Electrolyte effect in pulsed anodization of NiTi alloy to form a Ni-free oxide layer

Abstract Growing an oxide layer on an almost equiatomic NiTi alloy using anodization is challenging owing to the presence of Ni in the alloy. Ppulsed voltage can highlight the chemical reactions occurring with the electrolyte to obtain a Ni-free TiO2 layer. For a better understanding of this technique, we investigated the effect of electrolyte ionic species on the growth of a Ni-free TiO2 layer. The H+ and SO42− ions in the electrolyte withdraw Ni from the growing layer, whereas the coexistence promotes layer dissolution. Therefore, H2SO4 facilitated the formation of a Ni-free TiO2 layer with a groove-like surface, while (NH4)2SO4 led to a smoother surface than that in the presence of H2SO4. When NO3− is present in the electrolyte, it behaved as an oxidizing agent and promoted layer growth. Thus, anodizing in HNO3 resulted in the thickest layer with nanometer-sized pores as the vestiges of Ni removal owing to supplementary action by the H+ ions. However, NH4NO3 did not form such an oxide layer. The corrosion-protective function was dominated by the surface characteristics, and the HNO3-anodized surface exhibited inferior properties owing to pore corrosion.