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Protection of Aluminum Alloy (AA7075) from Corrosion

by Sol-Gel Technique

von der Fakultt fr Naturwissenschaften der Technischen Universitt

Chemnitz genehmigte Dissertation zur Erlangung des akademischen Grades

doctor rerum naturalium

(Dr. rer. nat.)

vorgelegt von

M.Sc. Ahmed Younis

geboren am 01.03.1979 in Kairo, gypten

eingereicht am 30.08.2011

Gutachter: Prof. Dr. Rudolf HolzeProf. Dr. Thomas Lampke

Tag der Verteidigung: 24.01.2012

http://nbn-resolving.de/urn:nbn:de:bsz:ch1-qucosa-83230

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Bibliographische Beschreibung und Referat

Ahmed Younis

Protection of Aluminum Alloy (AA7075) from Corrosion by Sol-Gel Technique

Die vorliegende Arbeit beschftigt sich mit der Entwicklung von Sol-Gel-Beschichtungen

durch Optimierung der Ausgangszusammensetzung und der Applikations-Parameter fr

den Korrosionsschutz der Aluminium-Legierung AA7075. Verschiedene Arten von

Silanen, z. B. Tetraethoxysilan (TEOS), Phenyltriethoxysilan (PTES) und Phenyltrim-

ethoxysilan (PTMS) sind verglichen worden: Der Sol-Gel-Film aus PTMS prpariert, weist

dabei die hchste Hydrophobizitt auf, was sich insbesondere in den Barriere-Eigenschaften

dieser Verbindung zeigte. Die Wirkung von Essigsure als Katalysator in Sol-Gel-

Prozessen wurden untersucht, um die optimale Katalysatorkonzentration fr den

Korrosionsschutz der beschichteten Proben zu ermitteln. Die Korrosionsbestndigkeit der

beschichteten Proben sinkt bei hheren Konzentrationen des sauren Katalysators durch die

Auflsung des Aluminiumoxids an der Substratoberflche. Allerdings fhrten zu niedrige

Konzentrationen des Katalysators zur Verlangsamung der Hydrolysereaktionen der Silane

und es bildete sich porse Sol-Gel-Schichten. Die Wrmebehandlung der beschichteten

Aluminium-Proben ist fr die Vernetzung des Films erforderlich. Eine Wrmebehandlung

bei 300 Cfr 2,5 Stunden ergab dabei den besten Korrosionsschutz. Hhere Temperaturen

fhrten zu einer Verschlechterung der Eigenschaften der Filme, was mit der Zerstrung des

organischen Teil des Films erklrt werden kann. Darber hinaus verursachen zu niedrige

Temperaturen einen geringeren Korrosionsschutz der beschichteten Aluminium-Proben.

Vermutlich ist die geringe Vernetzung des Sol-Gel-Films bei Temperaturen was fr als

300 C verantwortlich. Die beschichteten Aluminium-Proben wuden mittels Raster-

Elektronenmikroskopie (SEM), Energiedispersive Rntgenspektroskopie (EDX), Rntgen-Photoelektronenspektroskopie (XPS) und elektrochemischen Techniken charakterisiert.

Schlsselwrter: Aluminium-Legierung; Sol-Gel-Film; Sol-Zusammensetzung; Wrmebe-

handlung; Konzentration des Katalysators; elektrochemische Messungen.

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Abstract

A.A. Younis

Protection of Aluminum Alloy (AA7075) from Corrosion by Sol-Gel Technique

The present work pertains to the development of sol-gel coatings by optimizing the

composition and the application parameters for corrosion protection of aluminum alloy

AA7075. Different kinds of silanes e.g. tetraethoxysilane (TEOS), phenyltriethoxysilane

(PTES) and phenyltrimethoxysilane (PTMS) have been compared: the sol-gel film

prepared from PTMS shows highest hydrophobicity manifested by the best barrier property

of this compound. The effect of acetic acid as a catalyst on the chemistry of the sol is

investigated in order to estimate the best catalyst concentration for better corrosion

protection of the coated samples. The corrosion resistance of the coated samples is found to

be decreasing at higher concentrations of the catalyst due to the dissolution of the

aluminum oxide at the substrate surface in the acid sol. However, lower concentrations of

the catalyst lead to low hydrolysis reactions of the silanes and non-dense sol-gel films have

been formed. The heat treatment of the coated aluminum samples is required for cross-

linking of the film. The heat treatment at 300 C for 2.5 hours exhibits the best corrosion

protection. Higher treatment-temperatures lead to degradation of the properties of the film

which can be described in terms of destroying the organic part of the film. Moreover, low

treatment-temperatures cause low corrosion protection of the coated aluminum samples

which is presumably attributed to the low cross-linking of the sol-gel film at temperatures

less than 300 C. The coated aluminum samples are characterized by scanning electron

microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron

spectroscopy (XPS) and electrochemical techniques.

Keywords: Aluminum alloy; sol-gel film; sol composition; heat treatment; concentration of

the catalyst; electrochemical measurements.

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Die vorliegende Arbeit wurde in des Zeit von Januar 2008 bis November 2011 angefertigt.

Sie entstand unter der Leitung von Herrn Prof. Dr. W. Ensinger(TU-Darmstdt) und Herrn

Prof. Dr. Rudolf Holze (TU-Chemnitz).

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Acknowledgements

First and foremost I offer my thanks, obedience, and gratitude to God from whom I

received guide and inspiration.

I would like to express my gratitude to my supervisor Prof. Dr. Rudolf Holze (TU-

Chemnitz) whose expertise, valuable suggestions, comments, guidance and patience added

considerably to my knowledge. I deeply appreciate his vast knowledge in the field of

electrochemistry.

I would like to thank Prof. Thomas Lampke (TU-Chemnitz) for accepting to be the second

referee of this work.

I am deeply grateful to Prof. Dr. Karl Heinz Hoffmann and Prof. Dr. K. Banert (TU-

Chemnitz) for a guide and supporting me in my thesis.

I also wish to thank Prof. Dr. W. Ensinger(TU-Darmstadt) who gave me a chance to do

more experimental in his corrosion Lab.

I would like to thank several people without whom this research would have been

extremely difficult. Generally, I wish to give my grateful acknowledgements to all the

members of the institute of chemistry, Chemnitz University of Technology, who provide apleasant atmosphere where I spent one year of my happy time.

I wish to express my appreciation to the love and good wishes extended by Pro. Dr. Abdel-

Rahman and Prof. Dr. M.M.El-sabbah (Al-Azhar University), Prof. Dr. Alia Eldamati,

Prof. Dr. Mohamed Nour and Prof. Dr. Mohamed Hassan (National Institute for

Standards), Prof. Dr. Ahmed Hashem (National Research Center) and Prof. Dr. Abdel

Salam Makhlouf (Central Metallurgical Research & Development Institute) my Egyptian

professors who initially introduced me to the field of corrosion and discuss any problem

related to researching work.

My biggest and deepest gratitude goes to my mother for her constant inspiration, support

and encouragement. I would like to express my gratitude to my brothers and sister whom

theirsupports helped me along the way.

My sincere thanks with extreme love go to my children who were a great source of

motivation to finish this work in a short time. I am greatly indebted to the parents, brothersand sister of my wife for their love, affection and prayers.

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This dissertation is

dedicated to my dearparents, wife, lovely son,

daughters, and my

brothers, for their love,

caring, and supports me

through the last years.

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List of Abbreviation and Symbols

AFM Atomic force microscopy

E Cell potential (V)E Standard electrode potential (V)

F Faraday constant

FTIR Fourier transform infrared spectroscopy

G Gibbs free energy

G Standard free energy

jdiss Dissolution current density (A/cm2)

jcorr Corrosion current (A)

Ecorr Corrosion potential (V)

R (1) Resistance; (2) Gas constant 8.314 J/Kmol

RE Reference electrode

SEM Scanning electron microscopy

T Temperature (K)

t Time

XPS X-ray photoelectron spectroscopy

TEOS Tetraethoxysilane

PTES Phenyltriethoxysilane

PTMS Phenyltrimethoxysilane

Overvoltage (Overpotential)

DCP Direct current polarization

AcAc Acetic Acid

A. Acetone Acetylacetone

WE Working electrode

V Volt

Fig. Figure

Rp Polarization resistance

OCP Open circuit potential

IGC Intergranular corrosion

Contact angle measurement

SIMS Secondary ion mass spectroscopy

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OM Optical microscopy

EDS Energy dispersive x-ray spectroscopy

Charge transfer barrier for anodic or cathodic reaction

CR Corrosion rate

Ra Average roughness

RMS Root mean square roughness

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Table of Contents

Bibliographische Beschreibung und Referat:.................................................................. 2

