Effect of alloying elements on the structure and corrosion resistance of TCP conversion coating formed on different aluminum alloys

Trivalent chromium process (TCP) conversion coating has been widely used on aluminum alloys due to its high corrosion resistance and relatively low toxicity. Much work has indicated that the corrosion resistance of TCP conversion coating is highly dependent of aluminum alloy type, but the associated mechanism is still not clear. In this paper, based on a Cr-Zr-Ti conversion solution, the structure and corrosion resistance of the TCP coating on four typical aluminum alloys, namely 2024-T351, 7050-T7451, 6061-T6 and 5056-H18, have been investigated. It was found that the TCP coatings formed on 5056 and 6061 alloys showed higher corrosion resistance than those formed on 2024 and 7050 alloys, and the corrosion resistance was negatively related to the amount of Cu in the coating. Mg and Zn were present as oxides and hydroxides in the coating, beneficial to the compactness of the TCP coating. Compared with Cu, Mg and Zn, Si had less effect on the structure and corrosion resistance of the coating. It is suggested that the incorporation of the alloying elements from the alloy substrate into the TCP coating have contributed greatly to different structure and corrosion resistance of the TCP coatings formed on different aluminum alloys.