Fig. 1 Cross-sectional images of NiAl coating a and 5Hf-NiAl coating b, XRD patterns c

Fig. 2 Mass changes for the two coatings after hot corrosion in the mixed salt (75 wt% Na2SO4 + 25 wt% K2SO4) at 900 °C

Fig. 3 XRD patterns of the two coatings after hot corrosion in the mixed salt (75 wt% Na2SO4 + 25 wt% K2SO4) at 900 °C for 5 h and 140 h

Fig. 4 Cross-sectional images of the NiAl coating a and the 5Hf-NiAl coating b after hot corrosion for 5 h

Fig. 5 Higher magnification images and corresponding element mappings of the oxide scales of the NiAl coating a and the 5Hf-NiAl coating b, and internal oxides/sulfides of the NiAl coating c

Fig. 6 Cross-sectional images of the NiAl coating a and the 5Hf-NiAl coating b after hot corrosion for 140 h; higher magnification image and the corresponding element mappings of the oxide scale in the NiAl coating c

Fig. 7 Surface images of the NiAl coating a and the 5Hf-NiAl coating b after hot corrosion for 140 h

Fig. 8 EPMA mapping for the NiAl coating a and the 5Hf-NiAl coating b after hot corrosion at 900 °C for 140 h

Fig. 9 HAADF-STEM image and corresponding element mappings of the oxide scale/5Hf-NiAl coating interface after hot corrosion for 140 h, and SAED pattern of Ti sulfide

Fig. 10 Standard Gibbs free energy (\(\Delta G^\theta \)) of forming sulfides and oxides as a function of temperature (standardized to consuming 1 mol S2 or 1 mol O2)

Fig. 11 HAADF-STEM image and corresponding element mappings of the oxide of the 5Hf-NiAl coating after hot corrosion for 140 h, and SAED pattern of zone A and HRTEM image of zone B

Fig. 12 Schematic diagram showing the hot corrosion process of the two coatings in the mixed salt at 900 °C