高濃度硫酸中におけるステンレス鋼の腐食機構解析

抄録

The mechanism of corrosion of stainless steels in highly concentrated sulfuric acid was studied by the electrochemical method and surface analysis as EPMA and Laser-Raman Spectroscopy. Cyclic potential changes occured in this system about between -0.2V (SCE) (active state) and 0.2V (SCE) (passive state). The anodic reaction was determined as the metal dissolution in both the active and the passive state. The cathodic reaction in the active state near -0.2V (SCE) was mainly dominated by the reduction of hydrogen ion which was clearly indicated by the evolution of hydrogen gas. On the other hand, catholic reaction at the passive state about 0.2V (SCE) was determined as the reduction of molecular sulfuric acid resulting in the formation of sulfur and water. By the analysis of EPMA, precipitated sulfur was found on the specimen surface polarized at 0.2V (SCE) potentiostatically. The in-situ observation of corroding surface by Laser-Raman spectroscopy indicated the formation of precipitated sulfur on the metal surface at the corrosion potential. Cyclic changes in corrosion potentials were considered due to the following mechanisms; dissolved metal ion at the active state formed metal sulfate films on the metal surface, leading the corrosion potential to the passive state. Inversely at the passive state, water as the product of the cathodic reaction accelerated the dissolution of the metal sulfate film to move the corrosion potential to the active state.