Fatigue crack nucleation of pure copper single crystals in 1 M NaNO2 aqueous solution

The cyclic deformation of pure copper single crystals has been investigated to examine the corrosion fatigue crack nucleation in an oxide-film-forming NaNO2 solution. The corrosion fatigue tests were performed in a wide range of plastic strain amplitudes under a rest potential and a passive potential. It was found that the number of cycles to failure was reduced under the passive potentials, and crystallographic features of crack nucleation are clearly different between the tests conducted under a rest potential and a passive potential. Under the rest potential where oxide films were not formed, crack nucleation was observed at persistent slip bands (PSBs) and appeared identical with that in air. Under the passive potential, however, a number of cracks were nucleated by cleavage failure at PSBs, and the planes of the cracks were parallel to {110} planes. Crystallographic features of the cracks were essentially identical in nature to stress corrosion cracking (SCC) under monotonic deformation of copper single crystals. Variations in anodic current demonstrated that emerging PSBs caused film rupture and local anodic dissolution. The results confirmed that PSBs assisted the nucleation of SCC and thus reduction in corrosion fatigue resistance in a film-forming environment.