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

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℃. © 2021 Japan Society of Corrosion Engineering. All rights reserved.