Synthesis and characterisation of thin cerium oxide coatings elaborated by cathodic electrolytic deposition on steel substrate

Cerium oxide films are widely studied as a promising alternative to Cr(VI) based pre-treatments for the corrosion protection of different metals and alloys. This paper deals with the deposition and characterisation of cerium oxide films deposited onto mild steel substrates by cathodic electrolytic deposition via Ce(III) chloride aqueous and mixed water–ethyl alcohol solutions with and without hydrogen peroxide as a precursor. The major role of this precursor is to increase the deposition rate of the oxide films allowing smooth and adherent hydrated cerium oxide films to be obtained. However, the evolution of the composition of the bath with time leads to a non-reproducibility of the morphology and composition of the thin films. The deposited oxide films are composed of small particles of hydrated Ce(IV) oxide with a significant amount of amorphous phase. In the absence of a precursor, only the mixed water–ethyl alcohol solution permits cerium oxide films to be deposited. The influence of the deposition parameters such as applied cathodic current density, deposition time and composition of the bath was also investigated. The deposits exhibit a quite developed crack network, which is highly dependent on the deposition parameters and the drying process. Moreover, the incorporation of chloride ions during the deposition has been observed. The protection efficiency of these films was examined during extended immersion tests in 3% NaCl solution. The deposits allow the cathodic reaction kinetics to be reduced by acting as a cathodic inhibitor. However, the presence of large defects or cracks induces an accelerated local degradation of the steel substrate, removing partially the oxide deposit, whereas a thin crack network can rapidly be sealed by corrosion products without deterioration of the deposit.