Abstract:....... 3

Zeitraum, Ort der Durchfhrung: .... 4

Acknowledgments: ..... 5

Dedication:... 6

List of Abbreviation and Symbol:....7

Table of Contents:........ 9

Chapter I:........ 13

Introduction:..... 13

1.1 Literature Reviews:........ 13

1.1.1 Protection of pure aluminum from corrosion:...... 13

1.1.2 Protection of aluminum alloy from corrosion:.......... 14

1.2 Sol-Gel process:......... 15

1.3 Application of sol-gel process:...... 18

1.3.1 Spin coating:..... 18

1.3.2 Dip coating:....... 20

1.4 Corrosion and types of corrosion:.......... 21

1.4.1 Corrosion:......... 21

1.4.2 Electrochemical reactions in aluminum corrosion:...22

1.4.3 Aluminum as a passive metal:...... 23

1.4.4 Forms of corrosion:....... 25

1.4.4.1Uniform corrosion:.... 25

1.4.4.2Galvanic corrosion:....... 25

1.4.4.3Pitting corrosion:... 27

http://corrosion.ksc.nasa.gov/unifcor.htmhttp://corrosion.ksc.nasa.gov/galcorr.htmhttp://corrosion.ksc.nasa.gov/galcorr.htmhttp://corrosion.ksc.nasa.gov/galcorr.htmhttp://corrosion.ksc.nasa.gov/galcorr.htmhttp://corrosion.ksc.nasa.gov/unifcor.htm

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1.4.4.4 Crevice corrosion:. 30

1.4.4.5 Intergranular corrosion (IGC):...... 31

1.5 Thermodynamics:.. 32

1.5.1 Pourbaix diagram (potential-pH diagram):... 32

1.5.2 Gibbs free energy (G):35

1.5.3 Nernst equation:.......... ...... 36

1.6 Kinetic aspects of corrosion process:......... 37

1.6.1 Overpotential ():.. 37

1.6.2 Concentration polarization:........................... 38

1.6.3 Butler-Volmer equation:........... 40

1.7 Aim of the work:.... 41

Chapter II:...... 43

2. Experimental:....... 43

2.1 Sample preparation:... 43

2.1.1 Polishing process:. 43

2.1.2 Sol preparation: ....... 43

2.2 Electrochemical measurements:........ 44

2.2.1 Porosity determination:........ 46

2.3 Linear polarization resistance (Rp):............... 46

2.4 Determination of the film thickness by DekTak D8000:.. 47

2.5 Weight loss measurement:..... 49

2.6 Analytical techniques:............ 49

2.6.1 Scanning electron microscopy and energy dispersive x-ray spectroscopy

(SEM/EDX):............ 49

2.6.2 Infrared spectroscopy (IR):.......... 51

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2.6.3 Atomic force microscopy (AFM):... 53

2.6.4 Contact angle () measurement:...... 54

2.6.5 Secondary ion mass spectroscopy (SIMS):..... 55

2.6.6 X-ray photoelectron spectroscopy (XPS):........... 56

2.6.7 Optical microscopy (OM):.......... 57

Chapter III:.... 58

3. Results:............. 58

3.1 Study of the effect of PTMS coating on aluminum alloy:............. 58

3.1.1 Optimization of PTMS sol solution:......... 58

3.1.1.1 Study of the effect of Acetic Acid (AcAc):............ 58

3.1.1.2 Study of the effect of Acetylacetone (A. Acetone):........... 60

3.1.1.3 Study of the effect of water-propanol:........ 61

3.1.1.4 Study of the effect of heating temperature:................................................ 62

3.1.1.5 Film thickness and spin coating:..... 64

3.1.1.6 Film thickness and heat treatment:............. 65

3.1.2 Potentiodynamic tests:.......... 66

3.1.2.1 Potentiodynamic study with different electrolyte concentrations:..67

3.1.2.2 Potentiodynamic study with different pH values:........... 70

3.1.2.3 Potentiodynamic study with different temperatures:.. 72

3.1.2.4 Potentiodynamic study with different pH and temperatures:..... 75

3.1.3 Linear polarization resistance for samples coated with PTMS:... 75

3.1.4 Structural characterization of PTMS-coated samples:..... 77

3.1.4.1 SEM + EDX:.............. 77

3.1.4.2 Optical microscopic studies for samples coated with PTMS:........ 79

3.1.4.3 Secondary ion mass spectroscopy:......... 80

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3.1.4.4 X-ray photo electron spectroscopy:............ 81

3.2 Comparison between PTMS, PTES and TEOS as different thin coated film after

drying:........ 83

3.2.1 Potentiodynamic test after zero immersion time:...... 83

3.2.2 Potentiodynamic test after 1 day immersion time in case of Al alloy only:.. 84

3.2.3 Optical microscopic studies for samples coated with TEOS, PTES and

PTMS:........ 85

3.2.4 Polarization resistance:.......... 86

3.2.5 Atomic force microscopy:............. 87

3.2.6 Contact angle measurements:........ 89

3.2.7 Infrared spectroscopy:....... 90

3.2.8 Weight loss and inhibition efficiency:....... 95

3.2.8.1 Weight loss after immersion for 1 day in different pH electrolyte

solution:....... 95

3.2.8.2 Weight loss after immersion for 7 days in 0.05 MNaCl:....... 96

Summary and conclusion:................ 98

References:... 101

Selbstndigkeitserklrung:....... 110

Curriculum Vitae:.... 111

Publications and Conferences:. 112

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CHAPTER I

INTRODUCTION

1.1 Literature Review

1.1.1 Protection of pure aluminum from corrosion

Aluminum is the third most common element after oxygen and silicon on earth. It is an

important construction material for many applications in aerospace, aviation, automobiles,

household appliances, containers, and it is used for interconnects and electrodes in

electronic devices and integrated circuits [1 - 3] because of its good electrical and thermal

conductivities, low density, and high ductility. The corrosion resistance of aluminum is

based on its ability to form a natural oxide film on its surface in a wide variety of media [4

- 13].Nevertheless, it has been reported, that the oxide film can readily undergo corrosion

reactions in chloride-containing environments [14 - 17]. For this reason, its corrosion

mechanism in chloride solutions has been investigated repeatedly [18 - 23].

The protection of aluminum and its oxide films against corrosive chloride attack using

either inorganic oxidants, including chromate as in zinc chromate [5, 21, 24, 25],

molybdate as zinc-phosphate-molybdate composite and sodium molybdate dihydrate

(Na2MoO4 2H2O) [26 - 28] or tungstate [5, 25], organic compounds having polar groups,

such as oxygen, sulfur, and nitrogen [29 - 38], and heterocyclic compounds containing

functional groups and conjugated double bonds [38 - 41] as corrosion inhibitors has been

studied by many researchers.

In sol-gel processes reactions between solutes being present in small amounts and solvent

(high amount) in solution resulting in deposits, the process is known as chemical solution

deposition. The attractive features of both process and products have resulted in more than

35,000 publications [see e.g. 42 - 44]. The sol-gel process can be used to form nano-

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structured inorganic films (< 100 nm in overall thickness) which are more resistant than

underlying metals to oxidation, i.e. corrosion, showing good thermal and electrical

properties. The chemistry of the sol-gel process is well known [45 - 49]; sol-gel derived

products have numerous applications [50 - 53]. Sol-gel techniques can be used to prepare

a barrier coating showing low porosity preventing the surface of metal and alloy from

corrosion.

1.1.2 Protection of aluminum alloy from corrosion

Aluminum and its alloys are generally passive and corrosion resistant in aqueous solutions

except for pitting corrosion caused by reactive species, such as halide ions (e.g., chloride)

[54 - 56]. The passive film on the aluminum alloy surface is a poor electronic conductor

and cathodic reactions occur on the micron-size particles of impurity constituents or small

precipitate particles [57 - 69]. Various grades of aluminum alloy and metals were exten-

sively used to study the effect of alloying elements on the breakdown of the passive film

[57 - 66]. The presence of alloying elements in the microstructure such as insoluble

intermetallic particles (Al2Cu, FeAl3) or single elements (Cu, Si) leads to a formation of

local electrochemical cells between them and the aluminum matrix [57 - 74]. This causes a

highly localized attack by pitting in aggressive medium [54 - 56]. Various factors affect the

pitting corrosion in aluminum alloys such as the type of aggressive ion and its concentra-

tion, the pH of the media, the temperature or the structural characteristics of the oxide pas-

sive film [56 -74]. As an efficient replacement of highly toxic chromium-containing corro-

sion protection coatings thin layers of various materials obtained via sol-gel processing

have been proposed and evaluated [75, 76].

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1.2 Sol-Gel Process

It was discovered in the late 1800s and extensively studied since the early 1930s. Sol-gel

technology is proving to be very flexible with its variety of solution chemistries, and is

actively being researched as a replacement for chromate treatments. The sol-gel process

involves the growth of inorganic networks through the formation of a colloidal suspension

(sol) and gelation of the sol to form a network in a continuous liquid phase (gel). The

precursors for synthesizing these colloids consist of a metal or metalloid element

surrounded by various reactive ligands. Metal alkoxides are most popular because they

react readily with water.

The convenient solution to enhance the corrosion resistance of substrates is the deposition

of a protective barrier which can be achieved by several methods such as chemical vapor

deposition [77], physical vapor deposition [78], laser cladding [79], thermal spray [80] and

sol-gel [81]. Compared to other techniques, the sol-gel method seems simple as a

deposition procedure for preparing adherent and chemically inert metal oxide coatings at

low temperatures. Sol-gel techniques have several advantages besides being economical

and fast. So these can be used for all types of surfaces to control the size porosity of the

resultant thin film.

Sol-gel derived products have numerous applications [44, 82 - 86], for example, in the

synthesis of lightest materials and some of the toughest ceramics, in the field of optical,

electronics and energy. These thin films of respective material oxides are produced using a

spin or dip coating process. One of the largest application areas is thin films, which can be

produced on a piece of substrate by spin coating or dip coating is protective and decorative

coatings. Moreover, electro-optic components can be applied to glass, metal and other

types of substrates using these methods. Other coating techniques include spraying,

electrophoresis or inkjet printing.

