A number of observations1-3 have been made with regard to the pH and composition of the solution within the advancing crack of a stress corrosion cracking specimen of a high strength aluminum alloy. The consensus conclusion was that the solution is acid (about pH 3.2) and there is a buildup of aluminum ion concentration within the crack. This same conclusion was reached after the examination of pits and crevices on aluminum.4,6 These results are usually explained by the formation and hydrolysis of aluminum chloride, the majority of the experiments being conducted in chloride solution.
A study of the hydrolysis of aluminum salts including the nitrate, bromide, chloride, sulfate, perchlorate, and iodide indicated that the pH’s of all of the 0.1 N solutions were low, ranging from 2.80 to 3.76.7 On this basis, it would be expected that the composition of the solution in stress corrosion cracks in...
