Performance of Cr<sub>3</sub>C<sub>2</sub>-25(Ni-20Cr) and Ni-20Cr Coatings on T91 Boiler Tube Steel in Simulated Boiler Environment at 900℃

Abstract

The dangerous process in boiler steel tube is corrosion due to loss of metal by the attack of corrosive agents. In order to prevent this corrosion Cr₃C₂–25 (Ni–20Cr) and Ni-20Cr coatings were deposited on T91 boiler tube steel by high velocity oxy-fuel (HVOF) process. Hot corrosion studies were conducted on uncoated and HVOF-coated T-91 steel specimens after exposure to a molten salt (Na₂SO₄–60%V₂O₅) environment at 900℃ under cyclic conditions. Each cycle consisted 1 h of heating in tube furnace followed by 20 min of cooling in air. The weight change measurements were performed after each cycle in order to establish the kinetics of corrosion using thermogravimetric technique. X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDS) were used to analyze the corrosion products. The bare steel experienced higher weight gain due to the formation of unprotective Fe₂O₃ dominated oxide scales. The Cr₃C₂–25(Ni–20Cr) coating was found to be more protective than the Ni-20Cr coating.