Electrochemical Techniques in Corrosion

Journal of ASTM International, Vol. 5, No. 2Paper ID JAI101241

Available online at www.astm.org

Gerald S. Frankel1

Electrochemical Techniques in Corrosion: Status,Limitations, and Needs

ABSTRACT: The corrosion of metals occurs primarily by electrochemical processes involving metal oxida-tion and simultaneous reduction of some other species. The fundamental understanding of these pro-cesses has allowed the development of a number of electrochemical techniques for the study of thecorrosion phenomena and assessment of the corrosion rate. In fact, electrochemical techniques are soingrained in the field that many practitioners think of corrosion rates first in terms of current density ratherthan thickness or mass loss per unit time. Standard approaches for electrochemical corrosion rate deter-mination are commonly used in the field for on-line monitoring of systems and facilities. Electrochemistryalso provides powerful tools for developing fundamental understanding of corrosion phenomena. However,there are some limitations to the abilities of current electrochemical techniques and some needs for thefuture. This paper describes the status of electrochemical techniques, their limitations, where non-electrochemical methods are required, and future needs in the field.

KEYWORDS: electrochemical tests, corrosion

Introduction

As a result of the development of the fundamental understanding of corrosion electrochemistry, fast andaccurate potentiostats, and computer technology, a suite of electrochemical techniques exists for the studyof corrosion. These techniques provide the technologist with the ability to monitor corrosion rates inservice, giving early warning of conditions that could adversely affect performance and integrity. Theyalso provide the experimentalist with the ability to determine corrosion rate with high sensitivity, assessrate controlling mechanisms, and in some cases make life predictions. Furthermore, variations in electro-chemical techniques and innovative cell designs allow researchers to probe mechanisms and develop newand improved materials.

A number of excellent reviews describe the existing electrochemical techniques in detail and provideinstructions on their proper use �1–5�. It is not the purpose of this paper to present a comprehensivesummary of electrochemical methods. Interested readers are referred to other works. Instead, the focus willbe on the limitations of available techniques for assessing corrosion and the need for improved methods.A brief summary of the available test methods will be given, including different types of exposure andsome experimental design considerations. A generic view of corrosion problems will then be presented,using a rubric that includes the exposure of uncoated or coated metals in bulk solutions or under atmo-spheric conditions. The status, limitations, and needs of the available methods will be discussed in thecontext of that matrix. The focus will be on electrochemical methods, but a variety of non-electrochemicalmethods and mixed methods will be addressed. Owing to the limitations of electrochemistry, it is criticalto utilize a variety of approaches when studying complicated systems.

A number of important corrosion phenomena will not be addressed in this review. Stress effects andenvironmentally assisted cracking as well as oxidation and high temperature corrosion will not be covered.However, the electrochemistry of stress corrosion cracking is critical and still not understood. Furthermore,some high temperature corrosion topics such as hot corrosion are of immense concern in power generationapplications and are also not well understood.

Manuscript received May 17, 2007; accepted for publication January 17, 2008; published online February 2008. Presented atASTM Symposium on Advances in Electrochemical Techniques for Corrosion Monitoring and Measurement on 22–23 May 2007in Norfolk, VA; S. Papavinasam, N. Berke, and S. Brossia, Guest Editors.1
Fontana Corrosion Center, The Ohio State University, Columbus, OH 43210.
Copyright © 2008 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.

2 JOURNAL OF ASTM INTERNATIONAL

Available Test Methods

The applicability of various test methods depends on the exposure conditions. For the purposes of thisreview, two exposure conditions will be considered: immersion in solution and atmospheric exposure.Most full immersion environments are known in some detail a priori, such as seawater, process streams,or concrete pore solution. Atmospheric exposure conditions, in contrast, are much less well understood.The electrolyte responsible for atmospheric corrosion can be a thin layer in equilibrium with humid air andinfluenced by atmospheric pollutants or surface contamination from sources such as road salt. In outdoorexposure, dew, and precipitation play a major role in determining the local environment. Time-of wetness,defined as the time with T�0°C and RH�80 %, has been shown to be a critical parameter for outdooratmospheric corrosion �6�. However, wet layers can exist on contaminated surfaces at much lower humidi-ties owing to deliquescence of the salts on the surface �7�.

As a result of the complexity of the atmospheric corrosion environment, testing conditions are usuallyless connected to real exposures. The most common atmospheric exposure test is ASTM B117 “StandardPractice for Operating Salt Spray �Fog� Testing Apparatus,” �8�. However, the relevance of salt spray testperformance to real environments, including marine environments, has been discussed widely. In fact, theIntroduction section of the B117 standard states �8�, “Prediction of performance in natural environmentshas seldom been correlated with salt spray results when used as stand alone data. Correlation and extrapo-lation of corrosion performance based on exposure to the test environment provided by this practice arenot always predictable. Correlation and extrapolation should be considered only in cases where appropriatecorroborating long-term atmospheric exposures have been conducted.” Various cyclic tests have beensuggested to be more representative �9�. Microelectronic devices are often tested under conditions of hightemperature and humidity �T/H� with the added application of a bias �10�. The prediction of lifetime inservice from lifetime in more aggressive test conditions requires knowledge of acceleration factors. Theacceleration factors for microelectronic devices in T/H testing have been studied in some detail �10�, butacceleration factors for most corrosion tests relative to atmospheric exposures are not known, althoughvarious correlations have been made �9�. More comments on this situation will be provided below.

Non-Electrochemical Measurements

A number of non-electrochemical measurement techniques can be used to assess corrosion rate. Weightloss measurement, considered by some to be the “gold standard” of corrosion testing is certainly theeasiest. However, there are important issues to consider even for weight loss measurements. First, sincemass can be measured easily only to about 0.1 mg, the sensitivity of weight loss measurements is limited.Other issues include end-grain attack leading to different corrosion rates on different exposed faces,crevice corrosion associated with hanging or supporting the sample, and waterline attack if the sampleextends beyond the surface. Finally, weight loss measurements are usually performed after long exposuretimes so they provide an average rate over time as well as over the exposed surface. It should be noted thatthe quartz crystal microbalance can provide rapid and very sensitive �submonolayer� measurements ofweight loss for thin film samples �1�.

Corrosion rate also can be determined by dimensional changes of the exposed samples or analysis ofthe solution for dissolved species. Both of these approaches also have sensitivity limitations similar toweight loss. However, solution analysis can be particularly powerful for studying the corrosion of alloysbecause of the ability for chemical differentiation, which is only possible using electrochemistry byrotating ring-disk measurements.

A technique that has had more application in corrosion rate monitoring than in corrosion scienceinvolves the change in electrical resistance �ER� of a probe sample. The reduction of the cross-sectionalarea of a probe by corrosion is accompanied by a proportionate increase in the electrical resistance, whichcan be tracked easily. A major advantage of the ER technique is its applicability to a wide range ofcorrosive conditions including environments having poor conductivity or non-continuous electrolytes suchas vapors and gases. However, ER monitoring typically requires a relatively long exposure period for adetectible difference in probe resistance and electrically conductive deposits can affect the measurements.Thierry et al. compared the use of weight-loss coupons, ER, and polarization resistance measurements incooling water applications �11�. They found that, with the increasing extent of localized corrosion, the ER
measurements indicated corrosion rates in excess of those observed on the coupon. In addition, changes in

FRANKEL ON ELECTROCHEMICAL TECHNIQUES IN CORROSION 3

corrosivity of the water systems could be detected within a few hours by polarization resistance tech-niques, whereas the ER technique required a few days to measure the changes.

Electrochemical Measurements

As mentioned above, it is not within the scope of this paper to describe in detail the wide range ofelectrochemical methods available for studying corrosion. In this section, the primary electrochemicalmethods used for determining corrosion rate are addressed broadly.

Potentiodynamic polarization �PDP� over a potential range about ±200–250 mV from the open circuitpotential �OCP� results in a polarization curve that can be analyzed for corrosion rate, provided that therates of other anodic reactions such as those associated with redox reactions are small in comparison�which is a requirement of all electrochemical assessments of corrosion rate�. Typically presented in asemi-logarithmic plot, polarization curves provide corrosion rate by extrapolation of the linear cathodicand/or anodic regions to the corrosion potential or by fitting to the following equation �1,5,12�:

inet = icorr�exp�2.3�E − Ecorr�ba

� − exp�− 2.3�E − Ecorr��bc�

�� 1�

where inet is the current measured as a function of applied potential E, Ecorr is the corrosion potential, icorr

is the corrosion current density, and ba and bc are the anodic and cathodic Tafel slopes, respectively. Theequation represents an idealized form of the electrochemical data for the case of a mixed electrode inwhich there is only one anodic and one cathodic reaction taking place on the corroding surface. Bothreactions must be controlled by activation polarization and Ecorr must be far from both reversible poten-tials. Most commercial corrosion analysis software packages contain the capability to fit data to thisequation, but it has no agreed-upon name, nor does the technique of fitting data to this equation. It hasbeen proposed to call this equation the Wagner-Traud equation �13�. Wagner and Traud, in their 1938 paper�published in English in 2006 �14��, came close to deriving this equation, but did not take the final stepsto do so. They certainly at that time could not have envisioned the power of computers to utilize thisequation for determination of corrosion rates.

