Fig. 1 Schematic of experimental equipment

Fig. 2 Microstructures of a alloy 1, b alloy 2, c alloy 3 with different surface condition

Fig. 3 Cross section morphology and element distributions of alloy 2 after heat treatment at 1177 °C for 2 h (the color bar represents the degree of element enrichment)

Fig. 4 XRD analysis for surfaces of alloy 1, alloy 2, alloy 3 after corrosion

Fig. 5 Surface morphologies of a alloy 1, b alloy 2, c alloy 3 after corrosion at 700 °C for 400 h in FLiNaK salt and EDS analysis for d point A and e point B in a

Fig. 6 Cross section morphology and element distributions of alloy 1 after corrosion at 700 °C for 400 h in FLiNaK salt

Fig. 7 Cross section morphology and element distributions of alloy 2 after corrosion at 700 °C for 400 h in FLiNaK salt

Fig. 8 Cross section morphology and element distributions of alloy 3 after corrosion at 700 °C for 400 h in FLiNaK salt

Fig. 9 Cr element distributions of a alloy 1, b alloy 2, c alloy 3 from surface to substrate after corrosion at 700 °C for 400 h in FLiNaK salt

Fig. 10 TEM analysis for alloy 3 corroded at 700 °C for 400 h in FLiNaK salt: a FIB-SEM image and sampling position; b TEM image of subsurface area (5-6 μm away from surface, marked as “CD” in a; c SAED pattern for precipitates shown in b; d EDS analysis for precipitates shown in b

Fig. 11 TEM image and element distribution maps of rectangle area for alloy 3 corroded at 700 °C for 400 h in FLiNaK salt

Fig. 12 Schematic of M2C formation in decarburized layer: a microstructure and element distribution before corrosion test; b diffusion direction of elements during corrosion process; cM2C formation beneath corrosion layer