MEDIUM AGGRESSIVI1Y AND INHIBITIVE SPECIESIN PI1TING CORROSION OF STAINLESS STEELS

C.LEMAITREUniversite de Technologie de Compiegne - LG2mSURA 1505 du CNRS - B.P. 64960206 Compiegne Cedex - France

ABSTRACT. The presence of halide ions in an electrolyte can provoke local breakdown of thepassive film of stainless steels and initiate the localized corrosion by different exposedmechanisms, resulting in stress corrosion cracking, crevice corrosion orpitting. The chloride ionsplaya considerable role in these phenomena, but several oxyanions can be used to balance theinfluence of the halide ions presence and prevent the risk ofpitting corrosion. Their efficiencyis shown by the use of polarisation curves or impedance measurements, and calculated bystatistical studies of the pitting potential values. Several possible mechanisms can explain theiraction on the passive film. The results obtained show that there is a competition betweenaggressive and inhibitive species present in the medium. The corresponding mechanisms ­adsorption on active sites, diffusion and transport - are discussed, and it appears that thethickness of the film and the limiting step of the corrosion process depend on the mediumcomposition. If the austenitic or ferritic structure does not playa great role in these mechanisms,the steel cleanliness is an important parameter which can modify the properties of inhibitivespecies, and the sulphide-containing species - in the metal or in the medium - can provokechanges in the kinetics of the pitting corrosion or inhibition process, by modification of thelocalized stability of the passive film.

1. Introduction

The pitting of passive materials is due to the presence of localized defects whichdestroy the protective properties of the surface film created between the materialand the medium, with a small ionic conductivity. This film is in equilibrium with themetal and solution. It takes into account the potential drop between these twomedia. Thus, the equilibrium depends on the material, but also largely on thesolution, and the passive film characteristics can change if the temperature or thecomposition of the medium are modified.In fact, pit initiation corresponds to localized breakdown of this passive film, and

it appears that the pitting resistance depends on the nature of the solution, and onthe amount of different species which can be added in the medium. On stainlesssteels, halide anions are often very aggressive species, particularly the chloride ionsare well-known for that, but' other anions are inhibitive species, and most ofoxyanions can play this role. To understand the role of these species on the pitting,it is necessary to know the breakdown mechanisms of the passive films on stainlesssteels. This is the reason for that these processes are examined.

261

K. R. Trethewey and P. R. Roberge (eds.), Modelling Aqueous Corrosion, 261-278.© 1994 Kluwer Academic Publishers.

262

2. Passive film breakdown

2.1. STRUcruRE OF THE PASSIVE FILM

The passive film on stainless steels is due to the presence in the alloy of chromium.This element promotes the formation of MzOjM(OHh type of oxy-hydroxide films[1, 2]. The presence of water is necessary to obtain these compounds by hydrolysis.The total thickness of this layer corresponds approximatively to 5 nm [3, 4],depending on the conditions of manufacture and use. It is metastable [5] and canbe modified if the medium is changed. In particular, the potential drop between thematerial and the medium depends on the solution. The electric field in the film islarge, because the film thickness is small. It is then possible to obtain, under thislarge electric field, the diffusion of chemical species through the film [6, 7].

2.2. ADSORPTION

The first stage of a passive film breakdown is necessarily due to the adsorption ofan aggressive species of the medium at the film-solution interface [8]. Under theaction of the electric field, the chloride ions of the solution can take the place ofother anions in the film, as Oz- or OR [9]. This phenomenon conducts to formvacancies and promotes the decrease of the superficial stress between the film andthe liquid phase. Consequently, this first step of the localized corrosion process hastwo characteristics. First, vacancies are created, and are under the conditions fixedby a great electric field. Second, this field can be locally modified by the presenceof the aggressive species and the film equilibrium is changed.

2.3. PENETRATION

The second step now is the possible penetration of the aggressive species in the filmby a diffusion process due to the presence of the electric field [10]. Thisphenomenon promotes the formation of new species, which can be considered ascorrosion products. These compounds are made with ions of the film, and thepassive film is largely modified at that place. For example, one can have theformation of MCl3which takes the place of M(OH)3 in the film.

