Crevice corrosion is a form of localised corrosion and occurs under the same conditions as pitting, i.e. in neutral or acidic chloride solutions.
However, attack starts more easily in a narrow crevice than on an unshielded surface. Crevices, such as those found at flange joints or at threaded connections, are thus often the most critical sites for corrosion.
Any equipment likely to be exposed to an environment containing chlorides should be designed with as few crevices as possible.
In narrow crevices, capillary forces make liquid penetrate into the crevice. Oxygen and other oxidants are consumed for the maintenance of the passive layer in the crevice just as on the unshielded surface. However, in the stagnant solution inside the crevice, the supply of new oxidant is restricted, causing a weakened passive layer.
Small amounts of dissolved metal ions inside the crevice cause a decrease of the solution pH and the presence of chlorides facilitates the break-down of the passive layer. Thus the environment inside the crevice gradually becomes more aggressive and repassivation becomes less likely. As a result, crevice corrosion attacks often propagate at a high rate, thereby causing corrosion failure in a short time.
A higher chromium, molybdenum and nitrogen content in the steel increases the resistance to crevice corrosion. The relative resistance to crevice corrosion is often measured as a critical crevice corrosion temperature, CCT, in a selected media. The stainless steel grades 254 SMO, SAF 2507, and especially 654 SMO, have very high resistance to crevice corrosion.