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To understand the effect of Mo‐Nb additions on the electrochemical behavior of β‐titanium alloys in ambient temperature chloride solutions, characterization of the electrochemistry and passivity of a Ti‐15Mo‐3Nb‐3Al alloy (β‐21S) was undertaken. Both solution heat‐treated (SHT) and peak‐aged (PA) alloys exhibited passive anodic behavior in aerated and deaerated 0.6M normalNaCl , aerated and deaerated 0.6M normalNaCl adjusted to pH 1 with normalHCl , as well as aerated 5M normalHCl . X‐ray photoelectron spectroscopy (XPS) performed after exposure to neutral 0.6M normalNaCl indicated that both PA and SHT β‐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 normalHCl , all materials displayed active‐passive behavior except for SHT β‐21S which was spontaneously passive. This result suggests that the presence of the α‐phase is deleterious to the formation of a protective passive film on PA β‐21S in deaerated 5M normalHCl .

The repassivation characteristics of a titanium thin film evaporated from a high-purity Ti source as well as selected a (commercially pure Ti, Ti-5A1-2.5Sn), 13, and 13 + a titanium alloys (Ti-l5Mo-3Nb-3A1, Ti-15V-3Cr-3Al-3Sn, and Ti-3A1-8V-6Cr-4Zr-4Mo) were examined. Both the rapid thin film fracture and scratch depassivation methods were used in aqueous chloride solutions (0.6 M NaCl, 5 M HC1, 5 M LiC1, 5 M HC1 + 1 M T1C13). Bare surface open-circuit potentials followed the relationship E(VSCE) = -1.20 (pH 0) -0.043 pH based on the mixed potential established between the anodic * Electrochemical Society Active Member.

Examination of the electrochemical behavior of Ti-15 V-3 Cr-3 A1-3 Sn (Ti-15-3) alloy was performed as an initial step in understanding mechanisms for environmentally assisted cracking of ~-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 A1-2.5 Sn (grade 6 Ti). The latter roughly models a precipitates within the ~-tiianium matrix in PA Ti-15-3. Anodic polarization studies -in 0.6M NaC1 and 0.6M NaC1 adjusted to pH 1 with HC1 revealed that Ti-15-3 was spontaneously passive in each solution, with no observed pitting potential at potentials as positive as 2.1 V

The film rupture behavior on dynamically strained Ti-15 Mo-3 Nb-3 Al exposed to 0.6 M NaC1 has been examined by rapid data acquistion of anodic current transients. The anodic current transients resulted from dislocation intersection of the passive film, followed by film rupture, bare surface dissolution, and repassivation. The transient morphology during dynamic straining differs from that generated via conventional depassivation techniques (i.e., manual scratch and fractured thin film depassivation) which incorporate an electrode that does not experience active plastic straining following depassivation. During conventional depassivation testing, current transients increase relatively rapidly and decay with an approximately linear slope on the log i-log t plot. In contrast, the transients acquired during dynamic straining are characterized by a relatively slow current increase and a nonlinear current decay on the log i-log t plot. This nonlinear decay is not attributable to ohmic or capacitive effects. The difference between the anodic transient morphologies on dynamically strained and unstrained electrodes is attributed to the combination of many discrete dislocation intersections of the surface over a period which is much larger than the time required for repassivation of a single dislocation intersection. Additionally, atomic force microscopy revealed persistent slip on a limited number of slip planes, with slip offsets as large as 600 nm, which is consistent with the formation and emergence of superdislocations. Thus, film rupture results from surface intersection of a superdislocation comprised of individual dislocations which are spatially and temporally separated. Current transient modeling of superdislocation intersection agrees qualitatively with that observed experimentally. It is concluded that the repassivation behavior determined by conventional depassivation techniques may not be relevant for modeling of environmentally assisted cracking of dynamically strained electrodes in some cases. InfroductionRecent work has indicated that modern metastabletitanium alloys are susceptible to intergranular environmentally assisted cracking (EAC) in chloride environments. Film rupture has been shown to be a requirement for EAC of these alloys.4 Other evidence, such as a requirement for dynamic straining,1-3 a dependency on loading rate,2 and EAC immunity during static loading of 3-titani-* Electrochemical Society Active Member. * * Electrochemical Society Student Member. urn alloys"2 also suggest that film rupture plays a critical