What causes crevice corrosion? Crevice corrosion is
initiated by a difference in concentration of some chemical
constituents, usually oxygen, which set up an
electrochemical concentration cell (differential aeration
cell in the case of oxygen). Outside of the crevice (the
cathode), the oxygen content and the pH are higher - but
chlorides are lower. Chlorides concentrate inside the
crevice (the anode), worsening the situation. Ferrous ions
form ferric chloride and attack the stainless steel rapidly.
The pH and the oxygen content are lower in the crevice than
in the bulk water solution, just as they are inside a pit.
The pH inside the crevice may be as low as 2 in a neutral
solution. Once a crevice has formed, the propagation
mechanism for crevice corrosion is the same as for pitting
corrosion. The major factors influencing crevice corrosion
are: • crevice type: metal-to-metal, metal-to-non-metal •
crevice geometry: gap size, depth, surface roughness •
material: alloy composition (e.g. Cr, Mo), structure •
environment: pH, temperature, halide ions, oxygen
