Hot corrosion behavior of HVOF-sprayed Cr 3 C 2 -WC-NiCoCrMo coating

Abstract A novel binary carbide-based sprayed powder, consisting of hard phases (Cr 3 C 2 , WC) and matrix phase (NiCoCrMo), was designed to ensure the harsh environments (corrosion, wear, and high temperature) of power-plant boilers. A Cr 3 C 2 -WC-NiCoCrMo coating was deposited by high-velocity oxygen-fuel (HVOF) process using an agglomerated and sintered Cr 3 C 2 -WC-NiCoCrMo powder with a particle size of −45+15 µm. Hot-corrosion tests were performed under cyclic conditions using a NaCl-KCl-Na 2 SO 4 salt environment, at temperatures up to 500 °C. High-temperature oxidation was performed at 500 °C. For comparison, corrosion and oxidation tests were also performed on a commercial Cr 3 C 2 -25(NiCr) coating, and the corrosion resistance was evaluated by measuring the thickness of the corrosion scale. The properties of the specimens were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy with energy dispersive spectroscopy. The Cr 3 C 2 -WC-NiCoCrMo and Cr 3 C 2 -25(NiCr) coatings exhibited similar oxidation-resistance properties. The Cr 3 C 2 -WC-NiCoCrMo coating exhibited superior corrosion resistance to that of the Cr 3 C 2 -25(NiCr) coating. In the case of the Cr 3 C 2 -WC-NiCoCrMo coating, the thickness of the corrosion scale was approximately one-third that of the Cr 3 C 2 -25(NiCr) coating. The hot-corrosion resistance imparted by the Cr 3 C 2 -WC-NiCoCrMo coating may be attributed to the formation of alloy phases during the powder and coating manufacturing process, as well as to the formation of Cr, W, Ni, Co and Mo oxides, and Ni and Cr spinels.