Comparison of the Corrosion Behavior of Modern and Traditional High Strength Aluminum Alloys

In recent years, design requirements for new aircraft have required materials engineers to develop a stronger yet lighter aluminum alloy than the traditional AA7075 and AA2024 type alloys used previously. The aluminum-lithium alloy system is attractive to the aerospace industry due to the its lower density, increased elastic modulus, increased fatigue crack growth resistance, formation of strengthening phases, and increased corrosion resistance when compared to AA7075 and AA2024. Third generation aluminum-lithium alloys have proven to be a viable replacement for incumbent alloys AA7075 and AA2024 in aircraft structures. A key interest in these alloys is increased corrosion resistance and much research has been done to understand the corrosion mechanisms present in 3^rd generation aluminum-lithium. Despite this, no direct comparison of the bulk corrosion behavior observed in AA7075 to that observed in a 3^rd generation aluminum-lithium alloy, like AA2099, has been reported. To this end, B117 and G*% salt fog exposure tests were performed on AA7075, AA7050, AA2099, and AA2024 plate samples, in the longitudinal (L), short transverse (ST), and longitudinal transverse (LT) directions, to compare their corrosion behavior.  AA2524 sheet alloy was also compared in the longitudinal direction. Three tests were performed in the B117, an interrupted test of 72 hours, 120 hours, and a longer duration 168 hours test, with analysis of the results performed by optical microscopy, optical profilometry, and scanning electron microscopy. Optical microscopy of the AA7075, AA7050, and AA2024 samples generally showed localized shallow pitting with trenching around some secondary phase particles as well as areas of localized corrosion on the ST and LT directions. In contrast, AA2099 samples featured small pits across the sample surface with no evidence of trenching due to the alloys lack of secondary particles. Localized corrosion was not observed in the AA2099 samples at all exposure conditions. Analysis of the interrupted test samples by optical profilometry reveals that for a lower limit threshold of 20.0 microns and an upper limit threshold of 1500 microns, AA7075 and AA2024 featured more pits than the longitudinal, short-transverse, and longitudinal-transverse directions of AA2099. AA7075, AA7050, AA2524, and AA2024 samples were revealed to contain a larger number of pits that were slightly larger in size in all three directions. Examination of the samples exposed to G85 salt fog cabinet for 72 hours displayed localized corrosion in all directions for each alloy, with AA2099 having a fewer number of pits than the other alloys. These pits were deeper in depth than those observed in the other alloys. AA7050 and AA2524 also featured a lower number of pits than the more traditional alloys tested. Gravimetric data comparing the B117 and G85 cabinet exposures indicated that B117 had a more severe attack on all alloys, with the more traditional alloys exhibiting a higher mass loss than the modern alloys.  Examination of the pitting potential by cyclic polarization in aerated and deaerated solutions will also be presented.