Corrosion Testing of Ni-based HVOF Coatings in High Temperature Environments for Biomass Applications

This paper reports the corrosion behavior of coatings deposited by high velocity oxy-fuel (HVOF) spraying and representative boiler substrate alloys in simulated high temperature biomass combustion conditions. Four commercially available oxidation resistant Ni-base coating materials were selected: NiCrBSiFe, alloy 718, alloy 625 and alloy C-276. These were sprayed onto P91 substrates using a JP5000 HVOF thermal spray system. The corrosion performance of the coatings varied when tested at about 725°C in a molten salt (K2SO4-KCl) and gaseous HCl-H2O-containing environments. Alloy 625, NiCrBSiFe and alloy 718 coatings performed better than alloy C-276 coating, which had very little corrosion resistance resulting in degradation similar to uncoated P91 substrate. Alloy 625 coatings provided good protection from chlorine-assisted corrosion at 725°C, with the corrosion performance being comparable to wrought alloy 625, with significantly less attack of the substrate than uncoated P91. It is possible to conclude a tentative ranking based on a combination of the amount of coating remaining and the impact on the substrate following chlorine-assisted corrosion at 725°C. Alloy 625 performs best of these coating materials, with an overall ranking as follows: alloy 625 > NiCrBSiFe > alloy 718 >> alloy C-276.