INFLUENCE OF HEAT-TREATMENT AND GRAIN SHAPE ON EXFOLIATION CORROSION OF AL-LI ALLOY-8090

Exfoliation of aluminum-lithium (Al-Li) alloy 8090 (UNS A98091) in plate and sheet form was studied by measuring the compressive forces generated by voluminous grain boundary corrosion products. The forces were shown to be related closely to grain shape, with more elongated microstructures generating higher forces. Aging treatment of the alloy also was a factor. Corrosion product forces were ranked in the order: under-aged > over-aged > peak-aged. High susceptibility of the over-aged temper in 8090 differed from that of conventional, precipitation-hardened Ai alloys and was attributed to the formation of anodic phases in the Li-containing material. Stress corrosion tests were performed on the plate using double-cantilever beam specimens. The sheet was tested under constant load in the through-thickness direction. K-iscc values, the limiting values of stress intensify for stress corrosion to occur, were compared with the maximum corrosion product forces and supported the proposition that exfoliation proceeded by a stress-assisted corrosion mechanism.