1995 Volume 44 Issue 10 Pages 534-542
On the corrosion protection of carbon steel by sol-gel derived TiO2 coating under the illumination of light, the photoelectrochemical behavior of TiO2/steel system is investigated. The diffusion of Fe ions into TiO2 in the heat treatment of the coating process greatly degrades the photo-effect of TiO2 coating. An Fe oxide layer at the TiO2-steel interface, formed by oxidizing the steel above 500°C in air before coating, has been shown to retard the diffusion. The importance of α-Fe2O3 in the photo-response of TiO2/steel and the effect of its changing properties with oxidation temperature are discussed. The results on the effect of heat treatment condition in the coating process show that the TiO2/steel specimens exhibit remarkably less noble photopotentials when the heating temperature is increased above 300°C, which is related to the crystallization of TiO2 above this temperature. The ennoblement of photopotential caused by the further increase in heating temperature above 500°C is a result of Fe diffusion through the oxide layer at the interface. Under the optimum conditions for steel oxidation and TiO2 coating, the feasibility of localized corrosion prevention by TiO2 coating under illumination is clearly demonstrated by the fact that the photopotentials of TiO2/steel in deaerated solution are much less noble than the repassivation potential of carbon steel.