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Alkoxysilane is used as starting materials for network formation in the sol-gel process. The

low-temperature reaction rate allows scientists and researchers to incorporate organic

moieties into the inorganic network to provide a new class of materials which combine the

properties of reactants [87 - 89]. Organic components lead to an increased flexibility and

density while the inorganic part enhances hardness and durability. An effective coating

must be well-adherent to the substrate with a uniform and pore-free surface, which is

resistant to chemical/mechanical attack.

Sol-gel technique is depending on two steps hydrolysis and condensation reactions. The

mechanisms of hydrolysis and condensation, and the factors (organic radical of the OR-

group, solvent, catalyst, concentration and temperature) that have a bias the structure

toward linear or branched structures are the most critical issues of sol-gel science and

technology [90 - 97]. Hydrolysis reaction is achemical reaction in which the molecules of

H2O are splitting into hydrogen cations (H+) and hydroxide anions (OH) [98]. In

hydrolysis reaction, the reaction between OH from H2O and R from silanol can be proven

by using H2O18. The presence of isotope in silanol confirms the mechanism [99] as

described in the following equation:

Si-OR + H18OH Si 18OH + ROH (1)

Low hydrolysis reactivity of silicon precursor can be enhanced by either basic catalysis via

SN2 mechanism or by acidic catalysis via SN1 mechanism and with chelating ligands as

modifiers as well [100].

M(OR)4 + x H2O M(OH)x(OR)4-x+ xROH (2)

The SN2 mechanism is described in the following scheme [100]:

http://en.wikipedia.org/wiki/Chemical_reactionhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Cationhttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Anionhttp://en.wikipedia.org/wiki/Hydroxidehttp://en.wikipedia.org/wiki/Cationhttp://en.wikipedia.org/wiki/Hydrogenhttp://en.wikipedia.org/wiki/Chemical_reaction

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Hydroxide ions will attack silicon alkoxides by nucleophilic substitution following the SN2

mechanism. Usually, the hydrolysis of silicon alkoxides is slow because of the high

electronegativity of silicon compounds. An alternative reaction may occur in the acidic

environment via SN1 mechanism as in the following scheme [100]:

Condensation reaction is achemical reaction in which two molecules combine to form a

siloxane [SiOSi] bond with loss of water or alcohol [101]. In a condensation reaction

two partially hydrolyzed molecules can link to form:

(OR)3Si-OH + HOSi-(OR)3 (OR)3SiOSi(OR)3+ H-O-H (3)

or

(OR)3Si-OR + HOSi-(OR)3 (OR)3SiOSi(OR)3+ R-OH (4)

These types of condensation reaction continue to build larger and larger silicon-containing

molecules by the process of polymerization. Thus, a polymer is a huge molecule (or

macromolecule) formed from hundreds or thousands of units called monomers.

Polymerization of silicon alkoxide, for instance, can lead to complex branching of the

polymer, because a fully hydrolyzed monomer Si(OH)4 is tetrafunctional (can branch or

bond in four different directions see Fig. 1.1).

Fig. 1.1: Sketches showing hydrolysis and condensation of silanes

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The resulting solution/gel can then be converted to:

1- Xerogel by drying gel with shrinkage. Xerogel usually retains high porosity (25%),

high density and enormous surface area (150900 m2/g), along with very smallpore

size (1-10 nm) it consists of 60-90% of air (Fig. 1.2).

2- Aerogel is derived from a gel in which the liquid component of the gel has been

replaced with a gas.The solvent will be removed by hypercritical conditions. The

network doesn't shrink, a highly porous and low-density material consisting of

typically 90-99% of air (Fig. 1.2).

Fig. 1.2: A sketch of the general process [52].

1.3 Applications of Sol-Gel Process

1.3.1 Spin-coating

Spin-coating technique has been used for several decades for the deposition of thin films

on different substrates [102,103]. It is a procedure used to apply uniform thin films to flat

substrates. A machine used for spin coating is called a spin coater with version Laurell

WS-400B-6NPP/LITE, maximum rpm: 10000, substrate size: 150 mm, process chamber:

22 cm and sample hold-down: vacuum (see Fig. 1.3).

http://en.wikipedia.org/wiki/Porosityhttp://en.wikipedia.org/wiki/Gelhttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Gashttp://en.wikipedia.org/wiki/Liquidhttp://en.wikipedia.org/wiki/Gelhttp://en.wikipedia.org/wiki/Porosity

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Fig. 1.3: Spin coater machine

This method was first described by Emslie et al. (1958) and Meyerhofer et al. (1978) using

several simplifications. The spin coating process can be broken down into the four stages

shown in Fig. 1.4.

Fig. 1.4: The four stages of spin-coating process [104]

Initially, an excess amount of solution is being deposited onto the center of the surface. In

the second stage, the substrate is accelerated up to its final, desired, rotation speed, which

results in the rapid outwards flow of liquid caused by centrifugal force generated by the

rotating substrate. The spin off stage takes place for approximately 10 seconds after spin

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up [105]. In the fourth stage, evaporation primarily causes thinning followed by subsequent

shrinkage of the film. Spin speed is one of the most important factors in spin coating. The

speed of the substrate in round per minute (rpm) affects the degree of radial (centrifugal)

force applied to the liquid resin as well as the velocity and characteristic turbulence of the

air immediately above it [106].

1.3.2 Dip Coating

It is a popular coating method used to form a homogeneous thin film on the surface of

metals by immersing a metal into a tank containing the coating material followed by

withdrawal of the substrate from the tank. Although the thin film which formed by dip

coating doesn't have to be flat comparing to spin coating, but its thicker. This thin film can

be applied in industrial processes[107].

Dip coating process can be divided into five steps as shown in Fig. 1.5. First, the substrate

is immersed in thesolution of the coating material at a constant speed. The immersion time

is kept very low. Thin layer deposits itself on the substrate during withdrawal from the

coating material. The withdrawal is carried out at a constant speed to avoid any judder. We

can control the thickness of the coating by controlling the speed of withdrawing (faster

withdrawal gives thicker coating material). The excess of coating liquid will drain from the

surface at draining stage. The final stage is evaporation since the solvent evaporates from

the liquid to form this layer film over the surface of the metals [108].

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Fig. 1.5: Stages of dip coating process, a-e) batch

The process has many advantages [109]. It can be used to coat wood, metals and glasses.

Single or double sides of samples can be coated, maximum production flexibility,

flexibility in the glass substrates, e.g. various forms or species of glass (white glass, grey

glass, etc.) and formation of the highest homogeneity over the surface of samples.

1.4 Corrosion and types of corrosion

1.4.1 Corrosion

It is the electrochemical reaction between the surface of the metal or alloy and

environment. The corrosion of a metal is the result of two simultaneous reactions that are

in equilibrium. During oxidation, metal loses electrons as per the fundamentals reaction M

Mn++ ne- where M is the metal, Mn+is the metal oxidized to metal ions and ne -is the

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number of electrons which metal loses during the anodic process. On other hands,

reduction of metal takes place according to the following fundamental reaction as:

2H++ 2e H2 Hydrogen reduction reaction

4H++ O2+ 4e 2H2O Oxygen reduction in acid solution

2H2O + O2+ 4e 4OH- Oxygen reduction in basic or neutral solution

M3++ e M2+ Metal ion reduction e.g. Fe3++ e-Fe2+ [110]

The surface at which oxidation take place is called anode. It carries negative charges and is

designated by the sign (-); the resulting current is called the anodic current. Reduction

takes place on a surface called the cathode, designated by the sign (+); the reducing current

is called the cathodic current. There are four fundamental components in an

electrochemical corrosion cell, anode, cathode, electrolyte solution and electrical

connection between the anode and cathode for the flow of electron current. If any of the

above components is missing or disabled, the electrochemical corrosion process will be

stopped. Clearly, these elements are thus fundamentally important for corrosion

control [111].

1.4.2 Electrochemical reactions in aluminum corrosion

The fundamental reactions of the corrosion of aluminum in the aqueous mediums

have been the subject of many studies [19,112]. In simplified terms, the oxidation of

aluminum in water proceeds according to the equation (5):

Al Al3++ 3e- (5)

Metallic aluminum, in oxidation state 0, goes in solution as trivalent cation A13+ when

losing three electrons. This reaction is balanced by a simultaneous reduction in ions present

in the solution, which capture the released electrons. In common aqueous media with a pH

close to neutral such as fresh water, seawater, and moisture it can be shown by

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thermodynamic considerations that only two reduction reactions can occur: - reduction of

H+protons produced from dissociation of water molecules:

H2O H++ OH- (6)

3H++ 3e- 3/2 H2 (7)

Globally, the corrosion of aluminum in aqueous media is the sum of two electrochemical

reactions, oxidation and reduction:

A1 Al3++ 3e- (8)

3H++ 3e-3/2 H2 (9)

A1 + 3H+ A13++ 3/2 H2 (10)

or

A1 + 3H2O Al(OH)3+ 3/2 H2 (11)

This reaction is accompanied by a change in the oxidation state of aluminum which, from

the oxidation state 0 in the metal, is transformed into the oxidation state of alumina (+3),

and by an exchange of electrons, since aluminum loses three electrons that are picked up

by 3 H+. Aluminum corrosion results in the formation of aluminum hydroxide Al(OH)3,

which is insoluble in water and precipitates as a white gel, which is found in corrosion pits

as white gelatinous flakes.

