Abstract:Fluorine conversion film was synthesized on AZ31 magnesium alloy to improve its corrosion resistance as degradable implant material. The degradable process was studied by electrochemical impedance spectroscopy (EIS) in Hank’s solution for different immersion times. The formation mechanism and the degradation behavior of fluorine conversion film in Hank’s solution were discussed by investigating the variation of surface morphology and chemical composition. The results indicate that MgF₂ conversion film forms on AZ31 alloy through the fluoride treatment. The evolution of hydrogen results in the formation of a very small amount of blind micro-pores in the film. With increasing the immersion period in Hank’s solution, the corrosion resistance of MgF₂ conversion coating decreases gradually. During this process, magnesium hydroxide forms with the slow dissolution of MgF₂ layer, and Ca²⁺, HPO₄^2- and PO₄^3- ions deposit on the surface of the sample. The existence of micro-pore on MgF₂ conversion coating leads to the initiation of pitting corrosion after immersion for 15 min. After immersion for 7 d, MgF₂ layer suffers more serious corrosion damage due to the self-accelerated corrosion at the micro-pores. After immersion for 15 d, the pitting corrosion damage becomes more obvious. The in-leakage of a great amount of Cl^- ions enhances the localized corrosion of AZ31 substrate through the micro-pores in the film, where the accumulation of corrosion products results in the formation of some cystiform protuberance.

Key words: magnesium alloy; fluorine conversion coating; electrochemical impedance spectroscopy; degradation behavior; biomaterial