Cerium-based conversion layers on aluminum alloys

Mixed layers of Al and Ce oxides have been obtained in conversion coatings on two aluminum alloys (AA6061 and AA2618). The microstructure and the chemical composition of the protective films have been examined by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The corrosion resistance in NaCl solution was investigated and a comparison with a traditional chromate conversion coating on the same alloys is given. The coatings obtained by immersion in boiling solution of Ce(III) salts cover uniformly the alloys surface with a protective layer, whereas the H2O2-cerium conversion coating process produces a thick, but cracked layer with "dry-mud" morphology, which preferentially covers the intermetallic particles on the samples surface. The boiled cerium conversion coating treatments are as effective in corrosion resistance as the traditional chromating process, but the interest for industrial purposes is limited because of long application times. The H2O2-cerium conversion coating process produces effective corrosion resistance layers, thinner than the ones obtained by traditional processing, but very interesting for industrial applications due to the limited environmental impact. (C) 2001 Elsevier Science B.V. All rights reserved.