1.4.3 Aluminum as a passive metal

Aluminum is naturally passive and, therefore, does not need to be passivated, unlike

certain metals such as steel. A metal that can be passivated has undergone a chemical

treatment or contains an alloying element, which renders it passive against the medium. It

can be depassivated, or may not have been passivated. This does not apply to aluminum,

which is of course always covered by its natural oxide layer [113]. Because of this highly

electronegative potential, aluminum is one of the easiest metals to oxidize (see Table 1.1).

However, aluminum behaves as a very stable metal, especially in oxidizing media (air,

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water, etc.). This behaviour is due to the fact that aluminum, like all passive metals, is

covered with a continuous and uniform natural oxide film corresponding to the formula

A12O3, which is formed spontaneously according to equation 12,

2Al + 3/2 O2Al2O3 (12)

The free energy of this oxidation reaction, - 1675 kJ, is one of the highest, which explains

the very high affinity of aluminum towards oxygen. The major importance of this oxide

film for the corrosion resistance of aluminum was recognized in 1896 by Richards [114]; it

has been the subject of a large number of investigations since 1930 [115].

Table 1.1: Standard oxidation-reduction potential at 298 K

ElectrodeOxidation

reactions

Standards

potential (volts)Nature

Li/Li+K/K+Ca/Ca2+

Na/Na+Mg/Mg2+Al/Al3+Zn/Zn2+Fe/Fe2+Cd/Cd2+

Ni/Ni2+Sn/Sn2+Pb/Pb2+Pt/H2H

+Cu/Cu2+Ag/Ag+Hg/Hg+

Cl2/Cl

-

Pt/Pt2+Au/Au3+

Li = Li++ e-K = K++ e-Ca = Ca2++ 2e-

Na = Na++ e-Mg = Mg2++ 2e-Al = Al3++ 3e-Zn = Zn2++ 2e-Fe = Fe2++ 2e-Cd = Cd2+ + 2e-

Ni = Ni2++ 2e-Sn = Sn2++ 2e-Pb = Pb2++ 2e-H2= 2H

++ 2e-Cu = Cu2++ 2e-Ag(s)= Ag

++ e-Hg(l)= Hg

++ e-

2Cl

-

= Cl2(g)+ 2e

-

Pt = Pt2++ 2e-

Au = Au3+ + 3e-

-3.040-2.924-2.870-2.710-2.380-1.660-0.762-0.441-0.403-0.236-0.140-0.1260.000

+0.337+0.799+0.920

+1.359+1.200+1.498

more active

more noble

Aluminum has high negative standard electrode potential of - 1660 mV, obtained by

calculation with respect to the free energy -G, which has one of the highest negative

values, -1657 KJ mol-1. Therefore, aluminum is expected to be very unstable in the

presence of moisture. However, experience shows that this is not the case, because

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aluminum is covered by a natural oxide layer that modifies its behaviour compared to

thermodynamic prediction: aluminum is a passive metal. On the other hand, metals with a

positive potential as platinum and gold have a weak tendency to oxidation (Table l).

1.4.4 Forms of corrosion

1.4.4.1Uniform corrosion

This is also called general corrosion. The surface effect produced by most direct chemical

attacks (e.g., an acid) is a uniform etching of the metal. General thinning takes place until

failure (Fig. 1.6). This kind of corrosion is easily observable and hence easy to protect by

coating or painting. Uniform corrosion is preferred from a technical standpoint because it

is predictable and thus acceptable for design [116]. The corrosion occurs to the metals

because the tendency of all metals to return to their lowest energy state, which is usually

the oxidized state.

Fig. 1.6: Uniform corrosion attack on structural steel [117].

1.4.4.2Galvanic corrosion

It is an electrochemical process taking place between two dissimilar metals (different in

standard electrode potential values) which are in direct contact with each other, in the

presence of an electrolyte and an electron conductive path. This leads to deterioration of

anodic metal [116]. The nobility of a metal, i.e. its resistance to corrosion and oxidation in

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moist air, can be evaluated from galvanic series. In a galvanic cell, the metal with high-

potential value reacts as the anode and corrodes faster than it would alone. In contrast to

other metal which acts as the cathode and corrodes slower than it out of galvanic cell.

Fig. 1.7: Galvanic cell with all the elements required for corrosion [118].

Fig. 1.7 shows a general setup required for galvanic corrosion to occur. It consists of two

dissimilar metals with different potential values contacted together and immersed in an

electrolyte solution. The anode is the less noble of the two dissimilar metals. Fig. 1.8

shows the galvanic corrosion of aluminum in contact with a stainless steel screw.

Fig. 1.8: Corrosion of aluminum in contact with stainless steel screw [119].

The galvanic corrosion mechanism can be used to protect the metals from corrosion by

using the principle of a sacrificial anode which has more applications in ships, marine,

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pipeline, etc. Sacrificial anode can be used as the anode to protect the metal from corrosion

depending upon its potential value as the metal with high-potential value will corrode

faster.

1.4.4.3Pitting corrosion

It is a localized corrosion that leads to the creation of small holes in the metal [120]. The

pits are often found underneath surface deposits caused by corrosion product accumulation

(Fig. 1.9). It is considered one of the most destructive forms of corrosion as it leads to

equipment failure due to penetration.

Fig. 1.9: Pitting corrosion

The pitting corrosion occurs on the surface of metals as aluminum, stainless steel, etc.,

which has a metal oxide layer. From a purely thermodynamical point of view, aluminum is

active. However, in oxygen containingenvironment (air, water), aluminum is rapidly

covered with an oxide layers [113] which prevent aluminum or aluminum alloy from

further corrosion. The thickness of the oxide layer may vary as a function of temperature,

at ambient temperature the thickness of oxide layer is average between 23 nm. At high

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temperatures (673-723 K), the film thickness will be increased by 15-20 nm [121]. When

the oxide film isdamaged, e.g. by a scratch, new oxide will immediately form on the

surface of the metal because the rate of re-passivation is higher than the rate of corrosion

[122]. Higher rate of corrosion (compared to the rate of re-passivation) leads to further

corrosion. The pitting corrosion is initiated from a weak site in the oxide layer by a halide

ion attack [123,124]. Pits propagate according to the reactions:

Al3++ 3H2O Al(OH)3+ 3H+ (13(

Al3++ 3Cl- AlCl3 (14)

H++ 2e-H2 (15)

O2+ 2H2O + 4e-4OH- (16)

Oxidation of the aluminum anode leads to the formation of aluminum ion, which combines

with chloride ions to form aluminum chloride and free electron moves to the cathode

where reduces hydrogen cation to form hydrogen gas. Moreover, oxygen reduction also

takes place at intermetallic cathode resulting in the formation of hydroxyl ions (Fig. 1.10).

Fig. 1.10: Mechanism of pitting corrosion [125].

Formation of insoluble Al(OH)3will prevent oxygen to react with metal. The chloride ions

will migrate into the pit depending on its low radius to balance with positive hydrogen ions

as described in equation 17:

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H++ Cl-HCl (17)

Then AlCl3will be hydrolyzed as describe in equation (18):

AlCl3+ 3H2O Al(OH)3+ 3HCl (18)

Which make the medium more acidic and accelerate pit propagation. The reduction

reaction will cause local alkalinization around cathodic particles, which leads to dissolution

of aluminum particles.

Pitting type

It is more dangerous than the uniform type because it is very difficult to detect, predict and

design against. The types of pitting corrosion commence when the protective thin layer

(oxide film) is chemically or mechanically damaged and does not immediately re-

passivate. This damage of a protective oxide layer leads to the formation of pits with

different diameter and holes of varying dimensions, which can rapidly perforate the wall

thickness of a metal (Fig. 1.11).

Fig. 1.11: Variations in cross-sectional shape of pits [126].

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The shapes of pitting corrosion can only be identified through metallography where a

pitted sample is cross-sectioned and shape, in turn size and the depth of penetration can be

determined. The main reason for pitting corrosion is environment or use of aggressive

chemical species such as chlorides. Chlorides are particularly damage the passive film

(oxide layer) so pitting initiates with the breakdown of oxide layer. Factors that influence

the occurrence of pitting attacks are primarily the chloride content, acidity, and the

temperature. Another factor causing pitting is the presence of oxygen, which is responsible

for oxidation process or certain metal ions. At high oxygen, content increases the corrosion

potential (capability of passive-film weakening) of the metal and thus increases the risk of

pitting corrosion. Consequently, low oxygen content considerably reduces the risk of

pitting. Pitting corrosion can be prevented by controlling pH (~7), halogen concentration

and temperature.

1.4.4.4 Crevice corrosion

It is a type of localized corrosion that can be found within crevices where a stagnant

solution is present [127]. Crevice corrosion occurs at narrow openings or spaces between

two metal surfaces or between metals and nonmetal surfaces. A concentration cell form

with the crevice being depleted of oxygen. This differential aeration between the crevice

(microenvironment) and the external surface (bulk environment) gives the crevice an

anodic character. This can contribute to a highly corrosive condition in the crevice. Fig.1.12 represents one of the most kinds of localized corrosion because it occurs in the areas

that are not clearly visible. Therefore, crevice corrosion may lead to sudden devastating

failure of the metal in service.

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Fig. 1.12: Crevice corrosion of titanium flange at an interface with a nonmetallic gasket[128].

