Effect of Hf Element on the Hot Salt Corrosion Resistance of NiCoCrAlY Coatings

To improve the hot corrosion resistance of NiCoCrAlY coatings under high‑temperature salt‑laden conditions, two free‑standing coatings—NiCoCrAlY and NiCoCrAlYHfSi—were prepared by high‑velocity oxy‑fuel spraying. Their corrosion behavior was systematically investigated at 900 °C in a mixed salt environment of 75 wt.% Na ₂ SO ₄  + 25 wt.% NaCl. The results show that the addition of Hf and Si significantly enhances coating densification and structural integrity, effectively suppressing TGO thickening, crack propagation, and internal oxidation. Corrosion kinetics analysis indicates a lower corrosion rate for the NiCoCrAlYHfSi coating. Microstructural characterization confirms that Hf forms a dense HfO ₂ layer, which strengthens TGO adhesion through “pinning effects” and blocks corrosive ion penetration. Additionally, Hf effectively traps and immobilizes sulfur, inhibiting internal sulfidation. This study provides experimental and theoretical support for the application of Hf‑doped NiCoCrAlY coatings in severe high‑temperature corrosive environments.