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

Abstract

Corrosion of aluminum (Al) alloys represents a major issue in industrial applications, giving rise to safety concerns and enormous economic loss. Chromate conversion coatings (CCCs) have been widely used in industry over the past decades, providing exceptional corrosion protection for aluminum alloys at a relatively low cost. However, new environmental regulations prohibiting the use of chromium (VI) (Cr6+) have raised a growing interest in the development of alternative technologies to CCCs, but only titanium (Ti)/zirconium (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 affect their wide application. Here, a colored titanium/zirconium-based conversion coating, formed on aluminum alloy 6063 using a solution containing tannic acid (TA; C76H52O46), hexafluorozirconic acid (H2ZrF6), hexafluorotitanic acid (H2TiF6) and sodium metavanadate (NaVO3), was developed recently by the authors, enabling rapid film formation at room temperature and improved corrosion resistance. The influence of TA on the film-forming process of colored titanium/zirconium-based conversion coatings was investigated using the ultraviolet–visible spectrophotometry, X-ray photoelectron spectroscopy and infrared spectroscopy techniques, and the possible reaction process after adding TA, as well as the mechanism of rapid film formation and color rendering, was also explored. The results showed that the large number of organic complexes formed by TA with metal ions during the aging process of conversion solution and film formation was the main reason for the rapid film formation and color rendering of the coating.