Effects of Mn2+ on the chrome-free colored Ti/Zr-based conversion coating on 6063 aluminum alloy

The conversion coating with golden color and improved corrosion resistance had been prepared by adding Mn2+ in the Ti/Zr conversion coating solution. Comparing with that of conversion coating without Mn2+, the optimal treatment time of this conversion coating was much shorter and the corrosion resistance was obviously improved. The effect of Mn2+ on the formation of golden Ti/Zr conversion coating was thoroughly investigated by means of energy dispersive X-ray spectroscopy, SEM, XPS, and Raman and electrochemical workstation. The results showed that the conversion coating had a double-layer structure: the outer layer consisted of the metal-organic complex and the inner layer was mainly made up of Na3AlF6. Mn2+ was oxidized into MnOOH in solution and precipitated on the substrate surface which provided the nucleus to Na3AlF6 crystal and accelerated Na3AlF6 crystal formation and also made the microstructure of conversion coating change to the cubic. The mechanism of the formation of the conversion coating can be deemed as nucleation, growth of Na3AlF6 crystal, and formation of metal-organic complex. Copyright (c) 2015 John Wiley & Sons, Ltd.