Environmental and metallurgical factors affecting discoloration of titanium sheets in atmospheric environments

摘要

Titanium sheets have been applied for roofs, walls and monuments for more than 10 years even in seaside where stainless steels may suffer pitting. There have been no localized corrosion problems such as pitting or crevice corrosion reported so far, however, discoloration has occurred in some cases. Discoloration of titanium is caused by growth of titanium oxide films on the surface causing interferential colors depending on thickness of the films. Therefore, it does not deteriorate durability of titanium sheets for exterior materials, however it might spoil outward appearance. As an application of titanium sheets as exteriors has begun not so long ago, the mechanism of discoloration of titanium sheets in atmospheric conditions has not been clarified yet. In this study, environmental and metallurgical factors on discoloration of titanium were investigated. Concerning environmental factors, air-born salts and UV radiation did not enhance discoloration, whereas acid rains of pH below 4.5 were found to cause discoloration of titanium. On the basis of experimental results above, we have developed an acceleration test for discoloration, immersion test in artificial acid rain. For metallurgical factors, impurity elements, such as oxygen, iron and nitrogen, did not affect discoloration of titanium in discoloration test. It was revealed that formation of TiC on titanium surface drastically enhance discoloration of titanium. It is considered that dissolved titanium ions from TiC by acid rain would form titanium oxides precipitated on the surface causing discoloration of titanium. Based on the mechanism of discoloration of titanium, we have developed titanium sheets with reducing TiC precipitation, which have shown highly resistance to discoloration.