Effects of Nb and Zr Alloying Additions on the Activation Behavior of Ti in Hydrochloric Acid

The activation behaviors of air‐passivated commercially pure Ti, Ti‐50 wt % Zr, and Ti‐45 wt % Nb were investigated in hydrochloric acid (HCl) ranging from 0.1 to 10 M, and temperatures ranging from 13 to 52°C. In situ electrochemical impedance spectroscopy and X‐ray adsorption studies in 5 M HCl confirmed: ( i ) oxide dissolution occurred uniformly and ( ii ) rapid drops in open‐circuit potential (OCP) corresponded with the exposure of the underlying metal to solution. Activation energies varied from 56.5 ± 8, and 62.9 ± 5, to 103 ± 6.2 kJ/mol for Ti, Ti‐45% Nb, and Ti‐50% Zr, respectively. Activation energies, low dissolution rates and dependency of the log of the oxide dissolution rate on the log of hydrogen ion activity ranging from 0.4–0.6 suggest that cleavage of metal cation—oxide bonds occurs by a rate‐determining ion transfer step involving protonation of in oxides by one ion to form free . © 1999 The Electrochemical Society. All rights reserved.