Magnesium (Mg) and its alloys are emerging as a possible biodegradable implant material. The corrosion behavior of pure Mg, AZ31, and AZ91D were evaluated in various In Vitro and In Vivo environments to investigate their potential application of being biomaterials. Mg implants may degrade too quickly in the body, before the natural healing process is complete. Anodization is known to be an effective approach for slowing down the initial corrosion rate of magnesium (Mg) and its alloys. Anodization was investigated to slow down the initial corrosion of Mg in a simulated body corrosive environment. Pure Mg and AZ91D alloy were anodized and their corrosion resistance was compared in terms of anodization behavior and parameters such as applied voltage and current with different anodization time. Electrochemical impedance spectroscopy, DC polarization, and immersion testing were used to evaluate the corrosion resistance of Mg samples and further optimize anodization parameters. The results showed that anodization increased the corrosion resistance of both pure Mg and AZ91D samples. Further characterization showed the anodized layers on both pure Mg and AZ91D consisted of Mg, O and Si, in the mixture of MgO and Mg2SiO4.
The anodization of AZ91D was further investigated by studying the specific use of oxy-salts to improve the corrosion resistance of anodization coatings. Oxy-salts of silicate, phosphate, and carbonate were added separately to a sodium hydroxide alkaline electrolyte used for anodization. This modified process was investigated in terms of anodizing behavior, the surface properties of the film, and enhanced corrosion protection of the metal. Anodization of AZ91D using the silicate containing electrolyte generated sparks and increased the electrolyte temperature, and produced a thicker and more corrosion resistant layer than the other oxy-salts. In this process, MgO and SiO2 formed Mg2SiO4 at high temperature and silicon (Si) in the anodized coating was mainly presented in Mg2SiO4. Phosphorus (P) and carbon (C) were not found in the coatings from the phosphate and carbonate containing electrolyte anodizations. The effects of silicate concentration and anodizing time on the surface properties and corrosion resistance were studied in detail. A detailed chemical structure of the Si-containing anodized coating was postulated.
Moreover, this paper described the water-based bis-[triethoxysilyl] ethane (BTSE) silane modification on the surface of Magnesium-Yttrium (Mg-Y) alloy for adhesion promoting, crosslinking of resins, and corrosion protection. Surface characterization of ESEM, FTIR, and EDX showed the hydrolysis and condensation of silane resulted in a strong covalent bonding. In Vivo corrosion behaviors of the uncoated and coated Mg-Y alloy were evaluated in the acute implantation by using novel self-developed corrosion probes which were inserted into the body cavity and subcutaneous tissue of the mice. In Vitro experiments, compared to In Vivo results, artificial simulated body solutions were more corrosive. Based on the electrochemical experiments of potentio-dynamics polarization and electrochemical impedance spectroscopy (EIS), the epoxy-modified BTSE silane coating successfully increased corrosion resistance at the initial stage of implantation.