MODELING OF ANODIC CURRENT TRANSIENTS RESULTING FROM OXIDE RUPTURE OFPLASTICALLY STRAINED BETA+ALPHA TITANIUM

The film rupture behavior on dynamically strained Ti-15 Mo-3 Nb-3 Al e xposed to 0.6 M NaCl has been examined by rapid data acquistion of ano dic current transients. The anodic current transients resulted from di slocation 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 experienc e active plastic straining following depassivation. During conventiona l depassivation testing, current transients increase relatively rapidl y and decay with an approximately linear slope on the log i-log t plot . In contrast, the transients acquired during dynamic straining are ch aracterized by a relatively slow current increase and a nonlinear curr ent decay on the log i-log t plot. This nonlinear decay is not attribu table to ohmic or capacitive effects. The difference between the anodi c transient morphologies on dynamically strained and unstrained electr odes is attributed to the combination of many discrete dislocation int ersections 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 li mited number of slip planes, with slip offsets as large as 600 nm, whi ch is consistent with the formation and emergence of superdislocations . Thus, film rupture results from sc-face intersection of a superdislo cation comprised of individual dislocations which are spatially and te mporally separated. Current transient modeling of superdislocation int ersection agrees qualitatively with that observed experimentally. It i s concluded that the repassivation behavior determined by conventional depassivation techniques may not be relevant for modeling of environm entally assisted cracking of dynamically strained electrodes in some c ases.