2.4. ELECfROSTRICTION

If a corrosion product is formed in the film, its thickness decreases where the newcompound exists. But the potential drop metal/solution is not modified. Thus, theelectric film in the film increases and the equilibrium conditions are modified [11].This phenomenon provokes a large change in the values of mechanical stresses atthe interfaces, and the film can be broken [12].

263

2.5. DIFFUSION

The created vacancies during the first stages can also diffuse under the electric fieldin the film. They can coalesce to provoke the formation of a small porosity at thefilm-metal interface [13]. This process depends on the quantity of formed corrosionproducts and on the dissolution current of the metal. The induction time of pittingcan be explained by the time necessary to obtain a critical dimension for the pores.

3. Pitting Corrosion

If the metal contains structural defects, like inclusions, intermetallic compounds orsegregates, the pits initiate at these defects because they are "active sites" comparedto the bulk of the surface. The properties of the active sites with respect to localizedcorrosion, are dependent on mechanical and thermodynamic considerations,discussed later in this paper. If the metal has not defects, the pitting corrosion isinitiated when the passive film is broken down. Thus, the metal is directly under theaction of the medium.The resistance of stainless steels to pitting corrosion is influenced by metallurgical

considerations, but also by physico-chemical parameters, where the medium playsa great role [14]. To measure this resistance, several parameters can be used: pittingpotential, critical temperature or induction time. In this paper, only the pittingpotential is considered, but the other parameters are often used.

3.1. AGGRESSIVE SPECIES

3.1.1. Depassivation of Stainless Steels. The passive film of stainless steels containsOU- ions, and the film stability depends on the pH. Indeed, if the acidity of themedium corresponds to the destabilization of the passive film, the corrosion processof the stainless steels can be considered as similar to the plain carbon steels one. Acritical value of pH exists for each grade of stainless steel, and for each medium, forwhich the alloy is depassivated. It is called the depassivating pH, pHd' In the caseof localized depassivation, either crevice corrosion or pitting corrosion can beinitiated.

3.1.2. Pitting Potential Measurements. During the induction period, many active sitesare depassivated, but can also be repassivated if a critical potential or local pH isnot obtained: this phenomenon corresponds to unstable pits. The first stable pit isobtained for a given potential at the induction time t, or after a given time at thepitting potential Epit ' Thus, these two parameters are not independent. Indeed, theprobability of observing the first pit on a large surface area is greater than a smallone. It is therefore necessary to introduce a third parameter and to take intoaccount the probabilistic approach, for the creation of the first stable pit corresponds

264

to stochastic considerations about the life and death of unstable pits [15].To determine pitting potential, it is necessary to obtain the pitting probability

evolution with potential change [16, 17]. We have used a technique [18, 19] basedon potentiokinetic measurements from polarisation curves at a potential sweep ratey. For a sample i, the polarisation curve is determined, and the electrical circuit isopen when a stable pit is considered to be obtained, i.e when a threshold current ismeasured. The corresponding potential Ei is also measured, and the different valuesof Ei obtained from N tests are statistically treated. From the experimental data, thesurvival probability PEis deduced as a function of the electrode potential E: if nEisthe number of pitted samples at the potential E, one has:

PE = (N-nE ) IN (1)

The "elementary pitting probability" [18, 19] per unit area, ti3E, is then, if S is thesurface area of each sample:

from which one may determine the "pit generation rate", gEo:

° clWe clWegE= dt = Y dE

If Epil is a conventional pitting potential, for a given value of gOE' one have:

gOE = constantpit

(2)

(3)

(4)

3.1.3. Local depassivation by chloride Ions. It is now possible to use the gOE parameterto characterize the pitting resistance of stainless steels. The measurements weremade using different commercial grades in neutral solutions containing differentamounts of sodium chloride (Figure 1). They show that the pit generation rate canbe written in the following form [18]:

