大気腐食環境におけるチタンの変色挙動に及ぼす表面仕上げ状態と環境因子の影響

抄録

In order to know the discoloration mechanism of Ti in atmospheric environments, the influence of surface finish and environmental factor on the growth of surface films on Ti has been examined. Commercially available Ti plates with different surface finishes, a bright-anneal (BA) and an acid-etching (2B) finish, were used as specimens. A significant amount of C (15-30at%) was found as TiC in the surface layer of the BA plates. At potentials higher than 1.5V (vs. Ag/AgCl (3.33kmol·m^-3 KCl)) in neutral solutions, the growth rate of anodic oxide films on the BA plates was higher than that on the 2B plates. The high growth rate measured on the BA plates is ascribed to the fast oxidation of TiC precipitates in the surface layer of the plates. Under the illumination of UV light, a localized oxide growth was induced. The oxide growth was thought to be partly due to the formation of H₂O₂ by the photo-oxidation of water. In fact, the rapid growth of surface films by a dissolution-precipitation mechanism was observed on Ti and TiC exposed to a neutral solution containing H₂O₂. The film growth rate on the BA plates was much higher than that on the 2B plates in the solution. This is because TiC is easy to react with H₂O₂ to form soluble peroxo-titanium complexes. The presence of Cl^- ions in a solution accelerates the initial deposition of surface films stemming from hydrolysis of the soluble titanium complexes. It has been presumed that the discoloration of Ti is accelerated when Ti containing TiC is exposed to atmospheric corrosion environments including Cl^- ions under strong illumination of sunlight.