Corrosion mechanism of mild steel in burning sodium and its compounds

Several significantly damaged structures made of mild steel were found at leaked sodium flowing down parts in the actual FBR piping room where a sodium leakage incident was occurred, and in two full mock-up sodium leakage and combustion test cells. Each corroded material which was exposed to burning sodium and/or its molten compounds at elevated temperature for about 1-3.5 hours was analyzed by using optical and chemical measures in order to investigate the corrosion mechanism. The corrosion mechanism of mild steel in of that type reducing conditions of that type is discussed based on the results of material analyses and the chemical thermodynamics and the equilibrium theory. And two different corrosion mechanisms depending on oxygen potential are proposed. One is "Na-Fe double oxidization type corrosion" occurred under a low oxygen potential and another is "Molten salt type corrosion" occurred under a high oxygen potential.