Characterization of oxide layer formed on NiTi alloy by anodic oxidation in a molybdate electrolyte

NiTi shape memory alloy was potentiostatically anodized in a molybdate electrolyte containing ammoniac ion to form a thick and uniform oxide layer. The oxide layer was characterized by SEM, XRD, EPMA, XPS and potentiodynamic polarization test. It was found that the as-prepared coating is a thick (approx. 40 μm), smooth and dense oxide layer, and is free of cracks and porous defects. Moreover, the surface analyses revealed that the anodic oxidation at 60 mV potential (SCE) reduces significantly the proportion of Ni in the outmost surface layer, and the atomic ratio of Ti and Ni elements is raised from 1:1 to 9.7:1. The as-prepared coating was primarily composed of noncrystalline TiO₂ on the outermost surface with a small quantity of Ni(OH)₂. Potentiodynamic polarization tests in Fusayama artificial saliva pH 6.2 demonstrated that the oxide layer presents a significant increase in breakdown potential due to titanium enrichment.