Influence of local redox activity at Al alloys on corrosion

Imaging the surface of AA2024-T3 aluminum alloy using scanning electrochemical microscopy (SECM) allows identification of individual intermetallic particles (IPs) and determination of their locations on the surface. Following this, approach curves are used to examine the electrochemical properties of individual IPs and of regions of the surface devoid of IPs. It is found that the bare surface regions exhibit net blocking behavior. In this case, diffusive delivery of the solution-phase redox couple used for the SECM imaging is impeded due to proximity of the SECM probe tip to the surface. This produces decreasing currents as the probe tip approaches the surface. In contrast, the larger IPs exhibit positive feedback behavior. In this case, the solution-phase redox couple is repeatedly oxidized at the tip and reduced at the IP, giving an increasing current as the tip approaches the surface. The relevance of these observations to corrosion of these alloys is discussed.