ELECTROCHEMISTRY AND PASSIVITY OF TI-15 V-3 CR-3 AL-3 SN BETA-TITANIUM ALLOY IN AMBIENT-TEMPERATURE AQUEOUS CHLORIDE SOLUTIONS

Examination of the electrochemical behavior of Ti-15 V-3 Cr-3 Al-3 Sn (Ti-15-3) alloy was performed as an initial step in understanding mechanisms for environmentally assisted cracking of beta-titanium alloys in room temperature aqueous chloride solutions. Both solution heat-treated (SHT) and peak-aged (PA) Ti-15-3 were studied, in addition to SHT commercially pure Ti (grade 2 Ti) and Ti-5 Al-2.5 Sn (grade 6 Ti). The latter roughly models alpha precipitates within the beta-titanium matrix in PA Ti-15-3. Anodic polarization studies in 0.6M NaCl and 0.6M NaCl adjusted to pH 1 with HCl revealed that Ti-15-3 was spontaneously passive in each solution, with no observed pitting potential at potentials as positive as 2.1 V vs. SCE. However, an active/passive transition was observed on all alloys in both aerated and deaerated 5M HCl. In situ characterization of passivity by electrochemical impedance spectroscopy confirmed that passive film growth was linear with potential above primary passivation potentials in all solutions, and that the passive dissolution rate is limited by the charge-transfer reaction, especially at lower potentials. Similar passive current densities, active/passive transition potentials, and apparent oxide resistivities on SHT and PA Ti-15-3 suggest little effect of the alpha precipitate on the passive behavior of PA Ti-15-3. Auger electron spectroscopy and hydrogen evolution kinetics suggested less than stoichiometric incorporation of alloying additions into the passive film of Ti-15-3. X-ray photoelectron spectroscopy indicated that, to the limited extent that alloying additions are found in the passive film, they are oxidized. The valence states present are discussed. Predominantly TiO2 passive films may explain the similar passive electrochemical behaviors observed on the materials examined.