Modeling of Anodic Current Transients Resulting from Oxide Rupture of Plastically Strained β + α Titanium

Abstract

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

Effect of Variable Stress Intensity Factor on Hydrogen Environment Assisted Cracking

“…A value of 35 ls for 1=k à , where t 1=2 ¼ 0:693=k à ¼ 24 ms, is comparable to rise times (10 to 20 ls) and t 1=2 decay times (10 to 25 ls) reported by Kolman et al [40] for corrosion currents measured on circumferentially notched tensile bars tested at a constant extension rate. The value for C à LS was chosen based on the results of notched tensile tests conducted using cathodically charged specimens and reported by Gaudett and Scully.…”

“…The fitted value will be compared to the measured k * values obtained by Kolman, Gaudett and Scully. [40] The calculated magnitude of _ a à is within a factor of 2 of the maximum crack growth rates obtained in Gangloff's analysis [39] of HEAC crack growth rate data for a range of alloys exhibiting a wide range of hydrogen diffusivities.…”

Effect of passive film on mechanical properties of martensitic stainless steel 15-5PH in a neutral NaCl solution

“…In addition, the presence of passive films on the surface affect the mechanical properties of stainless steels [11][12]. This effect is closely related with mechanical and electrochemical properties Page 4 of 32 A c c e p t e d M a n u s c r i p t 3 of the passive films and the microstructure of stainless steels.…”

Stress corrosion cracking (SCC) and hydrogen-assisted cracking in titanium alloys

“…This can be studied through anodic current transients and atom force microscopy (Kolman et al, 1998), as well as several other surface characterization techniques. A current transient that lasts longer indicates an inability to repassivate quickly.…”