Abstract
The mechanism of transgranular stress corrosion cracking of pure copper in 0.05M NH4OH solution at 70°C was investigated by means of stress corrosion tests both under the constant strain rates from 1.0×10-4min-1 to 5.0×10-4min-1, and under potentiostatic anodic polarization in the range of-90.0mV to-10.0mV (SCE) at a constant strain rate of 1.0×10-4min-1. Stress corrosion cracking can be observed only at lower strain rates than 1.0×10-4min-1 which is slower than rate of tarnish film growth. Good adherent and protective film is formed over the potential range of -90.0mV to -10.0mV. The susceptibility to stress corrosion cracking is increased by anodic polarization in the potential range of -90.0mV to -50.0mV in which thick and black tarnish film is formed, but is decreased by anodic polarization in the potential range of -30.0mV to -10.0mV in which thin and dark brown film is formed. Weight losses of specimens both on the surface produced by straining and on the surface of static specimens were measured over the potential range of -70.0mV to -10.0mV. These weight losses decrease with the increase of applied potential, and the ratio of these weight losses is nearly constant and independent of applied potential and susceptibility to cracking. These results support the theory that crack propagates with the repetition of formation and rupture of the brittle tarnish films, and the distance of propagation equals the film thickness formed after each rupture.
