Role of grain-boundary precipitates and solute-depleted zone on the intergranular corrosion of aluminum alloy 7150

Aluminum alloy (AA) 7150 (UNS A97150) is known to be more susceptible to intergranular corrosion (IGC) in the T6 temper than in the T7 temper. The T6 temper exhibits two breakdown potentials during a potentiodynamic polarization scan, whereas the T7 temper exhibits only one breakdown potential. The corrosion morphology of the T6 and T7 tempers varies with time and potential. Using analytical transmission electron microscopy (TEM), the copper concentration in the grain-boundary precipitates of the T7 temper is seen to be much higher than for the T6 temper. However, the composition of the solute-depleted zone (SDZ) is similar in both the T6 and T7 tempers. Electrochemical tests on thin film compositional analogs of the T6 and T7 grain-boundary precipitates indicate that the breakdown potential for both is below the breakdown potential of the alloy. 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 localized microchemistry containing a low Cu ion concentration. However, dissolution of T7 precipitates results in a relatively high localized concentration of Cu ions. Dissolution kinetics of the SDZ is ennobled in the presence of Cu ions, which makes the T7 temper more resistant to IGC than the T6 temper.