evaluation of theories for the initial stages of …...evaluation of theories for the initial stages...
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EVALUATION OF THEORIES
FOR THE INITIAL STAGES
OF PITTING CORROSION
by .
Pierre G.R. Zaya, Ing. A&M, M.Sc.(Eng.), P;Eng.
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A Thesis
Submitted to the School of Graduate &tudies
in Partial Fulfilment of the Requirements
for the Degree of
Doctor of Philosophy
McMaster University
©. October 1984
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EVALUATION OF THEORIES lOR PIT7ING CORROSION
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DOCTOR OF PHILOSOPHY (1984)
(Metallurgy)
McMASTER U~IVERSITY
Hamilton, Ontario
TITLE: EVALUATION OF THEORIES FOR THE INITIAL STAGES OF
PITTING CORROSION
AUTHOR: Pierre Gabriel Robert Zaya
Ing~nieur Arts et H~tiers (Paris, France)
M.S~.(Eng.), (L~ndon, England)
SUPERVISOR: Professo~ M.B. Ives
NUMBER OF PAGES: xi, 217.
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ABSTRACT
It is assumed in this study that the protection
imparted by passive laye~ is dependent upon the balance
between the breakdown of the protective film and the healing
of the produced flaws. Therefore, the occurrence of a pit
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must coincide with a change in the local conditions in a
flaw which did ~ot heal.
Using this hypothesis, five proposed ~heories for-pitting corrosion are compared using data published in.the
literature. The processes assumed to occur during pit, <,
nucleation are simulated. The solution compositions and
potentials 'from reported experimental detertnations of
pitting potentials are used as ~tiai conditi~ns. The
po~ential and concentrations at the base of the flaw are
computed by using mass transport equations.
The values obtained are then used to compare.the
five models. For.each theory, an expression depending on\
potential and concent~ations at ~he metal surface was iden-
tified, so that where a theory is correct, thii expression
is smaller (or la'rger)' than a parameter., . Thill parameter,
should be constant, and independent of the particular set of
experimental conditions (e.g. pH, chloride concentration in
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the bulk solution), as all the calculations are m~de f9r
conditions corresponding to the pitting potential, i.e.
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conditions borderlining between pit~l~g and passivity.-
The particular Case of iron" in borate buffers is
considered here.' The computed results indicate that the
most probable mechanism controlling pitting is the adsorp- -. ~
tion of the chloride ions at the metal surface. The repas-
sivation by precipitation of ferrous hydroxide is always
thermodynamically fav9ured, but apparently slow.
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ACKNOWLEDGEMENTS"
I.wish to thank the following people for their help
with this work:
Dr. M.B. Ives, my-supervisor, for his support and
understanding during the various stages of the project and
for his generosity in facilitating encounters with
specialists in the field.
Dr. J. Kirkaldy and Dr. W.K. Lu" the members of my Ph.D.
Supervisory Committee, for their encouragement, critiques
and f~cusing suggestions.
The staff of the McMaster Computer Centre,
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for their
help in deciphering the "messages· of The Machine and for
their suggestions which facilitated and expedited its
operation.
Drs. R.C. Alkire, M. Janik-Czachor. B. MacDougall and
H.H. Strehblow who. at various times during the development,
of this work, hsve patiently listened to my ideas and came
•forward with valuable suggestions and counter-arguments.
This thesis would have been very different without their
help.
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Angela Bull and Gail MacLean, for their skill in typing
and their patience with the countless revisions.
And Phyllis Gordan and Claire ~o~ their love and,
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understanding during all those evenings .
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We dance around in a ring and suppose,
But the Secret sits in the midd~e and kpOW5 •
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TABLE OF CONTENTS
I INTRODUCTION
II CHARACTERISTICS AND THEORIES OF PITTING CORROSION
Stages of Pit Development
Characteristics of Pitting Corrosion
Defects in th~'Passive Film
•2-4 Dissoiution of th~Passive Layer
. I2-5 Film Breaking
2-6 Breakdown and Repair
2-7 Competitive Adsorption
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2-9
Ion Penetration
Local Changes in the Environment0
2-9-1 'Critical Concentration..2-9-2 Localized Acidification
2-9-3 Resistance Polarization
2-10 Salt Film ..,.....
2-11 Cmncluding Remarks
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CORROSION
techniques3-·2 Su'r£ace
III" METHODS OF INVEST~IO/T 0";31 PIT.TING). ~'~
3-1 Electrochemic 1 Methods
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3-3 Probabilis~ic Approach
3-3-1 Time Distrib~tion (stochastic approach)
3-3-2 Spatial Distr~bution
3-4 Mass~;ansfer Cal~ula~ions
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IV MECHANISMS OF IRON DISSOLUTION
.4-1 . Mechanisms Involving Only OH-
4-2 Mechanisms in the Presence ~f Cl-
4-3 Mechanisms on Scratched Electrodes.
4-4 Mechanisms Relevant to Pitting ~o~rosiqn• -.