(5)

where g* is a constant for a given material in a given solution, and lp is the inverseof the slope for the experimentally obtained lines.From these results, it is easy to fix the constant to obtain the conventional pitting

potential Epil ' The results obtained from a AISI 304 stainless steel are reported inFigure 2. It appears that if the Epil values are modified by the adopted conventiontheir evolutions with the chloride content are similar, with:

Epit = a In [Cl-j + b (6 )

265

100 .........--------------..,

!Eu

AISI 304 - Deaerated media

lei·)

• O,5M• 0,1 M• O,02M

500300 400

E (mY ISCE)

10' 4.I---_-..,..--.,.....-~--.,.....-_i200

Figure 1 - Chloride concentration influence on the pit generation rate in deaerated solutions (AISI304).

520

470

420w~ 370:>E 320=:Q.

270w

220

170,01 ,1

AISI304

Deaerated media

10

gE crit

(cm-2. sec-1 )

• 1• 0,2• 0,1• 0,05

[CI-] ( molll )

Figure 2 - Pitting potential evolution vs the chloride content in deaerated solutions (AISI304).

266

where a and b are parameters which have the potential dimension. This result mustbe compared with the <p values. The pit generation rate [19] can be written:

(7)

where k is a constant depending on the chemical kinetics, a is a transfer coefficient,q is tije electric charge of the electron, kB is the Boltzmann constant and T thetemperature in Kelvin. It appears that:

<p = kBT/n«q (8)

and the pit generation rate is:

g%=k[Cl-jn.exp(E/<p)

Thus, at the pitting potential:

g;it = k [Cl -j n. exp (Epit / <p)

and:

(9 )

(10)

(11)

n In [Cl-j + (Epit/<p) = In g;it - In k = constant (12)

then:

d(Ep't)~ = n<p

d(ln [Cl-j)

This is the slope of the line corresponding to the equation (6):

a = n. <p = kBT/«q

(13)

(14)

The measurement of a and <p permits the determination of the n value. It appearsthat generally a = 120 mY dec') and <p = 40 mY dec'I, thus n = 3, in sodiumchloride media. One can write now:

g*=k[Cl-j3 (15)

267

Thus it can be suggested that the corrosion process is regulated by the formation ofan unstable complex MCl3, where M is essentially iron and chromium.

3.1.4. Other co"osive species. Other halide ions than chloride can be also aggressiveto corrode stainless steel, and the role of fluoride was studied. It seems that thecorrosivity of this ion depends largely of the pH values: in acidic pH, it is possibleto consider that the medium contains hydrofluoric acid, and it is very easy todepassivate the alloy.The sulphide ion can also help to depassivate stainless steels. This phenomenon

can be observed by addition of manganese sulphide in the system metal/solution [20,21].Concerning the pitting resistance, it was also found that thiosulphate additions of

Na2S20 3 in neutral chloride media [22] provokes a decrease of the values of thepitting potential (Figure 3). These observations are in accordance with comparisonof results obtained by using stainless steels which contain different amounts ofalloyed sulphur. This point will be discussed later in this paper.

3.2. INHIBITIVE SPECIES

Only a few oxyanions will be considered in this paper to limit the scope, and alsobecause these inorganic species can be used at more elevated temperatures thanorganic ones.

3.2.1. Efficiency of inhibitive species: In uniform corrosion, the efficiency of aninhibitor is defined as following:

x 100 (16)

where Vo is the corrosion rate without inhibitor, and Vi is the one with inhibitor; ~

is the inhibitor efficiency. In the case of pitting corrosion, one can define a similarparameter:

(17)

where gE is the pit generation rate at the potential E. The value ~E is dependent onthe potential. Thus, conventionally, one can fix E = Epil, the Epil value correspondingto the one obtained without inhibitor. Then, one has:

~% = (18)

268

and now, two parameters permit to characterize the action of inhibitive species: thepitting potential value and the inhibitor efficiency.

