Modelling of carbon steel corrosion under oxygen depleted environment

摘要

In Japan, high level radioactive waste (HLW) produced by reprocessing spent fuels has been planned to be fixed in the glass solid body which is enclosed in the stainless steel canister which is further enclosed in the metal overpack. The overpacks are emplaced in tunnels at a deep underground facility. Carbon steel is the most important candidate material for the overpack, because the corrosion rate of carbon steel is known to be as low as 10 μm/year at an assumed deep underground site which is kept at oxygen depleted condition. The corrosion rate of carbon steel measured in the laboratory was found to be almost 10 μm/year at the initial stage, and decreases to a steady state corrosion rate in the order of 0.1-0.01 μm/year, changing with time according to an inverse parabolic rate law. A corrosion model assuming the diffusion of H_2O molecule through the precipitated corrosion film (Fe_3O_4) has been developed to rationalise the observed corrosion rate law under oxygen depleted environment. In this study, an improved model which takes account of the corrosion film dissolution has been proposed by assuming that the dissolution of precipitated Fe_3O_4 film is controlled by mass transfer process through adjusting diffusion layer in the solution. Digital simulation based on the improved corrosion model with suitable parameters such as the diffusion constant of H_2O in the corrosion film and concentration of dissolved Fe~(2+) species at the surface of the Fe_3O_4 corrosion film was found well to simulate the observed corrosion rate change with time at various pH values as a function of partial pressure of hydrogen gas and to predict a low and steady state corrosion rate after long time exposure.