Guide
Why does crevice corrosion start under gaskets and washers, and how do you design it out?
Crevice corrosion is localized attack that starts inside a narrow, shielded gap, such as the space under a gasket, a washer, a fastener head or between two lapped plates, where trapped water loses its oxygen, collects chlorides and turns acidic until it breaks down the passive film on stainless steel or aluminum. The open surface next to it can stay bright while the metal under the seal is eaten away, which is why the damage usually shows up only when the joint is taken apart.
By Saola Manufacturing engineering team · Published · Updated
This guide is for the engineer or buyer who has just found corrosion confined to a sealing face, a washer footprint or a lap joint and needs to know why, how to confirm it, and what to change on the drawing so the next lot does not repeat it. The short version: the crevice is the cause, the alloy only decides how tight a crevice it can survive, and most fixes are geometry and sealing choices that cost little if they are made before the part is machined.
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How do you recognize crevice corrosion and tell it apart from pitting or galvanic attack?
Crevice corrosion is confined to the shielded area: it follows the outline of the gasket, washer, O-ring or overlapping plate, while the exposed surface a few millimeters away often stays clean. Pitting appears on open surfaces, and galvanic attack needs two different metals in electrical contact in a common electrolyte.
Typical signs are a rust-brown or white ring that matches a washer footprint, etching or shallow trenches along the inner edge of a flange gasket, a stained line that bleeds out of a lap joint, and corrosion product packed into the threads under a nut while the bolt shank above it is bright. On stainless steel the stain is usually brown; on aluminum the corrosion product is white. Do not expect every metal to corrode in exactly the same place: SSINA notes that the location of metal loss, inside the crevice or just outside it, varies with the metal family, and ASTM G78 likewise notes that some crevice-related attack takes place immediately next to the crevice former rather than under it.
The distinction matters because the cures differ. The Specialty Steel Industry of North America (SSINA) notes that crevice attack can occur even when the rest of the exposed surface is not pitting, and that when two different metals form a tight crevice both crevice and galvanic corrosion can occur at once (SSINA, Pitting and Crevice Corrosion). If the joint is a single alloy, or stainless on plastic or rubber, the problem is crevice corrosion and a galvanic fix such as an isolating washer will not help; a soft washer can make the crevice tighter. If the joint couples aluminum to stainless, read the galvanic corrosion guide too, because both mechanisms will be running.
What causes crevice corrosion under gaskets, washers and lap joints?
A gap tight enough to let water in but too tight for it to circulate. Oxygen inside the gap is used up, chloride ions migrate in, the trapped liquid turns acidic, and the passive film inside the crevice breaks down while the open surface stays passive and acts as the cathode.
AMPP describes the starting conditions as depleted oxygen and inhibitor, a shift toward acidity and a build-up of aggressive ions such as chloride, with metal ions hydrolyzing and releasing protons that keep the crevice acidic (AMPP, Crevice Corrosion). Once started, the cell sustains itself. The factors that decide whether a given joint corrodes are:
- Crevice geometry. Narrow, deep gaps are the worst. The Australian Stainless Steel Development Association points out that metal against flexible plastic tends to make a narrower crevice than rigid metal against metal, so joints against plastic are the more aggressive crevices, and it names the space under a washer, between bolted plates and between a gasket and a flange (especially an absorbent gasket) as typical crevices (ASSDA, Crevices and corrosion).
- Chlorides. Coastal air, deicing salt, seawater, salty process water and chlorinated or bleach-based cleaners all supply the chloride that breaks down the film. SSINA lists higher chloride, higher temperature, lower pH and more noble potentials as the important environmental factors that favor localized attack, and ASSDA adds that strongly oxidizing liquids such as bleach, hydrogen peroxide or ozone make crevice attack more severe.
- Temperature. Every stainless grade has a critical crevice temperature below which crevice attack does not start in a given test solution. ASSDA gives a rule of thumb that above that temperature each 10 °C rise roughly doubles the corrosion rate (ASSDA).
- Alloy. Chromium, molybdenum and nitrogen raise resistance. SSINA gives the pitting resistance equivalent as PRE = %Cr + 3.3 %(Mo + 0.5 W) + x %N, where x is typically 16 or 30, and says it also gives a relative ranking of crevice resistance (SSINA), but warns that it reflects ideal conditions and ignores intermetallic phases, poor heat treatment and inferior surface condition.
- Absorbent or shrinking seal materials. An absorbent gasket holds liquid in the joint, and ASSDA warns that a sealant that does not wet the surface, or that dries and shrinks, forms its own crevice.
- Fabrication crevices. Intermittent (stitch) welds, incomplete root penetration, weld undercut and cracks, sticky labels, crayon markings and smears of grease or oil all create shielded spots before the part reaches service; ASSDA lists each of them.
