貴金属（Pd, Ru）添加クロム基合金の硫酸中における腐食

Abstract

Cr exhibited much better corrosion resistance to H2SO4 and HCl than those of Fe-Cr alloys and Ti at low temperature and low concentration. Because a wide passive potential range with a low passive current was observed for Cr in H2SO4 and HCl solutions. However Cr was severely corroded in high temperature or concentrated conditions. The corrosion resistance of Cr was improved by adding noble metals such as Pd and Ru.The corrosion behavior of Cr-Pd and Cr-Ru alloys in H2SO4 solutions was investigated by measurement of the polarization curves, the change in corrosion potential with time and the average corrosion rate. And then, surface filmes of Cr-based alloys immersed in H2SO4 have been studied by means of AES and ESCA.The corrosion resistance of Cr-based alloys in H2SO4 was improved by adding noble metals such as Pd and Ru. The addition of Pd and Ru to Cr increased the cathodic current, and decreased the anodic dissolution current in the active state. Accordingly the corrosion potential of the alloys shifted to the noble direction and the alloys were easily passivated. And the addition of Ru is more effective than that of Pd at a high temperature up to 400 K.The composition of the surface films formed on Cr-Ru and Cr-Pd immersed in H2SO4 were Cr oxide such as Cr2O3. Ru and Pd were concentrated in these films. SO2−4 existed in the surface oxide films. Pd in the surface oxide films exist in the form of Pd0, and Ru in the form of Ru2+. The effect of the Pd addition on the corrosion behaviour of Cr is caused mainly by cathodic depolarization, while the effect of the Ru addition is caused by cathodic depolarization and improvement of the passive films on Cr.