Use of rare earth metal salt solutions for corrosion protection of aluminum alloys and mild steel

The rare earth metal salt (REMS) compounds which are non-toxic and inexpens ive have been used successfully for the corrosion protection of commercial aluminum alloys as well as of mild steel. The resistance to localized corro sion of aluminum alloys such as Al 2024, Al 6061 and Al 7075 has been great ly improved by immersion in hot cerium salt solutions. Cerium oxides/hydrox ides were formed at sites where intermetallic compounds containing copper w ere located and eliminated the formation of local cathodes. REMS solutions were also used for sealing of anodized layer on Al alloys replacing toxic c hromates. In addition to increased corrosion resistance excellent paint adh esion was observed. Cerium nitrate and yttrium sulfate solutions produced t he most satisfactory results. Cerium salts were also applied successfully a s inhibitors and as pretreatment for the corrosion protection of carbon ste el in hot NH3/water solutions used in absorption heat pumps. Factorial desi gn experiments have been used to determine the optimum concentrations of Ce Cl3 and H2O2 as well as the treatment time used in the cerating process for mild steel. The corrosion protection provided by the cerated layer was fur ther improved by cathodic polarization in cerium chloride. A dual corrosion protection strategy employing cerating and addition of a REMS to the worki ng solution is expected to provide long-lasting corrosion protection of mil d steel.