Impedance spectroscopy response of aluminum-copper-silicon alloys

The effect of increased Cu and Si additions and heat treatment on the polarization resistance, oxide-layer/double-layer capacitance, open circuit potential and breakdown potential of Al-Cu-Si alloys was evaluated. Electrochemical impedance spectroscopy and DC polarization techniques were combined with mechanical property evaluations to determine the relationship between the microstructure of Al-Cu-Si alloys and the ability of their oxide layer to provide protection against corrosion in an aqueous 5% NH$\sb4$Cl solution. Alloys were investigated in both the solution-treated and artificially aged conditions. The solution-treatment procedure involved heating the alloys to 550$\sp\circ$C for 30 minutes and subsequently water-quenching. Artificial aging was carried out at 250$\sp\circ$C. Increased aging times of 1, 2, 8 and 32 hours at 250$\sp\circ$C were employed on Al-Cu(2%), Al-Si(2%) and Al-Cu(2%)-Si(2%) alloys in order to correlate the observed electrochemical behavior with the precipitation strengthening reactions which occur during age-hardening. These precipitation reactions govern the resultant microstructure and determine the mechanical behavior, electrochemical behavior and type of corrosion attack observed in Al-Cu-Si alloys. In general, an increase in the susceptibility to pitting corrosion was correlated to a decrease in the protective oxide layer thickness brought about as a result of increased Cu and Si additions in solution-treated alloys and the precipitation of Guinier-Preston zones in age-hardened alloys. Cu and Si atoms at the metal/oxide interface inhibit the diffusion of Al$\sp{+3}$ ions to the oxide/electrolyte interface thereby limiting the oxide-layer thickness. Increased aging times, leading to the formation of Guinier-Preston zones and a concomitant age-hardening peak, also decrease the total number of Al$\sp{+3}$ ions available for passivation and thereby decrease the oxide-layer thickness. Consequently, the oxide-layer/double-layer capacitance and the polarization resistance of the alloys are correlated to changes in the oxide-layer thickness and the loss in continuity of the oxide-layer with overaging. In addition, the open-circuit and breakdown potentials and the type of corrosion attack observed during anodic DC polarization scans was related to the type of alloying element and their distribution