Vacancy generation in electrochemical oxidation / dissolution of copper in NaNO2 solutions and its role in SCC mechanism

Interaction of copper with the oxide layers growing on it at various potentials during electrochemical polarization in NaNO₂ solution was studied. It was shown that anodic oxidation/dissolution of copper is accompanied with generation of vacancies at the oxide/metal interface, when cuprous oxide, partially coherent to copper substrate, becomes unstable. The ingress of vacancies into substrate results in a significant rearrangement of dislocation substructure and relaxation of stresses in the plastically deformed copper substrate. If the anodic oxidation/dissolution of annealed pure copper is continued in the absence of effective annihilation mechanisms for the generated vacancies, i.e., diffusion or dynamic plastic deformation, dislocation sources, such as Bardeen-Herring sources, start to operate close to the film substrate interface, which results in measurable plastic strain. The influence of anodic oxidation/dissolution on dynamic straining conditions of pure copper at various temperatures was also studied. The potential and current density for anodic dissolution in NaNO₂ solution were higher if the substrate was simultaneously dynamically strained. It was shown that excessive vacancies were generated under constant current anodic oxidation/dissolution at the potential of about 100 mV_SCE, which led to a significant increase in the steady state creep rate. In order to measure the strain introduced by electrochemical oxidation/dissolution in NaNO₂ solution, plastic deflection of copper strips in one-sided oxidation was studied. Deflection is due to plastic relaxation of elastic stresses stimulated by annihilation of vacancies near the surface layer, i.e., rearrangement of dislocation assemblies. The influence of oxidation/dissolution processes on dislocation substructures in copper was confirmed by low-temperature internal friction measurements and TEM. High-purity copper was used in the above mentioned experiments. The results are discussed with a possible model developed for TGSCC based on electrochemical oxidation/dissolution of copper, where the major influence of vacancies generated at the crack tip on crack initiation and growth is described.