Effect of microstructure on corrosion behaviour of Al-Cu-Li alloys

Abstract

The corrosion behaviour and its relationship with microstructure, especially the distribution of precipitates at grain boundaries and within grains, of two Al-Cu-Li alloys with different Cu/Li ratio were investigated using accelerated immersion testing, advanced microscopy characterization and quasi-in-situ corrosion observation. The Al-2.23Cu-0.96Li alloy with low Cu/Li ratio exhibited excellent intergranular corrosion resistance and its corrosion modes unusually remained consistent intragranular corrosion regardless of the ageing stage. The corrosion behaviour of the counterpart Al-4.03Cu-0.91Li alloy with high Cu/Li ratio was found to follow the general evolution regulation, from intergranular corrosion to intragranular corrosion with the ageing time. Microstructure characterizations demonstrated that the coarsening and discontinuous distribution of grain-boundary precipitates played the key role in lowering the intergranular corrosion susceptibility of the two Al-Cu-Li alloys. Further quasi-in-situ corrosion observation of the Al-2.23Cu-0.96Li alloy showed that the dealloying process of the grain-boundary precipitates led to the dissolution of the precipitate-free zone, which meant that the broadening of the precipitate-free zone along the grain boundary could reduce the intergranular corrosion resistance of the alloy. Besides, the thin T 1 precipitates that were uniformly distributed within the grains did not suffer preferential dissolution, but they still affected the intragranular corrosion propagation at the nanoscale. Specifically, the corroded T 1 precipitates could become {111} Al -oriented sidewalls of the intragranular corrosion channel. • The Al-2.23Cu-0.96Li alloy kept low IGC susceptibility regardless of the ageing stage. • Intragranular T 1 precipitates do not preferentially dissolve due to dealloying. • Nanoscale corrosion behaviour was investigated through quasi-in-situ TEM experiment.