Pitting and Crevice Corrosion

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Pitting and Crevice Corrosion

Text of Pitting and Crevice Corrosion

  • Pi#ng Corrosion and

    Crevice Corrosion (Corrosion Engineering)

  • Pi#ng Corrosion Lecture Outline Deni:on Examples Observa:on Mechanism Eect of Environment Eect of Alloy Composi:on Pi#ng Corrosion Other Alloys Impacts

  • Types of Corrosion

    Uniform Galvanic corrosion Pi#ng corrosion Crevice corrosion Intergranular corrosion Selec:ve leaching Erosion corrosion and fre#ng Environmentally induced cracking Hydrogen damage

  • Pi#ng Corrosion Deni:on Highly localized aNack occurring at a rapid penetra:on rate due to breakdown of a passive metal.

    The local sites of breakdown are oQen associated with microscopic defects in the metal or alloy.

    ASM deni:on: Corrosion of a metal surface conned to a point or small area that takes the form of cavi:es

  • Example of Pi#ng Corrosion

    Requirements for pi#ng corrosion: Alloy composi:on (stainless steel, nickel alloys, aluminum alloys, :tanium alloys, copper alloys)

    Passive lm Solu:on composi:on (Cl-, Br-, I-, F-)

    Surface heterogenei:es

  • Observa:on of Pi#ng Corrosion

    Visual observa:on: Density Diameter/Area Depth

    Pit vs. general corrosion

  • Observa:on of Pi#ng Corrosion - 2

    Electrochemistry: Change in environment aects the current response

    Higher passive current

    Signicant current increase before transpassivity

    Log(i co

    rr)

    MS-SHE

    Addi:on of chloride

    Pi#ng corrosion

    Passive

    Transpassive

  • Pi#ng Poten:al and Signicant Values Log(i co

    rr)

    MS-SHE Erep

    Transpassive

    Current response as a func:on of the poten:al applied

    Epit

    Metastable pi#ng

    Ac:ve pi#ng

  • Determine occurrence of pi#ng knowing the corrosion poten:al in the environment of interest

    Log(i co

    rr)

    MS-SHE Erep

    Transpassive

    Epit

    1.

    2. 3.

    4.

    Use of Pi#ng Poten:al

  • Mechanism Pit Chemistry The anodic reac:on (release

    e-) and cathodic reac:on (consume e-) are separated.

    Cathodic reac:on on passive lm:

    O2+2H2O+e-4OH- Anodic reac:on in pit: FeFe2++2e- Hydrolysis of metal ion: Fe2++2Cl-+2H2OFe(OH)2+2HCl Hydrolysis acidies the pit

    solu:on

  • Mechanism Ini:a:on 1 Mul:ple mechanisms suggested: Chloride adsorp:on. (a)

    FeOOH Fe3+ + 3OH-

    (b) FeOOH + Cl- FeOCl + OH-

    FeOCl + H2O Fe3+ + Cl- + 2OH- (c) Fe Fe2+ + 2e-

  • Mechanism Ini:a:on 2 Structural defects (grain boundaries or disloca:on pile ups). Weaken passive lm. Preferen:al passive lm dissolu:on exposing the bare

    material and ini:a:ng pi#ng corrosion.

    Passive lm Passive lm

    Passive lm Passive lm

  • Mechanism Ini:a:on 3 Chemical heterogenei:es. Such as Manganese Sulde (MnS) in Stainless Steels.

    Passive lm Passive lm MnS

    Passive lm Passive lm

    Weak passive lm Suggested mechanism:

    MnS dissolve preferen:ally

    Bare surface exposed Pi#ng corrosion ini:ates

    OR MnS/Matrix galvanic cell Matrix dissolve and undercut MnS

    Bare surface exposed Pi#ng corrosion ini:ates

  • Eects of Environment Chloride Pi#ng corrosion is highly dependent on the chloride concentra:on Pi#ng poten:al increases Repassiva:on poten:al stays constant

    Epit

    Erep

    Poten:

    al

    Pi#ng

    Poten

    :al

  • Eect of Environment Temperature 1 The pi#ng poten:al decreases as the temperature increases.

    Materials resistant to pi#ng corrosion at low temperature may become suscep:ble at high temperature

  • Eect of Environment Temperature 2

    Cri:cal pi#ng temperature CPT: temperature above which a signicant current density is measured when a xed poten:al is applied

    Time

    Tem

    pera

    ture

    Current

    density

    CPT

    100 A/cm2

  • Eect of Alloy Composi:on 1

    Pi#ng Resistance Equivalent Number (PREN) empirical equa:on to rank stainless steels

    PREN= Cr+3.3(Mo+0.5W)+16N

  • Eect of Alloy Composi:on 2 Alloy composi:on impact microstructure and defects.

