Investigation of the microstructure, growth mechanism, and corrosion behavior of a new Zr–Ti conversion coating on 6016 aluminum alloy

A novel Zr–Ti conversion coating (ZrTiCC) was fabricated on 6016 aluminum alloy. The coating thickness was evaluated, and the corrosion resistance of the coating was investigated through CuSO4 drop experiments to determine the optimum process. Potentiodynamic polarization results revealed the corrosion current density of ZrTiCC (4.821 μA/cm2) was lower than that of bare specimen (12.263 μA/cm2), and the δE of ZrTiCC (0.752V) was significantly lower than bare specimen (0.087 V). Scanning electron microscope and energy dispersive spectrometer was used to characterize the morphology and composition changes of the coating during its growth. The coating was proved to have a double-layer structure, and cracks appeared when the coating thickness continued to increase, which reduced the corrosion resistance. The growth mechanism of the coating was explained in detail. X-ray photoelectron spectroscopy was used to determine the components of the coating, which comprised Al2O3 at the base, with TiO2, AlF3, and ZrF4 at the top and ZrO2 evenly distributed throughout. Bode diagram revealed the impedance modulus of the coating at low frequency was increased to 8.6 × 104 Ω/cm2 after five days immersion in 0.5 M NaCl. The coating's self-sealing behavior and active protection may relate to the formation of SiO2, as well as the dissolution and regeneration of the upper coating (AlF3 and ZrF4) on 6016 aluminum alloy.