Effect of Molybdenum Content on the Corrosion of 9Cr Ferritic Steels in a Flowing Sodium Environment

In a previous paper the sodium compatibility of high purity (5∼15) Cr-1Mo-0.1C ferritic steels had been studied in order to know the effects of chromium content and the influence of sodium velocity, using a sodium loop constructed of Type 316 stainless steel. Molybdenum has been considered to be one of the alloying elements having higher corrosion resistance to sodium. In the present work, the compatibility of high purity 9Cr-(0∼6)Mo with and without 0.1C ferritic steels in a high velocity sodium environment has been studied to know the effects of molybdenum content. The sodium conditions used were 873 and 923 K, 1∼2 mass ppm oxygen, and 4.0 m/s in velocity. Since the compositions of ferritic steels were close to those in quasi-equilibrium in the sodium loop (as to the contents of Fe, Cr and Ni; Fe-∼6Cr-<1Ni at 923 K and Fe-∼7Cr-<1Ni at 873 K) and the chromium contents were a little higher than those in quasi-equilibrium, the corrosion loss was relatively small with the dissolution of small amounts of chromium and the slight deposition of nickel. Both corrosion loss and decarburization of the steels were decreased by the addition of up to ∼1 mass% molybdenum, but they were scarecely decreased by a further addition of molybdenum. Also, tensile tests of sheet specimens of the ferritic steels before and after exposure to sodium showed almost no beneficial effects of more than ∼1 mass% molybdenum additions. Thus, ∼1 mass% molybdenum additions are considered to be most favorable to the high purity 9Cr ferritic steels as other benefits such as strengthening of the steels by formation of intermetallic compounds composed of molybdenum, iron and other metallic elements.