INTERGRANULAR AND EXFOLIATION CORROSION STUDY OF Al-Li-Cu-Mg-Zr ALLOYS

Abstract
The 8090 type Al-Li alloys are prone to intergranular and exfoliation corrosion in the peak aged temper. In marine atmosphere exfoliation corrosion occurs within 2 years. The 48 H EXCO test give a good prediction of the corrosion susceptibility of 8090 type alloys in marine atmosphere. Under-ageing and over-ageing are two possibilities to suppress exfoliation susceptibility. Exfoliation corrosion is always intergranular on specimens examined in this study. Intergranular corrosion is usually explained by the existence of a continuous anodic path at grain boundaries. The anodic path in Al-Li-Cu-Mg alloys is first a Cu depleted zone. Intergranular and exfoliation corrosion only occur in Cu containing Al-Li alloys. The corrosion potential Eo of Al-Li alloys reflect the Cu content in solid solution and is not affected by Li addition. The pitting potential also reflect the Cu content of the solid solution, it is shifted 30 mV in the active direction by Li addition at the PA temper. Li addition prevent intergranular corrosion desensitization observed for 2XXX AA, in the PA temper.