Role of grain boundary precipitates and solute depleted zone in the intergranular corrosion of aluminum alloy AA7150

Aluminum alloys exhibit vastly different corrosion behavior depending on the temper. For instance, potentiodynamic scans on 7150-T6 and T7 indicated that the T6 temper exhibit two breakdown potentials whereas the T7 tempers exhibit only one breakdown potential. Potentiostatic testing of the T6 and T7 temper for 1 h and 24 h. The corrosion morphology of the T6 temper varied with time and potential. The T7 temper also exhibited a change in corrosion morphology with time under conditions of potetniostatic testing. Analytical TEM on the T6 and T7 temper proved that the copper concentration in the grain boundary intermetallics of the T7 temper was much higher than the T6 temper. The composition of the solute depleted zone (SDZ) in both the T6 and T7 temper was similar. In summary there is no clear understanding of the role of temper and microstructure on the corrosion of 7xxx series alloys.; The approach taken to shed light on this problem was to prepare compositional analogs of the grain boundary constituents and characterize their electrochemical behavior.; Electrochemical tests on the T6 and T7 grain boundary precipitate compositional analogs found that the breakdown potential for both was below the breakdown potential of the alloy. Therefore the effect of temper cannot be explained directly from the behavior of the grain boundary precipitates. The composition of the SDZ in both tempers was essentially identical, hence the effect of temper cannot be attributed to differences in the SDZ. An explanation for the effect of temper was developed based on indirect influence of the grain boundary precipitates on the SDZ behavior. The notion is that dissolution of the grain boundary precipitates controls the microchemistry at the grain boundary. The dissolution of the T6 grain boundary precipitates results in a microchemistry that has a little or no Cu ion concentration. However, the dissolution of the T7 precipitate may result in a microchemistry that contains a high concentration of Cu ions in solution. The dissolution kinetics of the SDZ is ennobled in the presence of Cu ions, the T7 temper is more resistant to IGC than the T6 temper.