Crevice corrosion occurs when two components are joined close together to form a crevice. Corrosion occurs as the crevice accumulates water.
If the crevice is small enough a differential oxygen concentration in the water can form. When this happens the base of the crevice becomes anodic to the upper region.
Crevice corrosion often occurs under bolts and rivet heads as well as in shielded areas and under dirt or sand deposits.
The implications of crevice corrosion were seen on April 28th, 1988, when Aloha Airlines flight 243 suffered extensive fuselage damage and was required to make an emergency landing.
The cause of the damage was found to have been metal fatigue that originated from a crack site caused by crevice corrosion.
Pitting corrosion is similar to crevice corrosion, where a localised change in the geometry of the metal allows for changes in water chemistry. This then creates small anodes which increase in size with further corrosion.
The initial pit may be due to a scratch or surface irregularity, but the formation of an anode at the base causes it to grow deeper.