npj Materials Degradation

Supplementary Information

The potentiodynamic polarization curves of bare alloys and coatings, tested in 44.53 wt. % NaCl – 55.47 wt. % KCl at 700°C in N2(g) vs. Pt reference electrode (RE) are shown in Supplementary Figure 1.

Supplementary Figure 2 is a light microscopy (LM) micrograph of the coating oxidized in air at 900°C using 5°C/min as the heating and cooling rates.

Backscattered-electron in scanning electron microscope (BSE-SEM) imaging of as-deposited HVOF–NiCoCrAlYTa is shown in Supplementary Figures 3 and 4.

Supplementary Figure 4 shows the etched condition of as-deposited HVOF–NiCoCrAlYTa.

The semi-quantitative chemical analysis via energy-dispersive X-ray spectrometry (EDS) of oxidized HVOF–NiCoCrAlYTa (air, 900°C, 24 h, 0.5°C/min) is reported in Supplementary Table 1.

The microstructure of the 310SS substrate of electroetched oxidized HVOF–NiCoCrAlYTa (air, 900°C, 24 h, 0.5°C/min) sample in an aqueous 10% oxalic acid electrolyte is shown in Supplemental Figure 5. The oxalic acid electroetching of sensitized In800H microstructure is shown in Supplemental Figure 6.

EDS analysis of HVOF–NiCoCrAlYTa pre-oxidized (air, 900°C, 24 h, 0.5°C /min) and exposed to NaCl–KCl (Figure 4) is reported in Supplementary Table 2.

Metallographic characterization of as-deposited HVOF–Amdry 510 (NiCrAl) tested with two consecutive polarizations in NaCl–KCl at 700°C in N2(g) atmosphere is shown in Supplementary Figures 7 and 8.

The nominal composition of substrate alloys and coatings tested are shown in Supplementary Tables 3 and 4.

Supplementary Figure Legends

Supplementary Figure 1. Electrochemical potentiodynamic polarization curves of bare alloys and coatings in 44.53 wt. % NaCl – 55.47 wt. % KCl at 700°C in N2(g) vs. Pt reference electrode (RE): (a) Amdry997-NiCoCrAlYTa, (b) Diamalloy4700-NiCoCrAl, (c) Diamalloy4008NS-NiAl, and (d) SPM4-6667-NiCoCrAlYHfSi.

Supplementary Figure 2. LM micrograph of cross-section HVOF–NiCoCrAlYTa oxidized in air at 900°C for 10 h with heating/cooling rates of 5°C/min. No grinding/polishing of the surface was performed. Scale bar is 2 mm.

Supplementary Figure 3. LM micrograph of cross-section as-deposited HVOF–NiCoCrAlYTa. Scale bar is 20 μm.

Supplementary Figure 4. Microstructure of 310SS near coating/steel interface, oxalic acid electroetched: (a) typical deformed layer because of surface cleaning with sand blasting before coating deposition; scale bar is 50 μm, (b) central band of small recrystallized structure after cold work because of sand blasting and high-temperature exposure during coating deposition of sprayed molten metal; scale bar is 100 μm, and (c) oxalic acid etching resolved sensitized structure, revealing carbide precipitates at the grain boundaries and bulk of the grains; scale bar is 20 μm.

Supplemental Figure 5. LM image of 310SS microstructure near coating/substrate interface showing sensitized structures with precipitation at deformation bands: (a) lower magnification; scale bar is 50 μm, and (b) higher magnification showing recrystallization and grain growth; scale bar is 20 μm. 10% oxalic acid electroetching.

Supplemental Figure 6. LM image of In800H microstructure near coating/substrate interface showing sensitized structures: (a) lower magnification; scale bar is 50 μm, and (b) higher magnification showing recrystallization and grain growth; scale bar is 20 μm. 10% oxalic acid electroetching.

Supplementary Figure 7. BSE-SEM microstructure of as-deposited HVOF–NiCrAl tested with two consecutive polarizations in NaCl–KCl at 700°C in N2(g) atmosphere; scale bar is 100 μm.

Supplementary Figure 8. BSE-SEM of top surface of HVOF–NiCrAl coating with subsurface dispersion of fine features: (a) lower magnification; scale bar is 50 μm, and (b) higher magnification; scale bar is 10 μm.

Supplementary Tables Legends

Supplementary Table 1. Semi-quantitative EDS composition in wt. % of the overall microstructure, and the light and dark phases identified in Figure 1.

Supplementary Table 2. Semi-quantitative EDS composition in wt. % of phases identified in Figure 4.

Supplementary Table 3. Nominal composition of commercially available substrate alloys.

Supplementary Table 4. Nominal composition of commercially available gas-atomized raw powders for coatings. Deposition technique (DT): high-velocity oxy-fuel (HVOF) and atmospheric thermal plasma spray (APS).

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