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JAEA Reports

Corrosion of iron buried in clay

Sumiyama, Morio*

JNC TJ8400 2000-009, 138 Pages, 2000/02

JNC-TJ8400-2000-009.pdf:3.0MB

To evaluate corrosion behavior of carbon steel, a candidate materials of overpack, buried in soil for a long time, the water pipes buried in freshwater clay for a long time we digged out and the soil environment and the corrosion weight loss of pipes have been researched. From the results, a corrosion model (an empirical equation), an oxygen reduction reaction rate-determing step type, of carbon steel buried in soil was introduced. The corrosion data of under ground pipe collected by the Japan Community Gas Associations was used to increase reliability of the corrosion model equation. These data are one of researches of corrosion behavior of carbon steel buried in soil for a long time studied by at home and abroad. 38 samples buried freshwater clay were selected in 171 samples. With estimating the corrosion velocities and the soil environment factors of the above data, the maximum depth of pit corrosion was calculated by the statistical method of the extreme values using the area of overpack as the recurrent time. The correlation between the soil environment factors and the corrosion weight loss was obtained by the correlation analysis. The corrosion model of the maximum depth of pit corrosion at 0.99 of cumulative probability was compared between the under ground pipe data and the above data. On the reference data and the above data, the corrosion model equation; H = aY $^{n}$ was compared with the maximum depth of pit corrosion at 0.99 cumulative probability. The data of water pipes and community gas pipes at 0.99 cumulative probability showed the reasonable values when these data were compared with the reference data. So that the model was proved as a good corrosion model m the neutral low dissolved oxygen environment.

JAEA Reports

Study on corrosion behavior of carbon steel buried in soil environment for a long Term (XI)

Sumiyama, Morio*

JNC TJ8400 99-042, 203 Pages, 1999/02

JNC-TJ8400-99-042.pdf:14.07MB

This study is concerned with a natural analogue research on the corrosion of carbon steel nominated overpack materiral. The corrosion of water pipes buried in freshwater clay similar to bentonite, which forms disposal environment, had been examined until 1998. The obtained data was applied to the corrosion model equation H=aY $^{n}$ and a reasonable n0.4 was obtained. An attempt to redict the corrosion of carbon steel for a long period was made using the corrosion model equation. The corrosion environment of carbon steel in bentonite, which was used in themal loading test filled with the clay (bentonite) and conducted in the rock of a drift, was investigated until fiscal 1998 to find the analogy on corrosion between the above water pipe soil and the bentonite environments. As a result, it was found that the bentonite environment showed a little lower resistivity than the water pipe soil environment, and that the bentonite was in the same range for probe measurement value as the water pipe soil including chloride. However, its pH is likely to become higher. Clay fraction and sulfate of bentonite were higher than those of the water pipe soil. This suggests that the initial bentonite was different from freshwater clay and that, if the dissolved oxygen in the bentonite becomes neutral and lowers after carbon steel has been buried for a long time, the bentonite would provide the same environment as the water pope soil. A simple comparison cannot be made in term of corrosion between the bentonite and the water pipe soil. environments, because in the bentonite environment, the temperature of the steel was higher, corrosion history was supposed to be complicated, and the buried time of the steel was short. The corrosion in bentonite was not remarkably larger than that in general water. Thus, it is supposed that, when dissolve oxygen in the bentonite environment becomes lower after carbon steel has been buried for a long time, its corrosion in the bentonite ...