Electrochemical microscopy: An approach for understanding localized corrosion in microstructurally complex metallic alloys

A method has been developed to map electrochemical data from constituent particles comprising intermetallic compounds found in aluminum alloys onto real or virtual alloy microstructures with micrometer resolution. This method, referred to as "electrochemical microscopy" facilitates presentation of phase-specific microelectrochemical data by presenting it visually within the context of alloy microstructure. In this paper, several examples of the approach are given using electrochemical reaction rates of the various phases in Al alloy 7075 at the prevailing alloy corrosion potential (-0.8V _sce). In near-neutral dilute chloride solutions, the resulting maps show expected high net anodic reaction rates at Mg₂Si particles and high net cathodic reaction rates at Al₇Cu₂Fe and Al ₃Fe particles. When the pH is varied from 2.5 to 12 and chloride concentration is varied from 0.01 to 0.6 M, the maps capture the changes in relative reaction rates. A comparison to corrosion morphologies developed during free corrosion experiments with the alloy show that electrochemical microscopy captures forms and intensity of localized corrosion as well as trends associated with systematic changes in environment chemistry.