Mechanism of Dissimilar Metal Crevice Corrosion of Superferritic Stainless Steels

Superferritic stainless steels (SS) can suffer from dissimilar metal crevice (DMC) corrosion when part of a crevice with certain other stainless alloys. For example, UNS S44735 is attacked when part of a crevice with type 316 SS (UNS S31600) but not when part of a crevice with type 304 SS (UNS S30400). To understand this phenomenon better, solutions which form inside such crevices were collected and analyzed with ion chromatography and capillary electrophoresis for their metal ion content. These analyses provided the data required to design bulk solutions for electrochemical kinetics measurements. Electrochemical measurements in these simulated crevice solutions were compared to measurements made in simple hydrochloric acid (HCl) solutions. The simulated crevice solutions were much more aggressive toward the materials than the simple HCl solutions. The superferritic SS was depassivated more easily in the simulated crevice solutions and exhibited a much larger active-to-passive transition in these solutions. In HCl solutions of the same pH, the superferritic SS passivated quite easily and could be depassivated only with great difficulty. These results were used to rationalize the observations of DMC corrosion of UNS S44735 when part of a crevice with type 316 SS and the failure of electrochemical measurements in simple HCl solutions to predict this attack. The origin of the specificity of the alloys that cause DMC corrosion of UNS S44735 was explained in terms of the effect of alloyed molybdenum on electrode kinetics of these alloys in the critical crevice solution. The roles of ohmic drop and chloride ion (Cl^–) concentration on the initiation and propagation of DMC corrosion also were considered.