Q:
What are Pitting Corrosion
and Crevice Corrosion?




A:
    PITTING CORROSION:
  • Highly localized holes may be well separated
    or so close they resemble a roughened surface.

  • Corrosion products can mask them. One area of
    surface becomes highly anodic or localized changes
    occur in corrodent composition (at crevices).

  • Microbiologically Induced Corrosion (MIC) is a
    primary pitting producer. With SS, halides, (especially
    chlorides
    ) destroy the protective oxide film and cause
    pitting corrosion.


    CREVICE CORROSION:

  • Narrow openings, gaps between two metals (riveted seams,
    bolt/washer
    ) or a metal and nonmetal (0-ring, gasket, etc.)
    Concentration cell is formed within the crevice. Tubercles --
    local mounds of corrosion products -- form on a surface and
    promote accelerated corrosion underneath, e.g., steel
    and cast iron water supply system - pipes, tanks.

  • The example below shows a heavily tuberculated 75 mm
    (3-inch) outer diameter steel mill water supply line.