Effects of straining on oxide films and passivity of copper in nitrite solution at ambient temperature

The effects of strain rate on interactions between copper and its oxide films have been studied. The electrochemical oxidation process of copper is accompanied by the generation of vacancies in the copper substrate. Diffusion of vacancies from the oxide/metal interface or annihilation of vacancies by dislocation reactions is essential for oxidation to continue. Sufficiently slow straining without breaking the passive film on copper leads to a re-arrangement of the dislocation sub-structure at the interface, which helps to consume the oxidation-generated vacancies. The balance of slow straining and oxidation/dissolution produces environmentally-enhanced plasticity in the copper substrate, and simultaneously, accelerated corrosion. These conditions occur as long as the strain rate is low, i.e., on order of less than 10⁻⁸ s⁻¹. The effects of slow straining on oxidation/dissolution of high purity copper were studied at various strain rates, even as low as 10⁻¹⁰ s⁻¹. The electrochemical oxidation behaviour of cold-deformed copper was recorded by dynamic polarization scans. The synergistic effects of oxidation/dissolution process and straining on the mechanical properties of metal, i.e., dislocation sub-structure of copper were studied by TEM. The results are discussed in conjunction with a TGSCC model (SDVC, Selective Dissolution – Vacancy Creep). Central to the TGSCC model is the concept of vacancies generated in the process of oxidation/dissolution taking part in substrate recovery and playing a key role in the stress corrosion crack growth behaviour.