Localized attack in which small anodic spots corrode rapidly while the surrounding surface stays passive or protected. Inside the pit, chloride concentrates and acidity rises, so the pit feeds itself and grows deeper rather than wider. Stainless steels, aluminum and poorly protected coated steel are the usual victims. A pit can perforate a pipe or tank wall with only a tiny loss of total metal, which is why average weight-loss rates understate the risk.
UT shows nominal wall everywhere, but the cut-out has pitting under the deposits nearly through to the OD.
Report pitting rate separately from average mpy. A 2 mpy coupon with a 30 mpy pit is a pitting problem, not a good result.
Also heard as
- pitting corrosion
- pits
Do not mix it up with
- Uniform corrosion: Uniform corrosion thins the whole surface evenly; pitting concentrates attack in a few spots that can perforate quickly.
Related terms
Crevice corrosion
Localized attack inside tight gaps under gaskets, deposits or lap joints where the solution stagnates.
PREN
Pitting resistance equivalent number: an index of a stainless alloy's chloride pitting resistance from its chemistry.
MIC
Microbiologically influenced corrosion: corrosion started or sped up by bacteria and other microbes.
Uniform corrosion
Corrosion spread evenly over a whole surface, thinning the metal at a fairly predictable rate.