Towards a better preservation of current and future outdoor architectural heritage; maximum suppression of discolouration in anodized and non-anodized titanium sheets

Outdoor atmospheric corrosion has been exerting continuous deteriorating effects on the cultural heritage, both in the matter of structural resistance and aesthetic quality. To this point, titanium as a modern metal with excellent mechano-physical properties, especially high strength, light weight, superior corrosion resistance has already gained many attentions in different engineering and design fields. In addition to its high atmospheric corrosion resistance, titanium sheets ability to provide a wide range of tasteful colours with high aesthetic quality have paved their way towards the field of modern art and architecture, to the point that its use for monuments and especially as faÁade material for exterior cladding design has been continuously increasing. On the other hand, the undesired growth of this titanium dioxide layer due to long-term exposure in harsh atmospheric conditions leads to the appearance of interference colours and spoils the building from the viewpoint of aesthetic appearance. In particular, acidity of the rainwater falls and its reaction with the impurities present on the surface of titanium sheets to eventually alter the thickness of the oxide film is the main cause of discolouration. This review provides a conceptual framework on different manufacturing issues which have to be precisely taken into consideration in order to minimize the presence of impurities on both anodized and non-anodized end-products, as well as principles of anodizing process to increase the thickness of the oxide layer in order to eventually limit the access of acid rains to the remaining impurities.