The objective of the research reported in this paper was to refine an existing model for chromate conversion coatings (CCC) formed on the surface of AA2024-T3, an Al-Cu aircraft alloy, by considering the composition and structure of the CCC formed on constituent intermetallic compounds (IMCs). To achieve this aim it was necessary to develop large-area samples composed of compositionally homogeneous thin films of the various IMCs found on the AA2024-T3 surface, which were galvanically attached to thin films of Al-4.2wt.%Cu (representative of the AA2024-T3 matrix). This was performed in a two-step process: disks of IMC compositions were formed by reactive arc melting (RAM), followed by femtosecond laser ablation of the RAM IMCs, resulting in the formation of homogeneous thin films. These thin films were used to analyze the formation of CCC on IMCs, the AA2024-T3 matrix analog and matrix-IMC galvanic couples. Secondary ion mass spectrometry depth profiling revealed significant variations in CCC film thickness and composition related to the underlying IMCs. The SIMS results indicated that the CCC formed on the matrix analog had the same average thickness as the average CCC formed on AA2024-T3, whereas those formed on individual q and S phases were only 9% and 12% as thick, respectively, when uncoupled to the matrix and 11% and 14% as thick when coupled to the matrix. The CCC thickness on the Al 20 Cu 2 (MnFe) 3 was found to be variable, having an islanded structure. Topographical maps indicate that the IMC/matrix boundaries are covered with CCC of a thickness approaching that of the matrix. Inhibition of CCC growth is related to the high copper content (and therefore insufficient aluminum) present in the IMCs compared to the matrix, which provides a more accurate model of CCC formation on AA2024-T3.

This study explores the repassivation ability of a scratch in a chromate conversion coating (CCC) on aluminum alloy, AA2024-T3, and hence evaluates the theory of migration of hexavalent chromium ions from the protected surface of the aluminum alloy to the exposed surface. To confirm that protection was indeed restored by hexavalent chromium ions, the repassivation of a scratch on pure aluminum exposed to a dichromate solution was studied. This forms the simplest subsystem model of the CCC on the alloy in which the CCC is replaced by pure hexavalent chromium and alloy with pure aluminum. Opencircuit potential measurements, synchrotron infrared microspectroscopy (SIRMS) and secondary ion mass spectroscopy (SIMS) have been used judiciously to evaluate the repassivation behavior. Results indicate that the dichromate ions have high mobility. The slow migration of Cr(VI) ions from the protected surface to the scratch is observed to result in repassivation, as seen from the steady increase in the potential in 0.05 M NaCl solution. The results obtained from SIMS and SIRMS confirm the migration of the oxyanions from the protected region to the metal surface exposed by the scratch. The SIRMS results indicate the formation of an Al(III)-Cr(VI) complex, proposed and shown to be formed in the pits.

The corrosion of aerospace aluminum alloys has been linked to the electrochemical behavior and chemical composition of constituent intermetallic compounds (IMCs). In this study, secondary ion mass spectrometry (SIMS), x-ray photoelectron spectroscopy (XPS), synchrotron infrared microspectrometry (SIRMS) and xray absorption near-edge structure spectroscopy (XANES) were used to characterize chromate conversion coatings (CCCs) formed on AA2024-T3 sheet and cast IMC analogs. The nascent surface film formed on IMC and matrix surfaces was characterized to reveal the influence of microstructure on CCC structure and composition. Copper-rich regions were found on the outermost CCC surface. Intermetallic compounds rich in Cu were found to be surface depleted of Cr and enriched in CN from the ferricyanide present in the CCC as an accelerator compound. By distinguishing particle morphology and chemistry, SIMS data demonstrated that heterogeneities in CCC composition were associated with IMCs.