1.4.4.5 Intergranular corrosion (IGC)

Intergranular corrosion (IGC) is the selective dissolution of the grain boundary zone, while

the bulk grain of metal or alloy is not attacked [127]. Thesusceptibility to IGC is

depending on the alloy composition and thermomechanical processing.Grain boundaries

are sites for precipitation and segregation, which makes them physically andchemically

different from the matrix. The structure of bulk grains is chemically different from the

grain boundaries. The accumulation of impurities is the cause of this corrosion. Exfoliation

is a form of intergranular corrosion, which has a close relationship with aluminum alloys

(Fig. 1.13).

Fig. 1.13: Intergranular corrosion of a failed aircraft component made of 7075 aluminum[129].

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1.5 Thermodynamics

1.5.1 Pourbaix Diagram (potential-pH diagram)

It is a graphical presentation of the thermodynamic equilibrium states of a metal-electrolyte

system. Pourbaix diagrams plotted electrode potential of the metal vs. pH of the

electrolyte.Oxidizing conditions are described by the top part of the diagram (high positive

electrode potential).Reducing conditions are described by the bottom part of the diagram

(high negative electrode potential). Acidic solutions are presented in the left side of the

diagram (pH lower than 7).Alkaline solutions are presented in the right side of the diagram

(pH higher than 7). In case of aluminum, only four species containing the aluminum

element will be considered:

two solid species Al and Al2O3 H2O (aq)

two ionic species Al3+and AlO2-(s)

The first equilibrium examines the possible presence of either Al3+or AlO2-expressed in

equation.

Al3++ 2H2O AlO2-+ 4H+ (19)

From equation (19), it can be indicated that there is no change in a valence of the

aluminum present in the two ionic species considered the associated equilibrium is

independent of the potential, and the expression of that equilibrium can be derived in the

following expression for standard conditions.

RTln Keq=RTln Q = - G0

reaction (20)

Where, R: universal gas constant (8.314 J mol-1 K-1), T: absolute temperature (K), Keq:

equilibrium const, Q: reaction quotient and G0: standard free energy. From equation (21),

the equilibrium constant can be calculated.

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Assuming that the activity of H2O is unity and that the activities of the two ionic species

are equal, one can obtain a simpler expression of the equilibrium in equation (21) based

purely on the activity of H+, equation and its logarithmic form, equation (22 and 23).

RT ln [H+]4= -G0reaction (22)

Log10[H+] = -pH = - G0reaction /4 2.303 RT (23)

Equation (23) can be simplified by adding the data forR= 8.314 J mol-1 K-1and T= 298 K

to be:

pH = 4.38 10-5G0reaction (24)

By using the standard thermodynamic data from the literature [130], it is possible to

calculate that the free energy of reaction is in fact equal to 120.44 k J mol-1when both

[Al3+] and [AlO2-] are equal.

G0= 3 96485 0.415 = 120.44 k J mol-1 (25)

pH = 4.38 10-5120440 = 5.27 (26)

From Fig. 1.14 we can see a dotted vertical line at pH = 5.27, which separates between two

ions pieces of Al3+at low pH and AlO2-at the higher end of the pH scale.

Fig. 1.14: E-pH diagram showing the soluble species of aluminum in water at 298 K.

Q =a x a

4H+AlO2

-

aAl3+ x a H2O

2 (21)

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Fig. 1.15 is to consider all possible reactions (see Table 1.2) between the four chemical

species, i.e. Al, Al2O3H2O, Al3+, and AlO2

-. The following Fig. 1.15 illustrates the results

of such computation for aluminum in the presence of water at 298 K when the activities of

all species considered were set at value unity. The Pourbaix diagram of aluminum,

presented in Fig. 1.15, indicates that Al2O33H2O, is the stable phase between about pH 4

and 9 [113]. Indeed, this film is considered to be responsible for the successful use of

aluminum in many structural applications. This diagram also predicts the amphoteric

nature.

Fig. 1.15: E-pH diagram of solid species of aluminum when the soluble species are at onemolar concentration (298 K).

Fig. 1.16:E-pH corrosion diagram of aluminum at 298 K.

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Fig. 1.16 showed the three possible states of a metallic material; a) corrosion region, metal

is stable as an ionic (soluble) product and therefore, susceptible to corrosion attack, b)

immunity region; it can be defined as the lack of attack on a metal when exposed to

corrosion it is considered to be totally immune from corrosion attack and safe to use, c)

passive region; it is the region which covered with an oxide or hydroxide layer that may

form on metal by compact and adherent to prevent all direct contact between the metal

itself and the environment.

Table 1.2: Has the total reaction of four species.

1.5.2 Gibbs free energy (G)

It is thermodynamic quantities, which can be used to predict whether a process will occur

spontaneous or not at constant pressure and temperature. It is also known as free enthalpy

[131]. The reaction with high negative value of Ghas the tendency to go and continue.

For example, consider the following reaction at 298 K:

Mg + H2O(l) + 1/2 O2(g) Mg(OH)2(s) G0= -596,600 J (27)

Cu + H2O(l) + 1/2O2(g) Cu(OH)2(s) G0= -119,700 J (28)

Au + 3/2H2O(l) + 3/4O2(g) Au(OH)3(s) G0= + 65,700 J (29)

Equilibria involving aluminum metal

3e-+ A += Al3e-+ Al(OH)3+ 3H

+= Al + 3H2O6e-+ Al2O3.H2O + 6H

+= 2Al + 4H2O3e-+ AlO2- + 4H

+= Al + 2H2O3e-+ Al(OH)2++ H+= Al + H2O3e-+ Al(OH)2

++ 2H+= Al + 2H2OEquilibria involving solid forms of oxidized aluminum

Al(OH)3+ H+= Al(OH)2

++ H2OAl2O3.H2O + 2H

+= 2Al(OH)2+

Al(OH)3+ 2H+= Al(OH)2++ 2H2O

Al2O3.H2O + 4H+

= 2Al(OH)2+

+ 2H2OAl(OH)3+ 3H+= Al ++ 3H2O

Al2O3.H2O + 6H+= 2Al ++ 4H2O

Al(OH)3= AlO2-+ H++ H2O

Al2O3.H2O = 2AlO2-+ 2H+

Equilibria involving only soluble forms of oxidized aluminum

AlO2-+ 4H+= Al3++ 2H2O

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From the values of G0in equation 28 and 29 we can see that the tendency of magnesium

to react with water and oxygen (equation 27) is higher than the tendency of copper to react

with water and oxygen in the same condition (equation 28). The corrosion tendency of

magnesium is higher than the corrosion tendency of copper in aerated water. Finally if the

value of G0 with positive value (equation 29) this meaning that the reaction has no

tendency to go at all, and this metal (gold) will not corrode to form Au(OH)3 in the

aqueous mediums.

The relation between Gin joules and electromotive force (emf) in volts, E, is defined by

G= nFE, where n is the number of electrons taking part in the reaction, and F is the

Faraday's constant (96485 coulombs/mol).

1.5.3 Nernst Equation

It is used to calculate the voltage of an electrochemical cell or to find the concentration of

one of the components of the cell [132].

Ecell=E0

cell- (RT/nF) lnQ (30)

Which,Ecellis cell potential under nonstandard conditions (V),E0

cell is cell potential under

standard conditions, R is gas constant (8.314 J mol-1 K-1), T is temperature (K), n is the

number of electrons in such a reaction equation, its related to the amount of charge

transferred when the reaction is completed, Fis Faraday's constant, 96485 coulombs/mol

and Q = reaction quotient, which is a function of the activities or concentrations of the

chemical species involved in a chemical reaction. Nernst equation can be written in a

simple method [133]:

Ecell=E0

cell- (0.0591/n) logQ (31)

Q can be calculated by divided the product values over the reactant values as here;

A + BS + T (32)

Qr = {St} {Tt}

t / {At} {Bt} (33)

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Nernst Equation Example; a zinc electrode is submerged in an acidic 0.80 M Zn2+solution,

which is connected by a salt bridge to a 1.30 M Ag+solution containing a silver electrode.

Determine the initial voltage of the cell at 298 K.

The cathodic reaction can be:

E0

red: Zn2+(aq) + 2e- = Zn (s) -0.76 V (34)

E0red: Ag+ (aq) + e-= Ag (s) +0.80 V (35)

From upon equation (34 and 35) we can see that this reaction can be happen only if Zn

became anode and exposed to oxidation (loss electrons) and Ag became cathode and

exposed to a reduction (accept electrons). So equation (34 and 35) needs to be

rearrangement as;

E0

oxid: Zn (s) = Zn2+ (aq) + 2e- +0.76 V (36)

E0

red: 2Ag+ (aq) + 2e-= 2Ag (s) +0.80 V (37)

The total reaction is,

Zn (s) + 2Ag+ (aq) = Zn2+(aq) + 2Ag (s) +1.56 V (38)

To calculate the cell potential from equation;

Ecell=E0

cell- (0.0591 /n) log Q (39)

Ecell=E0

cell- (0.0591 /2) log (0.76/0.8) (40)

Ecell= 1.56(-6.583 x 10-4) = 1.566 V (41)

1.6 Kinetic Aspects of Corrosion Process

1.6.1 Overpotential ()

It is the difference in theelectric potential of an electrode with nocurrent flowing through

it, atequilibrium,and with a current flowing. To measure overpotential the extra energy

needed to force the electrode reaction to proceed at a required rate. The overpotential

increases with increasing current density[134].

