III transients on partially sensitised material occurred only during CERT tests at high stress,
probably due to mechanical film rupture events. No continuous crack propagation set in after
these events. Type III transients in these cases therefore did not indicate imminent macroscopic
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cracking. Their significance as warning signs for proceeding crack growth therefore depended on
the experimental conditions.
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6 Comparison of macro and micro scale
measurements
6.1 Direct upscaling: Macrocapillary measurements For the direct upscaling from micro to macro scale SCC testing, measurements with unpulled
capillaries were performed: The glass capillaries were not pulled to tips, but left at their initial
diameter, cut to the appropriate length, and coated with the silicone rubber in the same way as for
microcapillaries. The result was a macrocapillary with a diameter of 900 µm, exposing surface
spots approximately 250 times larger than the normally used 60 µm micro capillaries. The
application of these macrocapillaries allows a direct comparison of small and large scale
measurements using an identical experimental setup. The same measuring procedure as for micro
capillaries could be applied. Measurements with the two capillary types could therefore be
performed under identical conditions.
6.1.1 Passive surface behaviour
Unstressed samples generally showed passive behaviour (Figure 66). Passive surfaces could also
be found on few stressed samples, but most stressed surface spots showed active behaviour when
scanned with the large capillary diameter of 900 µm. The potentiostatic current measurements on
passive spots showed current densities that were similar to those found in microcapillary
measurements (-0.2 – 0 mA/cm2).
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Figure 66: a) Potentiostatic current measurement with a 0.9 mm macrocapillary on a passive surface spot of an
unstressed sample (DOS 22%) at -150 mV/SCE in 0.01 M K2S4O6 (pH 2.2). After an initial high current response to
polarisation, the current drops and shows a slow trend to more positive values. b) Detail view of the current signal,
showing fluctuations below the nA-range.
6.1.2 Current signals of active surfaces
Potentiostatic current measurements on active surface spots are shown in Figure 67a – d. Active
spots showed current densities in the range of -0.1 – +0.05 mA/cm2. The current densities on
active spots are lower than on active micro cracks, due to the much smaller ratio of active to
passive surface. Active surface spots exhibited additional anodic current of several hundred nA
superimposed on the cathodic sum current of passive surface sites. Single active cracks in
microcapillary experiments typically provided currents below 100 nA. Multiple cracks were
therefore active during these measurements. Due to the overlap of several signals, no passive
phases appeared during these experiments. Only few single transients appeared, and in most
measurements, no individual growth events could be discerned. After the measurements, the
investigated spots exhibited a slight dark sulphide precipitation. SEM images confirm that
multiple cracks were active simultaneously on the surface spot (see Figure 68a and b).
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Figure 67: Typical current measurements with the macrocapillary on active surface spots, performed on fully
sensitised material (a and b, DOS: 22%) and partially sensitised material (c and d, DOS: 15%) at -150 mV/SCE in
0.01 M K2S4O6 (pH 2.2).
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Figure 68: SEM Images of the investigated surface spots after the measurements shown in Figure 67. Multiple
cracks are visible across the area exposed to the electrolyte. Dotted circles indicate the inner rim of the capillary.
6.1.3 Comparison with microcapillary measurements
The data obtained from the array measurement described in Section 4.5 were used to model the
current signal of a macroscopic measurement: It was taken advantage of the fact that during the
array measurement, scans were made on both passive and active surface spots. This allowed the
summation of all current signals to model the behaviour of the whole area. The total area exposed
during the array measurements was 99 times the surface area of the 60 µm capillary, giving a
value of 0.28 mm2. The area of the presented macrocapillary measurement was approximately
two times larger (0.65 mm2). The resulting current signal is similar to the measurements
performed with the macrocapillaries (Figure 69). This shows that the direct upscaling from micro
scale results to the macro scale is possible. Simultaneous crack growth at multiple active sites
leads to a raise in baseline current, and no single transients can be discerned.
Analysis in the frequency domain confirmed the similarities of the signals. PSD I curves were
calculated by FFT for signals measured with the macrocapillary as well as the simulated current
signal derived from the microcapillary array measurement. Calculation over the first 10 minutes
of these current measurements gives overlapping PSD curves (Figure 70).
