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

► A golden conversion coating has been prepared by adding tannic acid in the treatment solution. ► 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.