Crevice corrosion occurs where the flow of the working fluid is restricted. The restriction creates a difference in conditions between the crevice and the bulk metal surface, which sets up an electrochemical cell.
Inside the crevice the working fluid cannot be replenished, so after a small amount of local corrosion and re-passivation the trapped fluid becomes de-aerated. As the concentration of aggressive species such as chlorides builds up within the crevice, the corrosion rate accelerates.
Where crevices form
The most common crevices are the very small gaps and contact points around fasteners, joints and washers, where the fluid can wet the part but flow is limited or negligible. Another classic form is a sharp or re-entrant corner, where the flow of working fluid is severely restricted.
Non-uniform conditions similar to crevice corrosion can also develop underneath surface deposits and debris, although this may be more correctly classified as poultice corrosion.