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Figure 69: a) Potentiostatic current measurement on a highly sensitised sample (DOS: 22%), with a 0.9 mm
macrocapillary, at -150 mV/SCE in 0.01 M K2S4O6 (pH 2.2). b) Sum current of 99 potentiostatic current
measurements with a 60 µm microcapillary under the same conditions as above, resulting in a total area of 0.28 mm2.
Figure 70: PSD spectra of the current curves shown in Figure 67 (brown, dotted) and the overlay of 99 micro spots
shown in Figure 67 (black). All curves show similar values.
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6.2 Discussion of microcapillary and macro scale EN measurements 6.2.1 Comparability of macro and micro scale results
The macro and micro scale experiments introduced in Sections 4 and 5 had to be performed under
different conditions, due to the specific requirements and limitations of the two setups. The
effects of these differences on the comparability of results are listed below. It is clear that a direct
comparability of the results is limited. Nevertheless, it was possible to identify transient types that
appeared in both experimental approaches. This indicates that the underlying mechanisms of
micro crack growth are identical for both studied approaches, and are not influenced by the
differences in surface preparation, type of stress or measuring method.
Surface state
Initial grinding of coupon samples was necessary for microcapillary samples, because they
exhibited very rough surfaces after machining. While microcapillary measurements could be
performed on such rough surfaces, the identification of micro cracks on them would not have
been possible. The surfaces of round tensile specimens directly after machining were smoother,
and no additional surface treatment was necessary. A reproducible and uniform grinding of the
surface areas in the sample notch before a measurement was not possible.
While the causes for crack initiation varied between the different sample designs, the different
surface states were necessary for the comparison of early micro crack propagation on both scales.
A high number of active sites and immediate initiation were necessary on the micro scale.
Macroscopic measurements on such surfaces did not show any discernable signal types, as shown
in Section 6.1. On macroscopic samples fewer active sites were favourable for the identification
of different transients and their comparison to microcapillary measurements.
Sample Polarisation
Microcapillary measurements were performed under potentiostatic conditions, measuring only
the current while the potential was kept constant. A few OCP measurements on active surfaces
have been performed, shown in 4.1. Active surface spots exhibited potentials in the range of
-200 – -300 mV/SCE during the growth of a single micro crack. Similar potentials on the macro
scale were measured only after the onset of macroscopic cracking. Hence, a comparison of
potential signals between macro and micro scale experiments is not useful, as the ratio of active
and passive surface area was much higher for microcapillary measurements. Signal analysis was
therefore limited to current transients.
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Application of stress
Due to limitations of the experimental setup, only constant deformation could be applied during
microcapillary experiments, whereas CL or constant extension rates were possible in macroscopic
experiments. Crack formation under CL and constant deformation conditions could undoubtedly
be assigned to SCC, while under dynamic loading conditions single fracture events might be
interpreted as mechanical cracking, merely accelerated by additional corrosion. Local plastic
deformation occurred in macroscopic CL experiments at the notch grounds, but crack initiation
also occurred on notch walls, where local stress values below σy were calculated. Crack initiation
below σy was also observed in microcapillary experiments. But as described in Section 3.6.3,
plastic deformation was applied for most experiments. This compensated the decrease in local
stress due to relaxation during crack propagation and caused larger cracks.
6.2.2 Comparison of current transients during crack growth
As already introduced in Section 5.3, type II transients during macroscopic experiments closely
resembled the typical signal shape of micro crack propagation, measured with the microcell
technique (Figure 71a and b). Certain microcapillary measurements also exhibited single current
transients similar to type Ia transients, exhibiting fast rise and slower decay (Figure 71c and d).
These signals could be assigned to the formation of micro cracks. Single transients account for
crack propagation by a fracture of a grain boundary, which can be explained by microscopic film
rupture and anodic dissolution events. Type Ib transients, which would suggest the formation of
metastable pits, rarely appear during microcapillary measurements. Accordingly, these samples
did not exhibit pitting corrosion after microcapillary measurements.
