Influences Of Surface Treatment On Degradation Properties Of Biomedical Magnesium Alloys

Magnesium alloys have excellent biodegradable absorption properties, good mechanical and biocompatibility, however, the fast corrosion rate restricts its clinical application. Therefore, it’s important to study the degradation behavior of magnesium alloy in physiological environment and improve its corrosion resistance. In this paper, the influences of Zn alloying on the degradation behavior of magnesium. Then the effects of micro arc oxidation on the degradation behavior of pure magnesium and Mg-2Zn alloy were investigated, respectively. Furthermore, the behaviors of pure magnesium and Mg-2Zn alloy in NaCl solutions with different chloride ion concentrations were investigated.During immersed in simulated body fluid (SBF), the tensile strengths and elongations of pure Mg and Mg-2Zn alloy decrease obviously, and the decreasing rate of the latter is faster. The mass loss rate and hydrogen evolution rate of Mg-2Zn alloy is almost constant, while the rates of pure Mg are faster at first and then decrease with increasing the immersion time. Pure Mg tend to corrosion uniformly in SBF, whereas the local corrosion of Mg-2Zn alloy is serious. The corrosion potential Mg-2Zn alloy is slightly higher than that of pure Mg, and the lower corrosion current density and larger radius of high frequency capacitance arc indicates a better corrosion resistance of Mg-2Zn alloy.During immersed in SBF, the micro arc oxidation (MAO) reduces the tensile strengths and hydrogen evolution rates of pure Mg and Mg-2Zn alloy in different degrees, and the rates of Mg-2Zn alloy decrease more apparently. The pH value of immersion SBF for MAO sample is lower than that of untreated sample, and decreases with increasing immersion time. After MAO, the corrosion current density of pure Mg and Mg-2Zn alloy decreases 1-2 order of magnitudes, which means the lower corrosion rate in SBF. The pitting potential of MAO samples increases, so it’s more difficult for pitting corrosion to occur. Additionally, the high frequency capacitance arc radii of MAO samples are significantly larger, no obvious low frequency inductive arc are observed. Accordingly, MAO hinders the corrosion dissolution reaction of magnesium alloy and improves its corrosion resistance.The pitting corrosion of pure Mg, Mg-2Zn alloy and their MAO samples is more significant while increasing the concentration of chloride ion in NaCl solution. The tensile strengths, residual mass fractions and hydrogen evolution rates of pure Mg have no obvious differences in 100mmol/L and 153.85mmol/L NaCl solutions. On the contrary, the MAO samples of pure Mg is more sensitive to the concentration of chloride ion. The mass loss rates and hydrogen evolution rates of Mg-2Zn alloy and its MAO samples are faster with increasing the concentration of chloride ion. When the concentration of chloride ion is 100 mmol/L, the tensile strengths of Mg-2Zn and its MAO samples decrease rapidly firstly, and then the decreasing rates is slowed down with the increasing immersion time. While the concentration is 153.85mmol/L, the decreasing rates always keep rapid. With increasing the concentration of chloride ion, the corrosion current densities of pure Mg, Mg-2Zn alloy and their MAO samples are larger. Meanwhile, the high frequency capacitance arc radii of them and the charge transfer resistances are smaller, which cause the samples corrode faster.