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