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Figure 71: Comparison of potentiostatic current measurements with the microcapillary and macroscopic ECN
measurements. a) Macroscopic measurement, exhibiting type II transients. b) Microcapillary measurement,
exhibiting similar current transient groups. c) Single type Ia transient, exhibiting fast rise and slower decay, detected
during a macroscopic ECN measurement. d) Detail view of single current transients detected during a microcapillary
measurement.
Comparison of type II signals from macroscopic and microcapillary tests in the frequency domain
further confirms their similarity. Figure 72 shows that the current power spectral densities for
different type II signals (a) are overlapping with curves calculated from microcapillary
measurements (b). Macrocapillary measurements on the other hand, which were performed under
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identical conditions as the microcapillary experiments, exhibit clearly higher activity.
Macroscopic noise measurements, performed under different conditions therefore allow a better
comparison of characteristic signal shapes than the macrocapillary measurements.
Figure 72: PSD I curves for different type II transients measured during macroscopic EN measurements (a),
microcapillary measurements (b), macrocapillary measurements and the combination of several microcapillary
measurements (c).
Type III transients during macroscopic EN measurements were identified as important warning
signs for the onset of macroscopic cracking under CL conditions. The lack of similar transients in
microcapillary measurements is in accordance with the interpretation of these transients as
accelerated crack growth events, which were induced by the coalescence of micro cracks or
active loading. Neither one of these possible causes was present during measurements with the
microcapillary, as only single cracks were investigated, and constant deformation was applied.
Hence, microcapillary measurements provided the possibility to identify certain elements of a
macroscopic noise measurement, although different experimental conditions had to be applied. A
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complete macroscopic signal can not be modelled, because certain signal features, which were
attributed to crack coalescence events, did not occur in microcapillary measurements. Direct
modelling of a macroscopic signal based on microcapillary measurements was possible for the
measurements performed with the macrocapillary. These measurements illustrated how several
active micro cracks lead to an elevated current signal, which does not exhibit any discernible
single transients.
6.2.3 Mechanistic considerations
Based on experiments with the microcapillary and the macroscopic setups, the initiation of
IG SCC varied between different surface preparations. Cracks were either initiated by a chemical
attack of the surface by the aggressive electrolyte, or by mechanical rupture of a surface film, due
to dynamically changing stress during CERT experiments. Initiation points for chemical initiation
on polished surfaces were grain boundaries. On ground coupons, the dominating initiation points
were MnS-inclusions, further crack initiation was observed on surface defects and on surface
areas that did not exhibit any visible weakness. In these cases, grain boundary triple points or
local weaknesses of the oxide layer are possible reasons for crack initiation. Metastable pitting
was likely responsible for crack initiation on round notched samples. Most initiation points have
in common, that an initial local corrosion event or mechanical film rupture occurred, which in
turn exposed a vulnerable point on a grain boundary.
Independent of the surface preparation, a single crack propagation mechanism explains the
current signals measured with the microcapillary as well as the observed noise signals for
macroscopic CL and CERT experiments: Crack propagation was limited to a narrow metal zone
along the grain boundaries, which was defined by the concentration profile of chromium. Micro
cracks propagated in a stepwise process of film rupture, anodic dissolution and repassivation,
forming semi-elliptical micro cracks. Along a single crack front, multiple propagation and
repassivation events could occur in parallel, until complete repassivation occurred. Occasionally,
corrosion could re-initiate on a micro crack, even under constant deformation condition. The
reason for these re-initiation events might be attributed to diffusion processes inside the micro
crack or low-temperature creep at the crack tip. More research is needed to clarify this behaviour.
In the presence of other growing micro cracks, re-initiation can be readily explained by the
constantly changing surface stress conditions.
With proceeding crack growth, sudden signal steps on the macro scale indicated accelerating
crack growth: Coalescence of micro cracks caused accelerated metal dissolution, indicated by
large current transients. The resulting large crack fronts often exhibited additional active spots,
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which led to a rise in base line current and further potential drop. Crack growth continued in this
stepwise fashion, causing increasingly large signal transients. Finally, the decrease of sample
cross section reached the point, where continuous crack growth set in, leading to continuous
current rise and potential drop, until complete sample failure occurred.