PDP over a wide range of potential generates more information about the system than just the corro-sion rate. For instance, information can be obtained about the proximity of the OCP to regions of passivityor localized corrosion susceptibility. It is often possible to view more of the anodic polarization curve byde-aerating the solution, which can reduce the corrosion potential. PDP is a tool for laboratory investiga-tions, not corrosion rate monitoring, as it involves perturbation of the potential relatively far from thesteady-state corrosion potential.

The corrosion rate also can be determined from the polarization resistance �RP� using the Stearn-Gearyequation provided that the polarization resistance is similar to the charge transfer resistance and if the Tafelslopes are known �2,4�. The most common way to determine RP is by the linear polarization resistance�LPR� method, in which the potential is scanned about ±5–10 mV relative to the corrosion potential. Theslope �dE/di�, at the zero current potential is a measure of RP. An easier approach is a two-point measure-ment at potentials above and below the OCP. Or even simpler, a single measurement can be made at apotential either above or below the OCP and the slope dE/di can be determined using the �net current,potential� point of �0, OCP� since the i-E curve must go through this point. These simplified analysesassume that the polarization response is perfectly linear, and error will result if there is any deviation fromlinearity. The LPR method has been put to considerable use in corrosion monitoring as it involves rela-tively little potential perturbation. However, accurate assessment of corrosion rate requires knowledge ofthe Tafel slopes, which must be determined separately or assumed. On the other hand, calculated corrosionrates are usually not wrong by more than a factor of 2–3 if the Tafel slopes are both assumed to be100 mV /dec.

The electrochemical impedance spectroscopy �EIS� technique involves the application of a time-varying voltage and measurement of the current response. The ratio of two gives the frequency-dependentimpedance. Several books and papers have been written on EIS and its application to corrosion �15–21�;details will not be given here. Suffice it to say that the impedance represents a fuller description of thetransfer function for the response of a system to a perturbation, relative to DC methods. The low frequencylimit of the impedance magnitude can be related to RP and thus the corrosion rate using the Stearn-Geary
equation. Again, the Tafel slopes are required to do so. Constant phase elements �CPEs� are used widely


in the analysis of EIS corrosion data. The extra fitting parameter associated with the non-ideal capacitanceof a CPE improves the fit to the data. The CPE is a mathematical construct of convenience. However, it isnot surprising that a physical structure such as an electrochemical interface does not behave exactly like acombination of standard circuit elements, and no rationale need be given for the use of CPEs. Thesimplified Randles circuit with a CPE shown in Fig. 1 is commonly used to represent many corrodinginterfaces. EIS is a particularly useful technique for low conductivity electrolytes as the ohmic resistanceis determined explicitly. It also provides a good description of the response of paint-coated samples and issensitive to early stages of coating failure. One main difficulty with the technique is the proper selectionof an equivalent circuit. An equivalent circuit should always be based on a physical model of the corrodingsystem; addition of circuit elements simply to improve the fit is unacceptable. However, a number ofcomplex circuits could be rationalized as the detailed nature of the physical system often is not known.There is broad agreement about the two time constant model �Fig. 2� used to represent a defective coating,which is one of the main applications of EIS. On the other hand, good coatings typically exhibit one timeconstant during the early stages of exposure, and it often is not clear exactly when use of the two-time-constant defective coating model should be implemented when evaluating a time series of spectra.

The electrochemical noise �EN� technique involves the measurement of electrochemical events �i.e.,current or potential transients or both simultaneously� produced by the corrosion process. EN has beenreviewed by several investigators �21–29�. The most common approach is to measure current noise �uti-lizing a zero resistance ammeter� of two identical electrodes shorted together and the potential noisebetween the pair and a reference electrode �RE� or a third identical electrode. The ratio of the root meansquared deviation of the potential and current fluctuations is one measure of the noise resistance. Alter-natively, the data can be transformed into the frequency domain to generate a power density spectrum orevaluated using wavelet analysis. One problem with EN is the proper approach for accounting for theexposed area A of the sample. Cottis has described how current noise amplitude is proportional to A1/2,whereas potential noise amplitude is proportional to A−1/2 �26�. He recommends simply reporting the areaand not normalizing the noise resistance by area. However, to determine a corrosion rate, area normaliza-tion is required, so the usefulness of EN for determination of absolute corrosion rate is questionable. Onthe other hand, EN is particularly appealing for in situ monitoring as no applied perturbation is required.Even though the absolute corrosion rate cannot be obtained, the EN character is quite different for passiveconditions �low noise�, metastable pitting �random events of short duration�, and stable pitting �individu-alized events of longer duration� so it can be useful for assessing the onset of localized corrosion or stresscorrosion cracking for a stressed sample. Changes in conditions can be detected, triggering closer inspec-tion or sampling of passive probes immersed in the environment.

A multi-pronged strategy is being used to monitor the corrosion of a steel tank holding liquid radio-active waste at the Hanford Site �30�. A probe made from a thick-walled fiberglass pipe inserted into thetank holds multiple samples for measurements of LPR, ER, and EN at different positions within the tank.LPR is performed using two nominally identical electrodes with no reference electrode. EN is performedusing three nominally identical electrodes, one being a pseudo-reference electrode. Samples with differentconfiguration are tested, including a bullet-shaped electrode, two concentric rings, and stressed and un-stressed C-ring samples for sensing stress corrosion cracking �SCC�. The probe also contains reference

FIG. 1—The simplified Randles circuit with CPE.

FIG. 2—Nested two-time-constant model used to represent defective coatings.


electrodes designed to monitor the potential of the tank since all of the rate monitoring will be done onspecial samples and not the tank. A second passive probe has been installed with stressed and unstressedsamples that will be removed periodically for inspection. This approach to monitoring takes advantage ofeach of the multiple techniques utilized.

Electrochemical frequency modulation �EFM� is the last electrochemical method for assessing corro-sion rate to be discussed here. EFM uses harmonic analysis whereby a corroding system is perturbed by apotential waveform consisting of the combination of two sine waves of different frequencies �31�. Anonlinear system will respond to this excitation with a current that has components at harmonic andintermodulation frequencies. In practice, the method is very similar to an EIS measurement. A referenceelectrode and a counter-electrode �CE� as well as a computer-controlled potentiostat are needed to applythe waveform and measure the response. As with EIS, the magnitude of the excitation potential signal ison the order of 10–20 mV. The harmonic and intermodulation frequencies of the two excitation frequen-cies should not overlap and the frequencies should be low enough to avoid capacitive influences. Assum-ing that both the anodic and cathodic reactions obey Tafel kinetics and taking the first few terms of aTaylor series expansion of the exponential form of the current response, it can be shown that the intensitiesof the current response at certain harmonic and intermodulation frequencies create a set of equations withthe variables being the corrosion rate and the anodic and cathodic Tafel slopes. These equations can besolved to determine values of these parameters. EFM directly generates a value of the corrosion rate unlikethe LPR and EIS methods, which require knowledge of the Tafel slopes for the determination of thecorrosion rate from the polarization resistance through the Stern-Geary equation. Furthermore, idealizedresponses contain components at various frequencies having intensities of fixed ratios. Therefore, bycomparing the ratios from real data with theoretical values it is possible to determined so-called causalityfactors, which are measures of the quality of the analysis. Data analysis methods exist to handle the casesof �a� both anodic and cathodic reactions being under activation control �exhibiting Tafel kinetics�, �b�passive systems, and �c� cathodic reaction under diffusion control. However, the data analysis requires thatone of these conditions be preselected.

Kuş and Mansfeld recently took a critical look at the EFM technique �32�. They evaluated severaldifferent electrochemical systems with EFM as well as EIS and PDP. EFM agreed well with the othertechniques when the frequency range was entirely within the low frequency DC limit of the impedance,which is usually easy for systems with high corrosion rates. When the EFM frequency range was withinthe capacitive regions of impedance, the estimated corrosion rate was higher than that predicted by theother techniques. Kuş and Mansfeld recommended using EFM with caution �32�, but the same can be saidof all electrochemical techniques.

