Characteristic variation of pulsed‐anodized NiTi surface by the adjustment of voltage‐unapplied state

Pulsed anodization of superelastic NiTi alloy in nitric acid electrolyte is a novel surface modification process that enables the formation of an almost Ni‐free TiO2 layer on their surface. The core technology is the modulation of voltage‐unapplied state that is necessary for the chemical reaction with an electrolyte. Here, the effect of the adjustment of the voltage‐unapplied state on the layer characteristics was investigated. Prolongation of the voltage‐unapplied state led to the formation of a thicker TiO2 layer. This result was attributed to the elimination of Ni by nitric acid. As a result, nanometer‐sized pores appearing as vestiges of Ni elimination increased and enlarged concomitantly. Furthermore, by prolonging the voltage‐unapplied state, the amount of Ni ions released into a phosphate‐buffered saline (PBS) solution diminished significantly. Such prolongation enabled significant suppression of the dissolution of the layer itself; thus, Ni ion release from the surface region was considered to be reduced.