Growth of oxide layers on NiTi alloy surfaces through anodization in nitric acid electrolyte

The present study demonstrates the formation of an almost Ni-free oxide layer ~ 180 nm in thickness on a NiTi surface through anodization in 0.1 M HNO3 electrolyte. The anodic oxidation of Ti and release of Ni occurred simultaneously during anodization, resulting in the homogeneous growth of the Ti oxide layer on the alloy surface. However, the release of Ni induced the generation of small pits ~ 10 nm in size on the surface, which enlarged over time due to a process similar to pitting corrosion. Since the layer growth and pit development were competing reactions that occurred simultaneously, an oxide layer without pits could not be formed through anodization in the HNO3 electrolyte.