The ASSDA table shows why grade alone rarely rescues a bad joint: in the same ferric chloride test, every grade starts crevice attack at a lower temperature than it starts pitting (ASSDA puts the gap at 20 °C or more in this test), and the common austenitic grades start crevice attack well below room temperature.
| Grade | Critical crevice temperature, °C | Critical pitting temperature, °C |
|---|---|---|
| 304 | below −10 | 3 |
| 316 | −10 | 10 |
| 904L | 12 | 42 |
| Duplex 2205 | 13 | 33 |
| Super duplex 2507 | 38 | 78 |
How do you diagnose crevice corrosion on a failed part or a rejected lot?
Take the joint apart, photograph the attack against the footprint of the seal or washer, and check four things: the gap, the liquid, the seal material and the alloy as delivered. If the attack maps exactly to the shielded area and the grade and surface are as drawn, the joint design and environment are the most likely root cause, not the supplier's material.
Work through this checklist before raising a nonconformance against the parts:
- Map the damage. Photograph the parts before cleaning and mark the seal or washer outline on the photo. Does the attack start at the inner edge of the gasket or the washer bore and stop where the seal ends? Is the open surface clean? A yes to both points to a crevice cell. Attack that covers open faces too points to pitting or general corrosion, which changes the fix.
- Identify the liquid. Find the chloride source: coastal or road-salt exposure, washdown chemicals, chlorinated water, sweat from handling, or residue from a cleaning agent that was left in the joint at assembly.
- Inspect the seal. Is the gasket absorbent? Is it bulging past the flange or bridging a gap instead of filling it? Did a sealant shrink or debond? Is a plastic washer cold-flowing into a tight ring?
- Confirm the alloy. Check the material certificate against the drawing grade, and run a positive material identification (PMI) check on the failed part if 304 may have been substituted for 316; molybdenum is the element that separates them. A wrong grade makes a marginal joint fail sooner, but the right grade in the same joint may only fail later.
- Look at the surface. Heat tint near welds, embedded iron from tooling, and rough or torn machined faces under the seal all lower resistance; the passivation guide covers how free iron is removed from stainless.
- Section if the stakes justify it. A metallographic section through the attack shows depth and shape, which tells you whether the joint can stay in service.
To qualify a replacement alloy or a new joint before it ships, use a recognized test rather than judgment. ASTM G48 has six ferric chloride methods: Method B is a crevice test at a set condition (Methods A and B are designed to corrode Type 304 at room temperature), and Method F finds the critical crevice temperature of a stainless steel (ASTM G48-25). ASTM G78 is the guide for crevice testing iron-base and nickel-base stainless alloys in seawater and other chloride solutions, using crevice formers such as the nonmetallic segmented washer called the multiple crevice assembly (ASTM G78-20(2025)); it is the better fit when you want to test your own gasket or washer material against the alloy in your own water. SSINA cautions that these laboratory values rank alloys and are not directly transferable to service conditions.
Which design changes stop crevice corrosion, and which one fits your joint?
Eliminate the crevice first, then seal it permanently, then open it wide enough to drain, and only then upgrade the alloy. SSINA notes that sealing or eliminating tight joints during design can often avoid crevice corrosion, and that welding is the preferred solution where parts are immersed or water stands.
The order matters because each step down the list depends more on assumptions about the service environment. A crevice that does not exist cannot corrode; a crevice that exists but is filled with a durable, non-absorbent material is the next best thing; a better alloy only raises the threshold at which an existing crevice starts. ASSDA adds that in seawater at ambient temperature crevice attack has been seen at gap widths of about 0.9 mm for 304, 0.4 mm for 316 and 0.15 mm for 904L (ASSDA), so even a fairly open gap is not automatically safe in chloride service.
| Joint where attack appears | Preferred fix | If that is not possible |
|---|---|---|
| Lap joint, stitch weld or overlapping plates | Full-penetration butt weld or continuous seal weld on both sides | Fill the overlap with a permanent, wetting sealant; never leave stitch gaps |
| Flange gasket face | Non-absorbent gasket compressed flush with the bore, with no overhang or step | Choose a grade resistant to crevice corrosion in the operating environment |
| Washer or fastener head | Fewer fasteners, or welded or bonded attachment in wet zones | Install fasteners wet with sealant; use a grade suited to the crevice, not just to the open surface |
| O-ring or seal groove | Keep the wetted clearance in front of the seal as short as the design allows, and do not let deposits settle in it | Higher-molybdenum or duplex grade for the grooved part |
| Threaded joint under a nut | Thread sealant or anti-seize that fills the thread crevice | Upgrade both fastener and part grade together |
| Deposits, sediment, dead legs | Slope and drain so solids and water cannot settle | Scheduled cleaning; maintain flow |
Three rules apply to every row. A sealant must wet the metal and stay bonded, because ASSDA warns that a sealant which does not wet, or one that dries and shrinks, forms its own crevice. Flexible inert washers and building sealants work on surfaces that shed water, but SSINA advises against them where water pools or parts are immersed. And on aluminum the same design logic applies: a coating does not make an absorbent gasket in a wet joint safe, so pair the finish with a non-absorbent seal and a drainage path.
