Modeling of the Factors Contributing to the Initiation and Propagation of the Crevice Corrosion of Alloy 625

Crevice corrosion of alloy 625 in both chlorinated and nonehlorinated Amer ican Society for Testing and Materials (ASTM) artificial ocean water was modeled. Secondary current and potential distributions as a function of crevice depth were obtained f rom the mode l using the following crevice initiation criteria (i) E < EFIaa~, (it) ia ~ = i0 ~A/cm 2, and (iii) pH-0.46 at the site where the first two criteria are met. For alloy 625 in nonchlorinated ASTM artificial ocean water these criteria are satisfied at depths wh ich are near the mouth of the crevice. These depths are incompatible wi th the mass-trans-port conditions governing the deve lopment of the critical crevice solution. In the chlorinated system the mode l predicts that initiation occurs at greater depths wh ich are compatible with all the criteria including mass-transport considerations. The mode l results are in agreement with remote crevice assembly exper iments wh ich show that crevice corrosion occurs more readily in the chlorinated ASTM artificial ocean water. Propagat ion of the crevice corrosion of alloy 625 is shown to be ohmical ly controlled by the resistance between the mouth of the crevice and the initiation site. Crevice corrosion of alloy 625 is the result of aggressive solution chemistries that are possible only in crevices char-acterized by a combinat ion of nar row gap and extreme depth. 1,2 Crevices with nar row gaps and extreme depth ar