A Study On Photoelectrochemical Anticorrosion Of Metals

It is quite well known that passive films on most metals and alloys exhibit a semiconducting behavior, which can protect metals from corrosion to some degree. However, in the study of electrochemistry, the effect of illumination on corrosion of metals has been paid attention to until in recent years, and the present studies are mainly focused on stainless steel, copper and etc, and less on active metals such as carbon steel. Although the passive films on metals can partly resist corrosion, the protection is limited. Recently, a few studies have proposed that the photoinduced charge-transfer process at the semiconductor interface can be applied to metal corrosion prevention. Theoretically, the semiconductor used is not consumed in the process of cathodic protection, thus it has a long lifetime. In the long run, it can make use of inexhaustible solar energy. So far, the semiconductor used for metal corrosion prevention is wide band-gap semiconductor, which can only absorb the ultraviolet radiation-a small part in the solar spectrum. So it is very important and significant on how to widen the spectrum response range of the catalysts and to make fully use of solar light to protect metals.Firstly, the influence of UV illumination on passivity and pitting susceptibility on X70 pipeline steel in a borate buffer (pH=8.4) solution containing NaCl using potentidynamic polarization curves and incubation time measurement was studied. It is observed that photo-inhibition of pitting corrosion can be achieved for X70 steel on illumination under UV light, which gave rise to an enhanced resistance. The cause of the photo-inhibition effect was discussed by galvanostatic reduction cures, Mott-Schottky analysis and photocurrent transient measurements. The results indicated that the cause is the modifications in the electronic properties of the passive film, mainly resulting in a decrease in the bulk doping and an increase in the surface state density.Secondly, the photoelectrochemical anticorrosion of X70 pipe steel under visible light using TiO₂ and doped TiO₂ was investigated. It was observed that the open circuit potential of the photocatalysts in the solutions including 0.1 mol·dm^-3 ethanol under visible light was positive than the corrosion potential of X70 steel in 0.5 mol·dm^-3 NaCl solution, So the above photocatalysts can not perform photoelectrochemical anticorrosion for X70 steel under visible light.Finally, WO₃ film electrode with visible photoresponse was prepared by cathodic electrodeposition method and its photoresponse was investigated at different conditions. In the presence of formate the photoactivity of the photoelectrode was the highest, and increased with increasing pH and the formate concentration in the anode cell. The photoelectrochemical approach for copper corrosion protection in 4 mol·dm^-3 NaCl solution using the WO3 photoanode under UV light and visible light was also studied. Under this condition, the WO3 electrode can protect copper from corrosion for a long time under UV light, but shorter under visible light illumination.