The influence of electrolyte pH on anodic growth of Ni-Ti-O nanopores on NiTi alloy

The influence of electrolyte pH on anodic growth of Ni-Ti-O nanopores on NiTi alloy was investigated. Through manipulating HCl, NaCl, and NaOH concentrations in the electrolyte composed of 95 vol% ethylene glycol, 5 vol% H2O, and 0.15 M Cl−, the electrolyte pH was varied from 0.82 to 13.5. Although the pH has little influence on surface morphology and composition of the nanopores, it can significantly impact their growth rate. Initially, increase of the pH decreases the growth rate from 13.8 nm/s at pH = 0.82 to 5.7 nm/s at pH = 7.0. However, when the pH exceeds 7.0, its value increases with the pH and reaches up to 15.8 nm/s at pH = 13.5. The knowledge gained from this study should be valuable in the anodization of other valve metals and their alloys.