| Corrosion of metals causes enormous economic losses to mankind. The traditional methods of anti-corrosion for metals have some defects, such as the limitation of long-term stability for zinc coating. Therefore, the development of environment-friendly anti-corrosion technology for metals has important scientific significance. In recent years, a novel approach has been developed which makes use of TiO2 coating of photocathodic protection for metals with UV irradiation. However, the pure TiO2 coating cannot be used in the dark, because it suffers from the well-known charge recombination problem. The shortcoming of pure TiO2 coating can be circumvented by coupling another semiconductor that has a different energy level from TiO2, which can reduce the charge recombination in addition to storing excess electrons during the period of irradiation. The stored electrons can subsequently be released in the dark, thus maintaining a more negative potential and offering a cathode protection.In this dissertation, the approaches for preparing nano-sized TiO2 coatings and its composite coatings, as well as the photocathodic protection properties and applications of these coatings were overviewed. Preparation of nano-sized TiO2 coatings by sol-gel method was investigated, and preparation process parameters were optimized so as to improve the photo-electrochemical properties and enhance remarkablly the photocathodic protection ability. The photocathodic protection properties of nano-sized TiO2 bilayer and composite coatings coupled with SnO2,WO3,Sb2O5 and V2O5 were researched, and their photocathodic protection mechanisms were also discussed. The main conclusions are as follows.(1) Preparation and photocathodic protection properties of nano-sized TiO2 coatings.Nano-sized TiO2 coatings were successfully prepared on 304 stainless steel and low carbon steel substrates via sol-gel dip-coating process, separately. The performance of photo-electrochemical and photocathodic protection properties of the coatings was investigated by the electrochemical method. The results showed that the nano-sized TiO2 coating can not only provide a good photocathodic protection for 304 stainless steel in 3.0% NaCl solution with UV irradiation, but also provide a good photocathodic protection for low carbon steel in 2.5% NaHCO3 solution with UV irradiation.(2) Preparation and photocathodic protection properties of nano-sized TiO2/SnO2 and TiO2-SnO2 coatingsSnO2 was used as the electron pool materials. Nano-sized TiO2/SnO2 bilayer coating and nano-sized TiO2-SnO2 composite coating were prepared on 304 stainless steel substrates via sol-gel dip-coating process. The performance of photo-electrochemical and photocathodic protection properties of the coatings was investigated by the electrochemical method. The results indicated that the nano-sized TiO2-SnO2 composite coating has better photo-electrochemical properties than those of the nano-sized TiO2/SnO2 bilayer coating in 3%NaCl solution. Type 304 stainless steel with the bialyer coating can maintain cathode protection for 7 h in the dark after irradiation by UV illumination for 1 h, while the composite coating electrode can only maintain cathode protection for 45 min.(3) Preparation and photocathodic protection properties of nano-sized TiO2/WO3 and TiO2-WO3 coatingsWO3 was used as the electron pool materials. Nano-sized TiO2/WO3 bilayer coating and nano-sized TiO2-WO3 composite coating were prepared on 304 stainless steel substrates via sol-gel dip-coating process. The performance of photo-electrochemical and photocathodic protection properties of the coatings was investigated by the electrochemical method. The results demonstrated that the nano-sized TiO2-WO3 composite coating has better photo-electrochemical properties than those of the nano-sized TiO2/WO3 bilayer coating in 3%NaCl solution. Type 304 stainless steel with the bialyer coating can maintain cathode protection for 6 h in the dark after irradiation by UV illumination for 1 h, while the composite coating electrode can only maintain cathode protection for 30 min.(4) Preparation and photocathodic protection properties of nano-sized TiO2/Sb2O5 and TiO2-Sb2O5 coatingsSb2O5 was used as the electron pool materials. Nano-sized TiO2/Sb2O5 bilayer coating and nano-sized TiO2-Sb2O5 composite coating were prepared on 304 stainless steel substrates via sol-gel dip-coating process. The performance of photo-electrochemical and photocathodic protection properties of the coatings was investigated by the electrochemical method. The results exhibited that the nano-sized TiO2-Sb2O5 composite coating has better photo-electrochemical properties than those of the nano-sized TiO2/Sb2O5 bilayer coating in 3%NaCl solution. Type 304 stainless steel with the bialyer coating can maintain cathode protection for 4 h in the dark after irradiation by UV illumination for 1 h, while the composite coating electrode can only maintain cathode protection for 30 min.(5) Preparation and photocathodic protection properties of nano-sized TiO2/V2O5 and TiO2-V2O5 coatingsV2O5 was used as the electron pool materials. Nano-sized TiO2/V2O5 bilayer coating and nano-sized TiO2-V2O5 composite coating were prepared on type 304 stainless steel substrates via sol-gel dip-coating process. The performance of photo-electrochemical and photocathodic protection properties of the coatings was investigated by the electrochemical method. It was found from the experiments that the nano-sized TiO2-V2O5 composite coating has better photo-electrochemical properties than those of the nano-sized TiO2/V2O5 bilayer coating in 3%NaCl solution. Type 304 stainless steel with the bialyer coating can maintain cathode protection for 6 h in the dark after irradiation by Uvillumination for 1 h, while the composite coating electrode can only maintain cathode protection for 20 min.(6) The mechanism of photocathodic protection of nano-sized TiO2 coatings and TiO2 composite coatings for metals.By comprehensive analysis of charge distribution and charge balance of nano-sized TiO2 coating electrode, the mechanism of photocathodic protection of nano-sized TiO2 coating for metals was explored. Based on the charge and self-charge of the electron pool materials, a novel mechanism of photocathodic protection for nano-sized TiO2 bilayer or composite coatings coupled with electron pool materials for metals has been put forward.
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