希薄アンモニア水溶液中における純銅および銅-亜鉛合金の応力腐食割れ

抄録

In the preceding paper, it was reported that 99.999% copper undergoes transgranular stress corrosion cracking in dilute ammoniacal solutions under uniaxial tensile stress with constant load and the results are interpreted in terms of the tarnish rupture model. On the basis of these results, studies were made of stress corrosion cracking for five kinds of α-brasses containing up to about 10wt.% Zn in ammoniacal solutions by varying the concentration from 0.04M to 0.07M and the temperature from 70°C to 40°C. Stress corrosion cracking occurs on all of the α-brasses under the condition in which thick tarnish film (Cu₂O oxide film) forms. The transition from transgranular to intergranular cracking occurs at about 1wt.% Zn. The path of cracking is transgranular for alloys containing less than 1.3wt.% Zn, but intergranular for alloys containing more than 1.3wt.% Zn. The mixed mode of transgranular and intergranular cracking appears on the alloy with 1.3wt.% Zn. The crack propagation rates are estimated in terms of the tarnish rupture model. Crack propagation occurs at regular time intervals, and the distance of propagation equals the film thickness formed in a unit interval. The above results are consistent with the tarnish rupture mechanism of stress corrosion cracking as with 99.999% copper.