The sequential migration of rare earth elements (REE) in WE43 magnesium alloy during anodic oxidation treatment

Abstract

Rare earth elements (REE) like Y, Gd and Nd could migrate and enrich on WE43 magnesium alloy (WE43) surface during anodic oxidation. However, the specific mechanism is still unclear. In this study, WE43 alloy was modified by anodizing for 10 min, 30 min, 1 h and 2 h in KOH solution, respectively, followed by heating treatment for 2 h at 500℃. The obtained materials were named as AO-10, AO-30, AO-1 h and AO-2 h in turn. The surface morphology, phase compositions, surface potentials, REE contents of these materials were studied. In addition, the contents of REE in materials and KOH solution were investigated before and after electrolysis. The results found that surface morphology, phase composition were different with different electrolytic time. The phase compositions changed from Gd2O3 and Nd2O3 on AO-10 surface to Y2O3 and Gd2O3 on AO-2 h surface. Moreover, The contents of REE on material surfaces would first increase and then decrease with over time. Surface potentials confirmed that the regions rich in REE had lower potentials than Mg substrate. The contents of REE in materials and KOH solution further revealed that Nd ion was the first to ionize into the solution, followed by Gd, finally by Y. The migration rates of REE were different, which depends on their electric potentials. The migration rate of Nd was the fastest in all elements. It would preferentially migrate to material surface and entered into electrolyte solution in the oxidation process. Hence, the migration of REE from the inside to the surface of WE43 metal is sequential during anodic oxidation.