Influence of sensitisation
A variation of the degree of sensitisation in the studied range of 15 – 22% did not appear to have
a large effect on the microscopic cracking behaviour: Crack initiation for both material treatments
occurred mainly on MnS-inclusions. It could not be verified, if the sensitisation had an effect on
the occurrence of crack initiation on surface spots in absence of these inclusions. Crack
propagation showed a tendency towards shorter cracks for lower sensitisation. Macroscopic EN
measurements on the other hand showed clearly less activity on the partially sensitised material
and no transition to macroscopic cracking in the studied time frame. Further studies and the
measurement of local chromium profiles are necessary to clarify the influence of sensitisation on
IG SCC initiation and propagation on the different scales.
Early detection of IG SCC
SCC during CL experiments exhibited a high variability, and the prediction of the time until
macroscopic fracture and sample failure was generally not possible. The measurement of EN
signals allowed the detection of warning signs for accelerating crack growth, and for certain
environments, it might be possible to identify very early signs of cracking, and estimate the
dimension of growing micro cracks: Such a favourable situation would be the sudden transition
from a passive signal to type II transients. When the base line signal during passive behaviour is
stable, a sudden signal change can be detected, and the characteristic shape of type II transients
might be perceptible. The integration of type II transient groups would then allow the calculation
of crack lengths, based on a presumed dissolution width. Under these conditions, the stepwise
change from passive to active behaviour would allow a very early detection of IG SCC.
If these transients cannot be detected clearly, type III transients and large stepwise signal changes
can serve as warning signs. When these signals appear, IG SCC might already have caused
extended damage, and the transition to macroscopic cracking might follow soon. On the other
hand, a single type III transient might just be a unique event, caused by a fluctuation of stress, or
the rupture of a surface oxide film due to the impact of a particle.
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EN measurements are a suitable tool for the detection of earliest signs of corrosion, but the
meaning of a single transient always depends on the specific system, its mechanical and chemical
properties, and the fluctuations of experimental parameters.
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Summary and conclusions
In the present work, electrochemical microcapillary and macroscopic EN measurements were
combined to study the characteristic EN signals during initiation and early propagation of IG
SCC of stainless steel. Initial work focussed on the development of an experimental system that
allowed the study of IG SCC on two scales: Macroscopic study of IG SCC was already well
established, but the use of the microcapillary technique for the monitoring of single stress
corrosion cracks was a novelty. The chosen material, thermally sensitised AISI 304 stainless steel
with high sulphur content, proved to be a good choice for study, as it provided a high density of
micro crack initiation sites. Just as important were the use of a suitable electrolyte and the surface
treatment of the samples. The critical point was the initiation of IG SCC. It had to occur in a short
time frame, with minimal occurrence of other types of localised corrosion, which would interfere
with the measurements. As only constant deformation could be applied during microcapillary
measurements, an aggressive electrolyte had to be chosen. This in turn increased the risk of
“unwanted” corrosion processes to occur. Acidic solutions of potassium tetrathionate fulfilled
these criteria and proved to be well suited for the study of single micro cracks with the
microcapillary technique. Systematic current measurements were performed on active surface
spots where cracking appeared, to reveal typical signal patterns: Distinct active and passive signal
periods showed that crack growth proceeds discontinuously, even on the scale of single grain
boundaries. The characteristic current signal of a growing micro crack is composed of
superimposed transients with fast rise and slow decay. This signal shape can be explained easiest
by parallel events of film rupture, anodic metal dissolution along the sensitised grain boundaries,
and repassivation.
The influence of the surface state emerged later during the study. Grinding up to 4000 grit SiC
paper without further polishing led to surfaces that exhibited distinct active locations, on which
crack initiation occurred, while the rest of the surface remained passive. Immediate response of
active spots to the electrolyte allowed the identification of such positions in a matter of seconds.
Further polishing of surfaces was found to render all grain boundaries vulnerable to corrosion.
