Effects of Acidity on Passive Film-Breakdown and Repassivation Processes in Alloy-Molten Salt Interface

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In the previous research work which was carried out under Grant-in-Aid for Science Research (B-2, Project # 07555665) in 1995-1996, a strain electrode system for use in high temperature molten salt environments had been developed.However, the serious problem of how to prevent the leakage of molten salts through gap between the specimen (SUS304 plate) and crucible (alumina) was still left. In the present research work, a rod of SUS304 stainless steel was used as the specimen and the crucible which was made of the same materials as the specimen was utilized. The specimen and crucible were screwed together and then the screw part was sealed with alumina cement. By this method, the leakage problem was almost solved.The new strain electrode system was applied to investigation of the passive film breakdown and repassivation of SUS304 stainless steel in molten nitrates at 300 C. The following results were drawn ;1. In the elastic region, the current response to the applied strain was not observed, which indicates that the oxide film is not broken down by the elastic deformation.2. Plastic deformation provided the strain current due to the formation of slip steps on the surface which leads to the breakdown of passive films.3. When the strain was increased stepwisely by 2%, amount of electric charge, which was used for the passivation of newly formed surface, decreased with increasing the strain. Large plastic deformation increases the number of the slips teps per unit length, as a result, decreases the distance between the slip steps, which leads to reduction of newly-formed surface area.