Fig. 1 Oxide images on the surface of alloy 690 exposed to hydrogenated SCW at 400 °C for 1000 h

Fig. 2 STEM HAADF images of oxides on alloy 690 exposed to hydrogenated SCW at 400 °C for 1000 h

Fig. 3 Oxides formed on the surface of the alloy 690 exposed to hydrogenated SCW at 400 °C for 1000 h: a STEM HAADF image; b high-resolution transmission electron microscopy (HRTEM) image for the large crystallite; c corresponding FFT of HRTEM image; d HRTEM image for both the inner and outer layers

Fig. 4 High-magnification images for the oxide/alloy interface region of alloy 690 exposed to hydrogenated SCW at 400 °C for 1000 h: a and b for the thick inner layer under the small NiO; c and d for the thin layer under the large NiFe2O4

Fig. 5 STEM image for the thick inner layer under granular NiO on the surface of the alloy 690 expose to hydrogenated SCW at 400 °C for 1000 h and corresponding selected-area diffraction patterns (SADPs)

Fig. 6 EDX mapping of the oxides formed on the surface of the alloy 690 exposed to hydrogenated SCW at 400 °C for 1000 h

Fig. 7 STEM images and EDX mapping a and profile b spanning oxide/alloy interface on the surface of the alloy 690 exposed to hydrogenated SCW at 400 °C for 1000 h

Fig. 8 Schematic mechanism of oxide formation on alloy 690 in hydrogenated SCW