3.2.2. Action of oxyanions in chloride media

Chromate ions (Cr,O:c:) If one observes the gE curves in NaCI + Na2Cr203 solutions(Figure 4), one can see that for small amounts of inhibitor the gE curves are parallel,with a lJl value constant, but for large amounts of chromate, the slope is modified,and lJl is changed.The pitting potential evolution of neutral chloride and chromate containing media

is reported in Figure 5. It appears that this parameter increases if the oxyanion isadded in the solution, but that this increase depends on the chloride and theinhibitor amount, with a great change in the evolution if a critical ratio of contentsis obtained: the slope a is then modified. It is now possible to determine theconditions of a given efficiency, for example lJl = 99%, and to report thecorresponding concentrations on an "inhibition diagram", i.e. a 10g[CI-) =f(log[inhibitor]) diagram. One obtains a line which separates the contents in twoparts where the value of lJl is less or more than 99% respectively (Figure 6).One can say that, in the first part, there is "inhibition" and in the second there is

"pitting". This pitting is a decreasing one, because gE < gE·' but the inhibition is nottotal. The obtained line corresponds to the change of the a and lJl values observedon the experimental results. Thus in the pitting domain, the gE curves are paraJleland the slope a is maintained. Then, the n value is equal to 3, and the limiting stepof the corrosion process is not modified.By studying the inhibition diagram, it was demonstrated [22] that:

(19)

and this equation can be explained if one considers that the inhibitor is not Cr20{but is HCr20 7-. Its action consists to cover the possible active sites, with acompetitive adsorption between the chloride ions and the HCr20 7- on the same sites[23]. The equation (19) is an adsorption isotherm.

Molybdate ions (MoOl) The inhibitive action of molybdate ions is known forstainless steels, essentially in aerated conditions. But in deaerated media, one canobtain an inhibitor efficiency due to the molybdate species presence. Indeed, gEdecreases in aerated or deaerated conditions, but it appears new conditions: thecritical amount of inhibitor to have the beginning of a gE decrease. The resultsobtained in chloride solutions, concerning the pitting potentials and the inhibitiondiagrams [24] are reported in Figures 7 to 9, respectively in aerated and deaeratedmedia. In this later case, the experimental relationship at ~ = 99% is:

269

100"T"--------------....,AISI430

Deaerated NaCI 0,2 mol.l-1

iii 0

~>.5-==.B- -100

0,0100,005

[S203=) (mol.J-1 )

-200 +----...----.,r----.....,.---~0,000

Figure 3 - Influence of thiosulfate additions in deaerated chloride media on the pitting potential (AISI430).

100..----------------....,

AISI 304 in NaCI 0.1 moVl

Deaerated

- 10- 1i•N

Eu

10·IIII:Il

10-300 400 500

E(mVlSCE)

600 700

[M004--) (moVl)

.. 0

• 0,01• 0,02• 0,05

Figure 4 - Inhibition of the pitting initiation by the chromate ions in deaerated chloride containingmedia (AlSI304).

270

1100

1000ui

900(.)en soo>E 700

600:: 500DoW

400

300

20010- 2

[Cr04 -- )(mol. 1-1 )

• 0.05a 0.01• 0.02x 0

a 0.005

100[CI- ) moLl-1

Figure 5 - Influence of chromate additions in deaerated chloride media on the pilling potential (AISI304).

100

--!:'0E

1(f 1...Q.

1(f2

[ Cr04 - ) (mol. 1-1 )

Figure 6 - Inhibition diagram - Competition between the chloride and chromate ions in deaeratedsolutions (AISI 304).

550[Ilo04- ]

iii omol.l-1iii 450 • 0,02 mol.l·1~ • 0,05 mol.l·1

> • 0,1 mol.1-1

!. 350

.t:Gow

250 AISI430

Aerated media150

10' 2 1<)"1 100 10 1

[ CI-] (mol.l-1 )

Figure 7 - Influence of molybdate additions in aerated chloride mediaon the pitting potential (AISJ 430).

