The Effect of Cu and Zn Addition on the Intergranular Corrosion Resistance of Al-Mg-Si Alloy

Abstract

Aluminum alloys are widely used for automobile and aircraft industries because of their good strength-to-weight ratio. In general, Cu improves the strength of Al alloys but reduces localized corrosion resistance. For Al-Mg-Si alloys, it has been reported that Cu decreases intergranular corrosion resistance. In this study, the effect of Cu and Zn on intergranular corrosion resistance was analyzed by electrochemical measurements.As specimens, Cu-free alloy, Cu-added alloy, and Cu- and Zn-added alloy were prepared based on AA6061 composition. The extruded specimens were subjected to T6 heat treatment. Intermetallic compounds were characterized by SEM-EDS and TEM. Open-circuit potentials (OCPs) was measured in 0.1 M NaCl (pH 6.0). To assess the distribution density of the initiation sites of localized corrosion, the size of electrode area was 100 mm2, 9 mm2, or 1 mm2. In the electrochemical measurements, Ag/AgCl (3.33 M KCl) was used as the reference electrode.At the beginning of immersion, OCP was −1.2 V (vs. SHE) for all specimens, and OCP increased with time. After 4000 s, the OCP of Cu-free alloy became −0.5 V, the OCPs of Cu-added alloy and Cu- and Zn-added alloy became −0.4 V. After 3 h immersion, intergranular corrosion was generated on Cu-added alloy and Cu- and Zn-added alloy. In contrast, no intergranular corrosion was observed on Cu-free alloy. Based on intergranular corrosion length, it was shown that intergranular corrosion resistance of Cu- and Zn-added alloy is low compared with the others. This result suggested that Zn promotes the growth of intergranular corrosion. Moreover, the number of intergranular corrosion damages was assessed and found to be less on Cu- and Zn-added alloy than on Cu-added alloy. This result suggested that Zn decrease the number of initiation sites of intergranular corrosion. Additionally, 3-h immersion of Cu-added alloy and Cu- and Zn-added alloy was conducted with the electrode area of 9 mm2 and 1 mm2. For both alloys, intergranular corrosion occurred at 10 sites in 100 mm2 and 1 site in 9 mm2. And no intergranular corrosion occurred in the case of 1 mm2 electrode area. The distribution density of the initiation sites for intergranular corrosion on Cu-added alloy and Cu- and Zn-added alloy was estimated to be approximately 1 site per 9 mm2.