AN INVESTIGATION OF THE STRUCTURE AND CHEMISTRY OF A CHROMIUM-CONVERSION SURFACE-LAYER ON ALUMINUM

X-ray absorption spectroscopy (XAS), Fourier transmission infra red spectroscopy (FTIR), X-ray photo-electron spectroscopy (XPS), Auger profiling and microscopy were used to characterize chemically and structurally the surface oxide layer created by the Alodine 1200S process on 2024 and 7075 aluminum alloys. The thickness of the layer was found to range from similar to 2000 Angstrom to 1 mu m for processing times of 1-3 min. The top similar to 20 Angstrom of the fresh Alodine surfaces were composed of H2O, Cr, Fe and CN. Al and the other elements in the alloys generally were not components of the top similar to 20 Angstrom. The Fe and CN were introduced from the Alodine bath and were present throughout the surface oxide layer as Fe(CN)(6)(3-). The hydration of the Cr was evident in both the FTIR and XAS spectra. Prior to salt spray exposure the Cr was coordinated with oxygen as similar to 23 +/- 2% Cr6+ in tetrahedral coordination, Cr-O = 1.71 +/- 0.03 Angstrom, the balance as Cr3+ in octahedral coordination, Cr-O = 1.99 +/- 0.01 Angstrom. During salt spray testing similar to 500 Angstrom of the surface was eroded away and a large fraction of the remaining Cr6+ was chemically reduced to Cr3+ resulting in as little as similar to 7 +/- 1% Cr6+ in tetrahedral coordination. The other elements in the alloy, i.e. Al, Cu, Zn, Mn, Ca, Ti and Si were found in the eroded surface. The Cu and Zn were present and buried within the surface oxide layer essentially in the form of the same intermetallic compounds comprising the original bulk alloy. The Alodine process did not change them. The Zn environment changed slightly during salt spray testing.