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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