Crevice Corrosion in Stainless Steel & Nickel Alloys

Key Takeaways on Crevice Corrosion

Localised corrosion of this kind stays confined to occluded gaps; the open surface around it often stays bright and passive.

It starts below the pitting threshold, at a lower critical temperature and chloride level. That's why the crevice, not the open surface, sets the grade requirement.

What drives it is a differential-aeration cell. Oxygen depletes inside the gap. The pH drops through hydrolysis. Chloride then migrates in and breaks down the passive film.

Resistance tracks PREN (pitting resistance equivalent). Higher molybdenum and nitrogen raise the bar, which is why the grade ladder climbs from 304 to 316/316L, then to 904L, and finally to Incoloy 825 or duplex.

• It is defeated by two moves together: step up to a higher-PREN grade and design the crevices out.

What Is Crevice Corrosion?

Crevice corrosion hits a passive metal in a shielded, stagnant gap, an occluded spot like the space under a gasket, bolt head, washer, lap joint, deposit or heat-exchanger tube-sheet roll. The electrolyte trapped there can't mix freely with the bulk solution. That isolation lets a self-sustaining corrosion cell take hold, something the open surface nearby never has to deal with.

It belongs to the same family of localised attacks as pitting corrosion, chloride stress-corrosion cracking and galvanic corrosion, see the full map of types of corrosion. Unlike general (uniform) corrosion, which thins a surface evenly, crevice corrosion is hidden and geometry-driven: the bulk metal loss can be negligible while a perforation grows unseen inside the gap.

Its closest relative is pitting. Both come from chloride attacking the passive film, but pitting starts at a flaw on an open surface. Crevice corrosion is different: the geometry itself seeds the attack. That's why it shows up earlier, under milder conditions, on the exact same alloy.

How Crevice Corrosion Works: The Mechanism

The occluded geometry and differential-aeration cell

Inside a tight gap, the electrolyte just sits there. Whatever dissolved oxygen gets trapped inside is consumed by the passive film and never replaced, since diffusion through a narrow crevice happens too slowly. The metal inside the gap runs out of oxygen while the surface outside, exposed to free air, stays rich in it. That imbalance sets up a differential-aeration cell. The occluded metal becomes anodic, the open surface cathodic, and metal dissolution concentrates right there in the crevice.

Acidification and chloride migration

As metal ions dissolve inside the anodic gap, they hydrolyse in water and release hydrogen ions, so the pH falls sharply. The occluded solution can turn strongly acidic. To balance that positive charge, negatively charged chloride ions migrate into the crevice from the bulk solution. Low pH and concentrated chloride together are exactly what the passive film can't survive. It breaks down locally, and the attack becomes self-accelerating, or autocatalytic.

Crevice vs pitting: why it starts earlier

Because crevice geometry pre-concentrates the aggressive chemistry instead of forcing it to nucleate on an open surface, crevice corrosion kicks off at a lower temperature and a lower chloride level than pitting does on the same grade. That's the reason the critical crevice temperature (CCT) sits below the critical pitting temperature (CPT) for any given alloy. A designer has to qualify a grade against the crevice condition, not the open-surface one.

Factors That Influence Crevice Corrosion

Crevice risk climbs as gaps tighten, chloride goes up, temperature rises and pH drops. The table below lines up each factor with its effect and the practical fix for it.

Which Grades Resist Crevice Corrosion: The PREN Ladder

How PREN predicts crevice resistance

The pitting resistance equivalent number grades an alloy's ability to withstand localized chloride attack, based purely on its composition.

PREN = Cr + 3.3 × Mo + 16 × N

A higher PREN pushes up the critical temperature where both pitting and crevice attack kick in. Molybdenum and nitrogen are doing most of the work here, which explains why super-austenitic and duplex grades sit higher on the scale. Take 316L as an example: roughly 17% Cr, 2.1% Mo, and 0.05% N, giving a PREN near 24. 904L, with about 20% Cr, 4.3% Mo, and 0.05% N, reaches around 34. For the element-by-element breakdown of how that number gets built, see how PREN is calculated and the discussion of molybdenum and nitrogen. One thing worth keeping in mind: crevice attack sets in at a lower critical temperature than pitting does. Treat PREN as a ranking tool, not a guarantee, and check the grade against the actual crevice condition it will face.

304 and 316/316L: where they stop

304/304L has no molybdenum, and its PREN sits near 18. Chlorides find it fast, driving pitting and crevice attack, and it's prone to chloride stress-corrosion cracking as well. Adding 2–3% molybdenum lifts 316/316L to a PREN around 24, a step 304 simply doesn't have, and that's enough to cover mild chloride exposure and marine-adjacent service. Even so, 316 isn't immune to chloride-SCC, and its crevice margin runs out in hot, concentrated chloride. Both are austenitic stainless steels supplied under ASTM A240 / ASME SA-240.

