Supercritical water (SCW) is obtained at pressures above 22.1 MPa and temperatures above 648 K and is advantageous because of its great solubility in organic liquids. Waste treatment of toxic substances, such as the decomposition of polychlorinated biphenyle (PCBs) and their application to the recycling of plastics using SCW is very effective and has less negative influence on environmental destruction. However, SWC was found to corrode a reactive container remarkably, and as a result, the shortened longevity of the container obstructed its industrial use. Moreover, a large-scale reactive container is necessary for the recycling of waste plastics and resources. Therefore, the use of an expensive Ni based alloy in the container is difficult from an economical standpoint. Application of austenitic stainless steel, comparatively economical in Ni based alloy, is has been tried for the container material, but provides insufficient corrosion resistance under the conditions of SCW. In this study, improvement of the corrosion resistance by the nitride process of SUS316L alloy under the supercritical state is considered. (1) Under the influence of a nitrided process, SUS316L alloy endures during usage as a container material in the vicinity of 640K due to the decrease in the corrosion rate by transpassive corrosion. (2) There is a correlation between the corrosion speed and the permittivity for the nitrided SUS316L alloy. Thus, the longevity of a reactive container correlates to a value of the permittivity of solution. (3) Above the value of 25 permittivity, it is unsuitable for use as a reactive container because of the flaking off of the nitride layer of the SUS316L alloy surface.
