| Abstract: | The paper describes a study of stress corrosion in brass (62·8 per cent Cu, 37·2 per cent Zn) exposed to ammoniacal copper sulphate solution. The relation between the stress corrosion behaviour and the composition of the corrosive medium is discussed against the background of potential/ pH diagrams for the corroding system. The time to cracking was found to be very dependent on the pH. At pH 2·0 cracking did not take place within 1000 h of exposure. At about pH 7·3 the time to cracking was at its minimum value, 1–2 h. The results indicated that the process is governed by the formation of a black surface coating of copper oxide on the metal, but counteracted by the precipitation of basic copper sulphate. The cracks were generally transgranular in the absence of black oxide but intergranular in the presence of such a coating. It appeared that the shape of the cracks was influenced by the precipitation of basic copper sulphate. Reduction reactions likely to cause metal dissolution during stress corrosion are discussed for different pH ranges. According to the potential/pH diagram the formation of Cu(NH3)2+ ions is in general important. |
