High temperature corrosion behaviour of HVOF-sprayed Ni-20Cr coating on boiler steel in molten salt environment at 900°C

Abstract

Ni-20Cr coating was deposited on a commonly used boiler steel ASTM A213 347H by high-velocity oxy-fuel (HVOF) thermal spray process to enhance its high-temperature corrosion resistance. High temperature corrosion behaviour was investigated for the uncoated and HVOF sprayed Ni-20Cr coated boiler steel in a simulated boiler environment (Na2SO4-60%V2O5) at 900°C for 50 cycles. The information regarding this system of coating-substrate steel-coating process with regard to its high temperature corrosion is not available in the open literature. Each cycle consisted of one hour heating in the silicon carbide tube furnace followed by 20 min. cooling in air. Mass change technique was used to approximate the kinetics of corrosion. Techniques like X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive (SEM/EDS) were used to characterise the corrosion products. It was observed that the uncoated steel suffered corrosion in the form of spalled powder and peeling up of its oxide scale. On the contrary, the Ni-20Cr coating provided considerable resistance to the corrosion. The Ni-20Cr coating was successful in reducing the mass gain of the steel by 77% which may be due to the formation of oxides of nickel and chromium in its oxide scale