Corrosion behaviour of structural materials in sodium influenced by formation of ternary oxides

Abstract

Abstract Fast breeder reactors use liquid sodium metal as the coolant in their primary and secondary circuits. Corrosion behaviour and compatibility of structural materials in sodium at the operating temperature of the reactors has been found to be influenced by oxygen impurity in sodium even at parts per million level. The compatibility of structural materials in sodium influenced by oxygen in the liquid metals has been dealt with previously on the basis of formation of binary oxides of the solid metals. Since then many of the structural metals have been found to form ternary oxides in sodium. Several three phase equilibria and free energy of formation data for the ternary oxides of structural metals with sodium are reviewed in this paper. Threshold oxygen levels in sodium for the formation of binary oxides and ternary oxides on structural metals are derived from the relevant thermodynamic data. Many of the structural metals are found to form ternary oxides at much lower oxygen levels in sodium than required for the formation of respective binary oxides. Threshold oxygen levels in sodium for the formation of ternary oxides of major constituents of stainless steel are also presented. Corrosion behaviour of the structural metals and austenitic steels influenced by formation of ternary oxides is discussed.