Effect of oxygen on liquid sodium embrittlement of T91 martensitic steel

• The martensitic T91 steel can be embrittled by liquid sodium.
• The steel–sodium interface consists of an in situ built oxide layer of NaCrO2.
• Dissolved oxygen enhance wettability changing crack initiation.
• The crack path is found intergranular and interlath by TEM on FIB machined samples.

The liquid sodium embrittlement susceptibility of the T91 steel in a standard metallurgical state has been studied with varying dissolved oxygen levels. The experimental procedure consists of a pre-exposure wetting step in static sodium before mechanical testing in liquid sodium. The oxygen impurity plays the role of a wetting promoter that facilitates LME of the T91 steel rather than taking part in the fracture process. Brittle grain boundary decohesion is observed using transmission electron microscopy on focused ion beam prepared foils. This settles the issue about the susceptibility of steels to liquid metal embrittlement in liquid sodium.