Quasi-in-situ Observation and SKPFM Studies on Phosphate Protective Film and Surface Micro-Galvanic Corrosion in Biological Mg–3Zn– x Nd Alloys

The phosphate protective film and micro-galvanic corrosion of biological Mg-3Zn- x Nd ( x = 0, 0.6, 1.2) alloys were investigated by scanning and transmission electron microscopy, quasi-in-situ observation, scanning Kelvin probe force microscopy (SKPFM) and electrochemical tests. The results revealed the Mg–Zn–Nd phases formed in Mg–3Zn alloy contained with Nd. Adding Nd resulted in a significant decline in the cracks of the phosphate protective film and micro-galvanic corrosion of alloys, which were recorded by quasi-in-situ observation. In addition, the Volta potential difference of Mg–Zn–Nd/ α -Mg (~ 188 mV) was lower than MgZn/ α -Mg (~ 419 mV) and Zn-rich/ α -Mg (~ 260 mV), and the corrosion rates of alloys markedly decreased after the addition of 0.6 wt% Nd. The improvement in corrosion resistance of Nd-containing alloys was mainly attributed to the following: (i) the addition of Nd reduced the Volta potential difference (second phases/ α -Mg); (ii) the phosphate protective film containing Nd 2 O 3 deposited on the surface of the alloys, effectively preventing the penetration of harmful anions.