Titanium has been applied to roofs and walls of buildings. It is known that titanium used as architectural material discolors in atmospheric environments. This discoloration is mainly caused by interference color due to the growth of oxide film on the surface of titanium. Authors have proposed a discoloration mechanism, in which TiC begins to dissolve by acid rain, turning into an oxide, and an oxide film grows. In this paper, the formation and reduction mechanism of TiC was examined on the surface of titanium cold rolled sheets finished with vacuum annealing. An atomic bond between Ti and C in cold rolled sheets was detected with XPS, and it was found that C is unremovable by degreasing process. A large amount of TiC is formed on titanium surface vacuum annealed at lower temperature than 600°C. In this case, a discoloration index is as large as 25 after a discoloration accelerating test, in which titanium sheet is immersed in sulfuric acid solution for several days. On the other hand, when TiC is reduced with raising the vacuum annealing temperature to 650°C, C on the surface area diffuses into the inside of the titanium sheet and the surface C concentration decreases. Consequently, the discoloration index also decreases to onethird of that processed with lower annealing temperature. In addition, this modified annealing process was confirmed to be effective uniformly in the commercial large sized cold rolled coils. The discoloration level is equivalent to that of the polished surface titanium.
