Galvanic Interactions on a Coupled Microelectrode Array of AA7050-T7451 and 316 Stainless Steel in Chromate-Containing NaCl Solutions Under Thick Electrolyte Film and Cyclic Wet-Dry Conditions

Abstract

The electrochemical behavior of an AA7050-316SS galvanic couple in chromate-containing NaCl environments under relatively constant thick electrolyte films and wet-dry cycling was investigated utilizing the coupled microelectrode array (CMEA) approach. The CMEA approach provided a means to analyze the in situ electrochemical kinetics as a function of spatial location and time. In inhibitor-free environments, the total net anodic charge associated with galvanic current increased with increasing conductivity and aggressiveness of the environment. The AA7050 electrodes supplied more than half of the total net cathodic charge in relation to the 316SS (316 stainless steel) electrodes in the more aggressive environments. Under thick electrolyte films, chromate became less effective at suppressing cathodic kinetics on the 316SS and AA7050 net cathodes as the chloride concentration increased. Under wet-dry cycling conditions, the effectiveness of chromate was diminished when compared to thick electrolyte film conditions, due to the cycling of the equilibrium chloride concentration as electrolyte thickness changed upon onset of drying and wetting while the loading density of the salt remained constant. Furthermore, chromate exhibited a diminished ability to suppress cathodic currents on the AA7050 net cathodes in comparison to the 316SS electrodes. This study highlighted the importance of Cu-rich intermetallic particles and replated Cu on precipitation-strengthened Al alloys when considering the driving force of cathodes in sustaining anodic dissolution in typical Al alloy macro-galvanic systems exposed to atmospheric conditions.