Chromate in conversion coatings: a XANES study of its concentration and mobility

Samples of pure aluminum were prepared with four different areas on one surface: etched aluminum, etched aluminum exposed to various electrolytes, Alodine treated aluminum, and Alodine treated aluminum exposed to electrolyte. X-ray absorption near-edge structure (XANES) was used to measure both the distribution of chromium and the proportion of hexavalent chromium. The spatial distribution of the chromium species was mapped following exposure to a range of electrolytes: samples were exposed for 499 and 835 h to 18 MΩ deionized water, 0.5 M NaCl, 0.5 M Na₂SO₄, and 0.5 M Na₂SO₃. Immersion in water and sulfate had a minimal effect on the ionic state and location of chromium species. Immersion in chloride induced crevice corrosion attack, as indicated by a reduction in the Cr content at the interface between the exposed and protected Alodine treated areas. Small losses in Cr attributable to the loss of Cr(VI) were also found on the exposed Alodine treated areas. Exposure to sulfite caused the most dramatic changes. The total chromium content of the exposed Alodine decreased by 13%, and the amount of Cr(VI) decreased by 69% of its original value. There was a marked deposition of chromium as Cr(III) on the etched exposed aluminum close to the Alodine interface.