Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to, the gap or crevice between two joining surfaces. The gap or crevice can be formed between two metals or a metal and non-metallic material. Outside the gap or without the gap, both metals are resistant to corrosion. The damage caused by crevice corrosion is normally confined to one metal at localized area within or close to the joining surfaces. In this photo, a type 316 stainless steel tube and tube sheet from a heat exchanger in a seawater reverse osmosis (SWRO) desalination plant suffered crevice corrosion due to the presence of crevice (gap) between the tube and tube sheet.
What causes crevice corrosion? Crevice corrosion is initiated by a difference in concentration of some chemical constituents, usually oxygen, which set up an electrochemical concentration cell (differential aeration cell in the case of oxygen). Outside of the crevice (the cathode), the oxygen content and the pH are higher - but chlorides are lower. Chlorides concentrate inside the crevice (the anode), worsening the situation. Ferrous ions form ferric chloride and attack the stainless steel rapidly. The pH and the oxygen content are lower in the crevice than in the bulk water solution, just as they are inside a pit. The pH inside the crevice may be as low as 2 in a neutral solution. Once a crevice has formed, the propagation mechanism for crevice corrosion is the same as for pitting corrosion. The major factors influencing crevice corrosion are: * crevice type: metal-to-metal, metal-to-non-metal * crevice geometry: gap size, depth, surface roughness * material: alloy composition (e.g. Cr, Mo), structure * environment: pH, temperature, halide ions, oxygen A material's resistance to crevice corrosion is usually evaluated and ranked using the critical crevice temperature (CCT) in accordance with the ASTM Standard G48-03: Standard Test Methods for Pitting and Crevice...
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