IMPEDANCE-SPECTROSCOPY RESPONSE OF ALUMINUM-COPPER-SILICON ALLOYS

The effects of Cu and Si additions as well as heat-treatment on the ac impedance-spectroscopy response and anodic de polarization of Al-Cu-S i alloys were evaluated. The observed corrosion behavior was correlate d with the changes in microstructure and age-hardening response induce d through artificial aging of Al-Cu and Al-Cu-Si alloys. In the soluti on-treated condition, Si additions were found to lead to a very slight increase in corrosion resistance, while Cu additions were found to lo wer the overall corrosion resistance significantly Cu and Si additions also led to an increase in the open-circuit potentials of Al-Cu-Si al loys. Silicon accelerated age-hardening of Al-Cu-Si alloys and thereby affected their impedance spectroscopy response through changes in mic rostructure. The metastable Cu-rich precipitates that formed during ag ing interfered with the growth of the Al-alloy oxide layer and decreas ed its effective thickness. Appearance of the equilibrium CuAl2 precip itates may have resulted in the formation of structural and electrical defects in the oxide layer at or near these precipitates. As a conseq uence, the appearance of defects in the oxide layer during aging led t o an overall decrease in the corrosion resistance. These results were similar to those obtained on Al-Cu (2%) alloys, but the changes in the age-hardening and impedance responses of the Al-Cu (2%)-Si (2%) alloy s were accelerated. The type of corrosion attack observed is discussed in terms of the microstructural changes that occurred during age hard ening.