| Magnesium (Mg) and its alloys, due to their excellent mechanical properties and in vivo degradability, have attracted widespread attention from biomedical device researchers. However, the rapid degradation rate of Mg alloys in physiological environment limits their application. As a load-bearing implant, Mg device is prone to stress corrosion cracking (SCC) under the combined effect of physiological load and corrosive media, leading to premature material fracture failure. Therefore, this paper intends to prepare virous surface modification coatings, such as ceramic micro-arc oxidation (MAO) film, PLGA polymer coating with good biocompatibility and degradability, and MgF2 conversion layer, to improve the corrosion resistance and SCC behavior of biomedical Mg alloy. The microstructure, corrosion resistance, and phase composition of the Mg-Al-Zn and Mg-Zn-Zr substrates, as well as the surface modified layers, were measured by scanning electron microscopy (SEM), optical microscopy (OM), and X-ray diffraction (XRD). The corrosion behavior of uncoated and surface modified Mg substrates were analyzed by potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS). Slow strain rate tensile (SSRT) testing was performed in Hank's solution to obtain the engineer stress-strain curves, and the fracture morphology characteristics of different samples were analyzed. The results indicate that although the same modified layers on the surface of Mg-Al-Zn and Mg-Zn-Zr substrates exhibit similar corrosion rate, there are significant differences in the resistance to SCC. PLGA coating shows the best corrosion resistance, while lowest resistance to SCC in all surface coated samples. Compared the data of SCC susceptible indices, the MAO film is the best inhibitor of SCC for Mg-Al-Zn alloy, while the MgF2 conversion layer is preferable for Mg-Zn-Zr alloy. |
