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 chem ical composition of the protective films have been examined by scanning ele ctron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and secondar y ion mass spectrometry (SIMS). The corrosion resistance in NaCl solution w as investigated and a comparison with a traditional chromate conversion coa ting on the same alloys is given. The coatings obtained by immersion in boi ling solution of Ce(III) salts cover uniformly the alloys surface with a pr otective 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 ceri um conversion coating treatments are as effective in corrosion resistance a s the traditional chromating process, but the interest for industrial purpo ses is limited because of long application times. The H2O2-cerium conversio n coating process produces effective corrosion resistance layers, thinner t han the ones obtained by traditional processing, but very interesting for i ndustrial applications due to the limited environmental impact. (C) 2001 El sevier Science B.V. All rights reserved.