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When two different metals such as zinc and magnesium are completely connected together

in an aerated medium since, zinc is immersed in hydrochloric acid occur over a single

metallic surface, the Ecorr that results from this situation is a compromise between the

various equilibrium potentials of all the anodic and cathodic reactions involved. The

difference between the resultant potential (E) and equilibrium potential (Eeq) is called

polarization and quantified in terms of overpotential () as in equation (42):

=E-Eeq (42)

The polarization is said to be either anodic, when the anodic processes on the electrode are

accelerated by changing the sample potential in the positive (noble) direction or cathodic

when the cathodic processes are accelerated by moving the potential in the negative

(active) direction.

1.6.2 Concentration Polarization

It is the polarization component that is caused by concentration changes in the environment

near to the surface of metal as illustrated in the following Fig. 1.17. When a chemical

species participating in a corrosion process is in short supply, the mass transport of that

species to the corroding surface can become rate controlling. The concentration of

dissolved oxygen limits the cathodic reaction rate [135,136].

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Fig. 1.17: Concentration changes in the vicinity of an electrode causing a concentration

polarization effect

The rate of the corrosion can be controlled by the mass transport of species to the surface.

Fig. 1.18 showed that the mass transport to a surface is depending on three forces, which are

diffusion, migration and convection [136]. When there is no electric field passing this lead to

negligible the migration term which has effects only on charged ionic species. In case of

convection force, it is disappeared in stagnant conditions.

Fig. 1.18: Graphical representation of the processes occurring at an electrochemical inter-face.

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j =j0 exp -nF

RTreactionreaction - exp 1-

nF

RTreaction

1.6.3 Butler-Volmer equation

It is describes how the electric current on an electrode depends on the electrode potential,

considering that both acathodic and ananodic reaction occur on the same electrode:

(43)

where, jreaction is the anodic or cathodic current, : is the charge transfer barrier for the

anodic or cathodic reaction, usually close to 0.5, n is the number of participating electrons,

R is theuniversal gas constant (8.314 J mol-1K-1), T is the absolute temperature (298 K),

andF is Faraday constant 96485 C/(mole of electrons).

When reaction is cathodic, i.e. negative, the second term in the Butler-Volmer equation

becomes negligible and equation (43) can be written in a simple method as:

Jreaction=jcath=j0exp (-nF reaction/RT) (44)

reaction= cath = bclog10(jcath. /j0) (45)

where bcis the cathodic Tafel coefficient described in equation (46);

bc= -2.303RT/nF (46)

that can be obtained from the slope of a plot of against log current density, with the

intercept yielding a value forj0as shown in the following Fig. 1.19.

Fig. 1.19: Plot of overpotential() vs. log current density.

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reaction= anode =banode log10janode

j0

(48)

jreaction=janode = -j0 exp 1-nF

RTreaction

Similarly, when r is anodic, i.e. positive, the first term in the Butler-Volmer equation

becomes negligible and the anodic current density (janode ) can be expressed by equation and

its logarithm, with banodeobtained by plotting reactionvs. log |j|:

(47)

ba= 2.303RT/nF (49)

1.7 Aim of the work

Owing to host of favorable characteristics, aluminum and its alloys find a wide variety of

applications in the field of automobiles, aircraft industry, household appliances, storage

containers and electronic devices. These characteristics include low density, low cost,

lightweight, high ductility, high strength, high thermal and electrical conductivity and

corrosion resistance. Corrosion resistance of aluminum and its alloys is due to the

formation of a passive oxide film on exposure to air. This film is bonded strongly to the

surface and is stable in aqueous solutions over a wide pH range from 4 to 9. However,

mechanical work and polishing cause the formation of tiny pores in thin oxide layer,

thereby exposing the metal surface to a corrosive environment.

Various protective methods have been employed to produce anti-corrosive thin films, e.g.

paint and sol-gel (dip, spin, spray or electrodeposition) coating methods. Thin films can be

formed by mixing organic or inorganic silanes with suitable catalysts. The resistance of

coating depends on various factors such as, speed of stirring for mixing the reactants and

that of solvent with distilled water.

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The present work aims to prepare anti-corrosive coating to protect aluminum alloy

AA7075 from corrosion. Phenyltrimethoxysilane (PTMS), tetraethoxysilane (TEOS), and

Phenyltriethoxysilane (PTES) have been used in sol-gel coating systems. Spin-coating

method has been used to coat aluminum alloy samples by these coatings to study the effect

of these coatings on decreasing the pit formation and increasing the hydrophobicity.

Initially, parameters were optimized for PTMS coating alone, to achieve maximum

corrosion resistance.

Suitable speed of spin-coating machine was selected to produce a very thin coating which

can be used as a barrier to protect Al-alloy from corrosion. DekTak 8000 was used to

measure the thickness of coated layer in case of samples coated with PTMS. Dependence

of film thickness on spin speed, spinning time, drying time and drying temperature was

determined. The uncoated and coated samples with PTMS were subjected to potentio-

dynamic tests with different electrolyte concentrations, pH and varying temperatures. The

structural characterization of PTMS coating was done by SEM+EDX, optical microscopy,

SIMS and XPS.

Comparison of different coatings (PTMS, TEOS and PTES) was done using potentio-

dynamic tests after zero and 1 day immersion time. The occurrence of local corrosion was

investigated by optical microscopy. The linear polarization method was used for to obtain

the polarization resistance (Rp). The linear polarization was applied for 50 mV around the

open-circuit potential at a scan rate of 2.0 mV/s.

Structural characterization of coatings was done by Fourier transform infrared

spectroscopy (FTIR) and atomic force microscopy, thereby characterizing average

roughness, root mean square roughness and 3-D images. The weight loss and inhibition

efficiency was also determined.

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CHAPTER II

EXPERIMENTAL

2.1Samples preparation

2.1.1 Polishing process

The aluminum alloy (AA7075) was cut into discs (3 cm diameter, 0.5 cm thickness) and

polished with a Beta grinder/polishing machine (Buehler-Met GmbH) with SiC abrasive

paper grit 600 (Fig. 2.1). Samples were cleaned and degreased ultrasonically with ethanol

and distillate water for a few minutes. Finally, they were cleaned with ethanol and dried

with air prior to the spin-coating process.

Fig. 2.1: Grinder-Polisher Machine.

2.1.2Sol preparation

The coating solutions were prepared by mixing 2.5 ml phenyltrimethoxysilane (Alfa Aesar,

97%) or 2.5 ml phenyltriethoxysilane (Aldrich, 98%) or tetraethoxysilane (Alfa Aesar,

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98%), 1.25 ml acetic acid (Fluka, 99%, used as a catalyst) and 1.25 ml acetylacetone

(Sigma-Aldrich, 99%, used as stabilizing agent to prevent rapid condensation under

constant stirring for 5 minutes at ambient temperature). A mixture of 2 ml n-propanol

(Fisher, 99.9%, used as cosolvent to prevent precipitation) and 0.5 ml distilled water was

added (the silane film was dissolved in ethanol as quickly as it is formed so that no layer or

a mono layer is left on the surface, by this way, samples can be subjected to various

surface sensitive techniques). The solution was stirred at ambient temperature for 90 min to

complete hydrolysis and condensation reactions. The resulting solution was deposited on

the polished aluminum alloy samples with spin-coating at 4,000 rpm for 90 s. Finally, the

samples were heated at 573 K for 150 min in a furnace.

2.2Electrochemical measurements

Electrochemical measurements were carried out in a conventional three-electrode cell (Fig.

2.2) with the aluminum alloy samples (3 cm diameter and 0.3 cm thickness) as working

electrodes with exposed surface area 2 cm2(Fig. 2.3), an Ag/AgCl reference electrode, and

a platinum counter electrode. Cyclic potentiodynamic current vs. potential curves were

recorded in the range - 1.5 >EAg/AgCl > 0 V at dE/dt= 50 mVs-1at ambient temperature. A

relatively high scan rate was chosen to avoid additional corrosive damage of the sample.

Dissolution current density (jdiss) at 50 mV from open circuit potential (OCP) in the anodic

direction was selected as a reference point for comparison between coated and uncoated

samples as shown in Fig. 2.4. The electrolyte solution contained 0.05 MNaCl at different

pH-values or different concentrations of NaCl at fixed pH = 7. A Princeton Applied Re-

search Potentiostat (Parstat 2273) was used with Power Suite 2.58 software (Fig. 2.5).

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Fig. 2.2: The apparatus (cell) for potentiodynamic and potentiostatic testing.

Fig. 2.3: Working electrode with exposed surface area 2 cm2 inelectrochemicalcell.

-1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

OCP

+E= 50 mV

Cathode

Anode

j/Acm

-2

EAg/AgCl

/V

Fig. 2.4: Current density vs. potential plot of uncoated aluminum alloy in 0.05 MNaCl.

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Fig. 2.5: A Princeton Applied Research potentiostat Parstat 2273

2.2.1 Porosity determination

Potential scans with a Princeton Applied Research potentiostat Parstat 2273 and

PowerSuite 2.58 Software were used to measure the film porosity electrochemically. The

samples were used as working electrode in a three electrode setup with a platinum counter

electrode and a saturated AgCl reference electrode. The dissolution current density of the

samples was measured in the potential range -1.5 > EAg/AgCl > 0 V at dE/dt = 50 mV s-1at

ambient temperature. A mildly corrosive electrolyte solution and a relatively fast scan rate

were used in order to minimize corrosive damage to the sample. Under these conditions,

the current densityjdissatE=EOCP(open circuit potential) + 50 mV chosen for the porosity

evaluation depends primarily on the area of the exposed, uncoated metal. The average film

porosity can be calculated from the ratio of the maximum dissolution current densities of

coated and uncoated reference samples [137,138]. This method has been successfully

applied with different coating systems before and is described in detail elsewhere [139].

