Passivation Characteristics of Carbon Steels and Their Constituent Phases in Water which Simulates Water in Contact with Bentonite
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概要
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Anodic polarization curves of a commercial carbon steel with 0.12 mass% C and high purity carbon steels with 0.12 and 1.16 mass% C have been measured in solutions of 5.72 mol⋅m-3 Na2SO4+7.99mol⋅m-3 NaHCO3 (pH 8.3-10), which simulate the water in contact with bentonite, 5.72mol⋅m-3 Na2SO4 (pH 8-10), and 7.99mol⋅m-3 NaHCO3 (pH 8.3-10). The thickness and optical constants of passive films formed on the steels in a boric-borate solution of pH 8.45 have been determined by using in-situ ellipsometry. The results were compared with those obtained for pure Fe and bulk Fe3C, which simulate two constituent phases of carbon steel, i.e., ferrite and cementite, respectively. It was found that in 5.72 mol⋅m-3 Na2SO4+7.99mol⋅m-3 NaHCO3 solutions of pH 8.3 and 9, complete passivation took place on pure Fe and Fe3C but did not on the three carbon steels used. The current densities at potentials ranging from 0.3 to 1.1V (vs. SHE), where both pure Fe and Fe3C undergo passivation, increased in the following order: pure Fe<Fe3C<high purity 1.16% C steel<high purity 0.12%C steel<commercial 0.12%C steel. In the Na2SO4+NaHCO3 solution of pH 10, pitting occurred on the commercial 0.12%C steel after the passivation was completed. These results suggest that the formation of uniform and stable passive films is difficult on the 0.12%C steels having ferrite-pearlite structure, and impurities in carbon steel have a detrimental effect on the passivation and depassivation process. The passive current density in 7.99mol⋅m-3 NaHCO3 solutions of pH 8.3-10 and a boric-borate solution of pH 8.45 increased with increasing C content of specimens, indicating that carbon increases the dissolution rate of passive films. The results of ellipsometric analysis showed that the optical constants, N2=n2-k2i, of passive films formed in the boric-borate solution are strongly dependent on the C content of specimens; the valueof n2 increased from 2.45 for pure Fe to 2.75 for Fe3C with increasing C content, and at the same time the value of k2 decreased from 0.35 to 0.15.
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