550[Mo04-]

• OM

• O,05M

a 450 • O,02M

~ • O,01M

!.'= 350Go

ro.l

250 AISl430

Deaerated media15010"2 10" 1 100

mol.l·l10 1

[CI-)

Figure 8 - Influence of molybdate additions in deaerated chloride mediaon the pitting potential (AISI430).

271

272

log [MoO;-] ~rit=const + log [Cl-] 2 (20)

Sulphate ions (SOl) The sulphate anions also have an inhibitive effect on pittingcorrosion, as shown in Figure 10. It appears also an increase in the conventionalpitting potential values, which depends linearly on the ratio [S042-]1[Ct-]4/3 (Figure 11)in deaerated conditions.

3.2.3. Change in the passive films properties. Electrochemical measurements of theimpedance at the interface metal/solution in neutral chloride media show only thatthe passive film is a capacitance. But, if an inhibitive oxyanion is added in themedium with a sufficient amount, a transfer resistance appears. This changecorresponds with the existence of the two contents domains in the inhibition diagramfor the chromate or the molybdate ions [25].This can be discussed in terms of mechanisms. If the limiting step of the corrosion

process is the competitive adsorption in the pitting domain, theinhibition process canbe one of diffusion. This diffusion of matter through the reinforced passive film isdue to the coverage of the active sites by the inhibitive species, with a ratio morethan 99%, corresponding to the conventional limit for the value of the inhibitorefficiency in the pitting domain [23].

3.3. MATERIAL INFLUENCE

Different studies performed with austenitic or ferritic stainless steels have shownthat the stainless steels structure does not have an influence on the resultsconcerning the corrosion process. One observes that cp = 40 mV dec-] and a = 120mV dec-] for "standard" commercial grades (AISI 304; 304L; 316; 430; 434) inneutral chloride media. Thus, the passive film breakdown mechanism does notdepend on the structure of the stainless steel. However, the microstructure of theused grade plays a great role in the process. Indeed, inclusions are preferential sitesof pitting initiation, and their composition and properties have to be taken intoaccount in this initiation process.

3.3.1. Influence on the breakdown. Two parameters are important to explain the roleof inclusions: the expansion coefficient and their stability product in the corrosivemedium. The first parameter influence is due to the making of the steel. Duringcooling, it can create stresses which lead to the birth of micro-cracks close to theinclusion. The second relates to the anodic or cathodic properties of the inclusion.If it is soluble, it is anode, and a hole appears in its place, which can be the site ofthe pitting initiation. If it is not soluble, it is cathode, then the alloy surface is anodicnear the inclusion, with also a pitting initiation risk [6, 24]. For example, thepresence of sulphide manganese inclusions (MnS) in an AISI 430 grade has a

273

--.A:I­.o

01

[ Mo04- ] (mol.l-1)

, 1

Figure 9 - Inhibition diagram - Competition between the chloride and molybdate ions in aerated anddeaerated solutions (AI51430).

500

AISl430

w Deaerated media [SUIf8teJ~ 400 mol.l-1

>E I!I zero

j • 0,01

C • 0,05

i 300

20010"2 10· 1 100

Chloride content (mol.1-1 )

Figure 10 - Influence of sulfate ions addition in chloride containing media on the pitting potential indeaerated solutions (AI51430).

274

detrimental effect on the pitting potential (Figure 12), because the MnS particles aresoluble in the case of pH < 5.8. Then, we have in the medium, species such assulphide or thiosulphate ions, which act as depassivating species and promotelocalized corrosion initiation.In an AISI 430-Ti grade, the inclusions (Ti, S) are obtained, and not MnS, and

(Ti, S) inclusions are not soluble. It appears that:- conventional pitting potential is highest with (Ti, S) than with MnS inclusions- in the presence of chloride ions only, the slopes <p and a are not the same for thetwo steels, but the n value is maintained (n = 3). Thus, the corrosion process is dueto the formation of a soluble complex MCl3•