The result was extended intergranular corrosion, independent of the applied stress. Hence, surface
preparation heavily influenced crack initiation, and measurements were limited to certain specific
types of surface preparation. In these cases, it was possible to identify typical crack initiation
sites. Distinct active surface spots were found, mostly on the interface of bulk metal and MnS-
inclusions but also on positions without any visible weakness. The direct transfer of these
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observations to other surfaces is not necessarily valid. The specific requirements of surface
treatment also limited further study of the surface, e.g. by EBSD measurements.
Upscaling of the surface area showed different results, depending on the sample and experimental
conditions: macrocapillary measurements, performed under conditions identical to the
microcapillary experiments, could be directly modelled from current data measured with the
microcapillary: The simple addition of a sufficient number of microcapillary current signals, both
on active and on passive surface spots, approximated the typical signals of macrocapillary
measurements. A different situation was observed when round notched tensile samples without
any prior surface treatment were used in EN experiments. These samples showed markedly less
activity than ground coupon samples, which were used for the experiments with the micro- and
macrocapillary. And compared to capillary measurements, the signal characteristics of
macroscopic EN measurements varied greatly between experiments. The development of IG SCC
on a macroscopic scale could not be predicted precisely, especially under CL conditions.
Macroscopic ECN signals exhibited distinct transient types, which were assigned to the formation
of metastable pits, the initiation and propagation of micro cracks, and to accelerated cracking
events of whole grain boundaries. Transients indicating micro crack initiation and propagation
were also observed regularly on the micro scale. Accelerated growth steps were attributed to
coalescence of micro cracks, or, in case of CERT tests, mechanically induced film rupture. They
could therefore not occur under the experimental conditions applied for microcapillary
experiments. Large current transients which were assigned to these events are commonly
documented in the literature as signs of IG SCC. For monitoring purposes, these large single
transients can be easily detected and clearly discerned from baseline signal fluctuations.
However, they are not the very first sign of IG SCC. Under favourable conditions, for systems
with low background noise, type II transients indicating the very early stage of IG SCC initiation
might be detected.
Concerning the mechanism of IG SCC under the studied conditions, all observations indicate a
film rupture mechanism. However, the present data does not allow a final dismissal of other
mechanisms, particularly hydrogen embrittlement, as the characteristic current signals are not
fully conclusive. To finally resolve this question, electrochemical methods alone, even when
obtained on both micro and macro scale, might not be sufficient.
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Outlook
As the aim of this project was the combination of two experimental techniques, much effort was
put into the development of an experimental system suitable for both approaches. While the link
between the two techniques could be made, many aspects of the two approaches could not be
investigated in depth, leaving a number of open questions and possible topics for further studies.
The main limiting factor of the presented macroscopic work is the small amount of experiments
performed with the finally introduced tetrathionate electrolyte under suitable loading conditions.
These experiments were time consuming and prone to interferences, and usually a conflict arose
between stopping an experiment after the first transients for identification of corrosion damage,
or waiting until macroscopic cracking set in, to monitor the development of the EN signals over
time. But the few successful experiments under CL conditions already proved to produce EN data
that is likely not obtainable in CERT measurements; therefore the introduced system should be
studied further. Additional experiments are necessary to refine the presented interpretation of
current transients and clarify the role of pitting in crack initiation. Extended experiments, lasting
for weeks or months, could provide data on the initiation of macroscopic cracking for less
sensitised material, which did not exhibit large current jumps in the studied time frames up to
200 h.
Considering the microcapillary technique, it is tempting to find additional systems that allow the
monitoring of SCC, to confirm the presented results, or to expand the technique to different
cracking mechanisms. A comparison of the characteristic current signals of different SCC
mechanisms on the micro scale might lead to new insight and better identification of these
mechanisms on the macroscopic scale. Especially the study of hydrogen embrittlement on the
micro scale by introduction of hydrogen into a sample could provide useful information.
Another open question is the influence of surface preparation. This aspect was only brought up
briefly in this work, as difficulties were encountered during measurements on highly polished
surfaces. The influence of surface oxide layers on the electrochemical behaviour of a surface, and
the differences in surface structure between micro spots need further study to improve the
understanding and predictability of microcapillary measurements.
