Effects of tannic acid on film formation process of colored Ti/Zr-based conversion coatings

Corrosion of aluminum (Al) alloys represents a major issue in industrial applications, giving rise to safety concerns and enormous economic loss. Chromate conversion coatings (CCC) have been widely used in industry over past decades, providing exceptional corrosion protection at relatively low cost for Al alloys. However, new environmental regulations prohibiting the use of Cr(VI) have raised a growing interest in development of the alternative technologies to CCC, but only titanium/zirconium (Ti/Zr) conversion coatings have been used in industrial applications thus far. However, there remain some disadvantages to these coatings, such as colorless appearance and poor corrosion resistance, which affects their wide application. Here, a colored Ti/Zr-based conversion coating, formed on AA6063 Al alloy using a solution containing C76H52O46(tannic acid, TA), H2ZrF6, H2TiF6, and NaVO3, was developed recently by ours, enabling rapid film-formation at room temperature and improved corrosion resistance. The influence of TA on the film-forming process of colored Ti/Zr-based conversion coatings was investigated using UV-Vis, XPS, and IR techniques, and the possible reaction process after adding TA, as well as the mechanism of rapid film formation and color rendering, were also explored. The results showed that, the large number of organic complexes formed by TA with metal ion during the aging process of conversion solution and film formation are the main reasons for the rapid film formation and color rendering of coating.