Crevice corrosion occurs in locations where the flow of the working fluid is restricted. This in turn leads to a difference in conditions between the crevice location and the bulk metal surface, initiating an electrochemical cell. For instance, in the crevice, the working fluid cannot be replenished, so that after a small amount of local corrosion and re-passivation the fluid in the crevice becomes de-aerated. The corrosion rate is accelerated as the concentration of aggressive species such as chlorides increases within the crevice.
The most common form of crevices are very small gaps and contact points around fasteners, joints and washers, where the fluid can wet the part but flow rate is limited or negligible. The other classical form of crevice is in a sharp or re-entrant corner, where the flowrate of the working fluid is severely restricted. However, non-uniform conditions akin to crevice corrosion can also be created underneath surface deposits and debris, although this may also be more correctly classified as poultice corrosion.
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