Abstract
It was observed that tough pitch copper and 99.999% copper underwent transgranular stress corrosion cracking in 0.05M NH4OH solution at 70°C under uniaxial tensile stress with constant load. On the basis of this result, studies were made of the mechanism of stress corrosion cracking of pure copper in ammoniacal solution by varying the concentration from 0.03M to 0.07M and the temperature from 40°C to 70°C. The formation of tarnish film (Cu2O oxide film) which is protective against corrosion and has a good adherence to the copper surface is essential to stress corrosion cracking. There is a critical concentration below which stress corrosion cracking occurs. When concentration exceeds this value, the good adherent film does not form and thereby stress corrosion cracking susceptibility is lowered. Stress corrosion cracking can occur only at the critical concentration or at the lower concentration in which a good adherent film is formed. Crack propagation occurs at the interval of constant time; 2-4 hours in 0.05M at 70°C, 2-4 hours in 0.04M at 70°C and 3-4 hours in 0.05M at 55°C. Assuming that the distance of propagation equals the film thickness formed in this interval, crack propagation rate is estimated at 0.35×10-3mm/hr in 0.05M at 70°C, 0.28×10-3mm/hr in 0.05M at 55°C and 0.21×10-3mm/hr in 0.04M at 70°C, respectively. In other words, crack propagation occurs at the interval of about 4 hours and the distance of propagation is 1.4μm in 0.05M at 70°C, 1.1μm in 0.05M at 55°C and 0.8μm in 0.04M at 70°C respectively. The values of time to failure calculated for these crack propagation rates approximate to the values of observed time to failure. The above results are consistent with tarnish rupture mechanism of stress corrosion cracking.
