Preparation and properties of chrome-free colored Ti/Zr based conversion coating on aluminum alloy

A golden conversion coating on the surface of aluminum alloy was prepared by adding tannic acid and coating–forming accelerator in the treatment solution containing titanium and zirconium ions. The growth process, main component and corrosion resistance of the conversion coating were characterized by EDS, SEM, XRD, XPS, FIIR and electrochemical workstation. The results showed that the main components of the conversion coating were Na3AlF6 and the conversion coating owns a double-layer structure. The outer layer consists of metal–organic complex and the inner layer is mainly made up of Na3AlF6. The mechanism of the formation of the golden conversion coating can be deemed as nucleation, growth of Na3AlF6 crystal and formation of metal–organic complex. In potentiodynamic polarization test, the corrosion current density decreases to 0.283 μA cm−2 from 5.894 μA cm−2, which indicates an obvious improvement of corrosion resistance.