For a given crevice type, two factors are important in the initiation of crevice corrosion: the chemical composition of the electrolyte in the crevice and the electrical potential drop into the crevice. Researchers had previously claimed that either one or the other of the two factors was responsible for initiating crevice corrosion, but recently it has been shown that it is a combination of the two that causes active crevice corrosion. Both the drop of potential and the change in composition of the crevice electrolyte are produced by the oxygen depletion of the solution inside the crevice (oxygen consumption caused by the metal oxidation at the inner surface of the occluded cavity) and the separation of electroactive areas, with net anodic reactions (oxidation) occurring within the crevice and net cathodic reactions (reduction) occurring at the exterior of the crevice (on the bold surface). The ratio of the surface areas between the cathodic and anodic region is significant. Some of the phenomena occurring within the crevice may be somewhat reminiscent of galvanic corrosion: galvanic corrosion two connected metals + single environment crevice corrosion one metal part + two connected environments The mechanism of crevice corrosion can be (but is not always) similar to that of pitting corrosion. However, there are sufficient differences to warrant a separate treatment. For example, in crevice corrosion, one has to consider the geometry of the crevice and the nature of the concentration process leading to the development of the differential local chemistry. The extreme and often unexpected local chemistry conditions inside the crevice need to be considered. Galvanic effects can play a role in crevice degradation. Mode of attack Depending on the environment developed in the crevice and the nature of the metal, the crevice corrosion can take a form of: pitting (i.e., formation of pits), but note pitting and crevice corrosion are not the same phenomenon, filiform corrosion (this type of crevice corrosion that may occur on a metallic surface underneath an organic coating), intergranular attack, or, stress corrosion cracking. Stress corrosion cracking A common form of crevice failure occurs due to stress corrosion cracking, where a crack or cracks develop from the base of the crevice where the stress concentration is greatest. This was the root cause of the fall of the Silver Bridge over the Ohio River, in 1967 in West Virginia, where a single critical crack only about 3 mm long suddenly