An interesting development in electrochemical testing over the past decade has been the miniaturiza-tion of corrosion cells by the use of microcapillaries �33–36�. In this approach, a capillary pulled to adiameter typically from 1–100 �m is used as the electrochemical cell. �Note that considerable skill isrequired to fabricate and handle capillaries of diameter less than about 15 �m.� The capillary is attachedto a holder that fits into an optical microscope objective carousel. The microscope allows for precisepositioning of the end of the microcapillary on a particular spot on the surface. A silicone coating on theend of the microcapillary prevents leakage and the reference and counter-electrodes are positioned in aconnected reservoir. This approach has been useful for studying the effects of inclusions such as MnS insteel �36� and intermetallic particles in Al alloys �34�. Essentially all of the electrochemical techniquesdescribed above can be used with this cell, with the advantage that the microcell geometry provides highspatial resolution.

Experiment Design Considerations

Several factors must be taken into account when designing an electrochemical experiment for the study ofcorrosion rate or mechanism. Decisions need to be made regarding sample selection, surface preparation,masking to expose a certain area, specifics of the experimental cell, selection of the appropriate testenvironment, and choice of the best technique. A detailed discussion of these factors can be found else-where �1� and will not be covered here. However, it is important to note that the design of the experiment
and the preparation of samples often are the most critical parts of electrochemical experimentation.


Problems to Study and Approaches

A wide range of problems exist in the field of corrosion. The application of protective coatings is one ofthe main approaches to corrosion protection, and the issues associated with coated samples are quitedifferent than for uncoated electrodes. Furthermore, there are two broadly different types of exposureconditions: immersion in an electrolyte or solution and atmospheric exposure. Therefore, corrosion prob-lems can be classified by a matrix considering samples that are coated or uncoated and circumstances ofimmersion in solution or exposure to atmospheric conditions. Presently, the four possible combinations ofthis matrix will be considered in turn. For each, the unfilled needs for electrochemical testing will bediscussed. All the experiments were performed at room temperature.

Metal in Solution

For an uncoated metal immersed in an electrolyte, all of the electrochemical techniques described aboveare usually applicable. However, the choice of technique depends on the form of corrosion exhibited:uniform active dissolution, passivity, or localized corrosion. Special studies on the effects of stress, in-hibitors or microbes, for instance, might require yet other approaches. The conditions of active dissolution,passivity, and localized corrosion will be discussed in turn.

Uniform Active Dissolution—It is instructive to make a comparison between many of the techniquesdescribed above for a model system representing uniform active dissolution, Fe in 0.5 M H2SO4. Twonominally identical 99.9 % Fe electrodes of area 0.63 cm2 were immersed in 0.5 M H2SO4 along with a PtCE and SCE RE. The following series of experiments was performed: �1� LPR on each electrode, �2� ENusing the two electrodes, �3� another set of LPR measurements, and then on one of the electrodes �4� EFM,�5� EIS, and finally PDP over a relatively wide potential range. PDP was performed last as it alters thepotential furthest from OCP. For a system such as Fe in sulfuric acid, all of the electrochemical techniqueswork quite well. The polarization curve generated by PDP is shown in Fig. 3. The linear portion of thecathodic branch extends for more than a decade of current density allowing accurate manual determinationof the Tafel slope: 99 mV /dec. In contrast, the anodic portion of the curve is more complicated, even forthis simple system, and a bend in the curve is observed about 75 mV above OCP. The mechanism of irondissolution to form ferrous ions involves several steps �37� and can change with potential. Determinationof the anodic Tafel slope near OCP is difficult owing to the bend in the curve and extrapolation of thisregion to OCP intersects at a different current density than the extrapolation of the cathodic portion. Insuch an example, the value determined by extrapolation of the cathodic region is probably more accurate,204 �A /cm2 in this case. The anodic Tafel slope is determined to be 53 mV /dec by manual fitting. Analternative and probably more accurate approach is to fit the data to Eq 1, the equation representing idealactivation-controlled polarization �38�. The limits of the fitting must be properly set, however, and thebend in the anodic curve requires an upper limit of the fit to be quite close to the corrosion potential. The

FIG. 3—Potentiodynamic polarization curve for Fe in 0.5 M H2SO4. Solid line – measured curve; dashedline – fit to Eq 1.

results of the fit, shown as the dashed curve in Fig. 3, are anodic and cathodic Tafel slopes of 110 mV /dec


and 103 mV /dec, respectively, with a corrosion rate of 216 �A /cm2. At high potentials, the currentdensity becomes independent of potential owing to mass transport limitations and salt film precipitation.The first part of the bending of the curve, however, results from ohmic potential drop in solution betweenthe reference and working electrodes. This can be minimized by current interrupt methods, but leads toinstability at higher potentials during the onset of passivity. LPR results in a rather linear relationship nearOCP �Fig. 4�, and an RP of 107 � or 67 � · cm2 can be determined easily. EIS is also quite easy with thissystem, because the low frequency limit is reached at a frequency of about 1 Hz �Fig. 5�. Fitting the datato the simplified Randles circuit with a CPE shown in Fig. 1, an RP of 107 � or 67 � · cm2 is found, whichis identical to that determined by LPR. As mentioned above, the calculation of corrosion rate from thisvalue using the Stern-Geary equation requires Tafel slopes, but the answer will not be much differentassuming Tafel slopes of ±100 mV /dec. Using the Tafel slopes determined by fitting the PDP to Eq 1, thecorrosion rate from LPR and EIS data is determined to be 343 �A /cm2. This value is relatively close tothe rate determined from the PDP data, especially considering that most corrosion rate measurements areonly reproducible to within about a factor of 2 at best. Noise analysis at a sampling rate of 20 Hz for101.2 s �a total of 2024 points� results in the data shown in Fig. 6. After trend removal, the power spectraldensity of the current and potential traces were obtained and the noise resistance calculated from Rn

= �EPSD / IPSD�1/2 was found to be 172 �. This resistance is similar to the polarization resistances deter-mined by LPR and EIS, but as described above, area normalization and the determination of corrosion rate

FIG. 4—Linear polarization experiment for Fe in 0.5 M H2SO4. Tangent is drawn at point where i�0.

FIG. 5—EIS data for Fe in 0.5 M H2SO4. (a) Nyquist plot and (b) Bode plot. The lines are fits to the
simplified Randles circuit shown in Fig. 1.


from Rn is problematic. The EFM technique also works quite well with this system, and it provides valuesof Tafel slopes. The EFM data for perturbation frequencies of 0.2 and 0.5 Hz are shown in Fig. 7. The rawEFM data in Fig. 7�a� show the modulated current response from the two perturbation frequencies and thefrequency spectrum in Fig. 7�b� clearly displays the response at the harmonic and intermodulation fre-quencies. Analysis of the data results in a corrosion rate of 200 �A /cm2 and anodic and cathodic Tafelslopes of 84 mV /dec and 112 mV /dec, respectively. The causality factors are 1.926 and 2.903, which arevery close to the theoretical values of 2 and 3. The results of the various techniques are given in Table 1.It is clear that the available techniques provide a number of different approaches for determining corrosion

FIG. 6—EN data for Fe in 0.5 M H2SO4. (a) Original current and potential noise. (b) Current andpotential noise detrended and filtered with 0.01–1 Hz bandpass filter. (c) Fast Fourier transform (FFT) ofthe current and potential noise. (d) Rn determined by the square root of the ratio of the potential andcurrent FFTs.

FIG. 7—EFM data for Fe in 0.5 M H2SO4. (a) Raw data. (b) Frequency spectrum.


rate, but the results for this system are all relatively close.It should be noted that the corrosion rates for 99.9 % Fe in 0.5 M H2SO4 determined by these

electrochemical methods are about 10 times lower than the published rate for steel at room temperature inthis concentration �5 wt% � of sulfuric acid as determined by weight loss measurements, 1200 mpy or2.5 mA /cm2 �39�. The corrosion rate of about 200 �A /cm2 is representative of values measured usingelectrochemical techniques on Fe in this solution by more than 200 lab groups overseen by the authorduring lab exercises in university courses and short courses for professionals over the past ten years.Weight loss measurements performed over the period of days by the same groups typically result insomewhat higher values. These differences between electrochemical and weight loss measurements areinteresting and deserve consideration. Electrochemical methods, when interpreted correctly, generate anaccurate representation of the instantaneous corrosion rate for the conditions of the measurements. On theother hand, as mentioned above, weight loss measurements are the “gold standard” of corrosion ratemeasurements and generate an average corrosion rate across the sample surface for the measurementperiod. These methods might provide different results for various reasons. One difference could be fromcrystallographic texture. The electrochemical experiments were performed on the sides of a drawn wire orthe rolling surface of a plate. During immersion experiments, end grain attack was observed to be fasterthan on these surfaces. The rate of attack at the water line has also been observed to be much higher thanthe rate well below the surface. This discussion highlights some of the issues associated with corrosion ratetesting. Weight loss experiments typically do not differentiate the rates on the different sides of an exposedsample or at a waterline and electrochemical measurements might not be performed on all surfaces orconsider water line attack.

