Microbially induced corrosion of carbon steel in deep groundwater environment

Abstract

The metallic low and intermediate level radioactive waste generally consists of carbon steel and stainless steels. The corrosion rate of carbon steel in deep groundwater is typically low, unless the water is very acidic or microbial activity in the environment is high. Therefore, the assessment of microbially induced corrosion of carbon steel in deep bedrock environment has become important for evaluating the safety of disposal of radioactive waste. Here we studied the corrosion inducing ability of indigenous microbial community from a deep bedrock aquifer. Carbon steel coupons were exposed to anoxic groundwater from repository site 100 m depth (Olkiluoto, Finland) for periods of 3 and 8 months. The experiments were conducted at both in situ temperature and room temperature to investigate the response of microbial population to elevated temperature. Our results demonstrate that microorganisms from the deep bedrock aquifer benefit from carbon steel introduced to the nutrient poor anoxic deep groundwater environment. In the groundwater incubated with carbon steel the planktonic microbial community was more diverse and 100-fold more abundant compared to the environment without carbon steel. The betaproteobacteria were the most dominant bacterial class in all samples where carbon steel was present, whereas in groundwater incubated without carbon steel the microbial community had clearly less diversity. Microorganisms induced pitting corrosion and were found to cluster inside the corrosion pits. Temperature had an effect on the species composition of microbial community and also affected the corrosion deposits layer formed on the surface of carbon steel.

Keywords: carbon steel; deep biosphere; microbial corrosion; sulfate reducing bacteria; terrestrial biosphere.

FIGURE 1

(A) 16S rRNA gene copies ml^-1 groundwater or carbon steel (cm²)^-1. The error bars present SE of mean (n = 3). (B) dsrB gene copies ml^-1 groundwater or carbon steel (cm²)^-1. The error bars present SE of mean (n = 3).

FIGURE 2

Denaturing gradient gel electrophoresis (DGGE), (A) 16S rRNA (B)dsrB gene.

FIGURE 3

Phylogenetic tree of bacterial diversity, based on the 16S rRNA gene sequences obtained by 16S gene-based PCR-DGGE, in relation to cultured- and the closest uncultured relatives. Parentheses indicate the number of individual sequences within the OTU. Bootstrap values, calculated from 1,000 repetitions, are shown at branch points with >50% support. The scale bar indicates 0.08 nucleic acid substitutions. The tree is rooted by Aquifex aeolicus.

FIGURE 4

Phylogenetic tree of sulfate reducing bacteria, based on the dsrB sequences (amino acid) obtained by dsrB gene-based PCR-DGGE, in relation to cultured SRB and the closest uncultured relatives. Parentheses indicate the number of individual sequences within the OTU. Bootstrap values, calculated from 1,000 repetitions, are shown at branch points with >50% support. The scale bar indicates 0.08 amino acid substitutions. The tree is rooted by Archaeoglobus veneficus.

FIGURE 5

Surface of carbon steel.

FIGURE 6

Surface of carbon steel incubated at 6°C by FE-SEM. (A,B) After 3 months incubation. (C–F) After 8 month incubation.

FIGURE 7

Surface of carbon steel incubated at RT by FE-SEM. (A,B) After 3 months incubation. (C–F) After 8 months incubation.