Crevice Corrosion: Why Gaps and Deposits Corrode First

Crevice Corrosion: Why Gaps and Deposits Corrode First is a form of localised attack that occurs inside shielded, stagnant gaps such as gasket faces, threaded joints, washer contacts and the space beneath surface deposits, where a small trapped volume of electrolyte becomes chemically distinct from the bulk fluid. The mechanism is driven by oxygen starvation inside the gap and a self-accelerating fall in local pH. On passive alloys such as stainless steels, crevice corrosion typically initiates at lower chloride concentration and lower temperature than open-surface pitting on the same alloy, which is why hidden joints often corrode long before the exposed metal shows any sign of trouble.

The mechanism: differential aeration and acidification

Inside a tight crevice, dissolved oxygen is quickly consumed by cathodic reduction and cannot be replenished by diffusion because the gap restricts mass transport. The bulk surface outside stays oxygenated and becomes the cathode, while the oxygen-depleted crevice becomes the anode. Metal dissolves inside the gap, producing positively charged metal cations. To preserve charge balance, chloride ions migrate inward. Those metal chlorides then hydrolyse with water, releasing hydrochloric acid and dropping the crevice pH to values as low as 2 to 3. The low pH and high chloride destroy the passive film locally, so dissolution accelerates itself. This is the same differential-aeration principle behind attack under deposits, which is why the process overlaps with microbiologically-influenced-corrosion when biofilms and sludge create the shielded zone.

Why crevices beat pitting for the same alloy

Pitting needs a defect to break down passivity on an open surface exposed to the full bulk chemistry. A crevice supplies its own aggressive micro-environment: it concentrates chloride and generates acid regardless of how mild the bulk fluid is. The occluded geometry means the critical crevice temperature (CCT) sits well below the critical pitting temperature (CPT) for the same steel, often by tens of degrees, and initiation happens at lower chloride thresholds. In practice this ranking holds across the austenitic and duplex grades, so a joint that passes a general pitting check can still fail in the gasket seat. Compare the open-surface behaviour in pitting-corrosion to see why geometry, not just alloy, decides the outcome.

Alloy resistance ranking

Resistance scales with the Pitting Resistance Equivalent Number, PREN = %Cr + 3.3 x %Mo + 16 x %N. Higher PREN alloys tolerate more chloride and higher temperature before a crevice activates. The values below are indicative and depend on gap tightness and surface finish.

Typical sites in plant

Crevice corrosion favours any geometry that traps liquid and blocks oxygen. The recurring offenders across process and utility systems are predictable, which makes them easy to design out or inspect on a schedule.

• Flange faces and gasket seats, especially where the gasket overhangs or is recessed.
• Threaded connections, unions and poorly seated fittings.
• Under bolt heads, washers and clamp bars.
• Tube-to-tubesheet joints and roller-expanded areas in exchangers.
• Beneath scale, silt, biofilm and process deposits on tanks and pipe walls.
• Lap joints, spot welds, backing rings and incomplete weld penetration.
• Crevices formed by paint blisters, tapes and non-metallic seals.

Common sites and their fixes

Prevention: design it out first

The most durable control is geometric. Butt welds beat lap joints, continuous welds beat intermittent ones, and full-face gaskets beat recessed ones. Where a joint cannot be eliminated, seal the gap so electrolyte cannot enter, and specify an alloy whose critical crevice temperature comfortably exceeds the service condition. Keep surfaces clean: raising flow velocity and removing deposits denies the process its shielded start point. On coated or insulated equipment the same trapped-moisture logic drives corrosion-under-insulation, so inspection strategies for both should run together. Because crevice damage is hidden by definition, a condition-based inspection routine keyed to known sites is essential. Teams that log gasket changes, torque records and deposit-cleaning cycles as recurring work orders in Fabrico catch repeat offenders before a leak forces an unplanned shutdown.

Detection and monitoring

Because the attack sits under gaskets, deposits and fasteners, visual checks alone miss it. Disassemble a sample of flanged joints at turnaround and inspect the seat. Ultrasonic thickness mapping, borescope inspection of exchanger tubes and eddy-current screening find wall loss that never shows on the outer surface. Coupon and electrochemical probes placed in representative crevice geometries give early warning where opening a joint is impractical. Trending these results over successive outages tells you whether an alloy upgrade or a chemistry change is actually working.

Frequently Asked Questions

How is crevice corrosion different from pitting?

Both are localised chloride attacks on passive alloys, but pitting starts on an open, fully exposed surface, while crevice corrosion starts inside a shielded gap that creates its own acidic, high-chloride micro-environment. For the same alloy, the crevice form initiates at lower temperature and lower chloride because the geometry concentrates the aggressive species.

Which materials resist crevice corrosion best?

Resistance rises with molybdenum and nitrogen content, tracked by the PREN index. Duplex 2205, super-austenitic 254 SMO and nickel alloys such as C-276 tolerate far harsher chloride and temperature than 304 or 316L before a crevice activates.

Can a coating stop crevice corrosion?

A sound, continuous coating that keeps electrolyte out of the gap helps, but a damaged, blistered or poorly applied coating can create a new crevice and make things worse. Sealing the joint and choosing the right alloy are more reliable than relying on coating alone.

Where should inspection start?

Begin at the highest-risk geometries: gasket seats, threaded joints, fastener contacts, tube-to-tubesheet joints and any surface carrying deposits. These sites fail first, so a condition-based routine focused on them gives the best return. Book a Fabrico demo to see how recurring inspections are scheduled and tracked: Book a Fabrico demo.