Surface engineering, by detonation-gun spray coating, of 347H boiler steel to enhance its high temperature corrosion resistance

Surface engineering of ASTM A213 TP347H boiler steel was done by detonation-gun spraying of a Ni-20Cr coating with the aim to enhance the corrosion resistance of the steel. High-temperature corrosion behaviour was investigated for the uncoated and coated boiler steel in a simulated boiler environment (Na2SO4-60%V2O5) at 900 degrees C for 50 cycles. Each cycle consisted of 1 hour heating in the silicon carbide tube furnace followed by 20 minutes cooling in air. Measurement of mass change was used to approximate the kinetics of corrosion. X-ray diffraction (XRD) and scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS) were used to characterise the corrosion products. To ascertain the rate of corrosion the parabolic rate constants (k(p)) were obtained from the slope of the linear regression fitted of (cumulative weight gain/area)(2) versus number of cycles for the uncoated as well as the coated substrates. It was observed that the behaviour of uncoated steel was not parabolic and it suffered corrosion in the form of peeling up and spallation of its oxide scale. On the contrary, the Ni-20Cr coated steel followed a parabolic rate law and showed reduced rates of corrosion in comparison with its uncoated counterpart. The loss of material owing to high temperature corrosion of the steel was reduced appreciably after the application of the coating.