A SECM Study of Heterogeneous Redox Activity at AA2024 Surfaces

Scanning electrochemical microscopy (SECM) was used to spatially resolve the heterogeneous cathodic activity at AA2024 surfaces. Experiments used a 10 μm diameter Pt microelectrode in a solution containing the protonated form of (dimethylamino) methylferrocene [DMAFc⁺, i.e., the Fe(II) state]. The tip was brought near the alloy surface while held at a potential of 0.65 V vs. SCE, where DMAFc⁺ is oxidized to DMAFc²⁺ [i.e., the Fe(III) state]. The AA2024 substrate was held at -0.75 V, where the DMAFc ²⁺ produced near the probe tip is reduced back to DMAFc⁺ at regions on the surface that were cathodically active. Lateral variation in the DMAFc⁺ oxidation current at the tip arises either from a decrease of the diffusive delivery of DMAFc⁺ to the tip due to proximity to the surface or from a positive feedback mechanism in which the DMAFc²⁺ that had been oxidized at the probe tip was reduced at the substrate. The images show locally high redox reactivity which is attributed to second phase, intermetallic inclusions. Comparison of the SECM images with scanning electron microscopy-energy dispersive spectroscopy images shows that the regions of high redox reactivity correlate with the locations of the intermetallic particles.