Influence of the Chemical Composition of Ceria Conversion Coatings, Sealed in Solution of NaH₂PO₄ and Ca(NO₃)₂, on the Corrosion Behavior of Aluminum

The corrosion-protective influence of eco-friendly ceria conversion coatings deposited on Al-1050 alloy, additionally treated in mixed NaH₂PO₄ and Ca(NO₃)₂ solution, was studied. The main aim of this work was to investigate how the obtained mixed systems of coatings eliminates the negative role of cracks and pores on the surface formed after deposition only of ceria coating. For this purpose, the growth structure, main components and corrosion resistance of the so formed protective systems were investigated by SEM, EDS, XRD, XPS and electrochemical (PDP, Rp, etc.) methods. The results obtained show that the basic components of the conversion layers (before and after exposure in model corrosion media) are characterized by Al₂O₃, Al(OH)₃, CePO₄ and Ca₅(PO₄)₃(OH). Based on these results, the optimal conditions of immersion treatment(s) of Al substrate are established. At these regimes, the relationship of co-deposited Ce³⁺, PO₄³⁺ and Ca²⁺-containing components of the conversion layers determine the maximum values of their polarization resistance—a basic criterion for corrosion protection of Al. This effect is related to the formation of fill out of the defects of the conversion coatings and additional Ca₅(PO₄)₃(OH), CePO₄ AlPO₄ and Al(OH)₃ deposits, leading to the decrease of the corrosion rate.