Influence of Oxygen Concentration and Alkalinity on the Hydrogen Gas Generation by Corrosion of Carbon Steel
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Influence of Oxygen Concentration and Alkalinity on the Hydrogen Gas Generation by Corrosion of Carbon Steel A.Fujiwara1, I.Yasutomi1, K. Fukudome2, T.Tateishi3 and K.Fujiwara3 1 Radioactive Waste Management Center, 2-8-10 Toranomon, Minato-ku, Tokyo, Japan. 2 Kobe Steel Ltd., 1-6-14, Edobori, Nishi-ku, Osaka, Japan. 3 Kobelco Research Institute Inc. ,1-5-5, Takatsukadai, Nishi-ku, Kobe, Japan. ABSTRACT A repository for low and intermediate level radioactive wastes will contain a great deal of carbon steel as reinforcement materials and waste containers. These steel components are expected to generate hydrogen gas due to reductive corrosion of carbon steel under repository condition. In this study, we have investigated the influence of the environmental factors such as oxygen concentration and pH in the solution on the rate of the gas generation. This rate was measured with a gas flow type test system under the controlled oxygen concentration, which simulates the redox conditions from initial to final environment in a repository. In addition, the corrosion mechanism was examined through the electrochemical tests. As a result, it has been found that pH in the solution and the oxygen concentration considerably affected the generation rate of hydrogen gas in various way. As to the influence of pH of the solution, it has been found that the gas generation rate increased with the increase of pH between 11.8 and 14. Further, as to the influence of the oxygen concentration, when the oxygen concentration in the blowing nitrogen gas was 2 ppm or more, the gas generation rate increased with the oxygen concentration. This trend was possibly caused by local decrease of pH due to localized corrosion. The gas generation rate was very low when the oxygen concentration was in the passive region. The rate was equal to 0.01 µm/y or less if the corrosion occurs with an equation: 3Fe+4H2O→ Fe3O４+ 4H2↑ (hereafter we call this the equivalent corrosion rate). However the gas generation rate increased when the oxygen concentration was controlled to the extent that sufficient passive film cannot be produced and thus it shifts from passive state to active state. INTRODUCTION Hydrogen gas will possibly be generated in a underground repository by the corrosion of carbon steel such as concrete reinforcement and container material in the reducing underground water. If the rate of gas generation is high, gas will accumulates in a repository to cause deterioration of engineered barriers and pushing-out of the contaminated pore water. The solution being in contact with such carbon steel is influenced by the alkaline components (Na, K, etc.) contained in the concrete at the first stage of disposal, and is expected to be high alkaline close to pH14. The alkalinity is supposed to decline as time
passes by and to be stabilized around pH12 for a long time of period1). Further, right after the disposal facilities is shut down, it was in the oxidation condition under the influence of oxygen contained in backfill and the like, but the oxygen will be cons
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