Degradation behaviour of Mg-4Ag and Mg-5Gd alloys under in-vitro conditions and different time-frames

Abstract

The Mg-4Ag and Mg-5Gd binary alloys' degradation and corrosion properties were studied under in-vitro conditions (in a SBF solution of pH 7.4 at 37 °C) by using various analytical techniques and time-frame windows (up to 3 vs. 28 days), and compared with the pure Mg alloy. Different kinetics and mechanisms of the alloys' degradation were identified, influencing their corrosion rates' dynamics. The Electrochemical Impedance Spectroscopy (EIS) measurements, being performed for up to 3 days, revealed that the corrosion of all three alloys are under a kinetic-controlled mechanism, among which only pure Mg and Mg-4Ag show a repassivation ability in this time-frame. However, the corrosion rates of binary alloys after 28 days of incubation reaching a value of around 0.33 mm/y and a release of 64 mg/L Mg2+ ions, that was accompanied by a pH change up to 8.3, an be lower compared to the pure Mg alloy (0.62 mm/y, 117 mg/L of Mg2+, pH of 9.0). While thick and homogenous layers of differently-shaped and chemically secondary-phased Mg(OH)2 products were identified on both binary alloys, MgO products and an apatite structure are formed on the pure Mg alloy surface. The Mg-4Ag alloy seems to be biomedically the most appropriate regarding the in-vitro degradation process.