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

在1995年至1996年在科学研究资助计划(B-2,07555665项目)下进行的先前的研究工作中，已经开发了一种用于高温熔盐环境的应变电极系统，但如何防止熔盐通过试件(SUS304板)和坩埚(氧化铝)之间的缝隙泄漏仍然是一个严重的问题。在本研究工作中，使用了一根SUS304不锈钢棒作为试件，并使用了与试件相同的材料制成的坩埚。将试件和坩埚拧在一起，然后用氧化铝水泥封住螺丝部分。将新型应变电极系统应用于300℃下SUS304不锈钢在熔融硝酸盐中的钝化击穿和再钝化研究，得到了以下结果：1.在弹性区，没有观察到电流对外加应变的响应，这表明氧化膜没有被弹性变形击穿。塑性变形提供了应变电流，这是由于在表面形成滑移台阶而导致钝化膜的破裂。当应变递增2%时，用于新形成表面钝化的电荷量随着应变的增加而减少。大的塑性变形增加了单位长度的滑移数目，从而减小了滑移台阶之间的距离，从而导致新形成的表面积减小。

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.