Most technical persons in various industries are aware of the importance of the several forms of corrosion and their effects on the reliability of products or processes and thus the bottom line. Generally most people think about some of the well-known forms such as general corrosion, pitting or stress-corrosion cracking (SCC). However, crevice corrosion is often overlooked. This form of attack occurs when an area on a susceptible metal is partially, but not completely, closed off from full access to the liquid corrosive medium. Some typical crevice sites occur at the exposed, wetted edge of two plates of material (both don’t necessarily have to be metals) that are bolted together; under sand or debris on a wetted metal surface; at the gaps in a joint between two wetted metals that are tack welded together (rather than being joined with a continuous weld) or in the wetted interface spaces between the external threads on a bolt and the mating internal threads of a nut.

Crevice attack occurs rapidly in such localized confined spaces that are open sufficiently to let the corrosive liquid enter and start the corrosion process but are not open enough to let that corrosive liquid freely exit the space. After it starts the crevice corrosion process grows or propagates very much like pitting. For example crevice corrosion is self-accelerating – called autocatalytic – just like pitting. In addition the appearance of crevice corrosion inside the confined space is similar to a pitted surface. Crevice attack usually is more dangerous than pitting. Assuming otherwise equivalent conditions, it will initiate sooner and then continue penetrating the metal inside a crevice before attack occurs on a freely exposed surface where pits might form. This confined-area form of attack is also a major practical threat because it is often difficult to fabricate something without creating one of the many types of crevices in wetted areas. Of course being aware of the danger of crevices and then seeking to avoid them during design is a major way to minimize the problem.

If some potential crevices cannot be avoided there are actions that can be practical in certain cases. One is to close off the opening or “mouth” of the crevice with a suitable sealant or a continuous weld. Another approach is to provide for regular removal of sand, dirt or debris that may form crevices over time, e.g., by designing for complete drainage. If a crevice is unavoidable – assure that its opening is not small but is open enough to allow free interchange – in and out – by the corrosive liquid. Finally, selecting a more corrosion resistant metal can be an option. For example, crevice attack often occurs on commonly used austenitic stainless steels in a corrosive liquid containing chloride ions. In these applications picking another alloy with higher percentages of chromium, molybdenum and nitrogen in its composition provides the most resistance. Molybdenum is, by far, the most effective of these elements but also the most expensive

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