Effect of Pretreatment on the Intermetallics in Aluminum Alloy 2024-T3

The effect of surface treatments on the surface characteristics of aluminium alloy 2024-T3 before the appearance of filiform corrosion (FFC) is investigated. The nature of the surface prior to coating and initiation of FFC, with particular respect to the intermetallics is investigated in this work. The SKPFM (Scanning Kelvin Probe Force Microscopy), ToF-SIMS (Time of Flight Secondary Ion Mass Spectroscopy), XPS (X-Ray Photo Electron Spectroscopy) and SEM (Scanning Electron Microscopy) surface analysis techniques were used to characterize polished AA2024-T3 before and after etching or after etching with subsequent chromating treatments. The etching pretreatment is intended to remove surface intermetallics and increase the oxide layer thickness. In these respects, the treatment was partially successful: some, not all, of the particles were eliminated from the surface and the oxide thickness increased by about 25%. In addition, XPS depth profiling showed a copper and iron enrichment at the oxide-metal interface from this treatment. The oxide thickness is drastically increased following the application of the chromate conversion coating. Furthermore, XPS and ToF-SIMS analysis revealed that Cu and Fe were enriched on the oxide surface indicating that either Cu is complexed into the Cr-Al layer following chromating or that the chromating coating is insufficiently formed on the Cu-containing intermetallics. SKPFM analysis provided further information on the distribution of these complexes on seen by distinct points of high potential on the treated alloy surface.