Cyclic Hot Corrosion of High-Velocity Oxy Fuel Sprayed Coatings on Steel at 900°C

Chromium carbide nickel chrome (Cr3C2-NiCr), nickel chrome (NiCr), tungsten carbide cobalt (WC-Co), and UNS R30006 metallic coatings were sprayed onto ASTM SA213-T22 steel by the high-velocity oxy fuel (HVOF) process using liquid petroleum fuel gas (LPG) for applications in hot corrosion conditions. Microhardness, porosity, and roughness measurements were carried out to assess the coating characteristics. Hot corrosion studies were conducted on the uncoated and HVOF-sprayed specimens by exposure to molten salt at 900°C under cyclic conditions. The thermogravimetric technique was used to establish the kinetics of corrosion. x-ray diffraction (XRD), scanning electron spectroscopy/energy-dispersive spectroscopy (SEM/EDS), and electron probe microanalysis (EPMA) were used to analyze the corrosion products. All coatings showed better resistance to hot corrosion than the uncoated steel. The NiCr coating was found to be the most protective followed by the Cr3C2-NiCr coating. The WC-Co coating was the least effective. The formation of chromium oxide (Cr2O3), nickel oxide (NiO), nickel chromate (NiCr2O4), and cobalt oxide (CoO) may have contributed to the hot corrosion resistance shown by the coatings. The uncoated steel suffered corrosion in the form of intense spalling, cracking, and peeling of the scale.