The influence of microstructure on the electrochemical response and intergranular corrosion of Al-Cu-Li alloy AA2050

Abstract

Microstructure is one of the critical factors that can dictate the corrosion of aluminium alloys. When a heat treatable aluminium alloy undergoes ageing, the evolution of microstructure may cause extensive changes in the corresponding corrosion behaviour. When using electrochemical test methods, the corrosion response of Al-alloys can be assessed by relating the electrochemical response with microstructural evolution and features. This work presents an attempt to research the relationship between electrochemically determined pitting and the precipitate microstructure of AA2050, a contemporary Al-Cu-Li aerospace alloy. It is revealed that AA2050 has a peculiar corrosion behaviour with respect to other age hardenable Al-alloys. During low temperature ageing, the corrosion morphology of AA2050 changes from intergranular corrosion (IGC) to intragranular corrosion then back to intergranular corrosion with prolonged aging. To understand the mechanism of the corrosion morphology evolution, AA2050 samples with 11 different ageing conditions were tested. Electrochemical polarisation (potentiodynamic and potentiostatic), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Scanning Auger microprobe (SAM) analysis, microhardness and immersion testing were performed on the samples. The relationships between microstructure, electrochemical response and the corrosion morphology of samples with different heat treatment conditions were studied. As the corrosion mechanism of AA2050 is unique, this work makes an initial contribution to the salient points of the corrosion of such alloys. Several possible hypotheses are given to rationalise an apparent contrariness between the observed microstructure and the corrosion/electrochemical behaviours. Of key relevance, is that the alloy appears to be immune to conventional (nanostructure driven) pitting, owing to a persistently fine precipitate width, whilst solute segregation – studied to some degree herein – of Li, remains a challenge to detect and will be required in the future for a complete assessment of the alloy performance.