Passivity—The experimental techniques described above also can be applied to passive samples fordetermination of corrosion rate as will be shown in the following examples. For passive samples in anenvironment such as neutral aerated solutions, both the anodic and cathodic �oxygen reduction� reactionsare easily polarizable and the open circuit potential can be quite variable.

The same exercise described above for the Fe /H2SO4 system involving the application of multipletechniques was applied to a passive system, AA2024-T3 in 1 M Na2SO4. The areas of the electrodes were1.1 cm2. The polarization curve is given in Fig. 8. The anodic current only becomes independent ofpotential at several hundred mV noble to the OCP. At lower potentials the slope is high but not infinite:about 257 mV /dec, as determined by manual fitting. This can be considered a sort of Tafel slope eventhough the electrode was not dissolving actively. The cathodic Tafel slope, probably associated withoxygen reduction with diffusion limitation influences, is also high: 246 mV /dec. Extrapolations of the twolinear portions almost intersect at the corrosion potential, resulting in a corrosion rate of about0.2 �A /cm2. Fitting Eq 1 results in anodic and cathodic Tafel slopes of 152 mV /dec and 190 mV /dec,respectively, and a corrosion rate of 0.13 �A /cm2. The linear polarization experiment resulted in a curvethat is less linear than for the active dissolution case, as shown in Fig. 9, but still can be analyzed easilyfor determination of polarization resistance: 79 500 � or 72 300 � · cm2. The impedance spectrum lookslike a single time constant, but it trails to higher impedances at the lowest frequencies �Fig. 10�. It can befitted with different equivalent circuits, and two have been utilized in this analysis. The first uses separatecharge transfer resistances in parallel for the anodic and cathodic reactions with a Warburg impedance inseries with the cathodic resistance �Fig. 11� �21�. A parallel CPE is used to address the capacitance. The

TABLE 1—Analysis of electrochemical methods for determination of corrosion rate for Fe in 0.5 M H2SO4. Electrode areas�0.63 cm2.

LPR�using ba

and bc

from PDP�

EIS�using ba

and bc

from PDP� EN

EFM�active

dissolutionmodel�

PDP�extrapolating

cathodic region�

PDP�fitting to

Eq 1�

ba, mV /dec 84 53 110

bc, mV /dec 112 99 103

RP, � 107 107 172

RP, � · cm2 67 67

icorr, �A /cm2 343 343 200 204 216

second circuit is the nested two-time-constant model that is commonly used for defective coatings �Fig. 2�.


Both fit the data rather well, and the extracted polarization resistance is similar in the two cases: 1.71�105 � and 2.10�105 � or and 1.55�105 � · cm and 1.91�105 � · cm2, respectively. Using the Tafelslopes from fitting the PDP data to Eq 1, the corrosion rate can be determined to be 0.42 �A /cm2 andabout 0.18 �A /cm2 for the LPR and EIS data, respectively. The analysis of the EFM data depends on thetype of corrosion assumed. For active, diffusion controlled, and passive dissolution, the corrosion ratesfrom EFM are 0.60, 2.2, and 2.2 �A /cm2, respectively. These values are overestimates of the actual rate.As described above, EFM will overestimate the corrosion rate if the excitation frequencies are in thecapacitive region, as is the case for this example. The causality factors are 1.89 and 3.07, which are againclose to the theoretical values of 2 and 3 �31�. It is interesting that the active model gives the rate closestto that determined by the other methods. The EN data result in a value of RP that is considerably less thanthat determined by the other methods: 40 700 �.

The results of all the tests are shown in Table 2. There is much more distribution in the data than forthe active dissolution case. The data from the PDP and EIS methods were close. It is not clear why thepolarization resistance measured by LPR was less than half that measured by EIS. Furthermore, there is noagreement regarding the best equivalent circuit for a passive metal.

Advanced experimental techniques have been used to study passivity by measuring the current tran-sients associated with passive film formation. For an oxide that can be removed by cathodic reduction, theexperiment is a simple one. Following a cathodic reduction treatment, the potential is stepped into thepassive region and the anodic current is monitored as a function of time. The current will decay approxi-mately exponentially �40–46�, so, to obtain data over a wide range of current and time, it is necessary tohave a high resolution analog-to-digital converter or a series of data collection devices operating at

FIG. 8—Potentiodynamic polarization curve for AA2024-T3 in 1 M Na2SO4. Solid line – measured curve;dashed line – fit to Eq 1.

FIG. 9—Linear polarization experiment for AA2024-T3 in 1 M Na2SO4. Tangent is drawn at point where
i�0.


different current and time scales �43–45�. In the latter case, it is necessary to piece together the differenttime segments, which is not easy. With a wide dynamic range of data, it is possible to plot log currentdensity versus log time to determine the kinetics of the passivation process. If the sample area and peakcurrent are large, a limiting value and subsequent small shoulder in the decay can be observed. This hasbeen attributed to ohmic potential drop associated with the high initial current �47�. The ohmic potentialdrop decreases as the current decreases, so the potential at the electrode surface is not constant with timein that situation. Therefore, it is desirable to minimize ohmic potential drop by using a conductive elec-trolyte and an electrode configuration that minimizes the area of the exposed metal.

Most alloys of technical interest cannot be reduced cathodically, but repassivation can be studied byexposing an area of unpassivated metal. Many different techniques have been used to depassivate or baresmall areas of metal for repassivation studies, including scratching, breaking, guillotining, impingingparticles, and incident laser irradiation. Burstein and co-workers have developed and extensively used thescratching and guillotining approaches �43–45,47�. Scratching is easily accomplished using a hardnessindentation stylus and a rotating disk electrode. The stylus is pulsed against the surface to cut a groove into

FIG. 10—EIS data for AA2024-T3 in 1 M Na2SO4. (a) Nyquist plot and (b) Bode plot. The lines are fits tothe circuit shown in Fig. 11.

FIG. 11—Equivalent circuit with separate charge transfer resistances in parallel for the anodic andcathodic reactions, a Warburg impedance in series with the anodic resistance, and a parallel CPE.

TABLE 2—Analysis of electrochemical methods for determination of corrosion rate for AA2024-T3 in 1 M Na2SO4. Electrodeareas�1.1 cm2.

LPR�using ba

and bc

from PDP�

EIS�using ba

and bc

from PDP� EN

EFM�passivemodel�

PDP�extrapolation�

PDP�fitting to

Eq 1�

ba, mV /dec Infinity 257 152

bc, mV /dec 312 246 190

RP, � 79 500 190 000 40 700RP, � · cm2 72 300 173 000icorr, �A /cm2 0.42 0.18 2.2 0.2 0.13


the sample. A simpler approach is to scratch the surface manually with a sharp scribe. In this case, thescratch can be more reproducible if the sample has a short dimension, such as is achieved by mounting athin foil on edge into an epoxy mount. The manual scratching across the thin sample can then be accom-plished relatively quickly and reproducibly �48�.

To study the earliest stages of repassivation where the current densities are the highest, it is necessaryto create the fresh area as quickly as possible and to minimize the area of the fresh metal. A very small areaof fresh metal can be created extremely quickly �on the order of �s� by the thin film breaking experiment�49�. In this approach, a thin film deposited onto a brittle substrate such as glass or Si is suspended into thesolution �Fig. 12�. Breaking of the thin film electrode results in the creation of a fresh metal area of sizeequal to the cross section of the thin film. Current densities on the order of 1000 A /cm2 were measuredusing this technique on Al thin films �49�.

Localized Corrosion—Electrochemical testing can be used to assess the susceptibility to localizedcorrosion by determination of critical potentials, i.e., the breakdown, repassivation, and corrosion poten-tials, as well as the critical pitting or crevice temperature �1,50�. The critical potentials are typicallymeasured by cyclic potentiodynamic polarization as described elsewhere �1,50�. The relative values of thecritical potentials can be used as an assessment of the susceptibility to localized corrosion �50�. Thisassessment, however, is complicated by the several factors, including the broad dispersion of valuesmeasured in many systems.

