What Does Crevice Corrosion Mean?
Crevice corrosion refers to the attack of metal surfaces by a stagnant solution in crevices, for example around the edges of nuts and rivet heads. When dust, sand and other corrosive substances are deposited on surfaces, they create an environment where water will accumulate and corrode the part. It can happen between two metals or between a metal and a nonmetal. This causes damage to the metallic part, which is initiated by the concentration gradient in chemicals.
Oxygen causes an electrochemical concentration cell outside the crevice. This is a differential aeration cell where the air present is oxygen. In the crevice (the cathode), the pH and the oxygen content increases. However, this is the opposite for the chlorides; they are lower.
Corrosionpedia Explains Crevice Corrosion
For chlorides, the electrochemical concentration is higher on the inside, which worsens the corrosion. When a ferrous metal is present, the ferrous ions react with the chlorides to form ferric chloride, which attacks stainless steel. This makes the concentration of both the oxygen and the pH remain lower in the crevice than the concentration in the water solution that forms on the metal. The propagation mechanism is similar to that of pitting corrosion.
There are factors that influence crevice corrosion. These include:
- The type of crevice: either metal to metal or metal to nonmetal
- The geometry of the crevice: including the size of the gap, its depth and the surface roughness
- The composition of the metal: the structure of the alloy composition and can be Cr, Mo, or others.
- The environment: the pH, halide ions, temperature and oxygen1
The resistance of a material to crevice corrosion can be ranked and evaluated by its critical crevice temperature (CCT), but this has to be in accordance with the ASTM Standard G48-03. CCT is the minimum temperature in °C that can produce a crevice attack, and is found to be lower than the critical pitting temperature (CPT).
How can crevice corrosion be prevented?
- Replace riveted joints with welded butt joints.
- Eliminate crevices in lap joints through continuous welding and soldering.
- Drain existing solutions on surfaces and avoid creating stagnant conditions.
- Use solid and non-absorbent gaskets.
- Use higher alloys.