Development of a Suppression Method for Corrosion of Carbon Steel by Double-Layer Ni Metal-Ni Ferrite Film
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概要
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Water pH control or dissolved oxygen (DO) in water control is being applied to many thermal and nuclear power plants in order to reduce the corrosion of carbon steel (CS). In the former, a continuous pH adjuster, such as hydrazine, is injected into water during plant operation cycles, and in the latter, oxygen gas is injected into water and is controlled during plant operation. Then, we developed the Hitachi ferrite coating (Hi-F Coat) process to suppress the corrosion of CS in plant operation cycles without pH or DO control. In this process, a fine oxide film is formed on the CS base metal of the piping in solution at 363 K. In this study, we developed a method to form a double-layer Ni metal-Ni ferrite film to suppress the corrosion amount of CS in water. In this process, a double-layer film is formed on the base metal of the piping using four chemical reagents in solutions, Fe(HCOO)2, Ni(HCOO)2, H2O2, and N2H4 at 363 K. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, and Auger electron spectroscopy measurements of the film show a close-packed double-layer thin film formed in the amount of 3.6 g/m2. The inner layer is mainly Ni metal and the outer layer is Ni0.72+Fe0.32+Fe23+O4. After adding the four chemical reagent solutions, Ni2+ ions were reduced on the CS surface and a Ni metal film was formed on the base metal. Then, a Ni0.7Fe0.3Fe2O4 film was formed on the CS surface by oxidation of Fe2+ to Fe3+ with H2O2. We confirmed that the respective corrosion amounts were reduced to 1/5 and 1/2 compared with those obtained without and with oxygen injection at 423 K. Furthermore, we analyzed the film after a corrosion test. The double-layer film changed to a Ni0.7Fe0.3Fe2O4 single-layer film. There is the possibility that the transformation from the double-layer film to the single-layer film reduced the corrosion of CS by preventing the diffusion of Fe ions from base CS to high-temperature water.
著者
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Hosokawa Hideyuki
Energy and Environmental Systems Laboratory, Hitachi Ltd.
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Nagase Makoto
Hitachi Works, Hitachi-GE Nuclear Energy, Ltd.
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Fuse Motomasa
Hitachi Works, Hitachi-GE Nuclear Energy, Ltd.
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ITO Tsuyoshi
Energy and Environmental Systems Laboratory, Hitachi Ltd.
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