An approach for determining a more accurate value of repassivation potential is the scratching tech-nique described above �51�. By purposely removing the passive film, the initiation stage of pitting iseliminated and the repassivation potential can be measured directly. A sample can be scratched duringpotentiostatic polarization at various potentials or periodically during potentiodynamic polarization. Atpotentials below the repassivation potential, the current will quickly decay to the passive current levelfollowing scratching. When the sample is scratched at a potential above the repassivation potential, thecurrent will increase continuously. An electrochemical equivalent to the mechanical scratching experimentinvolves stepping the potential for a short period to a high potential �E1� to initiate localized attack, and

FIG. 12—Schematic of (a) sample cross section and (b) breaking electrode cell [49]. The arrow representsthe position where the sample is struck to initiate fracture.

then stepping down to a low potential �E2� �52�. If the localized attack repassivates and the current decays


at E2, then the cycle is repeated, but the value of E2 is increased during each step until repassivation at E2

no longer occurs. The highest value of E2 at which the repassivation occurs is then the repassivationpotential. This value represents repassivation of relatively small pits or crevices.

Localized corrosion of Al alloys is strongly related to microstructural features on the scale of �m totens of �m. The influence of local heterogeneities cannot be studied by most scratching methods becausethere is no control over the placement of the scratch relative to the microstructure. However, microstruc-tures are visible in atomic force microscopes �AFMs�, particularly by using the scanning Kelvin probeforce microscope �SKPFM� technique �53,54� and pressure applied by an AFM tip on the surface of asample during in situ contact mode rastering is a form of scratching on the micro scale. An AFM tip canbe placed directly on a feature of interest as determined by the topography or by ex situ SKPFM and theuse of fiducial marks. Furthermore, the force applied by a tip can be controlled exactly. In situ rastering ofan AFM tip on a surface at high forces has been termed AFM scratching �55–57�. AFM scratching involvesin situ contact mode rastering of an area at high force followed by reducing the force and increasing thescanned area size to assess the effect of the scratching. AFM scratching of pure Al and AA2024-T3 inchloride solution with and without the addition of chromate or vanadate provided understanding on thecritical concentrations of inhibitor needed for repassivation of the scratched surface under various condi-tions �56,57�.

A more complicated technique, called the Tsujikawa Hisamatsu electrochemical �THE� method, hasbeen described for the determination of a conservative repassivation potential associated with the repas-sivation of well-developed crevice corrosion �58,59�. Interest in the THE method has been spurred recentlybecause of efforts to predict the behavior of very corrosion resistant alloys for very long times in nuclearwaste storage applications �60�. This method has several steps. A potentiodynamic polarization scan to afixed current is used to initiate attack. This is followed by a galvanostatic hold at that current to grow theattack, and then stepwise decreasing of the potential under potentiostatic control to determine the repas-sivation potential, which is defined as the potential at which the current does not increase during thepotential hold period. Typically, this technique is performed using a sample to which a multiple creviceassembly �61� has been attached because the repassivation potential for a deeply grown crevice should bethe lowest repassivation potential, and thus the most conservative from a design perspective. In otherwords, if the corrosion potential were to remain below the repassivation potential determined by the THEtechnique on a creviced sample, then one could be relatively confident that localized corrosion would notoccur during exposure conditions. Several issues exist with the THE technique. First, the repassivationpotential determined by this method might be overly conservative for many applications �59�. Further-more, the repassivation potential could depend on the hold current density and the potential step period.Finally, the determination of the repassivation potential by the criterion described above is not alwaysclear. The current has been observed to stay constant for several potential steps and then increase again ata lower potential.

The study of metastable pitting has provided unique opportunities for understanding pitting corrosionsince metastable pits involve initiation, growth, and repassivation in short, discrete, and plentiful events�62�. It is easier to address the stochastics of pitting with the numerous metastable pits generated in a fewexperiments than by performing many experiments that generate stable pits. The signal associated witheach metastable pitting event can be analyzed to determine pit current density, which allows determinationof the electrochemical kinetics of dissolution in pits and assessment of the rate-controlling steps. Obser-vation of metastable pitting current transients requires low background current because the current asso-ciated with metastable pits is small. The smaller the background signal, the smaller the events that can beresolved. It is critical to avoid artifacts associated with crevice corrosion, which will swamp the metastablepitting signal. To get a low background current, it is necessary to use electrodes with small surface areas.Data collection must be at a high enough rate to distinguish the individual events.

Electrochemical monitoring of corrosion is useful for sensing the onset of localized corrosion througha decrease in RP or a change in the EN response. However, for localized corrosion the unknown active areamakes it impossible to determine the current density from RP even using assumed Tafel slopes. Anotherproblem is that a considerable amount of hydrogen evolution can occur within pits, especially for Al andAl alloys. It has been found that about 15 % of the total anodic dissolution current is consumed locally byhydrogen evolution in Al pits and crevices �63,64�. This means that the current measured at an applied
potential is only a fraction of the true anodic dissolution current. Finally, the determination of pit growth


kinetics at open circuit is a challenge, because by definition, no net current is passed at open circuit. Inmany monitoring applications, knowledge of localized corrosion initiation is sufficient and the exact rateof penetration or growth is not required. However, in fundamental studies of localized corrosion suscep-tibility or mechanism, or for lifetime prediction of components, the rates of localized corrosion growth andthe potential dependence are of interest.

A variety of experimental approaches address the problems listed above in different ways. In general,they involve either the formation of a single localized corrosion site, or non-electrochemical means fordetermination of the growth kinetics. One of the reasons that metastable pits are so interesting is thatmetastable pit current transients represent individual pitting events. Other techniques for forming a singlelocalized corrosion site include the exposure of a small area, laser irradiation of a small spot, implantationof an activating species at a small spot, or the use of artificial or single pit electrodes. Other than theartificial pit electrode technique, in which the whole exposed area is active, the other techniques listed stillrequire an assumption regarding the geometry of the active pit surface to determine the pit area and thusa pit current density.

Artificial pit electrodes are formed by imbedding a wire in an insulator such as epoxy �65–67�. Thelocal environment within an artificial pit electrode crevice should be identical to that formed in real pits.The whole exposed area is active so the measured current can be converted easily to pit current density.Furthermore, artificial pit electrodes have an ideal one-dimensional geometry allowing for easy modelingof transport. Artificial pit electrodes have been used extensively to study Fe and stainless steel behavior.Polarization curves for artificial pit electrodes of FeCrNi alloys with and without 2.7 % Mo in a 1 M Cl−

bulk solution showed that anodic polarization curve in the pit environment for the Mo-containing alloywas shifted to higher potentials �68,69�. The change in the corrosion potential was approximately equal tothe change in pitting potential measured on a standard electrode by potentiodynamic polarization �68,69�.This indicates that the effect of Mo on the pitting potential can be explained solely by its influence on thepit dissolution kinetics.

Non-electrochemical techniques are useful for the study of localized corrosion growth because theyeliminate several problems: the need to determine the current from a single site, assumptions regardingactive surface area, and complications associated with hydrogen evolution within the pits or intergranularregions �63,64�. The foil penetration method is a non-electrochemical method that measures the time forlocalized corrosion to perforate foils of varying thickness as a means to determine the growth rate of thefastest growing localized corrosion site �70–72�. The penetration time is determined by exposing one sideof a foil sample to an aqueous environment �Fig. 13�. A piece of filter paper and then a Cu foil are pressedagainst the back of the sample. Penetration of localized corrosion on the other side is sensed by a decreasein resistance between the sample and Cu foil resulting from wetting of the filter paper by the localizedcorrosion environment. This technique has been applied successfully to the study of pitting, crevicecorrosion, and intergranular corrosion in stainless steel, Al and Al alloys �70–72�. It even can assesslocalized corrosion kinetics under open circuit conditions �71�.

