Corrosion & Protection Center, UMIST
Engineering Department, Tarbiat Modares University
Natural cases of dealloying or stress-corrosion cracking in copper alloys normally occur in oxygenated solutions, where the cathodic reactant is O2 or Cu2+. Within cavities or under deposits, the local environment is enriched in cuprous ions (e.g. CuCl2-, Cu (NH3)2+) and the potential is close to the Cu/Cu+ equilibrium. Such conditions can be simulated macroscopically by stirring powdered Cu2O and Cu in NaCI or NH3 solutions, and the rate of dealloying can be monitored electrochemically at the Cu/Cu+ equilibrium potential. For a -Brass, the gross form of dealloying seen in chloride solutions does not normally cause stress-corrosion cracking, which is correlated with the superficial form of dealloying seen in ammonia solutions or in chloride solutions containing ammonium ions. The latter is especially significant, as the effect occurs at pH 5 where there is only a minute complexing action of ammonia. The results indicate that NH4+ ions have a special action in relation to dealloying, which is quite separated from the complexing action of NH3 in neutral or alkaline solutions. The action of alloyed arsenic in inhibiting dealloying seems to be similar to the action of NH4+ ions. Thin dealloyed layers formed in cuprous ammonia solutions have been shown to nucleate unstable cleavage fracture in thin foils of a-Brass.