How a crevice turns aggressive
A crevice is a gap wide enough to admit liquid but too tight to exchange it with the bulk flow. Inside, dissolved oxygen is consumed by normal corrosion and not replenished, so the shielded area becomes the anode and the freely aerated surface outside becomes the cathode. Metal ions dissolving in the gap hydrolyze and lower the pH, and chloride ions migrate in to balance the charge. The trapped solution becomes acidic and chloride-rich, the passive film breaks down, and the attack feeds itself — the same self-sustaining cell that drives pitting, but started by geometry rather than by a surface defect. There is usually an incubation period, so a joint can run clean for months before attack begins.
Where it shows up in plants
- Flange faces under gaskets, especially absorbent non-metallic gaskets that wick fluid
- Tube-to-tubesheet joints in heat exchangers
- Threaded connections, bolt heads, washers and lap or skip-welded joints
- Under deposits, sludge, biofilm and marine fouling
- Under disbonded coatings and tape edges
Stagnant flow, higher chloride content and higher temperature all make each of these worse.
How it is tested
Laboratory tests rank alloys; they do not predict service life. ASTM G48 includes a ferric chloride crevice test (Method B) and critical crevice temperature methods (Methods D and F). ASTM G78 is the guide for crevice testing of iron-base and nickel-base stainless alloys in seawater and other chloride-containing environments, using multiple-crevice assemblies. For a given alloy, the critical crevice temperature is typically lower than the critical pitting temperature, which is why crevice resistance usually governs alloy selection in chloride service. PREN gives a first-pass ranking before testing.
How to prevent it
- Design the gap out: welded instead of bolted or riveted joints, continuous instead of skip welds, full-penetration welds, and drainage that leaves no stagnant pockets
- Select an alloy with margin for the actual chloride level and temperature, not the nominal one
- Use non-absorbent gaskets and seal crevices that cannot be eliminated
- Keep surfaces free of deposits by flushing and cleaning at each outage
- Coat or line crevice-prone surfaces, such as restored flange faces, so the electrolyte cannot reach the gap
For the full mechanism, inspection clues and repair options, see crevice corrosion explained.