    Alloy composi:on changes the pH of the pit solu:on. Room temperature pH of concentrated salt solu:ons

    Salt 1N 3N Saturated

    NiCl2 3.0 2.7 2.7

    FeCl2 2.1 0.8 0.2

    CrCl3 1.1 -0.3 -1.4

  • Pi#ng Corrosion Other Alloys Nickel alloys are highly resistant to pi#ng corrosion

    Aluminum alloys: light alloys used for aeronau:c and automo:ve. Pi#ng ini:ates at microstructures (e.g. Cu-rich phases) linked to micro-galvanic cells

    Copper: annealed or half-hard tubes in cold tap water. Other possible as a func:on of pH, material condi:on and temperatures

    Other materials: Titanium, Zinc, Tin, Cadmium, Zirconium, Magnesium

  • Impact High localized stress

    leading to fracture (failed axle)

    Leaks with low amount

    of materials damage

    Explosion if under pressure

  • Lecture Review Require passive alloy and chloride solu:on Pi#ng poten:al:

    EcorrEpit : corrosion anode in the pit and cathode outside Solu:on in the pit contains chloride and low pH Compe::on:

    Crea.on>Diusion: corrosion Crea.on

  • Crevice Corrosion

  • Crevice Corrosion Lecture Outline Deni:on Examples Observa:on Mechanism Eect of Environment Eect of Alloy Composi:on Pi#ng Corrosion Other Alloys Impacts

  • Crevice Corrosion Deni:on

    Breakdown of passivity on a metal or alloy at a :ght crevice site due to the development of an aggressive crevice solu:on.

    ASM Deni:on: Localized corrosion at or immediately adjacent to an area that is shielded from full exposure to the environment due to close proximity between the metal and the surface of another material

  • Example of Crevice Corrosion Requirements for crevice corrosion: Creviced system combined to exposed area

    Alloy composi:on (stainless steel, nickel alloys, aluminum alloys, :tanium alloys, copper alloys)

    Passive lm Solu:on composi:on (Cl-, Br-, I-, F-)

  • Observa:on Visual:

    Expose creviced specimens and perform visual assessment of crevice corrosion

    Amount of feet having corroded

    Extent of corrosion

  • Observa:on 2 Electrochemically:

    Crevice poten:al to iden:fy when crevice will ini:ate

    Can crevice corrosion or pi#ng corrosion ini:ate?

    MS-SHE

    Passive

    Log(i co

    rr)

    Epit Ecrev

    Transpassive

  • Mechanisms

    Three models suggested: Acidica:on: similar to pi#ng corrosion

    IR drop: poten:al drop due to solu:on resistance

    Stabiliza:on of metastable pi#ng

  • Mechanism Acidica:on 1 1. Deple:on of oxygen

    in crevice 2. Separa:on of the

    anode and the cathode

    3. Chloride diusion and hydrolysis of metal ion

    4. Aggressive environment depassivate crevice

    M+

    M

    O2 OH- M+

    M

    O2 OH-

    M+

    M e-

    M(OH)a + aH+

    Cl-

    O2 OH-

    M+

    M e-

    M(OH)a + aH+

    Cl-

    O2 OH-

  • Mechanism Acidica:on 2

    Reduc:on Cathodic

    Chloride diusion

    Hydrolysis

    Oxida:on Anodic

  • Mechanism IR Drop 1. Current is owing

    through the crevice from inside to the mouth

    2. Due to the solu:on resistance, the poten:al drops:

    E=IR 3. As the poten:al

    decreases, it reaches the ac:ve peak

  • Mechanism Stabilization of metastable pitting

    Metastable pits die because the pit solu:on diuse out in innite bulk solu:on

    Crevice ini:ate because the metastable pit solu:on diuse out in a small crevice solu:on

    Neutral pH Low Cl-

    Acidic pH High Cl-

  • Crevice Former Proper:es eect Porous crevice former: Solu:on diuse out

    Small crevice length: Solu:on diuse out

    Wide crevice gap: Lower corrosion rate ANack near the mouth

  • Pi#ng vs. Crevice Corrosion : Epit in 1 M NaCl : Ecrev in 0.5 M NaCl : Ecrev in 1 M NaCl (ne) : Ecor in 1 M NaCl

    Can pi#ng corrosion occur?

    Can crevice corrosion occur?

    Which one is more likely?

  • Eect of Environment

    Chloride Temperature/pH

    What is the likeliness of crevice corrosion when you increase the chloride content?

    What is the likeliness of crevice corrosion when you decrease the pH?

    How do the cri:cal temperatures compare?

  • Examples of Crevice

    Flanges Rivets Rocks ?

  • Lecture Review Requirements: crevice, passivity, chloride Observa:on: visual or electrochemical Crevice poten:al above which crevice corrosion occurs Mechanisms: acidica:on, IR drop or metastable pit Compe::on:

    Crea:on (dissolu:on, hydrolysis) vs. Diusion Small crevice former: less crevice corrosion Loose crevice former: less crevice corrosion Ecrev < Epit : if possible, crevice corrosion ALWAYS occur

    before pi#ng corrosion High chloride High temperature BAD Low pH