Investigation of microbiologically influenced corrosion inhibition of carbon steel in compacted bentonite under subsurface environment

Nagaoka, Toru*; Hirano, Shinichi*; Kosaba, Takumi*; Amano, Yuki   ; Mochizuki, Akihito  

In-situ corrosion experiment of carbon steel coupons embedded in compacted bentonite were conducted in the subsurface environment at the Horonobe underground Research Laboratory. The experiment was carried out at the 250 m depth using boreholes. To simulate a small schale of geological disposal condition, compacted bentonite coupons in the test cells were made of 100% bentonite material (Kunigel V1) with different dry densities (1.0, 1.4, 1.6, 1.8 g/cm). After the installation, the test cells were collected after a certein period of time and measured the number of viable bacteria and analyzed microbial community stracture based on 16S rRNA gene. The carbon steel coupons were subjected to weight impairment measurements and analysis of corrosion products. The results showed that the weight of carbon steel coupons in compacted bentonite decreased at all conditions, but the amount of weight-loss tended to be greater at lower dry densities. The weight-loss was particularly large at the lowest dry density of 1.0 g/cm, and corrosion was suppressed at dry densities of 1.4 g/cm and higher. The corrosion rate tended to decrease in the later stages of the test compared to the early stages of the test. To evaluate the effect of microorganisms on the corrosion behavior, the number of viable bacteria in the collected bentonite samples was measured. The results showed that the number of aerobic bacteria tended to decrease with increasing dry density. In addition, 16S rRNA gene analysis showed that the composition of the microflora at high drying density was similar to that in the initial bentonite, while the microflora at low drying density was significantly altered. These findings suggest that microorganisms may be active in compacted bentonite at low dry density and promote corrosion.