Crevice corrosion: the attack that starts where you cannot look

What is crevice corrosion?

Crevice corrosion is localized attack that develops inside narrow gaps — under gaskets, washers and lap joints, beneath deposits and marine growth, in thread roots and rolled tube joints — where the fluid trapped in the gap becomes chemically different from the fluid outside. It is the sibling of pitting corrosion: the same occluded-cell chemistry, but triggered by geometry instead of by a defect in the passive film. That distinction matters, because you can buy your way out of many pitting problems with a better alloy — while a bad crevice defeats good alloys.

The mechanism: a gap becomes a cell

The sequence inside a working crevice:

1. The gap fills and stagnates. A crevice tight enough to admit liquid but too tight for exchange — figures in the fractions of a millimetre — traps a small volume of the service fluid.
2. Oxygen runs out inside. The trapped fluid's dissolved oxygen is consumed by the metal's normal passive behavior and cannot be replenished. Outside the gap, oxygen remains plentiful. A differential aeration cell forms: the starved interior becomes the anode, the aerated exterior the cathode.
3. The interior chemistry turns hostile. Metal dissolving inside the crevice hydrolyzes and drives pH down; chloride ions migrate in to balance the charge. The crevice fluid becomes acidic and chloride-concentrated — a different, far more aggressive electrolyte than the bulk.
4. The familiar disaster ratio takes over. A small anodic area inside the gap is driven by a large external cathode — the same area-ratio arithmetic as in galvanic corrosion — and attack penetrates rapidly, hidden from every visual inspection until the joint leaks.

Passive-film alloys — stainless steels above all — are the most vulnerable, because their protection depends on oxygen access that the crevice denies. Critical crevice temperatures (CCT) for a given alloy run consistently below its critical pitting temperature: crevices are the harder condition.

Where it shows up

What actually works

Design out the gap. Full-penetration butt welds instead of lap joints where the service is aggressive; sealed or eliminated crevices; drainage that leaves no standing liquid. The cheapest crevice to manage is the one that was never built.

Choose gaskets and fasteners deliberately. Non-absorbent gasket materials in good condition, correctly compressed; fastener and washer choices that avoid adding a galvanic couple to the crevice; sealants where the standard detail leaves a gap.

Alloy with crevice margin — judged by CCT, not brochure grade. Where crevices cannot be avoided in chloride service, selection should look at crevice-specific performance (CCT, service history in creviced configurations), with molybdenum-bearing and higher grades buying real margin — and none buying immunity under a wet deposit.

Keep surfaces clean. Deposit crevices are the commonest kind: cleaning programs, filtration and flow management remove the crevices that operations keep creating.

Seal and protect existing joints. For equipment in service, sealing vulnerable joints, resurfacing attacked faces with engineered polymer systems, and coating creviced regions — including the flange-face repairs this attack routinely necessitates — arrest the mechanism by taking the electrolyte out of the gap.

FAQ

How is crevice corrosion different from pitting? Same occluded-cell chemistry, different initiation: pitting starts at a point defect in the passive film on an open surface; crevice corrosion starts wherever a tight gap traps fluid — no film defect needed. Alloys and environments that pit usually also crevice-corrode, and the crevice case is generally more severe.

What gap size is dangerous? Tight gaps in the sub-millimetre range are the classic danger zone — wide enough to fill, too narrow to exchange. Exact susceptibility depends on alloy, chemistry and temperature; the design answer is to seal or eliminate gaps rather than to argue about their width.

Why did my flange face corrode under a brand-new gasket? A gasket interface is a designed crevice. If the service carries chlorides and the face alloy has little crevice margin, attack can start within one service interval — absorbent or under-compressed gasket materials accelerate it.

Does crevice corrosion happen under coatings? At coating edges, holidays and disbonded areas, yes — the gap between lifting film and substrate is a crevice. That is one reason edge retention and holiday testing matter in immersion service.

Can a crevice-corroded joint be repaired without welding? Frequently — machined or corroded flange faces and joint surfaces are rebuilt with engineered polymer composites and returned to service without hot work, when the damage and the engineering assessment allow it. The mechanism must be addressed at the same time: seal the gap or change the detail.

Creviced joints causing trouble in your plant?

Induscoat Canada rebuilds corroded flange faces and joint surfaces with engineered polymer systems, seals vulnerable details and applies protection selected for the service — without hot work, during your shutdown windows. Send photos of the joint through our request a quote page; we respond within 24 business hours.