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Zairyo-to-Kankyo Vol. 69 (2020), No. 12

ISIJ International
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ONLINE ISSN: 1881-9664
PRINT ISSN: 0917-0480
Publisher: Japan Society of Corrosion Engineering

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Zairyo-to-Kankyo Vol. 69 (2020), No. 12

Corrosion Resistance of Nitrided Gray Cast Iron (FC) in Salt Water Environment

Hisao Fujikawa, Souhei Nishisaka, Teruhiko Nishikawa

pp. 329-332

Abstract

Corrosion resistance of nitrided gray cast iron (FC) in salt water environment, using the salt spray test and the electrochemical measurement, is improved by an iron nitride layer formed about 10μm thickness on the surface. It was also clarified that the corrosion resistance is further improved by performing an oxide film treatment to form a thin oxide film of about several μm on the surface after the nitriding treatment. Nitriding treatment with NH3 alone without RX gas produces a higher concentration of nitrogen in the ε-Fe3N layer formed on the surface layer than the soft nitriding treatment in which CO is passed by RX gas in addition to NH3 during nitriding. The nitriding treatment with only NH3 without RX gas was improved in corrosion resistance due to the higher nitrogen concentration in ε-Fe3N layer. Also, by forming a thicker iron nitride layer or forming a dense oxide film on the surface layer, the graphite that appears on the surface layer is almost covered, and also the iron nitride layer is selectively formed at the interface of the graphite in the surface layer part. The oxide film treatment showed better corrosion resistance by forming a dense oxide film of several μm thickness at a low temperature of around 450℃.

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Corrosion Resistance of Nitrided Gray Cast Iron (FC) in Salt Water Environment

IG-Crack Susceptibility of Type-316 Stainless Steel in the Simulated PWR Primary Water

Mikio Takemoto, Kiyoko Takeda, Takeo Kudo

pp. 333-350

Abstract

Authors report susceptibility of IG-crack in the simulated PWR primary water and discuss the mechanism of the IG-crack of Type-316 stainless steel. First, the authors compare the crack propagation rates of the stainless steels in chloride solution, simulated BWR and PWR primary water. By referring the Wagner lengths in three solutions, we also discuss whether the anodic current density can be compensated by the cathodic reaction rate when wet corrosion is associated in the IG-cracks in these three solutions.Next we introduce stress relaxation of the Type-316 stainless steel at 340℃ and discuss the test methods required to reproduce the IG-crack in the simulated PWR primary water. In the third, we report IG-SCC test methods and test results performed by two our research laboratories and how the cold worked Type-316 stainless steel is highly resistant against the IG-crack in the simulated PWR primary water. We could not produce the IG-crack as long as we use notched specimens, however, the IG-crack propagated for the limited tests by the K-increase type loading of the specimens with fatigue induced pre-cracks. The authors propose a self-healing mechanism of the Type-316 stainless steel to explain how this steel is stable in the simulated PWR primary water.

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IG-Crack Susceptibility of Type-316 Stainless Steel in the Simulated PWR Primary Water

Investigation of Corrosion and Hydrogen Entry Behavior under Atmospheric Environment by Real-time Monitoring Techniques

Hiroki Harada, Masataka Omoda, Shinji Ootsuka, Takashi Kawano

pp. 351-358

Abstract

To develop high strength steel sheets with high resistance to hydrogen embrittlement, it is necessary to clarify the mechanism of hydrogen entry into steel. Hydrogen entry is caused by the corrosion reaction under atmospheric environment. However, the relationship between hydrogen entry and corrosion behavior, and the influence of environmental factors on the hydrogen entry such as temperature, relative humidity (RH) and salt deposition is not fully understood. In this study, simultaneous monitoring of both hydrogen entry and corrosion rate was employed to investigate the influence of environmental factors on hydrogen entry and corrosion behavior. The monitoring was conducted under the atmospheric exposure test.As a result, both the hydrogen entry and the corrosion rate increased with an increase of amount of salt deposition. In addition, the corrosion rate increased with an increase of RH, whereas the hydrogen had been permeated the most at the medium RH. The mechanism of the hydrogen entry at the medium RH range was assumed to be related to lowering pH in high [Cl-] environment.

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Investigation of Corrosion and Hydrogen Entry Behavior under Atmospheric Environment by Real-time Monitoring Techniques

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