Use of the scratching electrode technique to interpret the stress corrosion cracking behaviour of α-brass in ammoniacal copper sulphate solutions

Stress corrosion cracking (s.c.c.) of α-brass in Mattsson solutions [1 M (NH 3 + NH 4 +); 0.05 M CuSO 4] at a given strain-rate intensifies in pH order: 4.75 < 7.35 < 11.35. There is no cracking at pH 8.9 or when tarnishing at pH 7.35 is prevented by reducing [NH 3 + NH 4 +] to 0.25 M. A technique of continously scratching a shallow circular track on a rotating electrode has been used to investigate the electrochemical behaviour of bared metal surfaces in these environments with the object of examining the relative contributions of slip-dissolution and tarnish-rupture in the crack propagation mechanism. It is shown that the observed s.c.c. propensity correlates well with: (i) the physico-chemical nature of the reaction products, (ii) the bare surface current density at the applied potential and (ii) oxide growth kinetics on the bare surface. Thus, in non-tarnishing solutions s.c.c. does not occur if either the rate of protective oxide formation is sufficiently rapid [pH 7.35 (0.25 M NH 3)] or the whole surface is dissolving uniformly (pH 8.9); in the intermediate case (pH 4.75) cracking is observed. In tarnishing solutions [pH 7.35 (1 M NH 3) and pH 11.35] the crack propagation rate increases with the tarnishing rate. Qualitative agreement between s.c.c. behaviour and bare surface oxide growth rate, as determined by the scratching electrode method, supports a slip-dissolution model of propagation.