Diffusion of Corrosion Products of Iron in Compacted Bentonite
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DIFFUSION OF CORROSION PRODUCTS OF IRON IN COMPACTED BENTONITE. IDEMITSU K., H. Furuya and Y. Inagaki Kyushu Univ., 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, JAPAN. ABSTRACT Carbon steel is one of the candidate overpack materials for high-level waste disposal. The corrosion rate of carbon steel is reduced by the presence of buffer materials such as bentonite and seems to be affected by the diffusion of corrosive materials and corrosion products through the buffer material. The apparent diffusivities of corrosion product of iron were measured in some bentonite specimens in contact with carbon steel. The apparent diffusivities of iron were also measured without carbon steel for comparison. The apparent diffusivities of corrosion product were on the 2 order of 10-12 m /s and showed a tendency to decrease with increasing density of the bentonite specimen. There was no significant effect of silica sand on the apparent diffusivities. The 2 apparent diffusivities of iron in the system without carbon steel were in the range of 10-14 m /s and showed a tendency to increase with increasing silica sand content. The difference of the diffusivities between corrosion product and iron without carbon steel seems to be due to the difference of diffusing species. The color of the corrosion product was dark-green during contact with bentonite specimens and became red on exposure to air in a few minutes. Gas bubbles were also observed in the corrosion product. This suggests hydrogen generation during corrosion of the carbon steel. Thus the diffusing species seems to be in a reduced state, probably ferrous ion. On the other hand, the diffusing species of iron without carbon steel was probably a ferric hydroxide complex that was negatively charged. This suggests that ferrous ion could diffuse in the surface water adsorbed on bentonite, while ferric complex was excluded. INTRODUCTION Carbon steel is one of the candidate overpack materials for high-level waste disposal. The corrosion rate of carbon steel is reduced by the presence of buffer material such as bentonite[1,2] and seems to be affected by the diffusion of corrosive materials and corrosion products through the buffer material. In addition, carbon steel overpack will be corroded by consuming oxygen introduced by repository construction after closure of repository and then will keep the reducing environment in the vicinity of repository. Reducing condition will be expected to retard the migration of redox-sensitive radionuclides such as uranium by lowering their solubilities. Therefore, the diffusion of corrosion product of iron in buffer material is important to discuss the corrosion rate of overpack and migration of redox-sensitive radionuclides. The purpose of this paper is to study diffusion behavior of corrosion product of iron in compacted bentonites under reducing condition. The apparent diffusivities of corrosion product of iron were measured in some bentonite specimens with carbon steel. The apparent diffusivities of iron were also measured without carbon steel for compari
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