Crevice Corrosion: What It Is, Examples and How to Prevent It

Crevice corrosion is localized corrosion that attacks metal inside a narrow gap, such as under a gasket, washer or bolt head, between overlapping plates, or beneath a deposit. The liquid trapped in the gap loses its oxygen, turns acidic and concentrates chlorides, so the metal inside corrodes while the open surface next to it stays sound. It is most damaging to stainless steel, aluminum and other alloys that depend on a passive film, and because it is hidden inside a joint, it is often found only when the joint leaks or is taken apart.

Crevice corrosion is one of the classic forms of corrosion, and like pitting it is a type of localized corrosion. Below we explain how it works, where it occurs, which metals are most at risk, how to detect it, how it differs from pitting, and how to prevent it.

How Crevice Corrosion Works#

A dangerous crevice, metal-to-metal or metal-to-nonmetal, is wide enough to let liquid in but too narrow for it to circulate, often a fraction of a millimeter. The attack develops in stages:

1. Oxygen depletion. Inside the crevice, corrosion uses up the dissolved oxygen, and it can’t be replaced through the narrow gap. The open surface outside becomes the cathode and the metal inside becomes the anode.
2. Acidification. Metal ions dissolving in the crevice react with water and release hydrogen ions, so the trapped solution turns acidic even when the bulk water is near neutral.
3. Chloride migration. The positive metal ions draw negative ions in from outside to balance the charge. In most waters those are mainly chlorides, so the crevice fills with a concentrated, acidic chloride solution.
4. Breakdown and propagation. Once the chemistry is aggressive enough, the passive film fails and the metal dissolves rapidly. The process feeds itself, and the large open surface outside keeps supplying the cathodic reaction.

Tighter crevices, more chloride, higher temperature and a larger open surface around the crevice all speed it up. So does chlorinating seawater, which raises the corrosion potential of stainless steels.

Crevice Corrosion Examples: Where It Occurs#

• Gaskets and flanges. The gap under a gasket is one of the most common sites, especially on stainless steel in seawater or other chloride service. Fiber gaskets that wick water make it worse.
• Lap joints and supports. Skip-welded, riveted and bolted lap joints, back-to-back angles, and pipes resting on flat supports trap moisture. On carbon steel, the rust takes up several times the volume of the steel it replaces, so this pack rust pries plates apart and can break fasteners.
• Fasteners, washers and threads. Stainless fasteners in marine service often corrode under the head or washer while the exposed shank looks fine.
• Under deposits. Sand, scale, biofilm and fouling shield the metal and can host bacteria, so this attack often overlaps with microbiologically influenced corrosion.
• Tube-to-tubesheet joints. In heat exchangers, an expanded tube can leave a narrow gap behind the expanded section, where chloride-bearing cooling water attacks stainless tubes and tubesheets.

A typical example is a 316 stainless steel flange in a seawater cooling line with a compressed fiber gasket. The joint looks clean from outside, but when it is opened, the face under the gasket is etched and pitted, sometimes with a leak path cut across it.

Coated surfaces have their own under-film variant; see what you need to know about filiform corrosion.

Which Metals Are Most Susceptible?#

The metals most at risk rely on a passive oxide film, which crevice chemistry breaks down:

• Stainless steels. Type 304 is readily attacked in chloride service. Type 316 is better but still suffers crevice corrosion in seawater and other chloride waters. Duplex 2205 resists it better, and super duplex and 6% molybdenum super-austenitic grades are widely used in seawater, though even these have temperature limits in tight crevices.
• Aluminum alloys in chloride-bearing water and wet joints.
• Titanium and nickel alloys, which are highly resistant but not immune in hot, concentrated chlorides.
• Carbon steel, which is not passive in most waters but still suffers crevice attack and pack rust.

Chromium, molybdenum and nitrogen largely set an alloy’s crevice resistance, so stainless grades are ranked by Pitting Resistance Equivalent Number (PREN). ASTM G48 includes ferric chloride crevice tests, among them methods for finding the critical crevice temperature, the lowest temperature at which crevice attack starts under the test conditions. ASTM G78 is a guide for crevice testing in seawater and other chloride waters. These results rank alloys; they are not service limits.

