Environmental Factors Affecting Galvanic Corrosion of AA7075
Nearly 80% of cracks on military aircraft have been found to initiate near corrosion sites. In particular, these corrosion sites coincide with locations of dissimilar metals, such as within fastener holes, joining aspects of an aircraft together. Because aluminum alloys (AA) have high strength to weight ratio, they are used extensively within the aircraft skin. Unfortunately, recent data suggests that AA7xxx series may be more susceptible to stress corrosion cracking than previously thought. Previous research has shown there is indeed a relationship between stress corrosion cracking and the environment, however, there is a gap in knowledge of which driving conditions of the environment causes cracking. Thus, this work seeks to understand the relationship between the environmental parameters and intergranular corrosion (IGC) driving forces during a cyclic atmospheric exposure. In this study, atmospheric exposure tests with alternating immersions were performed to induce corrosion along the top surface and within the fastener hole of AA7075 panels coupled with stainless steel (SS) 316. An image analysis application was developed to characterize and quantitatively measure the corrosion damage occurring from metallographic samples prepared from the cross-section of AA7075 samples. The atmospheric conditions simulated field environments by cycling between high, intermediate, and low levels of relative humidity (RH). Also, a simulated seawater (SSW) solution at acidic, near-neutral, and basic pH was used on the panels. Additionally, potentiodynamic polarization tests were performed to study the electrochemical kinetics of corrosion in different pH levels of SSW. The cross-sectional analysis at different exposure times showcased 3 stages of IGC growth. RH cycles with a low intermediate RH presented nucleation of corrosion features. When the RH was below the deliquesce of NaCl, IGC penetration was visible, and near the deliquescence branching of the features was shown. It was observed that corrosion damage occurring on the top surface was dependent on the SSW droplet behavior following RH cycles, which linked to the presence of oxygen and salt concentration of the droplet. Deaerated electrochemical conditions highlighted how corrosion damage initiated within the fastener hole interface as well as within the tip of a localized corrosion feature.