The role of hydrogen peroxide in the deposition of cerium-based conversion coatings

Cerium-based conversion coatings are progressing as an effective alternative to hazardous chromate-based systems used in the treatment of metal surfaces. However, there is still considerable debate over the mechanism by which these coatings are formed. Here, titrations of cerium-based conversion coating solutions were carried out in order to model the reactions that occur at the metal-solution interface during coating, with a particular emphasis on investigating the role of hydrogen peroxide (H₂O₂). The titration curves obtained support the proposed formation of Ce(III) peroxo complexes such as Ce(H₂O₂)³⁺ as an initial step, followed by deprotonation, oxidation and precipitation to form peroxo-containing Ce(IV) species such as Ce^(IV)(O₂)(OH)₂. The precipitates resulting from titrations were characterised by Raman spectroscopy, X-ray diffraction and thermogravimetric analysis, confirming the presence of peroxo bonds, and nano-sized CeO₂ crystallites that decreased in size with increasing H₂O₂ concentration. Characterisation of cerium conversion coatings on aluminium alloy surfaces confirmed the presence of peroxo species in the coatings, thereby supporting the titration model.