The Role of Inclusions on Initiation of Crevice Corrosion of Stainless Steel II. Theoretical Studies

A mathemat ica l model for the in i t iat ion of crevice corrosion on 304 stainless teel in 0.1N NaC1 was developed. The central concept is that thiosul fate ions, formed by dissolut ion of MnS inc lus ions and t rapped wi th in the crevice, act in con-cert wi th chlor ide ions to cause passivity b reakdown when thei r concentrat ions exceed crit ical values. The model inc ludes cons iderat ion of migrat ion and di f fusion of the suppor t ing electrolyte, as well as ions produced by d isso lut ion of metal and MnS inclusions. Exper imenta l data reported separately in Part I were compared wi th theoret ical predict ions for the crit ical geometry at wh ich in i t iat ion occurred, and for the degree of cathodic protect ion required to prevent crevice corro-sion. A s impl i f ied model was developed to predict condi t ions under wh ich breakdown ould be expected to occur for var-ious cases of crevice geometry, potent ia l at the edge of the crevice, and MnS inc lus ion density. In Par t I of this invest igat ion (1), the role of sulf ide inclu-s ions in the in i t iat ion of crevice corrosion of stainless teel in 0.1N NaC1 was invest igated exper imental ly. In the pres-ent paper, Part II, these concepts are developed into a mathemat ica l model, wh ich is used to predict condi t ions under wh ich crevice corrosion is expected to init iate i