For future development of the microcell technique, an improvement of the cell design to include
the simultaneous visual monitoring of a surface spot seems to be an ambitious but promising
goal. The parallel recording of electrochemical and visual information during micro crack
propagation could significantly improve the understanding of localised corrosion processes. This
133
would however require a redesign of the microcell and likely the whole experimental setup.
Furthermore, the implementation of a dynamic loading device could open new possibilities for
the study of SCC on the micro scale.
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Acknowledgement
First and foremost, my thanks go to Prof. Dr. Sannakaisa Virtanen for giving me the possibility to
write this thesis. Her supervision and motivation during the whole thesis was invaluable.
I thank the examination committee for their time and effort.
I would like to thank my supervisors at PSI and EMPA, Hans-Peter Seifert and Thomas Suter, for
their guidance. Special thanks go to Stefan Ritter for his dedication to this project and his
continuous support.
The financial support of this thesis by the Swiss Federal Nuclear Safety Inspectorate (ENSI) is
gratefully acknowledged.
This thesis would not have been possible without the help and technical support from my co-
workers:
My colleagues at PSI, especially Beat Baumgartner, Leonard Nue, Roger Schwenold, Urs
Tschanz, and Patrick Simon. I thank Hans Leber for the EBSD measurements and an introduction
to metallography, and Klaus Reichlin for the finite element calculations of stress concentrations
on round notched samples.
And my colleagues at Empa, especially Alessandra Beni, Patrik Schmutz, Martin Sauder and
Ronald Lay. I thank James Derose for the many fruitful discussions and his help with proof
reading during writing.
Finally, I thank my family and friends for their continuous support over more than four years of
woe and distress:
My mother and father, for all their love and encouragement.
My brother and fellow outdoorsman, for having my back.
Katrin, Ursula, Anna, Raph and Katarina, for accompanying me on my adventures.
135
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Publication list M. Breimesser, S. Ritter, H.-P. Seifert, T. Suter, S. Virtanen, Electrochemical noise of SCC in austenitic stainless steels: A combined macro- and microelectrochemical approach, in: EUROCORR 2010, Moscow, Russia, 2010. M. Breimesser, S. Ritter, H.-P. Seifert, S. Virtanen, T. Suter, Application of the electrochemical microcapillary technique to study intergranular stress corrosion cracking of austenitic stainless steel on the micrometre scale, Corrosion Science 55 (2012) 126-132. M. Breimesser, S. Ritter, H.-P. Seifert, S. Virtanen, T. Suter, Application of electrochemical noise to monitor stress corrosion cracking of stainless steel in tetrathionate solution under constant load, Corrosion Science, accepted for publishing.
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Appendix
Appendix I: Signal interferences in macroscopic EN measurements
A series of different problems and interferences were encountered during macroscopic EN
measurements, usually appearing after a detail of the setup or the measuring procedure was
changed. Some of the interferences that occurred repeatedly could be identified and eliminated:
Bubble formation in the electrochemical cell
Depending on the weather, humidity and temperature in the laboratory, air bubbles appeared in
the electrochemical cell in some experiments. Generally, the incorporation of air in the electrolyte
during preparation should be avoided. Degassing of the distilled water by heating it to 80 °C for
2 h was done in some cases. During a measurement, the cell was checked regularly for bubbles. If
they appeared, they were removed to avoid blockage of the Haber-Luggin capillary (see Figure I-
1).
Figure I-1: Air bubbles can block the Haber-Luggin capillary partially or completely and therefore interfere with
potential measurement.
Blockage of glass frit between cell and reference electrode
If a saturated calomel electrode was used as a reference, it was observed that from one
experiment to the next the fluctuation of the potential signal could increase. It was found that the
potassium tetrathionate tended to block the frit that was used for the connection of the reference
electrode with the setup. If not corrected, this could lead to wrong potential measurements.
Cleaning of the frit surface or replacing the frit re-established conductivity.
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Figure I-2: A blocked frit between the electrochemical cell and the reference electrode heavily disturbs the potential
measurement. After removing the frit just before t = -44 h, the potential signal stabilises at the correct value.
