Hot Corrosion-Erosion Behavior of HVOF Sprayed NiCrFeSiB Coatings on Boiler Tube Steels

Fire side corrosion and erosion of metallic structural materials at elevated temperature in complex multicomponent gas environments that include particulates are potential problems in many fossil energy systems, especially those using coal as a feedstock. Combating these problems involves a number of approaches, one of which is the use of protective coatings. High velocity oxy-fuel thermal spraying with oxygen and liquid petroleum gas as the fuel gas has been used to deposit NiCrFeSiB alloy coatings on boiler tube steels designated as ASTM-SA210-Grade A1, ASTM-SA213-T11 and ASTM-SA213-T22. Hot corrosion studies have been conducted on the coated as well as uncoated specimens in molten salt (Na₂SO₄-60%V₂O₅) environment at 900°C for 50 cycles. The Thermo gravimetric technique is used to approximate the kinetics of corrosion. The coatings and alloy samples were exposed to actual service conditions of the coal fired boiler for 1000 hours. The extent of erosion-corrosion has been monitored by measuring the thickness loss and weight change of the samples after exposure. NiCrFeSiB coatings showed superior hot corrosion resistance in molten salt and in the real coal fired boiler due to the protective oxides formed on the surface, mainly consist of SiO₂ and Cr₂O₃.