Corrosion Behavior inside Crevice of Stainless Steels under Cyclic Corrosion Test: Estimation of Corrosion Environment inside Crevice

摘要

Cyclic corrosion tests, consisting of NaCl solution spraying, drying, and wetting process, were conducted using a crevice-structured spot-weld specimen of SUS436L in order to clarify the relation between corrosion behavior and amount of chloride accumulated into the crevice. The main component of chlorides accumulated into the crevice was sodium chloride coming from sprayed NaCl solution, although far less amount of metallic chlorides was also contained. The chloride content in the crevice depended on the concentration of spraying NaCl solution. When the concentration of spraying NaCl solution was 0.86 kmol·m-3, the amount of chloride reached about 20 mmol·m-2 and the maximum corrosion depth in the crevice increased as cycle number increased. When the amount of chloride reached about 20 mmol·m-2, the total amount of iron and chromium ions reached about 0.5 mmol·m-2 in the crevice. It is considered that pH in the crevice reached under depassivation pH of SUS436L, when the total amount of iron and chromium ions were 0.5 mmol·m-2. It is necessary that chloride which contains specific amount of metallic chlorides accumulate into the crevice for corrosion to proceed in the depth direction.