Another non-electrochemical approach to pitting involves the study of 2-D pits in thin film samples�64,72–75�. Pits in thin metallic films with thickness on the order of 10–1000 nm rapidly penetrate themetal, reach the inert substrate, and proceed to grow outward in a 2-D fashion with perpendicular side-walls. The measurement of pit wall velocity from the analysis of magnified images of the growing 2-D pits�Fig. 14� provides a simple and direct means for determination of pit current density via Faraday’s law,with no need for assumptions. Since the pit depth is limited by the metal film thickness, pits in thin filmsgrow at steady state with no increase in ohmic path or diffusion length with time as is the case for pits inbulk samples. As a result, 2-D pits in thin films exhibit a pit current density that is constant with time at

FIG. 13—Schematic of the lower part of the cell for foil penetration experiments [71].

a given applied potential. This steady-state aspect of pitting in thin film samples allows unambiguous


determination of the current-density/potential relationship. The typical form for the polarization curve of athin film pit includes a region at low potentials where the current density is almost linearly dependent onapplied potential followed by a region at high potential where the pit current density reaches a limitingvalue, and pitting is mass-transport limited. Another consequence of the constant pit depth is the findingthat the repassivation potential and the lowest pit current density at which a pit can grow are extremelyreproducible. It is therefore possible with pits in thin films to assess accurately the critical conditions forpit stability. One final advantage of working with thin films produced by physical vapor deposition is thatthe full range of compositional variety can be achieved with a single phase structure �typically nano-crystalline or amorphous� because of the non-equilibrium nature of the as-deposited microstructure.

The series of standard electrochemical methods described above was performed on a system under-going pitting corrosion, AA2024-T3 with electrode area of 1.1 cm2 in 1 M NaCl. The PDP curve is givenin Fig. 15. Since the sample was pitting at open circuit, the effective anodic Tafel slope was very low andthe current rapidly increased at potentials above the OCP. In standard electrochemical approaches theactive pit area is not known. Therefore, the reported current density is a nominal current density, normal-ized to the whole exposed area, and not representative of the actual rate of dissolution in the pits. By fittingto Eq 1, the apparent anodic and cathodic Tafel slopes were found to be 17 mV /dec and 390 mV /dec,respectively, and a nominal corrosion rate of 1.67 �A /cm2 was determined. Corrosion was under cathodiccontrol and the corrosion potential was pinned at a value close to the pitting potential. Figure 16 shows thelinear polarization results. The large difference in anodic and cathodic Tafel slopes resulted in a very

FIG. 14—Schematic drawings of the experimental setup and of the nature of 2-D pit growth in thin films[75].

FIG. 15—Potentiodynamic polarization curve for AA2024-T3 in 1 M NaCl. Solid line – measured curve;
dashed line – fit to Eq 1.


non-linear curve that is not amenable for easy determination of polarization resistance. An estimate resultsin Rp=812 � or 738 � · cm2. The EIS plots shown in Fig. 17 are also somewhat difficult to analyze.Owing to the non-steady-state nature of an electrode undergoing pitting, the stability requirement for thedetermination of a valid impedance is not fulfilled. The impedance magnitude decreases at frequencieslower than about 0.1 Hz and the phase angle becomes positive so that the data in the Nyquist plot lie in thefirst quadrant. This is likely the result of the increasing pit area and corrosion current with time during theEIS measurement. A number of different equivalent circuits can be used to fit these data. A specific modelfor fitting EIS spectra for a sample undergoing localized corrosion was proposed by Shih and Mansfeld�76�. In that model circuit components representing the pitted area are in parallel to components repre-senting the passive area. That model, however, will not result in decreasing impedance at low frequencies.The entire EIS spectrum in Fig. 17 can be fitted well to a two-time-constant nested model with CPEs if anegative exponent is allowed for the inner CPE. Given the non-steady-state nature of the data, it isprobably more reasonable to ignore the data below 0.1 Hz. The remaining data can be fitted to a nestedtwo-time-constant model with positive CPE exponent. The best fit is for an inner CPE exponent of �1,which does not make sense, so the exponent was set to 1. The values of the pore and double layerresistances are 3610 � and 2010 �, respectively, for a total low frequency impedance of 5620 � or5110 � · cm2. Using the apparent Tafel slopes from the PDP curves, nominal corrosion rates of1.15 �A /cm2 and 7.9 �A /cm2 for the EIS and LPR data, respectively, can be determined. Despite thedifficulties in the analyses, these values are quite close to the result of Tafel extrapolation. The Rp from

FIG. 16—Linear polarization experiment for AA2024-T3 in 1 M NaCl. Tangent is drawn at point wherei�0.

FIG. 17—EIS data for AA2024-T3 in 1 M NaCl. (a) Nyquist plot and (b) Bode plot. The lines are fits to the
circuit shown in Fig. 11.


LPR again seems to be low resulting in a high corrosion rate. However, the fit to the LPR data was veryproblematic. The EN data were analyzed with the same approach described above, resulting in a value ofRn=7723 �, which is reasonable. EFM seems to fail in this system as the causality factors are 3.20 and2.65, which are much different than the theoretical values of 2 and 3, respectively. Nonetheless, the valueof corrosion rate determined from the EFM analysis is 6.7 �A /cm2, which is still close to the valuesdetermined from the other methods. The results of all of the analyses are summarized in Table 3.

Unmet Needs for Electrochemical Testing of Samples Immersed in Solution—The analysis given aboveshows that all of the electrochemical techniques work quite well to determine corrosion rate for the caseof uniform active dissolution. The results are close, even if assumed values of the Tafel slopes are used.Many of the methods can be used for corrosion rate monitoring as only small �or no� perturbation from thesteady-state open circuit potential is required. The available techniques seem to meet the needs. Somemight express concern about the reproducibility of electrochemical measurements, which as indicated istypically within a factor of about 2–3. The measurements themselves are extremely accurate. The vari-ability is a result of real variation in sample-to-sample reactivity in many environments.

For passive metals, the available electrochemical techniques also are suitable for most needs. Theplans to bury high level radioactive waste in Yucca Mountain have brought to corrosion science a newproblem that still might require new methods. There is a need to predict passive film stability on metalcanisters for tens of thousands of years. Even ignoring the possibility of localized corrosion, the growthand stability of passive films must be understood. The canisters will experience one long thermal transient�heat up and cool down� in an environment consisting of rock dust and humid air or dripping dilutegroundwater. The thickness, composition, and stability of the passive film must be known and understood.Electrochemical approaches to this problem are being taken, but will not be reviewed here. However, thisproblem is extremely challenging.

Prediction and assessment of the localized corrosion phenomena present the greatest challenges toelectrochemistry and corrosion science. It is easy to determine the critical potentials, i.e., the corrosion,breakdown, and repassivation potentials, for a given metal in a given environment. Comparison of thecorrosion potential �and a prediction of what the corrosion potential will be in the future� with thebreakdown and repassivation potentials can be used to make an assessment of the likelihood for localizedcorrosion to occur. In particular, if the corrosion potential will remain below a conservative estimate of therepassivation potential for deep pits or crevices, one can be confident that localized corrosion will not bean issue. However, if this is not the case, there is no agreed-upon approach for prediction of componentlifetime. Corrosion scientists are often asked to quantify the benefit of a reduction in environmentalseverity �e.g., a decrease in chloride concentration� or a change in alloy. The field lacks the level ofunderstanding available, for instance, in the field of fracture mechanics where laws of similitude allowfield predictions from lab experiments. In some cases, such as in microelectronics, the mere initiation of apit could represent a failure, whereas in other situations failure would require complete penetration of acomponent such as a storage vessel. The standard electrochemical approaches do not determine the local-ized current density in pits or crevices. Pit growth kinetics can be assessed using methods described above,but they are time consuming and labor intensive. Improved methods are needed to measure rates oflocalized corrosion and to connect those measurements to field exposures. Another critical step that is notwell understood is the transition of pits and crevices to cracks, which tend to propagate at much faster rates

TABLE 3—Analysis of electrochemical methods for determination of corrosion rate for AA2024-T3 in 1 M NaCl. Electrode areas�1.1 cm2.

LPR�using ba

and bc

from PDP�

EIS�using ba

and bc

from PDP� EN

EFM�active

dissolutionmodel�

PDP�fitting to

Eq 1�

ba, mV /dec 116 17

bc, mV /dec 170 390

RP, � 812 5620 7723

RP, � · cm2 738 5110

icorr, �A /cm2 7.9 1.15 6.7 1.67

than pits or crevices.


Metal in Atmosphere

As described above, atmospheric conditions could be adsorbed water layers in equilibrium with humidityand pollutants, or could involve precipitation, including acid rain. Under most atmospheric exposureconditions, however, the layer of electrolyte on the surface is extremely thin. Electrochemical measure-ments clarifying the kinetics and mechanisms of atmospheric corrosion are complicated by problemsassociated with immersing a reference electrode �RE� and counter-electrode �CE� into the thin layer.