904L: the super-austenitic step-up

904L super-austenitic stainless (UNS N08904) carries ~25% nickel, ~4.5% molybdenum and ~1.5% copper, giving a PREN near 34. The extra molybdenum raises crevice and pitting resistance well above 316L, the high nickel lifts the chloride-SCC threshold so it outperforms 316L there, and the copper adds sulfuric-acid resistance. For the full picture of what 904L is, see the grade guide and the super-austenitic stainless steel family page.

Incoloy 825, duplex 2205 and super-duplex: for the hardest crevices

Incoloy 825 (UNS N08825) is a titanium-stabilized Ni-Fe-Cr alloy with 38–46% nickel, 2.5–3.5% molybdenum and 1.5–3% copper; it resists both reducing and oxidizing acids and is qualified for sour service to NACE MR0175 / ISO 15156, which is why it is chosen over 316L/904L in mixed-acid and sour-gas crevices, see Incoloy 825 explained. It is supplied under ASTM B424 / ASME SB-424. Where high strength or the most aggressive seawater crevices govern, duplex and super-duplex grades (2205 at PREN ~35, 2507 at ~43) are the catalog step beyond.

How to Test and Measure Crevice Resistance

Crevice resistance gets measured by the critical crevice temperature, or CCT: the lowest temperature at which crevice attack starts under a standard crevice former in a defined chloride solution. ASTM G48 is the reference method here. Method B covers the ferric-chloride crevice test, while Method D establishes the critical crevice temperature; both rely on a serrated crevice washer fitted to the coupon. CCT sits below the critical pitting temperature, which makes it the governing acceptance figure for occluded-geometry service.

On supplied material, resistance isn't just claimed, it's documented. The composition listed on the mill certificate confirms the PREN, and intergranular-corrosion testing per ASTM A262 confirms the alloy is solution-annealed, not sensitized. For the full test menu, see testing, MTC & PMI; for certificate levels, see EN 10204 3.1 mill test certificate. Every order ships with an EN 10204 3.1/3.2 mill test certificate.

How to Prevent Crevice Corrosion

Prevention works best when design and materials work together. Here's the priority order:

1. Design the crevices out. Use full-penetration butt welds instead of lap joints, continuous welds instead of spot welds, and seal or weld-back gasket faces so no stagnant gap remains.
2. Pair gasket and seal materials correctly. Avoid absorbent or chloride-holding gaskets. Choose non-wicking materials and the right surface finish so the joint doesn't trap electrolyte.
3. Step up to a higher-PREN grade. Once the crevice condition exceeds the grade's critical crevice temperature, move up the ladder, 316L → 904L → Incoloy 825 / duplex, chosen against the crevice, not the open surface.
4. Control cleanliness and deposits. Keep flow steady, eliminate dead legs, and clear out scale and settled solids that create under-deposit crevices.
5. Passivate and finish correctly. A smooth, passivated surface restores a uniform passive film and sheds electrolyte. See how to passivate. For the selection step, use choosing the right grade and the grade selector tool.

Where Crevice Corrosion Occurs: Real Examples by Application

Flange and gasket faces are the textbook case. Chlorides work their way under the gasket, and a 316 joint that looks perfectly sound on the pipe bore will perforate right at the seal. That failure mode is what pushes buyers to specify a higher grade.

Heat-exchanger tube sheets have their own weak spot: the rolled tube-to-tube-sheet gap. That gap is a built-in crevice, and it's what dictates grade choice in pressure vessels and heat exchangers.

Marine and offshore fasteners face a similar problem. Bolt, nut and washer contact points sitting in seawater are prime crevice sites. For that reason, marine and offshore hardware tends to sit high on the PREN ladder.

Then there's under-deposit attack in acid service. Settled solids and scale form occluded cells in chemical and acid processing and oil and gas streams, where reducing and oxidizing media end up combined.

Selecting a Crevice-Resistant Grade with Nandini

Specify the alloy against the tightest crevice in the design, not the open surface. That's the rule of thumb that prevents failures. Prove the choice with composition that delivers the PREN, and build in a critical-crevice-temperature margin over the service condition. Stepping up a grade almost always costs less than a perforated joint discovered in service.

Grades we stock for crevice service

Founded in 2003, Nandini Steel carries ISO 9001:2015 certification as both a stockist and supplier. We hold genuine mill plate from Aperam, Outokumpu, Jindal, POSCO, Nippon Steel and JFE. Our certified scope names 904L, Alloy 28, Incoloy 800 and Hastelloy specifically. Across the crevice-resistant ladder we carry, stock sits ex-stock and ready for immediate dispatch.

• 316L in sheet and plate, mild chloride service, A240 / SA-240.
• 904L coil, plus 904L plate and 904L sheet, the super-austenitic step-up for hot chlorides and acid.
• Incoloy 825 in coil, plate and sheet, mixed-acid and sour service, B424 / SB-424.

Every order ships with an EN 10204 3.1/3.2 mill test certificate. Pricing is quoted live on request, send an RFQ through contact with your medium, temperature, chloride level and crevice geometry, and we'll confirm the grade and stock.

Frequently Asked Questions