Crevice corrosion is localized corrosion which may occur in small areas of stagnant solution in crevices, joints and under corrosion deposits (i.e. under deposit corrosion).

Crevice Corrosion of Stainless Steel -

Crevice corrosion is the localized corrosive attack that occurs as a result of the occluded cell that forms under a crevice on the metal surface. On this stainless steel test coupon from an ASTM G48 Method B ferric chloride test, the crevice was a non-metallic block. Note that the test coupon was not attacked by corrosion except in the middle of the coupon where it was in contact with the block and on the edges were a rubber band used to hold the block in place also came in contact with the metal surface.

PREVENTION OR REMEDIAL ACTION

• redesign of equipment to eliminate crevices.
• close crevices with non-absorbent materials or incorporate a barrier to prevent of moisture penetration into crevice.
• prevent or remove builds-up of scale or other solids on surface of material.
• use of one-piece or welded construction versus bolting or riveting.
• select more corrosion resistant or inert alloy (note: see pitting corrosion for more information).

STANDARD TEST METHODS

• ASTM G-48 - test methods for pitting and crevice corrosion resistance of stainless steels and related alloys by the use of ferric chloride solution.
• ASTM G-78 - guide for crevice corrosion testing of iron-base and nickel-base stainless alloys in sea water and other chloride-containing aqueous media.

Evaluation of Crevice Corrosion

The principal reference for the evaluation of crevice corrosion is ASTM G78. The extent of crevice corrosion can be greatly influenced by the nature of the crevice and the technique utilized in the exposure test. Typically, tighter crevices promote greater localized corrosive attack. The use of serrated TFE or ceramic washers is one of the most common methods for obtaining reproducible simulation of crevice corrosion. These washers are bolted to the specimen using a corrosion resistant bolt with constant applied torque for each crevice washer assembly. In most cases, the rate of crevice attack in not constant. Initially, there is an incubation period where the attack rate is essentially zero. However, as the corrosivity of the crevice environment increases with exposure time, the local attack rate can actually increase with time in test. Therefore, multiple exposure periods may be needed to accurately assess crevice corrosion attack rates.