crevice corrosion
Look under a washer, inside a lap joint, beneath a barnacle, or under a scrap of dirt stuck to a metal surface, and you often find the metal there badly eaten while the open surface right beside it is untouched. Crevice corrosion is localized attack that thrives in these tight, shielded gaps where liquid can seep in but cannot freely flow or refresh. The trap only needs to be a fraction of a millimeter wide — wide enough to hold water, narrow enough to keep it stagnant.
The engine is a difference in oxygen (a differential aeration cell). At first metal corrodes equally inside and outside the crevice, but the crevice liquid is sealed off, so its dissolved oxygen is quickly used up and cannot be replenished. Now the oxygen-rich open surface becomes the cathode and the oxygen-starved crevice becomes the anode, where metal keeps dissolving. Just as in a pit, the growing positive metal ions draw in chloride and the trapped liquid turns acidic and aggressive — a self-accelerating pocket. Crevice corrosion is essentially pitting that geometry has pre-arranged for you: the shielded gap plays the role of the pit.
It is a menace for stainless steels and other passive-film alloys, because the acidified, oxygen-poor crevice is exactly the environment that dissolves their protective film. Prevention is largely a design job: avoid crevices altogether by using welded joints instead of bolted or riveted lap joints, use solid non-absorbent gaskets, slope surfaces so they drain and do not collect deposits, keep them clean, and where crevices are unavoidable choose a high-molybdenum alloy that resists the local acid attack. Good detailing beats good material here.
A stainless-steel bolt clamping two plates can stay mirror-bright on its exposed head while the shank hidden under the nut and washer is deeply etched: oxygen reaches the open head freely but is trapped and consumed under the washer, so the sheltered shank turns anodic and dissolves in its own acidified pocket.
The same bolt is both protected (aerated head) and attacked (starved crevice) — geometry alone sets up the cell.
Crevice corrosion often starts at lower chloride levels and milder conditions than open-surface pitting, so a design can be crevice-free clean yet fail simply because someone left a gasket, a deposit, or a lap joint that traps stagnant water.