Estudio de la corrosión de un acero en residuos nucleares líquidos simulados
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Comisión Nacional de Energía Atómica. Instituto de Tecnología Sabato
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Tesis para optar al título de Magister en Ciencia y Tecnología de Materiales
This work is part of a collaboration agreement between CNEA (National Atomic Energy Commission of Argentina) and USDOE (Department of Energy of the United States of America), entitled 'Tank Corrosion Chemistry Cooperation', to study the corrosion behavior of carbon steel A537 class 1 in different simulated non-radioactive wastes in order to establish the safety concentration limits of the tank waste chemistry at Hanford site (Richland-US). Liquid high level nuclear wastes are stored in tanks made of carbon steel A537 (ASTM nomenclature) that were designed for a service life of 20 to 50 years. A thickness reduction of some wall tanks, due to corrosion processes, was detected at Hanford site, beyond the existing predicted values. Two year long-term immersion tests were started using non radioactive simulated liquid nuclear waste solutions at 40 C degrees. This work extends throughout the first year of the 10 most corrosion significant chemical components: 5 main components (NaNO3, NaCl, NaF, NaNO2 and NaOH) at three concentration levels and 5 secondary components at two concentration levels. Measurements of the general corrosion rate with time were performed for carbon steel coupons, both immersed in the solutions and in the vapor phases, using weight loss and electrochemistry impedance spectroscopy techniques. Optic and scanning microscopy examination, analysis of U-bend samples and corrosion potential measurements, were also done. Localized corrosion susceptibility (pitting and crevice corrosion) was assessed in isolated short-term tests by means of cyclic potentiodynamic polarization curves. The effect of the simulated waste composition on the corrosion behavior of A537 steel was studied based on statistical analyses. The Surface Response Model could be successfully applied to the statistical analysis of the A537 steel corrosion in the studied solutions. General corrosion was not significant for A537 carbon steel in the concentration range of the solutions studied (pH 10-13) at 40 C degrees. The highest calculated corrosion rate for immersed samples, using the electrochemical impedance spectroscopy technique, was 25 mu m/yr, while the highest calculated corrosion rate for vapor phase coupons, using weight-loss measurements, was 51 mu m/yr. On the contrary, it was found that A537 carbon steel was highly susceptible to localized attack, due to pitting and crevice corrosion, in the solutions and at the temperature studied. The highest penetration rates produced by pitting attack, measured by optical microscopy examination after 11 month immersion, were 0.3 mm/yr and 0.4 mm/yr for the immersed and the vapor phase coupons respectively. The highest penetration rate produced by crevice corrosion on the immersed coupons, measured by optical microscopy examination after 11 month immersion, was 1,1 mm/yr. Stress corrosion cracking signs were not observed after 11 month immersion on the U-bend coupons.
This work is part of a collaboration agreement between CNEA (National Atomic Energy Commission of Argentina) and USDOE (Department of Energy of the United States of America), entitled 'Tank Corrosion Chemistry Cooperation', to study the corrosion behavior of carbon steel A537 class 1 in different simulated non-radioactive wastes in order to establish the safety concentration limits of the tank waste chemistry at Hanford site (Richland-US). Liquid high level nuclear wastes are stored in tanks made of carbon steel A537 (ASTM nomenclature) that were designed for a service life of 20 to 50 years. A thickness reduction of some wall tanks, due to corrosion processes, was detected at Hanford site, beyond the existing predicted values. Two year long-term immersion tests were started using non radioactive simulated liquid nuclear waste solutions at 40 C degrees. This work extends throughout the first year of the 10 most corrosion significant chemical components: 5 main components (NaNO3, NaCl, NaF, NaNO2 and NaOH) at three concentration levels and 5 secondary components at two concentration levels. Measurements of the general corrosion rate with time were performed for carbon steel coupons, both immersed in the solutions and in the vapor phases, using weight loss and electrochemistry impedance spectroscopy techniques. Optic and scanning microscopy examination, analysis of U-bend samples and corrosion potential measurements, were also done. Localized corrosion susceptibility (pitting and crevice corrosion) was assessed in isolated short-term tests by means of cyclic potentiodynamic polarization curves. The effect of the simulated waste composition on the corrosion behavior of A537 steel was studied based on statistical analyses. The Surface Response Model could be successfully applied to the statistical analysis of the A537 steel corrosion in the studied solutions. General corrosion was not significant for A537 carbon steel in the concentration range of the solutions studied (pH 10-13) at 40 C degrees. The highest calculated corrosion rate for immersed samples, using the electrochemical impedance spectroscopy technique, was 25 mu m/yr, while the highest calculated corrosion rate for vapor phase coupons, using weight-loss measurements, was 51 mu m/yr. On the contrary, it was found that A537 carbon steel was highly susceptible to localized attack, due to pitting and crevice corrosion, in the solutions and at the temperature studied. The highest penetration rates produced by pitting attack, measured by optical microscopy examination after 11 month immersion, were 0.3 mm/yr and 0.4 mm/yr for the immersed and the vapor phase coupons respectively. The highest penetration rate produced by crevice corrosion on the immersed coupons, measured by optical microscopy examination after 11 month immersion, was 1,1 mm/yr. Stress corrosion cracking signs were not observed after 11 month immersion on the U-bend coupons.