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

Abstract

Changes in the surface chemical state of a nearly equiatomic nickel–titanium (NiTi) alloy caused by immersion in aqueous solutions of HNO3 and H2SO4 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 TiO2 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 HNO3 or H2SO4 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.