Comparative high temperature analysis of HVOF-sprayed and detonation gun sprayed Ni-20Cr coating in laboratory and actual boiler environments

Abstract

High-velocity-oxy-fuel (HVOF) spray and detonation-gun (D-gun) spray techniques were used to deposit Ni–20Cr coatings on a commonly used boiler steel ASTM-SAE 213-T22. The specimens, with and without coating, were subjected to molten salt (Na2SO4–60%V2O5) deposition in a laboratory furnace at 900 C to determine hot-corrosion resistance. Specimens were also exposed to the superheater zone of a thermal power plant boiler at an average temperature of 700 C under cyclic conditions to ascertain their erosion-corrosion (E-C) behavior. Mass-change measurements were taken to approximate the kinetics of corrosion and erosion-corrosion. In the case of E-C, the thickness lost data were also taken at the end of the exposure. The exposed specimens were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy/energy dispersive spectroscopy (FE-SEM/EDS). The HVOF-sprayed coating was found to be intact during exposure to both given environments; whereas the D-gun coating showed spallation of its oxide scale during exposure to the molten salt environments. An overall analysis of the results indicated that the HVOF-sprayed Ni–20Cr coating should be a better choice for the given boiler applications