2.3Linear polarization resistance (Rp)

Rp can be defined as the slope of the potential-current density (dE/di) curve at the free

corrosion potential. The polarization resistance measurement was carried out by using the

same electrolytic cell shown in Fig. 2.2 to determine Rp, corrosion current (icorr) and

http://corrosion-doctors.org/Electrochem/linear.htmhttp://corrosion-doctors.org/Electrochem/linear.htmhttp://corrosion-doctors.org/Electrochem/linear.htmhttp://corrosion-doctors.org/Electrochem/linear.htmhttp://corrosion-doctors.org/Electrochem/linear.htmhttp://corrosion-doctors.org/Electrochem/linear.htm

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corrosion rate (CR). The corrosion current is related to polarization resistanceby Stern-

Geary equation (50),

Rp=B/ icorr (50)

where, Rp is polarization resistance, icorr is the corrosion current and B is Stern-Geary

constant, which can be calculated from equation (51)

B=BaBca/ 2.303(Ba+Bca) (51)

using the Tafel slopes Ba and Bca of the anodic and cathodic parts of Tafel plots in

millivolts per decade. Linear polarization measurements were performed at a scan rate

(dE/dt) = 2.0 mVs-1 with the samples previously immersed for 1 day in 0.05 M NaCl

solution. In all Rpexperiments, samples were immersed in the electrolyte solution for the

same time with different pH until equilibrium was established, then Rp, CR and icorrhave

been determined [140]. The scan range was 0.02 V and + 0.02 V vs. OCP, since OCP

was measured after 15 minutes by using Parstat 2273 machine with software PowerSuite

2.58.

2.4Determine the film thickness by DekTak D8000

Fig. 2.6 illustrates Dektak D8000 profilometer. The coated samples with Si film were cut

with a razor blade to form up and down area then the average roughness between them can

be measured with Dektak D8000 a typical resulting profile is shown in Fig. 2.7. The height

difference between a substrate and coating surface step was measured and showed that this

can be done with an acceptable error. The measured crater depths were used to calculate

sputter rates and the position of the metal/oxide interface. The geometry of the crater

(walls, bottom and bottom roughness) was also checked for its influence on the depth

resolution of the analysis [141]. The Dektak D8000 is a very high-precision measuring

instrument capable of measuring surface variations from 1 Ao to 2620 Aowith a vertical

resolution between 1 and 40 Ao. The main interest in this instrument is the measurement of

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surface variations by mechanical scanning of a diamond stylus. The thickness of the

coating layer was approximately 88.9 nm.

Fig. 2.6: DekTak D8000

0 50 100 150 200 250

-100

-80

-60

-40

-20

0

20

Heigth(nm)

Length (nm)

Fig. 2.7: Cut made across a spin-coated sol-gel film on aluminum alloy measured byDektak 8000.

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2.5Weight loss measurement

Tests were done in 250 ml glass beakers containing 150 ml of 0.05 MNaCl solution with

different pH-values (neutral, acidic and basic) at ambient temperature. The samples were

weighed before immersion in the test solutions. To determine the loss of weight, the

samples were removed after immersion for various numbers of days, washed with distilled

water, dried and then weighted again [142 - 145]. The corrosion rate (CR) is calculated

using the following equation [146,147]:

Corrosion Rate (CR) = 3.45 106 W / A D T (52)

where W is a weight loss in grams, A is the area of sample in cm2 (2 x 3.14 = 6.28 cm2),

D is the density of the sample in g/cm3(2.7 g/cm3) and T is time of immersion samples in

electrolyte solution by hours.

2.6Analytical Techniques

2.6.1Scanning electron microscopy and energy dispersive X-ray spectroscopy

(SEM+EDX).

It is a type of electron microscope that used electrons instated of light to study the surface

of the samples (uncoated and coated). Due to the very narrow electron beam, it has the

ability to show the surface samples in three-dimensional, which are useful for judging the

surface structure of the samples. It gives information about the surface morphology and

texture. In this thesis, the diameter of the samples was 3 cm and the thickness was 0.5 cm.

The corrosion behavior of the samples was monitored using scanning electron microscopy

(SEM-Fig. 2.8) during immersion in 0.05 M NaCl solution open to air and at ambient

temperature for different times. SEM images were obtained using a digital scanning

electron microscope Model XL30-FEG (field emission gun), Magen 500x (100 A).

Energy Dispersive X-ray Spectroscopy (EDS, also called EDXA for Energy-DispersiveX-

ray Analysis) is a technique that uses a scanning electron microscope for elemental

http://everything2.com/title/X-rayhttp://everything2.com/title/X-rayhttp://everything2.com/title/scanning%2520electron%2520microscopyhttp://en.wikipedia.org/wiki/Chemical_elementhttp://en.wikipedia.org/wiki/Chemical_elementhttp://everything2.com/title/scanning%2520electron%2520microscopyhttp://everything2.com/title/X-rayhttp://everything2.com/title/X-ray

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analysis or chemical characterization of a sample. As the SEM scans the surface of a

sample with an electron beam, when the sample emits x-rays, it will be collected by a

detector to measure both their numbers and energies. This analysis gives users a chance to

determine the elemental composition (by mounts or percents) of the sample [148]. EDS is

often used for characterization ofcoatingsandalloys, and evaluation ofcorrosion.

In a typical SEM, electrons are thermionically emitted from a tungsten or lanthanum

hexaboride (LaB6) cathode and are accelerated towards an anode. Using tungsten as

cathode because it has the highest melting point and lowest vapor pressure of all metals, by

this way allowing it to be heated for electron emission. A schematic sketch elucidating the

principle of working of SEM is shown in Fig. 2.9.

1) The electron source (gun) which producing a stream of monochromatic electrons. 2) The

stream is condensed by the first condenser lens. This lens is used to both form the beam

and limit the amount of currents in the beam. It works in conjunction with the condenser

aperture to eliminate the high-angle electrons from the beam. 3) The beam is then

constricted by the condenser aperture, which lead to eliminate some high-angle electrons.

4) The second condenser lens forms the electrons into a thin, tight, coherent beam and is

usually controlled by the "fine probe current knob." 5) A user selectable objective aperture

further eliminates high-angle electrons from the beam. 6) The beam passes through pairs of

scanning coils in the objective lens, which deflect the beam in a raster fashion over a

rectangular area of the sample surface. 7) The final lens plays an important role to focus

the scanning beam onto the part of the sample desired. 8) When the beam strikes the

sample for a few microseconds, interactions occur inside the sample and are detected with

various instruments.

http://en.wikipedia.org/wiki/Characterization_%28materials_science%29http://everything2.com/title/electronhttp://everything2.com/title/coatinghttp://everything2.com/title/alloyhttp://everything2.com/title/corrosionhttp://everything2.com/title/corrosionhttp://everything2.com/title/alloyhttp://everything2.com/title/coatinghttp://everything2.com/title/electronhttp://en.wikipedia.org/wiki/Characterization_%28materials_science%29

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Fig. 2.8: Scanning electron microscopy instrument

Fig. 2.9: Schematic diagram of an SEM.

2.6.2 Fourier Transform Infrared Spectroscopy (FTIR)

FTIR is a common technique for organic compound characterization. Nicolet IR 200

instrument (see Fig. 2.10) was used to measure liquid samples and solid sampleshave been

measured by using Nicolet iS10 FT-IR (see Fig. 2.11).All files were saved and analyzed

with EZ OMNIC program. FTIR was used to determine the composition and chemical

structure of the various components. The obtained data was used to support the proposed

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reactions. The chemical structure of the resulting coatings was determined by using

reflection absorption FTIR spectroscopy at a resolution of 4 cm-1. 16 additive scans were

carried out between 4000 to 400 wave numbers. Sodium chloride was used as standard.

The liquid sample was handled by sandwiching a drop of sample between 2NaCl windows.

To measure the samples by infrared spectroscopy, IR radiation is passed through a sample.

Some of this radiation is absorbed by the sample, and some of it is transmitted. The

resulting spectrum represents the molecular absorption and transmission, creating a

molecular fingerprint of the sample. As the same in fingerprint there are no two molecular

structures have the same infrared spectrum. This reason makes infrared spectroscopy

useful for several types of analysis.

Fig. 2.10: Infrared for liquid samples

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Fig. 2.11: Infrared for solid samples

2.6.3 Atomic Force Microscopy (AFM)

AFM has become a standard technique to image with high resolution the topography of

surfaces [149 - 154]. The probe consists of a sharp tip (made of silicon or silicon nitride,

which is a 3-6 m tall pyramid with 15-40 nm end radius) attached to a force-sensitive

cantilever. The tip scans across the surface move up and down with the contours of the

surface, and the cantilever deflects in response to force interactions between the tip and the

substrate see Fig. 2.12 and 2.13. Such deflection is monitored by moving a laser beam

from the cantilever onto a photodetector. The measured force is attributed to repulsion

generated by the overlap of the electron cloud at the probe tip with the electron cloud of

surface atoms.