How to Detect Crevice Corrosion#

Warning signs include rust-colored streaks from flanges, bolt heads or lap joints on stainless steel; weeping gasketed joints; blistered coating along joint edges; bulging plates or popped bolts on steel structures; and white deposits at aluminum joints. Because the damage is inside the joint, finding it takes more than a walk-down:

• Inspection on disassembly. The most reliable check is to examine flange faces and contact surfaces whenever a joint is opened, and to put known crevice locations on every outage scope.
• Nondestructive testing. Ultrasonic testing and radiography can sometimes find metal loss at a flange or lap joint, though the geometry makes results harder to read. Heat exchanger tubes are screened with eddy current or other tube inspection methods.
• Crevice coupons. Coupons fitted with crevice formers show whether a cooling water or process system can start crevice attack before the equipment does.
• Laboratory examination. Sectioning a removed part and analyzing the crevice deposits confirms the mechanism.

Crevice Corrosion vs. Pitting Corrosion#

Crevice corrosion and pitting corrosion are closely related: once growing, both are driven by an acidic, chloride-rich solution in a small, shielded space. The differences:

• Location. Crevice corrosion needs a joint, gasket, fastener or deposit. Pitting can start anywhere on an open surface, often at an inclusion or defect.
• Initiation. Crevice corrosion starts because the geometry lets the trapped solution change. A pit creates its own shielded space after the film breaks down at a small spot.
• Conditions. For a given alloy, crevice corrosion typically starts at a lower temperature and chloride level than pitting, so an alloy that resists pitting in a service can still suffer crevice corrosion there.
• Appearance. Crevice attack follows the crevice, often as an etched band under a gasket or washer. Pitting appears as separate pits.

How to Prevent Crevice Corrosion#

In Design

• Use full-penetration butt welds instead of lap, riveted or bolted joints where you can, with continuous rather than skip welds.
• Seal crevices that can’t be eliminated with a durable sealant or seal weld. A sealant that cracks or pulls away can leave an even tighter crevice.
• Design equipment to drain completely, without pockets, dead legs or ledges.
• In heat exchangers, specify tube-to-tubesheet joints that minimize the crevice, such as expansion over nearly the full tubesheet thickness, or seal welding.

In Material Selection

• Choose alloys for crevice resistance, not just pitting or general corrosion resistance. For stagnant or warm seawater, 316 stainless steel is often not enough.
• Compare alloys with PREN and critical crevice temperature data, and consider corrosion testing of production material for critical components.
• Use fasteners at least as corrosion resistant as the parts they join.
• Use non-absorbent gaskets, such as PTFE-based types, instead of fiber gaskets.

In Maintenance and Operation

• Remove deposits and fouling on a schedule, and keep flow high enough that solids don’t settle.
• Avoid stagnant lay-ups: drain and dry systems, or flush them with fresh water, before long shutdowns. Hydrotest water left in stainless piping is a well-known cause of under-deposit and microbial attack.
• Control chlorides, temperature and residual chlorine where you can.
• Replace gaskets rather than reusing them, and keep coatings and sealants on structural lap joints in good repair.

What to Do When You Find Crevice Corrosion#

The damage can’t be reversed, but the attack can be stopped. If the failure could become an insurance claim or lawsuit, preserve and document the parts, gasket and deposits before cleaning anything. Then measure the depth of attack and remaining wall, and decide whether to return the part to service, repair it or replace it. Reassembling the joint with the same gasket and materials usually brings the attack back, so the fix has to remove the crevice or the conditions that made it aggressive.

Frequently Asked Questions#

Is crevice corrosion the same as galvanic corrosion?

No. In crevice corrosion, one metal corrodes inside the crevice because the solution there differs from the solution outside. Galvanic corrosion is driven by contact between two different metals. The two can act together where a dissimilar fastener also forms a crevice. To see which metal of a pair corrodes, try our galvanic corrosion chart.

Does cathodic protection prevent crevice corrosion?

It can reduce it on submerged and buried structures, but its current doesn’t reach far into a tight crevice, and it does nothing for crevices exposed to the atmosphere. It works alongside good design and materials, not in place of them.

How fast does crevice corrosion progress?

It varies widely. Attack often starts only after an incubation period that can last months, then grows quickly because the process feeds itself. Only inspection and monitoring show the rate in a given system.

How Dreiym Engineering Can Help#

Our corrosion engineers help owners and operators find crevice corrosion, identify what is driving it and recommend the design, material and maintenance changes that stop it, inside piping, vessels and heat exchangers as well as on external surfaces. For buried and submerged structures, we design, install, test and commission cathodic protection systems. When a crevice has already caused a leak or failure, our forensic corrosion engineers investigate the cause for owners, insurers and attorneys, including on consulting-only (non-testifying) retentions. Contact us for a free quote.