3.3.2. Influence on the inhibition. Inhibition diagrams were compared by molybdateions as pitting corrosion inhibitors in deaerated chloride media, With AISI 430 andAISI 430-Ti grades. One observes that (Figure 13) the sulphide solubility has noinfluence on the slope of the obtained lines, but the inhibition domain is largest forthe AISI 430-Ti stainless steel: the corrosion inhibition will be obtained, for a givenchloride content, with less inhibitor than for the AISI 430.Thiosulphate additions have different effects [22]:- for the AISI 430-Ti grade, the pitting initiation resistance increases if S20t ionsare added in the chloride medium.- for the AISI 430 one, the thiosulphate presence has a detrimental effect on thisresistance.Thus it appears that these species promote the stabilization of a stable passive

film, but the sensitization to pitting of an unstable one: the inhibitive or aggressiveproperties of a given medium depend on the steel microstructure.

4. Actual developments

Current research concerning pitting resistance of stainless steels is reported indevelopments of different techniques which are reported in other papers in thisbook. One can take examples in either chemical or physical methods.

4.1. SURFACE ANALYSIS

Concerning the adsorption or diffusion process, the new microscopic techniques(tunnelling or force microscopy) can give information on the presence of certainspecies in the passive region. However, these methods are performed in a vacuum,and the absence of an electrolyte can provoke changes in the film reactivity.Nevertheless, they promise very interesting results.

275

10AISI 430 - Deaerated media

1-.8

:.:- 6:-Q.W:v 4d-

El ICI-) =O,lM2 • ICI-) =0.5 M

00,0 0,5 1,0 1,5 2,0

[504-] / [el-] 413

Figure 11 - Relationship between the critical concentrations of aggressive and inhibitive species, case ofchloride and sulfate ions (AfSf 430).

Source - B. Barowc, ECSC Research repon 721O/KB/31O.

700 "T""--~-~---------""

600

wg 500>E:= 400.t

300

10- 2 10"1

[ CI-] ( molll )

A=Aerated

D=Deaerated

Figure 12 - Microstructure influence on the pitting potential values.

276

10A = Aerated 0

o = Deaerated

-=;;PITTING'0

E~

:::r~ ,1

INHIB.,01 -F--.......- ......T"""'l,r;-'I"'T"I"T"""-~- ...........................t1~3 1~2 1~1

[ Mo04=J ( mol.1-1)

Figure 13 - Microstructure influence on the inhibition diagrams.

4.2. IN-SITU TECHNIQUES

In-situ techniques, essentially electrochemical ones, can also be used. In particularthe method of determining current transients corresponding to the initiation ofstable or unstable pits, or the method of the electrochemical noise.Photoelectrochemistry can also give information about the kinetics of the elementaryreactions at the passive film interfaces.

4.3. ACOUSTIC EMISSION

Classically used in hot corrosion, acoustic emission was also used to observe stresscorrosion cracking. This technique is very sensitive to hydrogen displacement.Different tests have actually as project to detect the pitting initiation by using theacoustic emission, and to correlate the obtained signal with the one registered by anelectrochemical method.All these studies seek to clarify the mechanisms of local breakdown of passive

films by species in the electrolyte, and to obtain a better resistance to the pittingcorrosion for commercial stainless steels used in different aggressive electrolytes.

5. Conclusions

It has been demonstrated that inhibitors like chromate, molybdate or sulphate ionsmodify the passive film, and change the reactivity at the interface film/solution, withthe possibility of a change in the limiting step of the corrosion process, if a criticalconcentration ratio is obtained for the aggressive and inhibitive species.

277

For stainless steels, the medium aggressivity is often considered by theobservation of the chloride content role. This ion has a detrimental effect on thepitting resistance. Many other species have different effects on the localizedcorrosion processes, particularly sulphur species, which can be present either in thesteel or in the medium. This work has confirmed that the redox state of the sulphurelement has a great influence on pit initiation, which can be explained by changesin the composition and in the potential values:- sulphide ions are aggressive species- sulphate ions are inhibitive species, as previously established by Leckie and Uhlig[26). .- thiosulphate aggressivity depends on the steel microstructure (i.e. the solubility ofthe sulphide inclusions, which can modify the medium reactivity) [27]The chloride content is not sufficient to describe the medium aggressivity. It is

necessary to consider the sulphur species and to know in detail the steel cleanlinessof the alloy used. Inclusions are the sites for pit initiation, and their properties arevery important in the localized corrosion kinetics, in the used medium.

