Abstract
This paper reports the corrosion behaviour 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 alloy coating materials were selected: NiCrBSiFe, alloy 718, alloy 625 and alloy C-276. These were sprayed onto P91 substrates using a JP5000 spray system. The corrosion performance of the coatings varied when tested at 525, 625 and 725°C in K2SO4- KCl and gaseous HCl-H2O-O2 containing environments. Alloy 625, NiCrBSiFe and alloy 718 coatings performed better than alloy C-276 coating at 725°C, which had very little corrosion resistance resulting in degradation similar to uncoated P91. Alloy 625 coatings provided good protection from corrosion at 725°C, with the performance being comparable to wrought alloy 625, with significantly less attack of the substrate than uncoated P91. Alloy 625 performs best of these coating materials, with an overall ranking at 725°C as follows: alloy 625 > NiCrBSiFe > alloy 718 >> alloy C-276. Although alloy C-276 coatings performed poorly in the corrosion test environment at ~725°C, at lower temperatures (i.e. below the eutectic temperature of the salt mixture) it outperforms the other coating types studied.
