Influence of Aging at 200 C on the Corrosion Resistance of Al-Li and Al-Li-Cu Alloys

Abstract

The influence of heat treatment on the microstructure and corrosion resistance of an aluminum-lithium alloy and two aluminum-lithium-copper alloys was investigated. Aging the Al-Li alloy resulted in the precipitation of δ′ with precipitate-free zone (PFZ) formation along the grain boundaries. The intragranular precipitation of δ′ did not influence either the morphology of pitting or the magnitude of the pitting potential. Pits appeared to consist of aggregates of submicron-sized cuboidal volumes. Their faceted shape suggests the strong influence of crystallographic factors. Anodic polarization of the Al-Li-Cu alloy in the T8 condition in aqueous solutions containing chloride ions resulted in blistering of the passive film and the formation of pits that were hemispherical in shape. Aging the Al-Li-Cu alloy resulted in the precipitation of Cu-rich phases with PFZ formation along the grain and subgrain boundaries. Energy dispersive x-ray (EDX) microchemical analyses indicated the PFZ was depleted of copper. Anodic polarization of aged samples in chloride ion media resulted in localized attack along the subgrain and grain boundaries. Such attack did not occur in solutions free of chloride ions. The electrochemical tests and microchemical analyses suggest that the boundary corrosion was caused by the pitting corrosion of copper-depleted zones.