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V PROPOSED MOD~L FOR PIT INITIATION
5-1 Definition.of the~pr~blem
5-2 Chemical Syste~.
5-3 Transport Equations
5-4 Boundary Conditions. .•
5-5 . comptitat~ro~ of ctj:'yent-. " .':..
...,'.
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.-"">. ~_..., - .
.... ~...-:.- ...'~~ ..
•5~6 Charge of the Ele~tricai Double Layer
5-7; Numeri~al Values of the Parameters
5-~-1 Anodic Dissolution of Iron
5-7-2 Other Parameters
5-8 Numerical, Solution "f the Eqcatlons
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: 5-9 Possible Criteria for Pitting•
5-9-1 Repassivation
5-9~2' Salt Film Precipitation
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',. 5-9-3 Adsorption
5-9~4 Critical Chloride Concentration
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. VI' RESULTS
6-1 General Results
6-2 Setisitivity of the Results to ~6i r~pbt Values.' :
6-3 Precision
6-4 Stability . ',j
6-5 Testing of the Program." ....'c,"
6-6" Pi.tt~ng. in N110H-NaCI Solutions
6-7 Pitting in NaOH-NaCI-B(OH)3 Solutions
6-7-2
6-7-1 Sources of Data
:C'\Analysis o~ the Res~lts...............' ~.,
VII DISCUSSION; 0
7-1 Ideal Solution
7-2 One Dimensional Diffusion
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7-3 Inert Film
7-4 No ~onvection~:.
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No ·Precipit.ation
Supporting Electrolyte
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7-8 Steady State
.' 7-7 Chemical Reactions
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7-9 Breakdown and Repair'; .
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7-10 Tem~erature.
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7-11 Fixed.G~ometry
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7-13 Comparison of Criteria
VIII CONCLUSIONS
IX FUTURE ,WORK \ ,
X REFERENCES
-APPENDIX A: DESCRIPTION OF THE COMPUTER PROGRAM
A-1 Program MAIN
A-2 Sub'routine DRIVER
A-3 Subroutine CALC
A-4 SUbroutin~IN
A-S Subroutine DERIV'
A-6 Subrou'tine INTEG
A-7 Subroutine CONC
A-a Subrou'tine DCDT
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I • APPENDIX B: LIST OF SYMBOLS .' • \
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. CHAPTER I
- ,IliTRODUCTION
It has been estimated that the cost of corrosion in
the United States is approximately 70 billion dollars a
year, ot4.2% of the Gross National Product (Payer, Boyd,
Dippold and Fisher, 1980). Comparable percentages of the
GNP have been obtained in other countries where~evaluations
have been undertaken (Behre~s, 1975i Hoar, 1971~ Rene and)
Uhlig , ..1974). Pitting corrosion is certaiqly responsibleI'
for a re~pectable fraction of this cost, as judged by its•
impact on certain sectors, such as the pulp and paper indus-
try'. One chemical manufacturer found that over a four-year
period, pitting corrosion was respons~ble for about 8% of,all failures of the metallic piping .and equipment (Collins
and Monack, 1973).
Pitting can be defined as a form of corrosion "in
which only small areas of the metal surface are attacked
whilst th'e remainder is largely unaffected" (Shreir, 1976a).
This can occur· on many common· metals such as iron_, nickel,,titanium, aluminum, zin~, an·d· t.heir alloys'. The danger of
this type of attack is that it is impossible to monitor Its
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progress by visual inspection until the metallic part has
been completely perforated.
The s~udy of pitting cort~iiori has received great
iapetus from 'the development of commercial potentiostats and
their systematic use in experimental electrochemistry since,
the early .1960' s. (Macdonald, 1977). These instruments, by
controlling the po~ential between the sample under study and
a reference electrode in the solution of interest, can
rapidly lndicate some of the conditions necessary for the
occurrence of pitting corrosion. Numerous experimental
results have been obtained: Shreir (1976b) gives a list of
269 articles published on the subject between 1960 and
1974.
The present work is motivated by a desire to gain
theoretical insights by using the already accumulated exper-
imental data. Starting with the assumption that pitting
proceeds through processes of breakdown. and repair, the
elemental physico-chemical processes 'are'simulated, using
contemp9rary numerical'techniques. These permit' the solu-
tion of the relevant differen~ial equations with a minimum
•of restrictive assumptions.
1By simulating the behaviour of
a passive metal. during a poten'tiostatic experiment, it is
possible to use the experimental pitting potentials and the~ ,
•solutions in which they were determined to compute local
electrochemical conditions. These results in turn allow the
quantitative evaluation of five different theories
As iron is the
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attempting to explain pitting corrosion.
After a short description of the characteristics of
corrosion pits, the different theoretical models which have
been proposed to elucidate this phenomenon are discussed.
The mai~ approaches of. current research are then reviewed in
order ·to situate jJ'e methodology used here.
metal which has been selected for the application of the
proposed methodology, the concluding section of the litera-
ture review discusses the most important theories of iron
dissolution •
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