Fig. 1 Simplified phase diagram of the Al-Cu-Mg-Er alloy with 0.19 wt% Er addition

Fig. 2 Intergranular corrosion sketch of the Al-Cu-Mg-Er alloys: a size of the tested samples; b maximal depth of the IGC

Fig. 3 Secondary phases in three under-aged alloys: a Al-Cu-Mg alloy; b Al-Cu-Mg-0.19Er alloy; c Al-Cu-Mg-0.34Er alloy; d composition of AlCuFeMnSi phase; e composition of Al8Cu4Er phase; f composition of θ phase

Fig. 4 Characteristics of microstructures in under-aged Al-Cu-Mg-Er alloys: orientation map of a Al-Cu-Mg, b Al-Cu-Mg-0.19Er, and c Al-Cu-Mg-0.34Er

Fig. 5 Grain boundaries of the Er-alloyed Al-Cu-Mg alloys: a under-aged Al-Cu-Mg alloy; b under-aged Al-Cu-Mg-0.19Er alloy; c under-aged Al-Cu-Mg-0.34Er alloy; d over-aged Al-Cu-Mg alloy; e composition of S-phase in d

Fig. 6 IGC morphologies of under-aged Al-Cu-Mg-Er alloys: a Al-Cu-Mg alloy; b Al-Cu-Mg-0.19Er alloy; c Al-Cu-Mg-0.34Er alloy

Fig. 7 OCP test results of Er-alloyed Al-Cu-Mg alloy in the under-aging state

Fig. 8 Potentiodynamic polarization test results of Er-alloyed Al-Cu-Mg alloy in the under-aging state

Fig. 9 Immersion test results of Er alloyed Al-Cu-Mg alloy in the under-aging state: a Al-Cu-Mg alloy; b Al-Cu-Mg-0.19Er alloy; c Al-Cu-Mg-0.34Er alloy

Fig. 10 Maximal pit depth and its average value of Er-alloyed Al-Cu-Mg alloy at under-aging state

Fig. 11 Corrosion morphologies of local region around secondary phase in the Al-Cu-Mg(-Er) alloy in the under-aged state after immersion tests in 3.5% NaCl solution for 2 h: a Al-Cu-Mg alloy; b Al-Cu-Mg-0.19Er alloy

Fig. 12 Depth of corrosion pits induced by secondary phases in the Al-Cu-Mg-0.19Er alloy