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

Abstract

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.

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“…Furthermore, similar methods are proposed for the corrosion protection of zinc [174] and low carbon steel [175]. Recently, Tsanev et al [176] proposed phosphatation of the CeCC layer deposited on AA1050 alloy by exposure for 15 min to 0.5 M NaH 2 PO 4 with the addition of 0.1 M Ca(NO 3 ) 2 at 85°C. Considering the amphoteric properties of the AAO layers and the CeO 2 solubility in definitely acidic media, Portolesi et al [177] propose two buffering solution compositions enabling the final sealing of the AAO/ CeCC layers at nearly neutral pH, namely: Na 2 HPO 4 /KH 2 PO 4 and Na 2 B4O 7 / HCl.…”