The initiation of crevice corrosion in stainless steels

There are currently at least four different models for the initiation of crevice corrosion on stainless steels: (1) passive dissolution leading to gradual acidification and general breakdown; (2) inclusion dissolution causing thiosulphate accumulation and assisting breakdown; (3) IR drop within the crevice forcing the metal into the active slate; 4) stabilisation of metastable pitting by the occluded crevice geometry. A technique using two coupled electrodes was used to measure crevice corrosion initiation potentials and induction times for 316L Stainless Steel under open circuit conditions in 1 M NaCl with sodium hypochlorite added as an oxidant. The ability of each model to explain the results was tested and the metastable pitting model was found to be the most suitable for these conditions. If metastable pitting occurs equally both inside and outside the crevice area, and any pit within the crevice has a probability (0..1) of initiating crevice corrosion, then the induction time is simply the time lapse before random initiation of a pit at a favourable site within the crevice. The role of metastable pitting can also be tested using temperature as a variable: the critical crevice corrosion temperature (CCT) was measured for 904L stainless steel and shown to coincide approximately with a steep increase in the metastable pitting frequency following potential steps at constant temperatures. (C) 1997 Elsevier Science Ltd.