Surface corrosion behavior and reaction product film deposition mechanism of Mg-Zn-Zr-Nd alloys during degradation process in Hank's solution

The surface corrosion behavior and reaction product film deposition mechanism of biodegradable as-extruded Mg-2Zn-0.6Zr-xNd alloys with different Nd addition were investigated using electron backscattered diffraction (EBSD), scanning electron microscopy (SEM), confocal laser scanning microscopy (CLSM), X-ray diffraction (XRD), electrochemical test and immersion test. The results showed that 0.2-0.6 wt% Nd addition led to significant grain refinement of the alloys and the formation of fine MgZn and T phases. Excessive Nd addition (1 wt%) significantly resulted in large size Zr-rich precipitates and second phases as cathodes, causing micro-galvanic corrosion acceleration. The electrochemical measurements, immersion tests and corrosion morphologies proved that the alloy with 0.6 wt% Nd addition had the best corrosion resistance and exhibited a uniform corrosion mode, which were attributed to small grain size, fine second phases and formation of compact CaHPO4·2H2O film. Moreover, corrosion models with different Nd contents were proposed to interpret the corrosion product film deposition mechanism.