Effects of Interfacial Iron Oxides on Corrosion Protection of Carbon Steel by TiO2 Coating under Illumination

On the approach of corrosion protection of metal by TiO2 coating under illumination, the effects of iron oxides on the photoelectrochemical response of TiO2/carbon steel were studied. The iron oxides, magnetite (Fe3O4), maghemite (γ-Fe2O3) and hematite (α-Fe2O3), were prepared on the carbon steel by using both the chemical conversion method (blackening treatment) and the heat treatments. From the examinations for the structures and the photoelectrochemical behavior of sol-gel derived TiO2 coating, it is found that the interfacial α-Fe2O3 demonstrates a significant photoelectrochemical response while the interfacial Fe3O4 and γ-Fe2O3 show poor performance. Although a vivid photoeffect for the TiO2/α-Fe2O3/steel system was observed, the Fe was still found to diffuse into TiO2 during the heat treatment of TiO2. Consequently, by changing the TiO2 coating procedure, particularly the heat treatment of TiO2 to suppress the diffusion of Fe ions, the photoeffect of TiO2/α-Fe2O3/steel could be improved. For TiO2/α-Fe2O3/steel under illumination, the stability of iron oxides was also studied. It was found that α-Fe2O3 was stable in aerated solution, while in a thoroughly deaerated solution α-Fe2O3 was reduced slightly by the photo-excited electrons.