Effect of anodic T phase on surface micro-galvanic corrosion of biodegradable Mg-Zn-Zr-Nd alloys

A complete understanding of the role of Mg-Zn-Nd (T) phase in surface micro-galvanic corrosion of Mg alloys has not been fully developed. Therefore, the microstructure evolution and corrosion behavior in simulated body fluid (SBF) of Mg-2Zn-0.6Zr-xNd alloys (x = 0, 0.2, 0.6 and 1 wt%) were investigated. It was found that the MgZn phase in Mg-2Zn-0.6Zr alloy changed to Mg60Zn32Nd8 (T-2) and Mg35Zn40Nd25 (T-3) phases after adding 0.2-1 wt% Nd. With the formation of T phases, the grain size increased first and then decreased. By using scanning kelvin probe force microscopy (SKPFM), the Volta potentials of second phases relative to Mg matrix were measured. T-2 and T-3 phases with a relative Volta potential of about -400 mV acted as micro-anodes, so that they were corroded preferentially during the corrosion process. Because of the appropriate amount of anodic T phases and their discontinuous distribution, the alloy with 0.2 wt% Nd addition showed a uniform surface corrosion characteristic and exhibit the best corrosion resistance.