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

在1995-1996年科学研究资助（B-2，项目# 07555665）下进行的先前研究工作中，已经开发了用于高温熔盐环境的应变电极系统。然而，如何防止熔盐通过试样（SUS304板）与坩埚（氧化铝）之间的间隙泄漏，仍然是一个严重的问题。本研究采用SUS304不锈钢棒作为试样，采用与试样相同材料的坩埚。将试样与坩埚用螺钉固定在一起，螺钉部分用氧化铝水泥密封。该方法基本解决了泄漏问题。将新型应变电极系统应用于SUS304不锈钢在300℃硝酸液中钝化膜击穿和再钝化过程的研究，得到了以下结果：在弹性区，没有观察到外加应变的电流响应，这表明氧化膜没有被弹性变形破坏。塑性变形提供了应变电流，这是由于在表面上形成滑动台阶导致被动膜的击穿。当应变逐步增加2%时，用于新表面钝化的电荷量随应变的增加而减少。大的塑性变形增加了单位长度滑动阶数，导致滑动阶数之间的距离减小，从而导致新形成表面积的减少。

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.