Abstract
The hydrolytic behavior of cations plays an important role in the crevice corrosion of aluminum and its alloys. Hydrolysis equilibrium reactions can either consume or produce H+ thereby altering pH. An external cathode electrolytically coupled to a crevice can also influence the pH developed in a crevice. In this study, simulated crevice experiments were performed with pure aluminum, solution heat treated (SHT) Al-3Li and SHT Al-3Cu to determine the effects of Al3+, Li+ and Cu2+ hydrolysis on steady state pH. Simulated crevice experiments were carried out with aerated bulk solutions, deaerated bulk solutions and with no bulk solution to determine the effect of a remote cathode on the steady state pH response. The pH response was interpreted in terms of distribution diagrams constructed from formation quotients and mass action equations for the appropriate hydrolysis products. Finally, the results of the above experiments were used to assess the roles of hydrolysis and the external cathode in determining the steady state pH measured in the ternary alloy Al-3Cu-2Li (AA 2090). In all experiments crevice acidification occurred when the bulk solution was aerated. When the bulk solution was deaerated or when no bulk solution was present a mildly alkaline crevice pH developed. Analysis of distribution diagrams shows that Al3+ hydrolysis can generate an acidic to neutral crevice solution. Lithium hydrolysis does not occur until a pH of 11 and is not an important process at the pH values observed here. However, lithium dissolution can assist in generating mild alkalinity. Evidence also suggests that some Cu2+ hydrolysis occurs contributing to the alkaline pH observed for isolated crevice in SHT Al-3Cu and SHT 2090.
