Localized corrosion process of Al-Zn-Mg-Cu-Zr alloy: Transitions from pitting corrosion to intergranular corrosion

The localized corrosion mode and process dependence on ageing conditions for Al-7.82Zn-1.99Mg-2.41Cu-0.12Zr alloy were investigated. Intergranular corrosion (IGC) is the dominant corrosion mode for UA temper alloy due to the small size (8.48 & mu;m) and low distribution continuity (L-GBPs/GB=0.75) of grain boundary precipitates (GBPs), whereas pitting corrosion is the dominant corrosion mode for T74 and T77 temper alloys because the large size (>18.5 & mu;m) and discontinuous distribution (L-GBPs/GB<0.5) of GBPs inhibit the occurrence of IGC. The corrosion mode of T6 temper alloy evolves with immersion time from pitting corrosion to IGC. Pitting corrosion is initiated at the site of Al7Cu2Fe particles. Al7Cu2Fe particle with larger size shows a higher potential difference value of 58.4 mV with the matrix, implying that larger Al7Cu2Fe particle is preferred to be the initial sites for pitting corrosion. At the initial stage of corrosion, the surface of T6 temper alloy is attacked by pitting corrosion. Al and Mg elements in the matrix around Al7Cu2Fe particles are preferentially dissolved. As the immersion time prolongs, the surface is attacked by intergranular corrosion.

Automated summary

The localized corrosion modes and their dependence on ageing conditions were investigated for an Al-7.82Zn-1.99Mg-2.41Cu-0.12Zr alloy. Intergranular corrosion was dominant in the UA temper due to small grain boundary precipitates (GBPs) and their low distribution continuity. Pitting corrosion was dominant in the T74 and T77 tempers due to larger GBPs and their discontinuous distribution. The corrosion mode of the T6 temper evolved from pitting corrosion to intergranular corrosion with immersion time.