Short answer
Crevice corrosion shows as rust bleeding from a joint, dark pitting at a swage mouth, staining under a fitting, or roughness where a surface should be smooth. It forms in oxygen-starved gaps — inside swages, under tape and sealant, and at deck penetrations — so the visible stain is usually smaller than the damage beneath it.
Stainless steel resists corrosion because of a passive oxide layer that needs oxygen to maintain itself. Seal that surface inside a tight, damp gap and the layer cannot rebuild, so corrosion proceeds in the one place you cannot see it.
On a yacht the classic crevices are the mouth of a swage terminal, the gap under a tang or fitting, the inside of a furler foil, anything wrapped in rigging tape, and a chainplate slot packed with sealant. Warm, humid conditions accelerate it, which matters more in tropical northern Australia than in the temperate south.
Judge significance by what the stain implies rather than by its size. A rust weep emerging from a sealed joint means material is being lost inside that joint. Surface tea staining on an otherwise clean, open fitting frequently means nothing at all.
Rigging tape deserves particular suspicion. Tape applied to protect sails traps salt water against the fitting it covers. Remove it at inspection rather than inspecting around it. Related reading: how to inspect standing rigging.
Key facts
- Mechanism
- Oxygen-starved gap prevents the passive layer rebuilding
- Typical sites
- Swage mouths, under tangs and fittings, inside furler foils, under tape, chainplate slots
- Accelerated by
- Warmth, humidity and permanently damp fittings — worse in tropical Australia
- Key principle
- The visible stain is smaller than the damage behind it
Full background: read the guide