Factors influencing the deposition of Ce-based conversion coatings, part I: The role of Al3+ ions

The mechanism of deposition of cerium-based conversion coatings and the influence of Al3+ ions was modeled using titrations of cerium-based conversion coating solutions with and without added Al3+ ions. Precipitates resulting from these titrations were characterised by Raman spectroscopy and thermogravimetric analysis. Cerium peroxo species and precipitated aluminium compounds were detected in the precipitates. The titrations indicated that cerium and aluminium compounds precipitate independently. The coating deposited onto AA2024-T3 was studied using Raman spectroscopy and optical and scanning electron microscopy. Similar species were observed to those in the precipitates. Changes to the properties of the coating during ageing were monitored using contact angle measurements and Raman spectroscopy. A model is presented that proposes a mechanism for the deposition of the coating over the matrix of AA2024-T3.