Crevice Corrosion

Crevice corrosion, more so than pitting, is the limiting condition which often prevents the use of conventional austenitic stainless steel in chloride environments. The attack usually occurs in small volumes of stagnant solution under gasket surfaces, lap joints, marine fouling, and solid deposits and in the crevices under bolt heads and the mating surfaces of male and

female threads. The mechanism involves oxygen depletion in the crevice followed by chloride ion concentration and increase in acidity (decrease in pH) within the crevice. In a neutral, pH 7, chloride solution service the liquid within a crevice may contain 3–10 times as much chloride as the bulk solution and have a pH of only 2–3.

Susceptibility to crevice corrosion increases rapidly with temperature. Molybdenum and nitrogen additions to nickel–chromium–iron alloys improve their resistance to crevice corrosion. Together with the use of appropriate materials, design practice to minimize crevices and maintenance procedures to keep surfaces clean are required to combat the problem.

The usual laboratory measure of resistance to crevice corrosion is the critical crevice corrosion temperature, or CCCT, which is the highest temperature at which an alloy resists crevice corrosion in a given environment. For a given environment the CCCT is usually significantly lower than the CPT. Crevice corrosion resistance as measured by the ferric chloride test relates, to a degree, to performance in seawater. The results for a number of
alloys2 given the temperature for initiation of crevice corrosion in 10% ferric chloride (FeCl3 6H2O) per ASTM G 48 Practice B and pitting reseistance equivalent (PRE) N Cr 3.3% Mo 30(%N) are as follows:

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STAINLESS STEELS
James Kelly
Rochester, Michigan

Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
Copyright  2006 by John Wiley & Sons, Inc.

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