10-Crevice Corrosion and Pitting

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Text of 10-Crevice Corrosion and Pitting

  • Crevice Corrosion and Pitting

    Corrosion Engineering

    ChE 291

  • We will discuss the following topics

    Why study corrosion? An introduction

    Basic concepts of corrosion science and engineering

    Charged interfaces

    Review of thermodynamic principles for corrosion

    Electrochemical cells and Galvanic corrosion

    Poubaix Diagrams

    Kinetics of Corrosion

    Concentration Polarization and Diffusion

    Passivity

    - Introduction; Electrochemical Basis

    - Theories of Passivity; Surface Analysis Techniques for the Examination of Passive Films

    - Properties of Passive Oxide Films; Passivity in Binary Alloys; Passivity in Stainless Steels; Anodic Protection

    Crevice Corrosion and Pitting

    - Crevice Corrosion Initiation and propagation; Testing; Area Effects; Protection

    - Pitting Critical pitting potential; Effect of chloride ions, inhibitors, Mechanism of pit initiation and propagation;

    protection potential; metastable pits and repassivation; Effect of molybdenum and sulfide inclusions;

    temperature

    - Occluded corrosion cells (OCC) on iron, aluminum, and copper; differences between pitting and crevice

    corrosion

    - Detection of corrosion pits

  • What is crevice corrosion? Localized corrosion that occurs within narrow clearances or under shielded metal surfaces; can occur in geometrical clearances (e.g. under gaskets or seals; between overlapping metal sheets; etc Can also occur under deposits (corrosion products; dust particles; barnacles in seawater)

    Alloy 625 after crevice corrosion testing at 65C in natural seawater (a) +200 mV vs. Ag/AgCl for 24 h(b) +400 mV vs. Ag/AgCl for 24 h

    The crevice was provided with a compressed gasket

  • How does crevice corrosion initiate?

    Crevice corrosion initiates due to the operation of a differential oxygen cell Oxygen reduction occurs both on the metal surface which is exposed to the bulk

    electrolyte and also on the portion of the metal surface which is contained within the crevice

  • How does crevice corrosion initiate?

    Metal exposed to the bulk electrolyte is in contact with an open supply of oxygen from the atmosphere, so as O2 is consumed near the external metal surface

    When O2 molecules are consumed within the narrow clearance of the crevice, they are not easily replaced due to the long narrow diffusion path formed by the crevice

  • How does crevice corrosion initiate?

    Variation of the oxygen concentration within crevices on a Cr-containing stainless steel

    Oxygen becomes depleted within the crevice Oxygen concentration cell is formed between the metal surface outside the crevice and the metal surface within the crevice

  • How does crevice corrosion initiate?

    Metal exposed to the lower concentration of oxygen has a more negative potential for oxygen reduction than does the metal exposed to a higher concentration of oxygen

    Limiting current density iL for O2 reduction within the crevice is decreased relative to that for the bulk solution; limiting current density is proportional to concentration

  • How does crevice corrosion initiate?

    Electrode potential for metal within the crevice becomes more negative with time before leveling off

  • How does crevice corrosion initiate?

    Establishment of steady-state potentials for open iron and iron within crevices in 0.6 M NaCl

    Electrode potential for metal within the crevice becomes more negative with time before leveling off

  • How does crevice corrosion initiate?

    Limiting cathodic density is less for the creviced iron than for open iron

    Limiting cathodic rate is suppressed to a constant value for crevice heights between 0.13 and 0.50 mm

    Crevice height of 0.50 mm ~ thickness of oxygen diffusion layer near the metal surface

    When crevice height ~ thickness of oxygen diffusion layer, diffusion of oxygen into crevice is impeded; rate of oxygen reduction is decreasedCathodic polarization curves for open iron and

    iron in crevices in 0.06 M NaCl

  • How does crevice corrosion initiate?

