Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environmental level. Such stagnant microenvironments tend to occur in crevices Process such as those formed under gaskets, washers, insulation material, fastener heads, surface deposits, disbonded scoating, threads, lap joints and clamps. Crevice corrosion is initiated by changes in local chemistry within the crevice:
a. Depletion of inhibitor in the crevice
b. Depletion of oxygen in the crevice
c. A shift to acid conditions in the crevice
d. Build-up of aggressive ion species (e.g. chloride) in the crevice
As oxygen diffusion into the crevice is restricted, a differential aeration cell tends to be set up between crevice (microenvironment) and the external surface (bulk environment). The chronology of the aggravating factors leading to a full blown crevice is illustrated here.
-Initially, oxygen content in the water occupying a crevice is equal to the level of soluble oxygen and is the same everywhere.
The cathodic oxygen reduction reaction cannot be sustained in the crevice area, giving it an anodic character in the concentration cell. This anodic imbalance can lead to the creation of highly corrosive micro-environmental conditions in the crevice, conducive to further metal dissolution. This results in the formation of an acidic micro-environment, together with a high chloride ion concentration.
-Metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products.>
All forms of concentration cell corrosion can be very aggressive, and all result from environmental differences at the surface of a metal. Even the most benign atmospheric environments can become extremely aggressive as illustrated in this example of aircraft corrosion (courtesy Mike Dahlager). This advanced form of crevice corrosion is called 'pillowing'.
The most common form is oxygen differential cell corrosion. This occurs because moisture has a lower oxygen content when it lies in a crevice than when it lies on a surface. The lower oxygen content in the crevice forms an anode at the metal surface. The metal surface in contact with the portion of the moisture film exposed to air forms a cathode.
A special form of crevice in which the aggressive chemistry build-up occurs under a protective film that has been breached is called "filiform corrosion." Another important for of crevice corrosion occurs under insulation.