6. Acknowledgements

Different results published in this paper were obtained by students preparing a thesisin our laboratory, A. Abdel Moneim and R. Djoudjou, and by the research teamcoached by B. Baroux in the Ugine research center. The author thanks thesecollaborators, the Ugine society, and the Prof. G.Beranger which partipates to thediscussion.

References

1.0kamoto G., CO". Sci., Vol. 13, (1973), 471.

2.Hoar T.P. and Jacob W.R., Nature, Vol. 216, (1967), 1289.

3.Sato N., Electrochem. Acta, Vol. 16, (1971), 1683.

4.Charbonier J.e., Maitrepierre P., Noval P. and Namdar Irani R. , Proc. 3rd Internat.Con! Solid Surfaces, Vienna, (1977).

5.Beranger G. and Lemaitre c., Mem. Sci. Rev. Met., (Fr) , Vol. 12, (1988), 675.

6.Baroux R, Beranger G. and Lemaitre C, " The stainsless steels", Ch 5, Editions dephysique, Paris, (1993).

7.Strehblow H.H. Proc. 9th Internal. Congo on Metallic Corrosion, Toronto CNRC Ed.Ottawa, (Can.), Vol. 2, (1984), 99.

278

8.Strehblow H.H. and Titze B., Corr. Sci., Vol. 17, (1977), 461.

9.MacCafferty E., J. Electrochem. Soc., Vol. 128, (1981), 39.

1O.Okada T, J. Electrochem. Soc. Vol. 118, (1971), 529.

I1.Bohni H. and Uhlig H.H., J. Electrochem. Soc., Vol. 111, (1964), 156.

12.Sato N. and Cohen M., J. Electrochem. Soc., Vol. 111, (1964), 512.

13.MacDonald D.D, 1. Electrochem. Soc., Vol. 134, (1987),41.

14.Uhlig H.H. and Gilman J.R., Corrosion, Vol. 20, (1964), 289t.

15.Williams D.E., Wescott C. and Fleischmann M, Proc. 9th Internat. Congo on MetallicCorrosion, Toronto CNRC Ed. Ottawa, (Can.), Vol. 2, (1984), 173.

16.Shibata T and Takeyama J., Vol. 33, (1977), 243

17.Shibata T and Takeyama J., Proc. 8th ICMC, Frankfurt/ Main, (Ger.) Dechema Ed.Vol. 1, (1981), 146

18.Baroux B. and Sala B, 8th European Corr. Cong., Nice, (Fr) Paper P53, (1985).

19.Baroux B., Corr. Sci., Vol. 27, (1988) 969.

20.Crolet J.L., Seraphin L. and Tricot R., Mem. Sci. Rev. Met., (Fr), Vol. 74, (1977), 647.

21.Djoudjou R., Thesis, Universite de Compiegne, (France), 1993.

22.Djoudjou R., Lemaitre C and Beranger G., Corrosion Reviews, in press, (1994).

23.Lemaitre C, Baroux B. and Beranger G., Werkstoffe und Korrosion, Vol. 40, (1989),229.

24.Lemaitre C, Abdel Moneim A., Djoudjou R., Baroux B. and Beranger G., Corr. Sci.,Vol. 34:11, (1993), 1913.

25.Farah L., Lemaitre C and Beranger G., " Modification of passive mms" , EFC seriesInstitute of Materials Publ., (London), (1994), 198.

26. Leckie H.P. and Uhlig H.H., J. Electrochem. Soc. Vol. 113, (1966), 1262.

27. Sedriks AJ., International Metals Reviews Vol. 28, 5, (1983), 295.