Sol-gel derived Ti-Fe oxide coating for photoelectrochemical cathodic protection of carbon steel

Abstract

On the novel approach of corrosion protection of carbon steel by TiO2 coating under illumination, photoeffect of TiO2/C-steel system is affected by the structural and electronic properties of the interfacial Ti-Fe oxide. In the present work, Ti-Fe oxide with a wide range of composition, from TiO2 to α-Fe2O3, was prepared by sol-gel method. By studying the characteristics of the Ti-Fe oxides and their influences on the photoeffect of TiO2/C-steel system, a multi-layer coating, expressed as from outer coating layer to substrate: TiO2 (amorphous)/TiO2 (anatase)/Ti-Fe oxide/α-Fe2O3/C-steel, was proposed for more effective photoelectrochemical cathodic protection of carbon steel. TiO2 (anatase) layer without Fe contamination exhibits a good photoeffect and behaves like the source of photo-excited electrons. The inner α-Fe2O3 layer acts as both a barrier to retard the diffusion of Fe into TiO2 and a way for the transfer of photo-excited electrons from the TiO2 (anatase) coating to the substrate. The outer layer of amorphous TiO2 inhibits the oxygen reduction and reduces the photopotential to a more less noble value. The intermediate layer of amorphous Ti-Fe oxide provides deep levels for Fe2+/Fe3+ redox couple, for which Fe3+ is reduced to Fe2+ by the photo-excited electrons under illumination. The state of Fe2+ left after stopping illumination contributes to phenomenon that the electrode potential is maintained at considerably less noble value until all the Fe2+ is re-oxidized by oxygen reduction.