The influence of Gd content on the microstructure, mechanical properties, corrosion behavior and corrosion film deposition mechanisms of as-extruded Mg–Zn–Mn–Sr–Gd alloys for biomedical applications

The microstructure, corrosion behavior and corrosive film formation of as-extruded Mg–2Zn–0.4Mn–0.1Sr– x Gd alloys ( x = 0, 0.5, 1 and 2 wt%) in Hank’s solution were studied. Microstructure was investigated by scanning electron microscopy (SEM), scanning Kelvin probe force microscopy (SKPFM), X-ray photoelectron spectroscopy (XPS). Corrosion behavior was evaluated by immersion and electrochemical tests. The microstructural observations indicated that the addition of 0.5–2 wt% Gd led to the formation of Mg 3 Zn 3 Gd 2 phase and grain refinement. SKPFM results showed that the Gd-rich (Mg 3 Zn 3 Gd 2 ) phase was nobler than the α-Mg matrix in Gd-containing alloys and formed micro-galvanic corrosion. The alloy with 0.5–1 wt% Gd addition had better corrosion resistance and displayed uniform corrosion behavior than that Gd-free alloy. However, excessive Gd addition (2 wt%) decreased the corrosion rate and caused severe localized corrosion due to massive cathode Gd-rich particles. Combined with mechanical properties and corrosion resistance, the alloy with 1 wt% Gd exhibited a potential for degradable implant applications. The obtained superior corrosion resistance was due to the refined microstructure and the compactness of Gd-containing corrosion products film. In addition, a systematic discussion of corrosion behavior and corrosion film formation was provided by means of schematic illustration.