Environmental and Metallurgical Factors Affecting Discoloration of Titanium Sheets in Atmospheric Environments

Effects of air bone salts, ultra violet (UV) irradiation, acid rain, impurity elements and TiC precipitated in surface layers of titanium on discoloration of titanium sheets were investigated to clarify the mechanism of discoloration of titanium in atmospheric environments. Air bone salts and UV irradiation did not enhance discoloration of titanium, but synthetic acid rain below pH 4.5 induced discoloration. The discoloration was largely enhanced by precipitation of TiC in surface layers of titanium sheets. It is considered that dissolved Tin+ from TiC by acid rain would deposit on titanium surface, inducing interferential color. Resistance to discoloration evaluated in accelerated discoloration tests was found to be remarkably improved by decreasing precipitation of TiC in surface layers of titanium sheets.