A rather simple way to mimic atmospheric corrosion is by the droplet cell, which has small dimensionsand allows the study of resistive electrolytes such as pure water �77�. A circular area of a flat sample isexposed through a hole in a piece of protective tape �Fig. 18�. A circular piece of filter paper of the samesize is placed securely into the exposed hole and a small �typically 10–20 �l� droplet of water is placedon the filter paper using a calibrated pipetter. This wet filter paper acts as the electrolyte. A piece of wovenPt mesh is placed on top of the wet filter paper, and a reference electrode is held against the back of thePt counter-electrode. This simulates atmospheric corrosion, in which a thin water layer forms on thesurface. As in atmospheric corrosion, soluble species on the sample surface and pollutant gases in the airare dissolved into the water droplet, which provides some conductivity.

Mansfeld and Kenkel described the use of atmospheric corrosion rate monitors, composed of inter-digitated fingers of different metals, that track the galvanic current flowing through the cross section ofstacks of alternating dissimilar metal plates �78�. For monitors exposed to the weather, the maximumcurrent was found to flow at the point just before complete drying owing to the increase in oxygendiffusion rate with decreasing solution layer thickness. Mansfeld used similar devices composed of asingle material to make measurements of polarization resistance �79�. The measurements were two-electrode measurements with no reference electrode. Lyon and co-workers measured electrochemicalkinetics in layers as thin as 50 �m using an essentially bulk electrochemistry approach involving acapillary connection to a remote RE and CE �80–84�.

The problem of introducing an RE into the thin water layer responsible for atmospheric corrosion wassolved by Stratmann and Streckel, who demonstrated that a Kelvin probe �KP� vibrating above a sampleprovides a measure of the corrosion potential �85–87�. In brief, the changing capacitance between thevibrating probe and the sample generates a current if the potential of the probe and sample are different.The local potential is determined by adding a backing potential to the sample that either nulls out thesignal or modifies the signal in a way that can be analyzed. Stratmann and Streckel made corrosion ratemeasurements by monitoring the rate of decrease in the oxygen partial pressure �86�. They also used amodification of the KP apparatus to control the potential and measure the current to a coplanar counterelectrode, thereby achieving the ability to measure polarization curves �87�. This arrangement might becalled a Kelvin probe potentiostat �KPP�. They used the KPP to measure cathodic polarization curves onPt under thin layers of 1 M Na2SO4. The limiting current density for oxygen reduction was found to vary

FIG. 18—Schematic drawing of droplet cell.

inversely with the thickness of the electrolyte layer for layers with thickness between about 10 �m and


100 �m. A new approach for making KPP measurements has been described recently �88�. In this ap-proach, the distance between the tip and sample is controlled, allowing simultaneous tracking of thesample topography or measurement of the thickness of an electrolyte layer as it dries.

Electrochemical measurements under thin electrolyte layers were also reported by Nishikata, Tsuru,and co-workers �89–93�. They first performed EIS measurements on samples composed of two identicalelectrodes of steel, stainless steel, or Cu embedded in epoxy under thin electrolyte layers during cyclicwet-dry conditions �90,91�. They subsequently developed a three-electrode approach using a coplanarchloridized Ag RE embedded in epoxy between the Fe working electrode and the CE �89�. The RE allowedfor the measurement of cathodic polarization curves, and they found the same dependence of limitingcurrent density on layer thickness as did Stratmann et al. Nishikata, Tsuru, and co-workers also studiedpitting corrosion of stainless steel under thin electrolyte layers �92–94�. The corrosion rate associated withpitting was assessed by EIS during cyclic wet-dry cycling on different stainless steels �92�. Corrosionpotential measurements were made on a second embedded working electrode using yet another sampledesign in which an agar-filled hole in the epoxy allowed ionic connection to a remote standard RE �93�. Adecrease in the corrosion potential by hundreds of mV coincided with a sharp decrease in polarizationresistance and the onset of pitting corrosion as a critical chloride concentration was reached during drying.The coplanar Ag /AgCl RE and agar-filled hole used in the work of Tsuru et al. suffer from problemsassociated with changing Cl concentration and non-uniform current distribution.

Exfoliation corrosion �EFC� is a form of localized corrosion that occurs in wrought Al alloys withelongated grain structures during exposure to aggressive atmospheres. The susceptibility to EFC typicallyis assessed by non-electrochemical techniques. ASTM Standard G34, “Standard Test Method for Exfolia-tion Corrosion Susceptibility in 2xxx and 7xxx Series Aluminum Alloys,� also known as the EXCO test,involves immersion in an oxidizing acidic chloride solution and comparison of the resulting surface tostandard photographs �95,96�. Such test conditions are very aggressive and different than typical exposureconditions. Other test methods involve intermittent spraying of acidified salt solutions onto the surface ofthe specimens �97–100�. The behavior of Al alloys in these accelerated environments has been correlatedto long-term exposure in less aggressive natural environments �101–103�. As a result, these tests are usefulfor assessing susceptibility to EFC attack. However, they do not provide quantitative measurements ofsusceptibility or growth kinetics, which are required for predictive modeling of corrosion development.Liddiard and co-workers have used a deflection technique to quantify exfoliation extent and determineEFC kinetics �104�. In this technique, the effective remaining load-bearing section of a sample exhibitingEFC is determined from its compliance under four-point bending. The rate of EFC can be assessed fromperiodic measurements. The deflection technique is valid only when the thinning of the specimen duringcorrosion is uniform. A new technique, exfoliation of slices in humidity �ESH�, was developed recently forthe determination of EFC susceptibility and quantification of EFC kinetics �105,106�. In this technique,slices of plate are pretreated by potentiostatic polarization in chloride solution to develop localized corro-sion sites. Subsequent exposure to high humidity after pretreatment of properly oriented and unconstrainedsamples results in the development of EFC at the edges of the slices. With time the EFC grows inwardsand the EFC kinetics are determined by measuring the width of the central unattacked region of thesamples. ESH test results have been shown to be representative of different EFC behavior of plates duringoutdoor exposure �106�.

Unmet Needs for Electrochemical Testing of Samples in Atmosphere—At present there is no goodelectrochemical technique for electrochemical testing of samples exposed in atmosphere. Electrochemicalmethods for samples in corrosive atmospheres are difficult due to the thin layer electrolyte. A counter-electrode can be coplanar, but this can result in non-uniform current distributions. Placement of thereference electrode is particularly problematic, and the use of the Kelvin probe and coplanar referenceelectrodes such as Ag /AgCl were discussed above. Standardized procedures for electrochemical testing ofsamples in atmosphere should be pursued.

Furthermore, it is becoming evident that complex atmospheric chemistry can influence the localenvironment for a sample exposed outdoors in ways that are not simulated in lab exposure tests. Theinteraction of sunlight �of wavelength 243 nm� and ozone results in the formation of uncharged OHradicals that can interact with chloride in seawater aerosols above or near the ocean to create ultimately Cl2

�107�. The presence of a small amount of aggressive radicals or chlorine gas could have a large influence
on atmospheric corrosion. The understanding of the atmospheric chemistry is still evolving and the influ-


ence of the environment on corrosion and the electrochemistry of this phenomenon are totally unknown.A major need in the field of atmospheric corrosion is a fundamental understanding of the mechanisms

and acceleration factors associated with the critical parameters in lab exposures. Accelerated tests are usedwidely in the electronics industry, and the acceleration factors associated with temperature, humidity, andbias �the typical stresses in the electronics industry� are relatively well understood �10�. Parts or simulatedparts are exposed to aggressive conditions and some property is measured or mean time to failure �MTTF�is determined. Electronic components are amenable to the measurement of failure time. The goal is todevelop an acceleration factor �AF� for a given test, where AF=MTTF�field�/MTTF�test�. Expressionsexist for the AF for each of the stresses. The effects of the stresses are typically considered to be inde-pendent, though that is not always the case. Regardless, the total AF is usually a product of the individualAFs. This approach works relatively well for electronic components allowing extrapolation from theaggressive accelerated test environment to service environments, which are typically much more benign.Such understanding does not exist for atmospheric corrosion in general. For instance, there is no under-standing that allows the behavior in ASTM B117 exposure to be used for prediction in some otherenvironment. This understanding is complicated by factors such as the atmospheric chemistry described inthe last paragraph. Nonetheless, it would be very valuable to have a scientific basis to predict corrosionrates and lifetimes for any environment from the results of an accelerated exposure test.

