Cathodic Protection for Nuclear Waste Packaging Under Gamma Ray Irradiation by Using TiO 2 Coating Combined with Glass S
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CATHODIC PROTECTION FOR NUCLEAR WASTE PACKAGING UNDER GAMMA RAY IRRADIATION BY USING TiO2 COATING COMBINED WITH GLASS SCINTILLATORS
RYUTARO FUJISAWA and SHIGEO TSUJIKAWA The University of Tokyo, Faculty of engineering, Department of Metallurgy, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113 JAPAN
ABSTRACT
The photoelectrochemical behaviours of a TiO2 single crystal and TiO2 coating were studied, for the purposes of cathodic protection of stainless steels and Cu via the TiO2 coating combined with glass scintillators under gamma ray irradiation. It was confirmed that a TiO2 coating could protect 304 stainless steel cathodically from crevice corrosion under illumination. A logarithmic relationship between the photopotential of single crystal TiO2(rutile) and light intensity was found, moreover, the photopotential was found to be least noble when wavelength equals 375 nm. Under illumination by gamma rays combined with the glass scintillators, the electrode potential of single crystal TiO2 was found to shift in the less noble direction by about 200 mV. Therefore, the technique of cathodic protection by TiO2 coating is considered to be applicable to protect the packaging metal from corrosion for a long time.
INTRODUCTION
The photoelectrochemistry of TiO2 has received great attention since Honda and Fujishima discovered its ability to cleave water under illumination [1]. The benefit of introducing TiO2 coating into corrosion science is that the potential of photo-irradiated TiO2 is shifted in the less noble direction to approach its flatband potential. The photoelectrochemical reaction which occurred was not decomposition of TiO2 but oxidation of H20 by the photoexcited holes in the valence band. These properties enable TiO2 to protect some metallic materials from corrosion by having TiO2 acting as a nonsacrificial anode. The technique of cathodic protection by TiO2 coating under illumination was first demonstrated by this laboratory [2]. The photopotential of stainless steel was found to be driven into the passive region by a TiO2 coating deposited via the sputtering method under illumination in chloride solution. This photopotential shifted in the less noble direction with increasing coating thickness over a range from 10 nm to 100 nm[2]. In a similar technique, J.Yuan applied this cathodic protection by a TiO2 coating deposited via a sol-gel method on Cu as shown in Fig. 1 [3]. Given the proper heat treatment condition, the TiO2 coating effectively protects Cu from corrosion in 0.3% NaCI solution over the pH range from 4 to 12 under
Mat. Res. Soc. Symp. Proc. Vol. 353 01995 Materials Research Society
736
illumination, for the photopotentials of the TiO2-coated Cu in both deaerated and aerated solutions are driven into the immunity domain as shown in the E-pH diagram of Cu.
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8 10 12 14 pH Fig. 1 Photopotentials after two hours of illumination for TiO2-coated Cu plotted in the E-pH diagram of Cu at 25"C. Heat treatme
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