Interference induced by thermocouple
The thermocouple that was used to measure the electrolyte temperature induced interferences in
both potential and current signals. Temperature measurements were therefore performed only at
the beginning and at the end of an experiment. During a noise measurement the thermocouple
was kept unplugged.
Figure I-3: Characteristic interference caused by the thermocouple during a noise measurement. These peaks
disappeared after disconnection of the thermocouple.
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Vibration of the cell due to electrolyte flow
While the electrolyte pump itself often did not cause any interference, the outflow of electrolyte
to the storage tank could significantly distort the signal. When a thin outlet tube was used, the
electrolyte tended to collect in the tube, due to surface tension, and then suddenly flow out. This
caused periodic vibrations of the whole cell and distorted both current and potential signal. The
transients were unevenly spaced, and their amplitude varied. An example is shown in Figure I-4,
but the appearance of this interference could vary.
This interference was identified by removing electrolyte from the cell during a running
experiment, to stop flow out. The interference vanished, and it reappeared when the cell was
completely filled again and electrolyte flow out of the cell continued.
The use of a larger outlet tube and adjusting the length and angle of the tube enabled a more
continuous electrolyte flow and reduced this interference. However, depending on the positioning
of the outlet tube, this interference often reappeared.
Figure I-4: Characteristic signal interference caused by periodic electrolyte outflow. Each current and potential
transient was directly correlated with a quantity of electrolyte flowing out of the outlet tube.
Interference peaks from the loading unit
The loading unit occasionally caused characteristic interference peaks. These signals showed
constant amplitude in negative direction and lasted only one data point, even at a data acquisition
frequency of 100 Hz. Changing of the detection limit also changed the amplitude of these
transients. Peaks were not evenly distributed, but occurred in high frequency for limited time
periods and then disappeared again for hours or days. They appeared in the potential and current
measurement, but not necessary in parallel with each other.
These peaks were caused by control signals from the Cormet system to the loading unit and
interfered directly with the ECMNoise device through the grounding of the faraday cage. The
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interference could be minimized by changing the grounding of the setup: The Faraday cage,
which was in contact with the loading unit, was disconnected from the ECMNoise ground.
Instead, it was grounded on the mains ground. Additionally, the insertion of a 400 Ω resistor in
the ground of the Faraday cage shielded the measuring setup against these interference signals.
Due to their constant amplitude and duration, these peaks had a very characteristic shape and
could be easily identified and removed from the data.
Figure I-5: a) A cluster of characteristic fast interference peaks from the Cormet loading unit. Peaks appeared in
both potential and current signal for a limited time. b) Signal after manually removing the interference peaks.
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Appendix II: Attempt to correlate crack growth direction with grain orientation by
EBSD measurements
EBSD measurements were performed on the systematically scanned surface introduced in
Section 4.5, to establish a map of grain orientation and active surface spots. It should be
evaluated, if crack initiation and early propagation could be attributed to a certain grain
orientation, and if specific grain boundaries showed a higher susceptibility towards IG SCC.
EBSD measurements of good quality can only be obtained, if the top layer (10 – 50 nm) of the
specimen is strain-free and clean from contamination. The array specimen did not meet these
requirements after the microcapillary measurements. The surface was polished with 3 µm and
1 µm diamond suspension 6 min each. The sample was then vibration polished with silica
emulsion for 3 h. After this polishing procedure, an EBSD measurement on the array was
performed (Figure II-1):
Figure II-1: Microstructure of the scanned surface area, shown in inverse pole figure with overlay of scanned micro
spots. Active spots and micro cracks are indicated in black. Passive spots and micro hardness imprints are indicated
grey.
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The EBSD measurement still exhibits areas with low quality, but already several µm of material
were abraded from the surface, and as a consequence, most micro cracks were removed, and the
positions of grain boundaries on the polished surface are shifted. Only on a few positions, grain
boundaries showed orientations similar to the cracks (Figure II-2), but the assignment of cracks to
these grain boundaries is difficult.
