抄録
The passivity of titanium, especially the passivity in boiling H2SO4, HCl, and HNO3, and oxide films formed on passivated surfaces are investigated. Titanium was naturally passivated in dilute H2SO4 (≤0.1 wt%) and HCl (≤0.2 wt%), and in HNO3 with any concentration up to concentrated acid. The passivated surfaces were covered with thin oxide films, and titanium attacked by H2SO4 and HCl with higher concentrations was covered with the titanium hydride layer.
Basic electrochemical data, e.g., corrosion potential vs. time curves, anodic polarization curves and Flade potential vs. pH curves, were obtained in those acid solutions with various concentrations. In the reducing acid solutions, the polarization curves have a typical shape with the transition from the active to the passive region. The relations between Flade potential and pH of the solutions were expressed as Ef(h)=0.19∼0.20 pH for H2SO4 and Ef(h)=−0.05∼0.19 pH for HCl.
The structure and composition of oxide films formed on naturally and electrochemically passivated surfaces were studied by means of electron microscopy and diffraction. They were composed of thin continuous films derived from the anodic oxidation of metallic titanium and fine deposited oxide. Three modifications of TiO2, i.e., rutile, anatase, and brookite, were detected, and further a difference in oxide modification was found in different acids.
