In a freshwater environment, accelerated corrosion of carbon and stainless steels is frequently observed. Here, an immersion study was conducted using nine types of steels in a freshwater pool for 22 mo. Accelerated corrosion was observed in carbon and Cr-containing steels and cast iron, whereas no visible corrosion was observed in stainless steels, even after 22 mo. Microbial community analysis showed that, in general corrosion, Fe(II)-oxidizing bacteria were enriched in the early corrosion phase, Fe(III)-reducing bacteria increased in the corrosion-developing phase, and sulfate-reducing bacteria were enriched in the corrosion products during the final corrosion phase. In contrast, in the 9% Cr steel with localized corrosion, the family Beggiatocaea bacteria were particularly enriched. These microbial community compositions also differed from those in the water and sediment samples. Therefore, microbial communities are drastically altered with the progression of corrosion, and iron-dependent microbial energy metabolism contributes to an environment that enables the enrichment of other microorganisms.

The influence of chloride ion concentration on the passivation film and corrosion product film of martensitic stainless steels (SS) (13% Cr, modified 13% Cr-1% Mo, modified 13% Cr-2% Mo, and 15% Cr SS) have been investigated using immersion tests and electrochemical measurements in sweet environments at temperatures of 150°C and 180°C in the presence of carbon dioxide (CO 2 ). The corrosion rate of conventional 13% Cr SS was found to increase with increasing chloride ion concentration at 150°C. However, the corrosion rates of modified 13% Cr SS and 15% Cr SS were independent of the chloride ion concentration at 150°C. The corrosion rates of modified 13% Cr SS increased with increasing chloride ion concentration above 1,000 ppm at 180°C. The corrosion rate of 15% Cr SS slightly increased at high chloride ion concentration at 180°C. Chromium-enriched corrosion product films were produced on all alloys considered, except for 15% Cr SS, at 180°C. A passivation film was maintained for 15% Cr SS at 180°C. In addition, the pitting corrosion behaviors were investigated by potentiodynamic polarization curves and open-circuit potential measurements. The behavior of the current density of anodic curves corresponded to the immersion test results. Pitting corrosion on modified 13% Cr-2% Mo SS is considered to repassivate because of low potential compared with pitting potential. KEY WORDS: CO 2 corrosion, high temperature, immersion test, martensitic stainless steel, modified martensitic stainless steel, pitting potential ISSN 0010-9312 (print), 1938-159X (online)

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The External Pressure Balanced Reference Electrode (EPBRE) system was applied to the electrochemical measurements in high temperature and high pressure aqueous solutions. Irreversible potentials arisen in the EPBRE system, such as liquid junction potential and thermal junction potentials were studied with comparison to the internal reference electrode system in 0.1 M KCl solutions at high temperatures. It was demonstrated that the potential measured by EPBRE system can be converted to the potentials referenced to standard hydrogen electrode potential at high temperature and also at 25 . The contribution of liquid junction potential and thermal diffusion potentials to the measured values was also evaluated in the EPBRE system.

The presence and activities of microorganisms on metal surfaces can affect corrosion. Microbial communities after such corrosion incidents have been frequently analyzed, but little is known about the dynamics of microbial communities in biofilms on different types of stainless steels, such as austenitic, martensitic, and duplex stainless steels. Here, we conducted immersion experiments on 10 types of stainless steels in a freshwater environment, where microbiologically influenced corrosion was observed. During 22-month of immersion, severe localized corrosions were observed only on martensitic S40300 stainless steel. Microbial community analysis showed notable differences between non-corroded and corroded stainless steels. On the surfaces of non-corroded stainless steels, microbial communities were slowly altered and diversity decreased over time; in particular, relative abundance of Nitrospira sp. notably increased. Whereas microbial communities in corrosion products on corroded stainless steels showed low diversity; in particular, the family Beggiatoaceae bacteria, iron-oxidizing bacteria, and Candidatus Tenderia sp. were enriched. Furthermore, sulfur enrichment during localized corrosion was observed. Since there was no enrichment of sulfate-reducing bacteria, the sulfur enrichment may be derived from the presence of family Beggiatoaceae bacteria with intracellular sulfur inclusion. Our results demonstrated slow and drastic changes in microbial communities on the healthy and corroded metal surfaces, respectively, and microbial communities on the healthy metal surfaces were not affected by the composition of the stainless steel.