Influence of niobium and zirconium alloying additions on the anodic dissolution behavior of activated titanium in HCl solutions

Ti-45 wt % Nb (30 atom % Nb), and Ti-50 wt % Zr (34.4 atom % Zr), with the alloying elements in solid solution, exhibited reduced active anodic dissolution rates in comparison with commercially pure titanium (C. P. Ti) in deaerated hydrochloric acid (HCl) at 37 degreesC. Furthermore, the alloying additions of niobium (Nb) and zirconium (Zr) to Ti were shown to reduce the anodic charge density required to transition from active anodic dissolution to passivity. The improved resistance of Ti-45% Nb, and Ti-50% Zr to active anodic dissolution in comparison to Ti was not attributable to spontaneous passivity induced by enhanced cathodic kinetics, as in Ti alloys containing noble metals, such as palladium. Instead, improved active dissolution resistance is attributed to strong covalent bond formation between neighboring Ti, Nbi and Zr atoms through the sharing of unpaired d level electrons. The extent of increases in covalent bond strengths is quantified by comparing the bond orders between atoms. A relationship between i(crit), i(corr), and bond order of selected Ti-based alloys is presented. The correlation obtained here takes into consideration surface enrichment of the Ti alloy surfaces with Nb and, possibly, Zr due to the preferential dissolution of Ti in the active state. (C) 2001 The Electrochemical Society. All rights reserved.