A study on compatibility of iron-based alloy in liquid metal

A study is made on the basic mechanism of the development of the compatibility of liquid-metallic structural material on the assumption that liquid metal acts as high-temperature heat transfer medium. A non-isothermal sodium loop made of 316 stainless steel is used to study the process of corrosion and mass transfer in heat-resistant steel and alloys at the highest temperature region (600-700degC) of flowing sodium containing 1-2 ppm oxygen. Samples used are austenitic stainless steel, precipitation-hardened Ni alloy, and high-Cr ferrite steel. Observations are made of the behaviors of their major components, Fe, Ni, Cr and Mo. Results show that austenitic steel and Ni alloy suffer from selective elution of Ni and Cr and a high rate of corrosion soon after being immersed in sodium. The corrosion rate then gradually decreases until it reaches a constant which depends on the original composition of the sample. This tendency is seen in all other samples except ferrite steel. An equation for the downstream effect is derived, and the corrosion rate at the zero downstream position is determined for each sample. Results suggest that the corrosion rate increases with the Ni and Cr contents in the original samples. It is concluded that the downstream effect results from a decrease in the corrosion rate caused by corrosion products eluted upstream and adsorbed on the surface of the specimen downstream. (N.K.)