Crevice corrosion

A crevice in a metal surface at a joint between two metallic surfaces or between a metallic and a nonmetallic surface or a crevice beneath a particle of solid matter on a metallic surface provides conditions that are conducive to the development of the type of concentration-cell corrosion called crevice corrosion. Crevice corrosion can progress very rapidly (tens to hundreds of times faster than the normal rate of generai corrosion in the same given solution). For example. a sheet of stainless steel can be cut (corroded) into two pieces simply by wrapping a rubber band around it, then immersing the sheet in seawater or dilute ferric chloride solution. The open surfaces will pit slowly, but the metal under the rubber band will be attacked rapidly for as long as the crevice between the rubber band and the steel surface exists.
In a metal-ion concentration cell, the accelerated corrosion occurs at the edge of or slightly outside of a crevice. In an oxygen-concentration cell, the accelerated corrosion usually occurs within the crevice between the mating surfaces.
Any layer of solid matter on the surface of a metal that offers the opportunity for exclusion of oxygen from the surface or for accumulation of metal ions beneath the deposit because of restricted diffusion is a probable site for crevice corrosion.
Differential aeration beneath solid deposits or at cracks in mill scale is a frequent cause of crevice corrosion in boilers and heat exchangers. 
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