Research On Degradation Mechanism Of Biomedical Magnesium Alloy In Physiological Environment And Its Surface Modification

In current project, the degradation mechanism of biomedical magnesium alloy was systematically investigated via experiments in simulated physiological environment. Surface modifications of AZ91 magnesium alloy by plasma immersion ion implantation (PIII) and deposition of Al₂O₃/Al bi-layer coating were carried out aiming at enhancing its corrosion resistance.By degradation performance and electrochemical behavior in the four designed test solutions, the influences of aggressive ions in physiological environment including Cl^-, HPO₄^2-, HCO₃^- and SO₄^2- on the degradation behavior of AZ91 magnesium alloys were systematically investigated and the corresponding mechanism were discussed and disclosed. The results demonstrate that chloride ions can induce porous pitting corrosion on magnesium alloy and the HPO42? can slow down the degradation rates dramatically. HCO3? stimulate magnesium dissolution progressively, while can induce fast surface passivation which can suppress development of pitting corrosion. SO42? is also aggressive to magnesium alloy to some extent.The structure, composition and phase distribution in degradation products layer of AZ91 magnesium alloy exposed in simulated body fluid were investigated systematically. The degradation products are amorphous and are mainly composed of MgO/Mg(OH)₂, magnesium/calcium phosphates, magnesium/calcium carbonates, Al₂O₃ and Al(OH)₃. The non-uniform corrosion results in non-uniform structure of degradation product layer. In serious corrosion regions, very thick corrosion product layer with high content of Ca and P forms. The regions with tiny corrosion possess very thin product layer with a small amount of P and nearly absence of Ca.The degradation rate, magnesium ions leakage rate and the induced changes in pH as well as the corrosion mechanism were disclosed by immersion in simulated body fluid and electrochemical test. During the early stage of exposure, high rates of degradation and magnesium leakage accompanying by progressive enhanced pH valued are present. After a period of exposure in SBF, the degradation rate and magnesium ions leakage rates are much lower than those during earlier stage. In simulated body fluid, weak pitting corrosion will occur. However, the pitting corrosion is self-limited to some extent.Zr and Al plasma immersion ion implantation can only enhance the corrosion resistance of magnesium alloy to some extent. Dense and uniform Al₂O₃/Al bi-layer coating is successfully deposited on AZ91 magnesium alloy using a filtered cathodic arc deposition system. The as-prepared coating can enhance the corrosion resistance of treated alloy in long term and bonds strongly with the substrate. The coating is not sensitive to fracture and delamination when subjected loads.