Fluoride concentration effect on the anodic growth of self‐aligned oxide nanotube array on Ti6Al7Nb alloy

Abstract

AbstractThe formation of nanotube oxide layers on Ti6Al7Nb alloy in H3PO4 acid solutions containing fluoride ions is presented here. Among several parameters influencing the quality of nanotubes formed anodically such as potential, time of anodizing, fluoride ions concentration and scan rate of polarization, in particular, the last two seem to be the most responsible for the structure and morphology of nanotubes. The effect of fluoride ions concentration on the morphology of nanotubes on the two‐phase (α + β) Ti6Al7Nb implant alloy, has been evaluated in our work. The formation of nanotubes was performed by polarizing the Ti6Al7Nb alloy samples in 1 M H3PO4 containing 0.2%, 0.3% and 0.4% wt. Hydrofluoric acid (HF) to 20 V using scan rate 500 mV/s and then holding them at that potential for further 2 h in the same electrolyte. Nanotubes of diameter ranging from 50 to 80 nm, with thicker walls over β‐phase grains than over α‐phase grains, were obtained. During the formation process, which includes two stages–the first, potentiodynamic and the second, potentiostatic (20 V)–different electrochemical behavior was observed in electrolytes of various fluoride concentration. The clear relationship between the highest currents and the biggest diameter of nanotubes for 0.3% wt. HF containing electrolyte observed during the first stage of anodizing is explained with regard to electrochemical characteristics of alloying elements and transport of electrolyte anions in oxide layers. Copyright © 2010 John Wiley & Sons, Ltd.