On the Growth of Thin Anodic Oxides Showing Interference Colors on Valve Metals

Abstract

All valve metals are potential candidates for anodic oxidation, an electrochemical technique that can produce the growth of nanometric oxide films at the metal surface, exhibiting fascinating colors due to their interference with light. Yet, comprehensive works comparing their oxidation kinetics and the characteristics and functionalities of oxides produced are hardly found, in spite of the technological importance of such information. In fact, these films lie in the field of optical coatings, whose engineering opens applications in technological areas such as communication, sensing, imaging, and display. Moreover, the possibility to modify their electronic structure allows to produce dielectrics, as well as ionic conductors or semiconductors, or memristors. The availability of a wide range of properties can increase the variety of functions that can be covered by such films, which requires not only different oxide thicknesses, but also different compositions, and therefore know-how on the widest set of valve metal oxides possible. This review article will present the main characteristics of anodic oxide films formed on the most common valve metals – i.e., titanium, aluminum, niobium, tantalum and zirconium, with reference to the relationship between the properties achieved and the potential applications. Additionally, a comparison will be presented among oxides produced on different metals in same anodizing conditions to discuss the effects of anodic oxidation parameters on the resulting oxide main features. Keywords: Anodic oxidation, interference color, metal oxide, Thin film, valve metal.