Influence of the Pre-Treatment and Post-Treatment Operations on the Surface Chemistry and Corrosion Behavior of Cerium-Based Conversion Coatings on Aluminum

Abstract

We investigated the corrosion-protective action of chemically deposited in green Ce solution conversion Cerium oxide coatings on technically pure aluminum (Al 1050) that were subsequently immersion-treated in phosphate solutions. A comparative study was conducted regarding the effect of: the type of pretreatment of the Al substrate; the conversion treatment of the Al substrate in Ce-containing or P-containing solutions; the consecutive conversion treatment of the Al substrate in Ce-containing and P-containing solutions. The obtained results indicate that the pre-treatment of the aluminum substrate and the additional phosphate treatment of the Ce-oxide conversion coating deposited on Al 1050 have a strong impact on corrosion-protective effect. We established that the phosphate treatments lead to a transformation of the chemical composition, chemical state of the elements, structure and morphology of the Ce-oxide coatings formed on Al substrates. Oxides and phosphates of Al and Ce are formed, providing an effective barrier to the diffusion of Chloride ions towards the metal surface and increased corrosion resistance (Rp) of Al 1050 to general and pitting corrosion. We have established a marked increase in Rp for one of the model systems (Al(NaOH)/ CCOC(Ce+Cu)/PhL(NH4H2PO4)) during extended corrosion tests, which was related to the formation of poorly soluble surface corrosion products.