What should the drawing, RFQ and PO say so crevices are not built in?
State the service environment, the exact alloy, the joint design and the seal or sealant by material, and forbid the fabrication habits that create crevices. A supplier machines what the drawing shows; it cannot guess that a lap joint will sit in salt spray.
- Environment line. For example, “SERVICE: OUTDOOR COASTAL, WASHDOWN WITH CHLORINATED CLEANER.” This one line tells everyone which joints are critical.
- Grade, not family. “316L” or “DUPLEX 2205 (UNS S32205)”, never “stainless”; add a positive material identification requirement if a substitution would be costly.
- Joint notes. “CONTINUOUS SEAL WELD BOTH SIDES, NO STITCH WELDS”; “INSTALL FASTENERS WET WITH SEALANT”; “GASKET: NON-ABSORBENT, COMPRESSED FLUSH WITH BORE.”
- Sealing faces. Call out the surface finish on gasket and O-ring faces and mark them as no-scratch, no-label zones; the surface roughness guide covers how to specify it.
- Cleanliness. “NO ADHESIVE LABELS, MARKER OR CRAYON ON PART; REMOVE OIL AND CHIPS FROM BLIND HOLES AND GROOVES BEFORE PACKING.”
- Verification. If crevice resistance is a requirement, name the test and the pass rule, for example a critical crevice temperature per ASTM G48 Method F for the material lot, and agree it in the quote.
How is a crevice-sensitive part handled on a Saola order?
Stainless and aluminum parts are machined by CNC turning and milling at our lead CNC plant in Vietnam, and any surface finish is applied at a partner plant we manage. The quote names the production site and country of origin for each step.
Machined bodies, flanges, seal grooves and threaded features run on CNC milling and CNC turning at T.K Viet Nam Precision, our lead CNC plant. When a drawing carries a crevice-sensitive joint, the engineering review flags joint notes that look ambiguous, such as a weld symbol that allows stitch welding or a gasket face without a finish callout, and asks before quoting rather than after. Anodizing, plating and other surface finishing are done at partner plants we manage, which may not be in Vietnam; the quote states where. Gaskets, O-rings and sealants are supplied to your specification or by you, and the material grade, the inspection plan and any corrosion test you name are agreed in writing. To start, send the drawing with the environment line and joint notes through the quote form.
Frequently asked
What is the difference between pitting corrosion and crevice corrosion?
Both are localized breakdowns of a passive film driven by chlorides. Pitting starts on an open surface at a local weak spot; crevice corrosion starts inside a shielded gap where the trapped liquid has changed chemistry. In ferric chloride tests crevice attack begins at lower temperatures than pitting for the same alloy, so a part that never pits on its open faces can still corrode under its gaskets.
Will switching from 304 to 316 stop crevice corrosion?
It raises the threshold but does not remove the cause. In ferric chloride ranking tests 316 still starts crevice attack below room temperature, so in warm chloride service a tight crevice in 316 can still corrode. Fix the joint first; move to a duplex or higher-molybdenum grade only where the crevice cannot be eliminated or sealed.
Does aluminum suffer crevice corrosion too?
Yes. Aluminum also depends on a thin oxide film, and a wet, chloride-bearing gap under a gasket, washer or lap joint can break it down in the same way. A finish does not cancel the joint design: an absorbent seal that keeps the joint wet is still a risk. Keep the joint draining and use non-absorbent seals.
Do plastic or rubber washers prevent crevice corrosion?
Not by themselves. They break galvanic contact, but a soft washer clamped on stainless makes a very tight crevice. They are acceptable on surfaces that shed water; in pooled water or immersion, weld, seal permanently or remove the joint instead.
Is crevice corrosion a supplier defect?
Usually not, if the parts match the drawing grade and finish. It is normally a joint-design and environment problem. The supplier side to check is a wrong or substituted grade, heat tint or embedded iron on wetted faces, and labels, marker or residue left in grooves.
Sources
- Crevices and corrosion, Australian Stainless Steel Development Association, accessed October 2026
- Pitting and Crevice Corrosion, Specialty Steel Industry of North America, accessed October 2026
- ASTM G48-25, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution (scope and significance), ASTM International, 2025
- ASTM G78-20(2025), Standard Guide for Crevice Corrosion Testing of Iron-Base and Nickel-Base Stainless Alloys in Seawater and Other Chloride-Containing Aqueous Environments (scope and significance), ASTM International, reapproved 2025
- Crevice Corrosion, AMPP (Association for Materials Protection and Performance), accessed October 2026