AFM can image both conductors and nonconductors with atomic resolution, because it

measures the interatomic force between the atom on the surface and the tip. The mean

roughness, RMS roughness and the 3-D morphology of the coatings obtained from atomic

force microscopy (AFM). In this research, JPK NanoWizard II (Berlin - Germany) was

used to capture the images. An area of 5 m was scanned at a scan rate of 2.543 Hz.

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Fig. 2.12: Scheme of atomic force microscopy

Fig. 2.13: Atomic force microscopy instrument

2.6.4 Contact Angle ()Measurement

It is the angle between the solid surface and the tangent drawn from the droplet to the

touch of the solid surface. It is used to measure the interfacial energy (it is the sum of free

energy of all the molecules present in the interface between different materials) of the

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surface where liquid and solid are in contact. The contact angle () measurement was used

to evaluate the hydrophobic nature of coating by using a goniometer CAM 100 (KSV 100

software) surface analysis system with high speed CCD camera for image capture a light

source, sample stage and lens as shown in Fig. 2.14. Deionised water was used for

evaluation of the properties of the coated substrate. Angle measurements were done in

triplicates. Water was taken in the syringe and drop (~10 l) was allowed to fall on the

substrate and left and right contact angles were measured for 10 s. The average contact

angle was found out by averaging the left and right contact angles.

When a liquid has a high surface energy, it will form a droplet on the solid surface [155]

having high hydrophobicity and helps against corrosions by water. When a liquid has a low

surface energy, it will spread out over a greater area and the film formed by the liquid will

be hydrophilic and has a poor corrosion protection performance. Surface energy

calculations were not carried out.

Fig. 2.14: View of contact angle instrument

2.6.5 Secondary Ion Mass Spectroscopy (SIMS)

Secondary ion mass spectrometry (SIMS) was used to obtain information about the ele-

ment distribution within the coating layer [156 - 161] using a double focusing sector field

instrument (Cameca ims 5f) with 17 kV O- primary ions (Fig. 2.15). As the layer also

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contains oxygen, the use of cesium primary ions would have been advantageous, but

suffered from sample charging effects, even with an additional gold coating on top.

Sputtered positive secondary ions were detected from the inner 60 m of an area of

150150 m.

Fig. 2.15: Secondary ion mass spectrometer Cameca ims5f

2.6.6 X-ray Photoelectron Spectroscopy (XPS)

It is an important technique which is used to determine the electronic state and chemical

composition of the sample. Extensive research has been performed on silane films with

various techniques; including XPS [162 - 164]. The measurement of samples by XPS was

done by exposure to X-ray photons under high vacuum. Photons penetrate several

nanometers (10-100) deep into the superficial layer of the sample to excite photoelectrons.

They interact with atoms in the surface regions causing electrons to be emitted by the

photoelectron effect. The kinetic energy of the photoelectrons emitted from the sample

surface is analyzed. The emitted photoelectrons have kinetic energies,Ekingiven by:

Ekin= hEb- s (53)

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where h is the energy of the photon,Ebis the binding energy of the atomic orbital from

which the electrons originates and spis the sample work function (assuming conductive

samples).

The diameter of the analyzer and the pass energy in the X-ray source play a major role in

determining the full width half maximum for a given photoelectric line. X-ray

photoelectron spectrometry measurements were performed using 5.85 eV pass energy with

a step width of 0.05 eV for the detail spectra and 187.85 eV with a step width of 0.8 eV for

the survey spectra. The binding energies of the elements of interest were calibrated with

respect to the C1s photoemission line 284.5 eV [165]

2.6.7 Optical Microscopy (OM)

OM was used to evaluate uncoated and coated surface coverage. The morphology surface

has been characterized by an optical microscope [60], Model Olympus BX 50 with a

camera Nikon DXM 1200.

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Chapter III

Results

3.1

Study the effect of PTMS coating on aluminum alloy

3.1.1 Optimization of PTMS sol solution

3.1.1.1 Study the effect of Acetic Acid (AcAc)

The effect of AcAc can be easily explained in terms of protonation of alkoxy groups and

providing better leaving groups, which lead to the substitution of OR groups by OH group

[166]. Metal alkoxides react with organic acids according to:

M(OR)n+ xRCOOH = M(OR)n-x(OOCR)x+ xROH (54)

It has been noticed that acetate groups attack the alkoxy groups on the alkoxide. This

enables the control of both hydrolysis and condensation reaction [167].

Si(OR)4+ nAcOH Si(OR)4-x(OAc)x+ xROH + (n-x)AcOH (55)

However, excess of acetic acid leads to the formation of corresponding ester:

C2H5OH + CH3COOH = CH3-COO-C2H5+H2O (56)

The excess of water can prompt further hydrolysis of silicon alkoxide, which can result in

the formation of unwanted products. For this reason, an optimized amount of acid has to be

added to the solution to control the rate of hydrolysis and condensation reactions and to

inhibit unwanted reactions to occur. To investigate the effect of acid catalyst, several sol

solutions of PTMS containing different amounts of acetic acid were prepared (Table 3.1).

Table 3.1: Different sols containing different amounts of acetic acid (AcAc).

SolPTMS

/ml

A. Acid

/ml

Acetone

/ml

Propanol

/ml

Water

/mlEcorr/mV

jdiss

/A cm-2

Porosity

/%

1 2.5 0 1.25 2 0.5 - 840 20.2 37.52 2.5 0.62 1.25 2 0.5 - 1151 0.55 1.023 2.5 1.25 1.25 2 0.5 - 1130 0.0008 0.00154 2.5 1.87 1.25 2 0.5 - 1130 0.06 0.11

5 2.5 2.49 1.25 2 0.5 - 1192 0.08 0.156 2.5 3.1 1.25 2 0.5 - 1228 0.2 0.37

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Fig. 3.1 shows the current density-potential plots of uncoated and coated aluminum alloy

samples after heating at 573 K for 150 min. This Figure can be used to estimate an amount

of acetic acid which is required to add to sol-gel solution. Comparing the dissolution

current density (jdiss) of uncoated sample (53.9 A cm-2), it can be seen from Fig. 3.1 that

sol 3 (addition 1.25 ml of AcAc) improves the porosity of the coating on aluminum alloy.

The dissolution current density of sol 3 is considered to be the lowest value (porosity =

0.0015 %). Whereas, the absence of acetic acid (sol 1) makes a thin coated layer easy to

corrode (porosity = 37.5 %).

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

j/

Acm

-2

EAg/AgCl

/ V

uncoated

Sol 1

Sol 2

Sol 3Sol 4

Sol 5

Sol 6

Fig. 3.1: Studying the effect of AcAc (ml) on the current density of PTMS-coated samplesin 0.05 MNaCl solution after heating for 150 min. at 573 K.

The relation between current density (jdiss) vs. amount of AcAc (ml) can be illustrated in

Table 3.1. In case of absence of AcAc the corrosion current density records the highest

value owing to on the large porosity.

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3.1.1.2Study the effect of Acetylacetone

Table 3.2 illustrates the effect of acetylacetone (A. Acetone) on the sol solution. Addition

of acetylacetone (> 1.25 ml) leads to the formation bulky material resulting in the films

with large porosity. So it is so easy for corrosive medium to penetrate the surface of coated

layer through porosity and react with the surface of aluminum alloy resulting in the

corrosion process.

Table 3.2: Different sols containing different amount of acetylacetone (A. Acetone).

SolPTMS

/ml

A. Acid

/ml

A. Acetone

/ml

Propanol

/ml

Water

/mlEcorr/mV

jdiss

/A cm-2

Porosity

/%

7 2.5 1.25 0 2 0.5 - 1197 0.11 0.28 2.5 1.25 0.62 2 0.5 - 1071 0.03 0.049 2.5 1.25 1.25 2 0.5 - 1127 0.006 0.001310 2.5 1.25 1.87 2 0.5 - 1138 0.11 0.211 2.5 1.25 2.49 2 0.5 - 1151 0.20 0.412 2.5 1.25 3.1 2 0.5 - 1119 0.28 0.4

Fig. 3.2 shows dissolution current density of sols 7-12 for PTMS-coated samples heat

treated at 573 K. the best result is concerted in sol 9, which obtained by adding 1.25 ml of

A. Acetone, which lead to decrease the porosity of coated thin layer.

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

1E-12

1E-11

1E-10

1E-9

1E-8

1E-7

1E-6

1E-5

1E-4

1E-3

0.01

0.1

j/Ac

m-2

EAg/AgCl

/ V

uncoated

Sol 7

Sol 8

Sol 9

Sol 10

Sol 11

Sol 12

Fig. 3.2: The effect of A. Acetone (ml) on the current density of PTMS-coated samples in0.05 MNaCl solution after heating for 150 min. at 573 K.

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3.1.1.3Study the effect of water-propanol

The effect of water-propanol on the porosity of thin film has also been investigated. Fig.

3.3 illustrates the relation between jdiss and EAg/AgCl for many sols 13-18 prepared with a

different amount of water : propanol. Fig. 3.3 shows the difference in dissolution current

density. Higher amount of water leads to enhancement of the porosity due to the higher

hydrolysis rate which prevents a highly cross-linked coating and results in the less

protection performance. Fig. 3.4 shows thejdissof coated samples prepared using different

amounts of water and propanol (by volume). The amount of water added is optimized in

order to balance the hydrolysis and condensation reactions which in tur