Crevice Corrosion
Crevice corrosion is a localized form of corrosion that starts in a narrow gap, shielded area, or occluded space where electrolyte becomes stagnant and oxygen is depleted, causing the crevice to become anodic relative to the adjacent open surface. It is a geometry-driven, differential-aeration mechanism that accelerates attack inside tight gaps at threads, under bolt heads, under gaskets, and in blind holes.
In CNC machining, crevice corrosion risk is highest wherever the part creates an unflushed pocket: blind tapped holes, deep counterbores, sharp internal corners, threaded inserts, dowel-pin interfaces, and assembled joints that trap coolant or moisture. Even high-alloy stainless steels are vulnerable if the gap stays wet and oxygen-starved, especially in chloride-bearing environments. The preferred shop-floor controls include designing out crevices, using full-penetration butt welds, sealing lap joints continuously, ensuring self-draining geometry, and applying solid non-absorbent gaskets or coatings where geometry cannot be changed. For millwork, the same failure appears at metal fasteners under trim, concealed brackets, or adhesive lines that hold moisture. The core rule is simple: if you can trap liquid, you can trap corrosion. Inspect stagnant zones after machining, cleaning, anodizing, assembly, and field installation to prevent hidden damage.
How is crevice corrosion different from pitting corrosion?
Both are localized and often involve passive-film breakdown, but crevice corrosion requires a defined occluded geometry such as a gap, joint, or shielded area; pitting can begin on an otherwise exposed surface, and once a pit deepens it can behave like its own crevice.
What are the minimum conditions for crevice corrosion?
A physical crevice and a stagnant electrolyte are the key prerequisites; the crevice must be tight enough to limit renewal of the solution but open enough to admit liquid in the first place.
What is the first-line mitigation strategy?
Design out the crevice if possible; if not, make the joint self-draining, seal the gap, minimize deposits, and choose an alloy or coating system with improved crevice resistance for the actual service environment.