CREVICE CORROSION C064
Sometimes called concentrated cell corrosion, this corrosion occurs any time liquid flow is kept away from the attacked surface. It is common between nut and bolt surfaces, under O-rings and gaskets, and between the clamps and stainless steel shafts we find in many split seal applications.
Salt water applications are the most severe problem because of the salt water low pH (8.0 to 9.0) and high chloride content.
Here is the mechanism:
- Chlorides pit the passivated stainless steel surface (the ceramic).
- The low pH salt water attacks the active layer that is exposed below the pits.
- Because of the lack of fluid flow over the attacked surface, oxygen is not available to re-passivate the stainless steel.
- Corrosion continues unhampered under the rubber and tight fitting clamp.
- The inside of the O-ring groove experiences the same corrosion as the shaft or sleeve.
The solution to crevice corrosion problems with stainless steel, in salt water applications, is to coat the surfaces of the parts in contact with zinc oxide.
The zinc will act as the sacrificial anode