These results show that the combination of electrochemical microcapillary measurements with
EBSD measurements to study IG SCC in the present system was severely limited. The two
methods had opposed requirements concerning surface: They could not be performed on the same
surface, and any changing of the surface between the two measurements severely alters the
results.
It might be possible to follow the course of larger cracks, which are allowed to grow deeper into
the material and are therefore still visible after surface polishing. The correlation of initiation and
early crack growth with grain orientation, which was the main point of these experiments, was
not possible.
Figure II-2: Detail view of three surface positions. Capillary positions and formed cracks are indicated as black
lines. Dotted lines indicate possible grain boundaries, along which the initial cracks might have formed.
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Appendix III: Integrated charge values and resulting theoretical crack lengths and
crack growth rates for selected current transients Table III-1: Type I transients.
Experiment (peak number)
Charge (µC)
Volume of dissolved
metal (µm3)
Crack wall area (µm2) for const. width
of 80 nm
Surface crack length (µm), calculated for
semi-elliptical cracks
Theoretical crack growth rate (µm/s)
RTL 49- 0.058 2.1 27 8.5 1.4
RTL 49- 0.013 0.48 6.0 4.0 2.7
RTL 49- 0.026 0.96 12 5.7 1.0
RTL 49- 0.008 0.29 3.7 3.2 2.1
RTL 49- 0.12 4.4 55 12 1.4
RTL 49- 0.053 1.9 24 8.1 1.5
RTL 49- 0.01 0.37 4.6 3.5 2.4
RTL 49- 0.002 0.074 0.9 1.6 3.2
RTL 46 0.062 2.3 28 8.8 0.88
RTL 46 0.018 0.66 8.3 4.7 0.59
RTL 46 0.040 1.5 1.8 7.1 1.0
RTL 46 0.011 0.40 5.1 3.7 0.52
RTL 46 0.0015 0.055 0.7 1.4 2.7
RTL 46 0.0007 0.026 0.3 0.9 3.1
RTL 46 0.19 6.8 85 15 0.29
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Table III-2: Intervals of 2 h, exhibiting superposition of type II transients.
Experiment, time interval
Charge (µC)
Volume of dissolved
metal (µm3)
Crack wall area (µm2) for const. width
of 80 nm
Surface crack length (µm), calculated for
semi-elliptical cracks
Theoretical crack growth rate (µm/s)
RTL51, 26 h – 28 h; 150 5500 69’000 430 0.066
RTL51, 28 h – 30 h; 120 4400 55’000 380 0.053
RTL51, 30 h – 32 h; 150 5400 68’000 430 0.059
RTL51, 40 h – 42 h; 190 6800 85’000 480 0.067
RTL51, 42 h – 44 h; 200 7400 93’000 500 0.070
RTL51, 44 h – 46 h; 210 7700 96’000 510 0.071
RTL44, 198 h- 200 h 1200 42’000 530’000 1200 0.17
RTL44, 200 h- 202 h 950 35’000 440’000 1100 0.15
RTL44, 202 h- 204 h 100 38’000 480’000 1100 0.16
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Table III-3: Type III transients.
Experiment (peak number)
Charge (µC)
Volume of dissolved
metal (µm3)
Crack wall area (µm2) for const. width
of 80 nm
Surface crack length (µm), calculated for
semi-elliptical cracks
Theoretical crack growth rate (µm/s)
RTL 47- 0.42 15 190 23 1.8
RTL 47- 0.68 25 310 29 4.1
RTL 47- 0.50 18 230 25 6.2
RTL 47- 0.42 15 190 23 4.6
RTL 47- 0.37 13 170 21 4.3
RTL 47- 0.36 13 170 21 3.0
RTL 46 9.1 330 4200 106 0.39
RTL 46 9.1 330 4200 106 0.89
RTL 43- 1.3 48 600 40 5.8
RTL 43- 5.4 200 2500 81 1.4
RTL 43- 2.1 78 970 51 2.3
RTL 43- 0.44 16 200 23 3.3
RTL 43- 4.8 170 2200 77 3.5
RTL 43- 0.81 30 370 32 2.1
RTL 43- 1.3 46 570 39 2.2