    Evans diagrams for the crevice corrosion of iron

    If iron within the crevice were not coupled to external open iron (an isolated crevice) then the corrosion rate would be given by icorr,1

    When the crevice iron is coupled to external iron, the initial corrosion rate is given by the larger value icorr,2

  • How does crevice corrosion propagate? Propagates by changes in the electrolyte composition within the crevice Crevice electrolyte will become acidic in nature and will also contain concentrated amounts of cations discharged from the metal or alloy

    Expressions for the pH of various crevice hydrolysis reactions

    A local internal pocket of acidity develops within the active crevice even though the external electrolyte has a neutral pH of 7

  • Example 1

    Chromium contained within a crevice of 0.5

    cm2 area and 0.2 mm in height corrodes at the

    rate of 1 A/cm2. What is the pH within the

    crevice after 10 days if Cr corrodes as Cr3+, and

    Cr3+ ions hydrolyze to form Cr(OH)3? Assume

    that all Cr3+ ions produced by crevice

    corrosion remain within the crevice.

  • Example 1 Solution

    Amount of Cr3+ ions produced

    Volume of the crevice

    Concentration of Cr3+ within the crevice

  • How does crevice corrosion propagate?

    With the accumulation of H+ ions and metallic cations within an active crevice, Cl ions then migrate from the bulk electrolyte to the crevice electrolyte in order to maintain charge neutrality

    Propagation stage of crevice corrosion involves formation of a highly corrosive internal electrolyte which is acidic and also concentrated in chloride ions and dissolved cations of the metal or alloy

    Initiation stage of crevice corrosion can be quite prolonged (months to years), but propagation may proceed rapidly due to the highly corrosive crevice environment which is formed

  • How to test for crevice corrosion?

    A crevice corrosion electrochemical cell

    (remote crevice assembly)

    A split electrode is used in which the metal in the crevice is physically separated (but electrically connected) to the open metal outside the crevice; studies mechanism of crevice corrosion

    Crevice corrosion current can be measured as a function of time for various parameters, such as crevice dimensions, alloy composition, and electrolyte concentration

  • How to test for crevice corrosion?

    Effect of chromate concentration on the crevice corrosion of iron in 0.6 M NaCl; Crevice height = 0.25 mm, area of crevice iron = 7.9 cm2, area of

    outer iron electrode = 50 cm2

    For a fixed chloride concentration in the electrolyte, increasing concentrations of chromate decrease the crevice corrosion rate

    Crevice corrosion can be inhibited by using an appropriate minimum amount of CrO42inhibitor

  • How to test for crevice corrosion?

    Composition of CrNiMo alloys and of crevice electrolytes formed in natural seawater after immersion for 160170 days; pH measured in separate experiments using synthetic crevice electrolytes

    As a result of crevice corrosion, the crevice electrolyte contained dissolved cations of each of the major alloying elements for each of the alloys

  • What are area effects in crevice

    corrosion?

    Rate of crevice corrosion increases with an increase in the area of open metal outside the crevice

    Effect of the area outside the crevice on the weight loss of an Fe-17% Cr steel inside a crevice in natural seawater.

  • What are area effects in crevice

    corrosion?

    Polarization curve for the limiting cathodic current density for oxygen reduction intersects the anodic polarization curve of the creviced metal (in the propagating electrolyte) at increasingly higher values of current

  • How to protect metals from crevice

    corrosion? Use of corrosion inhibitors, such as chromates, dichromates,

    nitrites, or phosphates

    Cathodic protection (using anodes located outside the

    crevice)

    Design considerations to minimize the existence of crevices

    Materials selection: titanium and its alloys and Mo-containing

    alloys such as alloy 625 or alloy C-276 are more resistant to

    crevice corrosion (at ambient temperatures) than

    conventional stainless steels; Presence of nitrogen is also

    beneficial to crevice corrosion resistance in alloys which

    contain molybdenum

  • How to protect metals from crevice

    corrosion?

    Cathodic polarization of type 304 stainless steel in 0.05 mm crevices in 0.6 M; Numbers indicate the distance (in cm) from the crevice opening

    Cathodic protection (using anodes located outside the crevice)

  • How to protect metals from crevice

    corrosion?

    Continuous welds in (b) are preferable to intermittent welds in (a) because intermittent

    welds introduce additional crevices

    Design considerations to minimize the existence of crevices

  • How to protect metals from crevice

    corrosion?

    Discharge valves can be re-designed as in (b) to prevent crevice corrosion under deposits

    Design considerations to minimize the existence of crevices

  • What is Pitting?

    Cross-sectional view of a corrosion pit on Al 6061 formed by anodic polarization in 0.1 M NaCl

    Form of localized corrosion in which the attack is confined to a small fixed area of the metal surfaceDue to localized breakdown of a passive film, usually by chloride ionsPits can serve as sites to initiate str