Corrosion rate evolution of magnesium alloys in simulated body fluids: environment and alloying elements effects

• EV31A and WE43B showed cathodic second phases leading to micro-galvanic coupling. • Chloride-rich environment enhanced corrosion rates and alkalization. • EV31A and especially WE43B exhibited lower corrosion and alkalization rates. • Phosphates improved the passive film stability and corrosion rates decreased, consequently. Mg-alloys are attractive for bioresorbable implants in which a certain tensile strength is needed after installation. However, the success in the application of biodegradable materials strictly depends upon degradation rate, which corresponds to corrosion rate in the case of metals. Therefore, this study investigated corrosion kinetics of EV31A and WE43B alloys in simulated body fluids up to 14 d of immersion. Weight loss (WL), hydrogen evolution (HE) and pH measurements were performed. These tests were carried out in a limited amount of test solution (15 mL/cm 2 ), to simulate the almost stagnant conditions which can locally occur inside the human body. In chloride-containing solutions, a decrease in corrosion rate was noticed with respect to commercially pure magnesium (Mg-CP) due to the mere addition of alloying elements such as Y and rare earths (RE). Micro-galvanic coupling effect was observed, and the effect of second phases was further assessed by means of scanning Kelvin probe force microscopy (SKPFM). Lower corrosion rates at early stages were observed for Y-RE alloy as well as solution alkalization. In a buffered phosphate-containing solution, differences between corrosion rates of Mg-CP and Mg-Y-RE alloys were significantly lower, due to the increase in the protectiveness of passive film. Alkalization was ever observed, but kinetics were strongly dependent upon the alloy and the environment. Localized corrosion and inhomogeneous general corrosion were noticed for all testing conditions. Average corrosion rates measured by means of WL and HE techniques gave comparable results, and an accurate estimation of overall corrosion rate of alloys for biomedical devices - which require long term exposure assessments - was achieved.