Coated Metal in Solution

Most of the techniques described above are not suitable for the study of metals covered with a protectiveorganic coating. The primary electrochemical technique for this application is EIS. As mentioned above,EIS is well suited for painted samples as it is sensitive to the early stages of coating degradation, wellbefore defects are visible by eye. However, it is a technique that is prone to artifact and misinterpretation,especially by ill-trained or inexperienced practitioners. So, for electrochemical evaluation of coated metalsin solution, the major need is better training in the proper use and interpretation of EIS. However, evenexperienced practitioners must make critical decisions in the analysis of EIS data regarding the choice ofequivalent circuit for fitting of the data. It is possible for some data sets to develop multiple equivalentcircuits that are reasonable and physically justifiable. Sometimes it is not clear when a second timeconstant must be incorporated in a set of data taken, for instance, as a function of exposure time.

Potentiostatic pulse testing �PPT� is a related technique that has been reported to be useful for moni-toring the earliest stages of degradation of paint coatings �108,109�. A square pulse of 0.1–2.0 V is appliedto a coated sample in a two-electrode cell with the reference electrode also acting as the counter-electrode.The pulse can be in the form of a square wave so that the frequency of the square wave and the samplingfrequency will determine the limits of the frequency response. As in EIS, large amplitude signals can beused for systems with very protective coatings. The current response is fitted to an equivalent circuit, as inEIS. Values of charge transfer �Rct� and pore �or defect� resistances �Rd� on the order of 1011 � · cm2 anddouble layer capacitance �Cdl� on the order of 10−11 F /cm2 have been successfully determined with thistechnique �108�. This range of impedance is well beyond the typical range of standard EIS equipment.However, it is not possible to get all of the circuit components simultaneously by PPT when the timeconstants are very different �109�. On the other hand, the unavailable components �e.g., the solutionresistance �Rs� and coating capacitance �Cc�� are typically not critical for the determination of the overallcorrosion resistance or the early stages of coating degradation. PPT can be sensitive and cost efficient if apicoammeter is used instead of a potentiostat, and PPT data can be very useful for fast prediction ofcoating performance at the early stage of the coating failure. However EIS provides a fuller description ofthe coated electrode response.

The Kelvin probe was described above as a tool that could be used for measuring or controllingpotentials for samples under a thin layer of electrolyte. By using a small probe needle and appropriatescanning motors, it is possible to map potentials on a surface. The scanning Kelvin probe �SKP� is a usefultool for studying coated samples �110�. The Kelvin Probe measurement cannot be performed in fullimmersion, so SKP is not suitable for studying immersed samples. However, using a reservoir of solutionadjacent to a coating delamination, it is possible to measure quantitatively the delamination rate of organic
coatings at defects �110�. In this way, the behavior under immersion conditions can be simulated. SKP


potential profiles have quantified delamination of organic coatings on galvanized steel and have led to thedevelopment of galvanizing methods with much lower delamination rates and thus improved corrosionprotection �111�.

One critical aspect of coatings, particularly for Al alloys, is the ability to heal defects spontaneously.This self-healing phenomenon, also called active corrosion inhibition, can be achieved by both pigments inorganic paints as well as conversion coatings. The artificial scratch cell is an approach that allows for clearassessment of various aspects of active corrosion inhibition �112�. An untreated surface �representing ascratch� is placed in close proximity to a treated or coated surface, separated by a thin layer of electrolyte,typically an NaCl solution. The two surfaces can be shorted electrically or not, and the thin electrolytelayer can be exposed to air at its edge or not, by using a partial shim or complete o-ring. In this fashion,the untreated surface is exposed to electrolyte that has been altered by exposure to the treated surface. Byinsertion of reference and counter-electrodes, it is possible to monitor the changes in corrosion resistanceof both surfaces in situ. The initially coated sample might degrade with time, and the initially uncoatedsample might improve with time as it interacts with the electrolyte. After the experiment the electrolyte canbe analyzed to determine what has been released by the coated sample, and the surfaces of the electrodescan be analyzed. Surface analysis of the initially uncoated sample provides information regarding thetransport of inhibiting species from the coated sample through the thin electrolyte layer. The artificialscratch cell has been used to study chromate conversion coatings and Ce-containing coatings �112,113�.

Unmet Needs for Electrochemical Testing of Coated Samples Immersed in Solution—The EIS methodis very well suited for the evaluation of coated samples immersed in solution. Sensitive, affordable, anduser-friendly EIS systems have made the technique available for use by technologists. However, it is notas widely used for testing of coated samples as it could or should be, due largely to a resistance to changeby practitioners in industry. It is likely that EIS will gain acceptance with time as more people use it andfeel comfortable with it. As described above, the nested two-time-constant equivalent circuit is well suitedfor painted samples. An issue, however, is when to apply the two-time-constant model to analyze data fora sample that slowly degrades with time.

Coated Metal in Atmosphere

There are many applications in which painted samples are exposed to the atmosphere, including automo-tive, aerospace, and construction usages. This important corrosion exposure condition is the one with thefewest options for electrochemical measurements, or actually for any kind of meaningful measurements.The difficulty is that this condition combines the complications of a coated sample with the limitations ofatmospheric exposure. Another coating class that is of interest for corrosion protection is inorganic con-version coatings, which are often used in combination with organic overcoats but can also be used asfree-standing protection without a topcoat. Furthermore, many coated systems have multiple layers, forinstance, a sacrificial protection layer covered by a conversion coating and then multiple organic layers.These layers can contain species that interact with the water permeating the coating and alter the localenvironment. So in the absence of a through-coating defect, the environment at the metal surface wherecorrosion occurs is different than the outer environment, either test or real life, that is measured orcontrolled.

As described above, a common approach for examining the corrosion protection provided by a coatingis exposure in an aggressive atmospheric environment such as the salt fog generated in a chamber asdescribed by ASTM B117 �8�. Samples are typically inspected after a period of exposure. The extent ofcorrosion is assessed by a metric such as the number or area of defects per unit area observable by eye, orthe extent of delamination at a scribe. A pass/fail ranking is assigned based on some criterion. Salt fogtesting has been criticized as not being representative of or correlating with many real world environments�98� and as quoted above, the B117 standard itself provides several strong statements about the appropriatelimits for usage of test results.

Electrochemical testing can be combined with salt fog testing to get a more quantitative measurement.In this approach, samples are exposed to the salt spray environment and periodically removed for testingby EIS. The EIS testing is done by immersion in solution using standard approaches and the sample is thenreturned to the salt spray environment. Such an approach leads to concerns regarding whether the periodic
removal and immersion influences the sample �93�.


EIS has also been investigated as a faster and possibly more discriminating alternative to salt spraytesting. A matrix of inorganic conversion coatings on Al alloys was tested by salt spray exposure and EISduring immersion in 0.5 M NaCl �114�. The resistance to salt spray was assessed by the number of pitswith an arbitrary value of five pits on the 194 cm2 sample considered a failure. EIS was performed on aparallel set of samples and the coating resistance �Rc� was used as a figure of merit to assess coatingperformance. Both tests effectively assessed the extent of corrosion protection; the probability of achievingpassing a 168-h salt spray exposure increased as Rc increased. For each alloy, threshold Rc values wereproposed for which a given coating can be expected to attain a passing result in a 168-h salt spray test.

Unmet Needs for Electrochemical Testing of Coated Samples Exposed to Atmosphere—This categoryhas the most needs. All of the needs described above for metals exposed to atmosphere also are relevanthere. However, the techniques developed for studying the behavior of metals under thin film electrolytesare not applicable for coated samples. There is a severe lack of scientific underpinning to the prediction oflifetime for coated samples in atmospheric exposure conditions, which results largely from a lack of usefultools.

Summary

Electrochemistry techniques are invaluable for studies, monitoring, and prediction of metallic corrosion.They are available at affordable prices and with user-friendly software that makes their utilization straight-forward. This review has considered the specific cases of uncoated and coated metal exposed to bulkelectrolyte environments or atmospheric corrosion conditions. This simple matrix provides a useful frame-work for considering the status, limitations, and needs of electrochemical techniques in corrosion. Thegreatest limitations, and needs are in the areas of localized corrosion and atmospheric corrosion. Inlocalized corrosion there is a need for methods to determine localized, not nominal, corrosion rates and forlaws of similitude that will enable the prediction of component lifetime in the field from short-termlaboratory experiments. In atmospheric corrosion there is a need for sensitive, simple, and standardizedapproaches for determination of corrosion rates, both for uncoated and coated metal samples.

Acknowledgments

The author gratefully acknowledges Y. Zhai and S. Taira for performing the experiments reported hereinand is indebted to R. Buchheit for his input and comments.

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Electrochemical Techniques in Corrosion

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