XPS study on surface modification of NiTi alloy by acidic solution immersion and subsequent heating in air

Changes in the surface chemical state of a nearly equiatomic nickel–titanium (NiTi) alloy caused by immersion in aqueous solutions of HNO 3 and H 2 SO 4 as well as subsequent heating in air at 723 K were analyzed using X‐ray photoelectron spectroscopy (XPS). An XPS analysis using angle‐resolved technique and a mathematical deconvolution technique revealed that a passive layer formed in an ambient atmosphere contained TiO 2 as a major state and Ni(OH) 2 and NiO as minor states. The Ni(OH) 2 on the alloy remained in the region even when heated in air at 723 K. Therefore, the resulting layer became a Ti‐oxide layer with Ni segregated region at the surface, which was NiO formed via dehydration of Ni(OH) 2 . However, immersion in an aqueous solution of HNO 3 or H 2 SO 4 enables Ni(OH) 2 state to dissolve in the passive layer of a NiTi alloy; thereby, the Ni segregated region rarely appeared in the oxide layer by heating. The Ni segregated region at the surface becomes an obstacle for the inward diffusion of oxygen; thus, the annihilation of such a segregated region results in an increase in the thickness of the oxide layer.