The Effect of Cu Addition on Corrosion Resistance of Al-Si-Mg-Cr Alloy

Two kinds of alloys with and without the addition of Cu, Al-7%Si-0.3%Mg-0.3%Cr and Al-7%Si-0.3%Mg-0.3%Cr-1.5%Cu, are studied in this work. The addition of Cu can notably improve notably the strength of Al alloy but it reduces its corrosion resistance. In this study, the electrochemical workstation is used to measure the open circuit potential and polarization curve of alloys and immersion corrosion is carried out. SEM and TEM images are taken before and after immersion corrosion to observe the pitting and intergranular corrosion of the alloy. Results show that the addition of Cu accelerates the immersion corrosion rate of Al alloy by 26.8% to 269.2%. This affects the peak ageing and overageing samples the most. The influence is less evident for underaged samples. At the same time, the addition of Cu aggravates the aggregation of pitting corrosion in the primary step of corrosion of Al alloy and the intergranular corrosion around and within the pitting hole. beta ''-Mg5Si6 precipitates and theta '' -Al3Cu precipitates are observed in Al-7%Si-0.3%Mg-0.3%Cr1.5%Cu alloy. The Cu atoms occupy Si3/Al site of beta '' and segregate at the edge of beta ''. It is believed that the deterioration of corrosion performance essentially is attributed to the Si-enriched particles, Al13Cr4Si4 phase and the Cu-enriched precipitates, beta '' -Mg5Si6 precipitates and theta '' -Al3Cu precipitates.