Fireside corrosion degradation of HVOF thermal sprayed FeCrAl coating at 700–800 °C

• FeCrAl coating was exposed in simulated co-fired combustion gases with deposits for 1000 h.
• Median metal loss of FeCrAl coating at 700 °C was ~ 85 μm/1000 h.
• An outer Al-rich mixed oxide layer and an inner Cr-rich mixed oxide layer formed.
• Sulphur penetrated the oxide layers and was present at the coating-inner-scale interface.
• Significant metal losses at 750 and 800 °C, suggesting a change in corrosion mode above 700 °C.

To reduce the emissions of greenhouse gases, the power generation industry is increasingly moving towards higher operating steam temperatures (and pressures) and more efficient ultra super critical (USC) boilers. However, higher operating temperatures coupled with biomass derived fuel can lead to aggressive corrosion damage to the superheater reheater tubes. This paper presents a systematic evaluation of high velocity oxy fuel (HVOF) thermal sprayed FeCrAl coating onto a 9% Cr boiler steel. The coated samples were exposed in a series of laboratory-based fireside corrosion tests in simulated coal-biomass co-fired combustion gases for 1000 h at 700, 750 and 800 °C. The tests were carried out using the deposit-recoat test method to simulate the environment that anticipated from air-firing 20 wt.% cereal co-product (CCP) mixed with a UK coal. The exposures were carried out using a screening deposit containing Na2SO4, K2SO4 and Fe2O3 to produce alkali-iron tri-sulphates, which had been identified as being the principal cause of fireside corrosion on superheaters and reheaters in pulverized fuel power plants. Pre- and post-exposure dimensional metrology was used to quantify the metal damage in terms of metal loss distributions. The exposed samples were examined in an environmental scanning electron microscope (ESEM) to characterize the oxide scales and damage. At 700 °C, FeCrAl coating provided suitable protection (median metal damage of ~ 85 μm) to the steel substrate; however, at 750 °C, the median metal loss of the coating was ~ 260 μm and ~ 305 μm at 800 °C. Sulphur was detected at the coating-scale interface and an aluminium-rich mixed oxide formed at the outer scale and a chromium-rich mixed oxide formed in the inner layer. The concentration of aluminium in the coating depleted to ~ 6 at.% following the 1000 h exposure.