8-Pitting and Crevice Corrosion

8/12/2019 8-Pitting and Crevice Corrosion

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CH.7.CH.7. PPITTINGITTING ANDANDCCREVICEREVICECCORROSIONORROSION


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Pittin CorrosionPittin Corrosion

Initiated by localized breakdown ofpassive film, usually on MnS by Cl-

Difficult to detect pits because of

their small size Pitting requires an extended

initiation period. Once started, a pitpenetrates the metal at an

increasing rate

to that of crevices

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Localized Corrosion: ClLocalized Corrosion: Cl-- EffectsEffects

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Process of Pittin CorrosionProcess of Pittin Corrosion

Film breakdown

metastable pitting (pit.

stable pitting: pit growth

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Models on Passivit Breakdown b ClModels on Passivit Breakdown b Cl--

Adsor tion ModelPenetration Model

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GrowthGrowth mechanism of pitting corrosionmechanism of pitting corrosion --

Pitting corrosion occurs by the breakdown ofpassive i m. C - is etrimenta in rea ingthe passive film.

Mixing electrolytes in the pit with bulksolution is highly restricted ; major

O2OH

-O2OH

-

Cl-

Cl-

IR drop

concentration difference is built up; low pHand high [Cl-].

Large cathode area(external surface) andsmall anode area it .

M+Z

Salt FilmCl-

Cl-

+

H+

External surface is generally passivated dueto high O2 + high pH or cathodicallyprotected.

M+ZM+Z

H+

H2e-

e-

i) Rapid metal dissolution(anodic dissolution).ii) Increase in [Cl-]iii) Hydrolysis reaction by cation leads to

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Evaluation of Tendenc to PittinEvaluation of Tendenc to Pittin

Film breakdown potential or pitting potential

.

Minimum [Cl-

] causing pitting. Measurements of the number and the depth of pits in a suitable

. . .

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Pittin PotentialPittin Potential EE

Potentiodynamic Method

Measure the current density with changing thepo en a gra ua y a a cons an scann ng ra eand determine the critical potential (Epit) at which

the current density increases abruptly due to filmbreakdown.

Epit

potential for pits is obtained when themeasurements are done in reverse direction. If

no pitting occurs, the curve traces the same pathon reversing. On the other hand, if the alloy has

Erp

un ergone pttng, t en we w get a ysteressloop due to the film breakdown and pit formation.

With reducing the potential, repassivation of pitsstarts and finishes until we reach the potential Erp

E

.

a. E


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A metal in which Ecorr is lower than Erp would not undergo pitting corrosion.

. ,

the same applied current density) is directly related to the resistance of the alloy to thepropagation of crevice corrosion.

The difference in sweep rate can vary the value Epit; Higher the sweep rate, the Eb.

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Effect of Chloride ConcentrationEffect of Chloride Concentration

Dependence of Epit on [Cl-]

Epit = A log [Cl-] + B

Epit = 0.168 - 0.88 log [Cl-] (304 SS)

Epit= 0.05 - 0.124 log [Cl-] (Al)

Minimum conc. of [Cl-] necessary for pitting

Alloy Cl (N)

Fe 0.0003

5.6Cr-Fe 0.017

11.6Cr-Fe 0.069

- .

24.5Cr-Fe 1.0

29.4Cr-Fe 1.0

18.6Cr-9.9Ni-Fe 0.1

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Effect of Inhibitor on PittinEffect of Inhibitor on Pittin

Pitting corrosion depends not only on the [Cl-], but also on theconcentration of unaggressive anions. In general, unaggressive anions

Increase in Epit Many anions present in the solution containing Cl- ions act as inhibitors, ie.,

SO 2- OH- CO 2- NO3- all of these anions shifts E to more ositivevalues. As the potential of passive metal is shifted in the noble direction, theamount of Cl- adsorbed on the surface is increased. The anions inhibit theadsorption of Cl- by a competitive adsorption on the surface of passive film.

- -

log[Cl-] = A log [ inhibitor ] + B

log[Cl-

] = 1.62 log[OH-

] + 1.84log[Cl-] = 1.88 log[NO -] +1.18log[Cl-] = 0.85 log[SO4

2-] - 0.05log[Cl-] = 0.83 log[ClO4-] - 0.44

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;OH-< NO3- < SO4

2- < ClO4-


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Decrease in the number of pits

Environmental effects on corrosion behavior of stainless steels.

Neutral electrolyteNo Cl-

a) Deaerated SO42-

M+ H2

EOH-/O2ECORR,B B

Cl-

inhibitor

b) Aerated SO42-

Active Dissolution

M OCORR,CCEpit

Metal-Passive

Ecorr > E ic) AeratedEH+/H2

EM+/MECORR,AA

Metal-Pitting

MxOySO4

2- + Cl-

log i

d) Aerated SO42-

+ Cl- + inhibitor

MxOy

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Metal-Passive


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Effect of Allo in ElementsEffect of Allo in Elements

S Mn Al

C

r

Ni

PRE = %Cr + 3.3 (%Mo + 1/2 %W)+ 16~30 %N

Cr,

Ni WMo,

Ti

Cr, Ti

, ,

C

r

S, Mn

r, ,

Mo, Cu,

Nb, Ti

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Influence of Solution Tem eratureInfluence of Solution Tem erature

Increasing the solution temperature causes Epit to shift in the active direction,indicating an increased tendency toward pitting.

Critical pitting temperature (CPT)CPT is determined by measuring anodic current of an alloy with increasing thesolution temperature either at open circuit or at a selected potential applied to thegven a oy. , e curren ens y ncreases a rup y. ower mp es grea ersusceptibility.

E

Increasing Mo in

18Cr-8Ni Stainless

Steel

900

m Pitting

Pitting

0%3%

5%

100

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Temperature (0C)30 65


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Mechanism of Crevice CorrosionMechanism of Crevice Corrosion

Anodic reaction : MAnodic reaction : M M+ + eM+ + e--

Cathodic reaction :Cathodic reaction :

OO22 + 2H+ 2H22O + 4eO + 4e-- 4OH4OH--

e-

Hydrolysis reaction of metal ion inHydrolysis reaction of metal ion increvice :crevice :

MM++ClCl-- + H+ H22OO MOHMOH + H+ H++ ClCl--

e-

Low pH, high ClLow pH, high Cl-- conc.,conc.,

depassivation and IR drop lead todepassivation and IR drop lead to

e-

an accelerating or autocatalytican accelerating or autocatalyticcorrosion in the crevice.corrosion in the crevice.

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Crevice Corrosion on LNG ShiCrevice Corrosion on LNG Shi

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Crevice Corrosion Test AssemblCrevice Corrosion Test Assembl

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Miti ation of Crevice CorrosionMiti ation of Crevice Corrosion

Do not form crevices. Use alternative joining technology such asweldin or brazin .

Use sealants to avoid moisture penetrating into the crevice.

Design to remove stagnant areas where moisture and deposits can.

with no residual moisture.

No materials which will hold moisture near the metal surface.

pressure washing, etc.

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ReferencesReferences

KAIST , http://corrosion.kaist.ac.kr

Com rehensive Treatise of Electrochemistr Vol. 4. ElectrochemicalMaterials Science, Plenum Press, 1981.

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8-Pitting and Crevice Corrosion