ELECTROCHEMISTRY AND PASSIVITY OF A TI-15MO-3NB-3AL BETA-TITANIUM ALLOY IN AMBIENT-TEMPERATURE AQUEOUS CHLORIDE SOLUTIONS

To understand the effect of Mo-Nb additions on the electrochemical behavior of beta-titanium alloys in ambient temperature chloride solutions, characterization of the electrochemistry and passivity of a Ti-15Mo-3Nb-3Al alloy (beta-21S) was undertaken. Both solution heat-treated (SHT) and peak-aged (PA) alloys exhibited passive anodic behavior in aerated and deaerated 0.6M NaCl, aerated and deaerated 0.6M NaCl adjusted to pH 1 with HCI, as well as aerated 5M HCl. X-ray photoelectron spectroscopy (XPS) performed after exposure to neutral-0.6M NaCl indicated that both PA and SHT beta-21S formed a predominantly TiO2 film. Auger electron spectroscopy (AES) and cathodic kinetics suggest that the Mo and Nb alloying additions are incorporated into the oxide in amounts less than that found in the alloy. The predominance of the passivating TiO2 may explain the similarity of the electrochemical behavior observed. However, in deaerated 5M HCl, all materials displayed active-passive behavior except for SHT beta-21S which was spontaneously passive. This result suggests that the presence of the alpha-phase is deleterious to the formation of a protective passive film on PA beta-21S in deaerated 5M HCl.