Formation and characterization of zirconium based conversion film on AZ31 magnesium alloy

Magnesium alloys have great potential in biomedical applications due to their unique combination of satisfactory mechanical property and decent biodegradability. However, their poor corrosion resistance limits their applications in biomedical fields. In this work, we employ a chemical conversion deposition method to prepare a Zr-based conversion film on the surface of AZ31 magnesium alloy to serve as a passivation layer. The mechanism for the film formation was studied and it showed the deposition process consists of four steps: substrate dissolution, nucleation, film growth, and film equilibrium. The film is mainly composed of Zr(OH)4/ZrO and Mg(OH)2/MgO with small amount of MgF2 and ZrF4. The protective performance of the Zr-based film was investigated by electrochemical and immersion tests in simulated body fluid (SBF). Electrochemical results showed a significant decrease in the corrosion current density (Icorr), a positive shift of corrosion potential (Ecorr), a bigger capacitive loop diameter and higher impedance values for the Zr-coated substrate as compared with an uncoated one. Immersion results indicated the corrosion rate of the Zr-coated sample was ∼20% lower than that of an uncoated one. All above results corroborate the great potential of Zr-based coating in enabling AZ31 alloy for biomedical applications.