Modelling fireside corrosion of thermal sprayed coatings in co-firing of coal/biomass

This paper presents a development and evaluation of coating materials for advanced fossil fuel plants and addresses issues related to coal/biomass-derived flue gases. A selection of candidate coatings: 625, NiCr and NiCrAlY were deposited on superheater/reheater materials (T91) using high velocity oxy-fuel (HVOF) spraying. A series of laboratory-based fireside corrosion exposures have been carried out on these coated samples in controlled atmosphere furnaces for 1000h. The tests were carried out with the deposit-recoat test method to generate the exposure conditions; the gaseous environment simulated that anticipated from air-firing 20wt% cereal co-product (CCP) mixed with a UK coal. The exposures were carried out using various mixtures of Na2SO4, K2SO4, Fe2O3 and kaolinite to produce different deposition fluxes at a test temperature of 650 degrees C. After the exposures, the samples were examined by environmental scanning electron microscope/energy dispersive X-ray analysis to characterise the damage. Pre- and post-exposure dimensional metrology was used to quantify the metal damage in terms of metal loss distributions. In all three coatings, the deposit targeted at forming undiluted alkali-iron tri-sulphate was found to be the most aggressive, causing the most corrosion damage to all alloys in simulated air-fired combustion gases. A corrosion model was proposed to predict the incubation time at different alkali deposition fluxes. The transition from incubation to propagation was found to be dependent on the chromium content of the alloys. The HVOF NiCr coating, with 46wt% chromium, was found to be the best performing coating with the longest incubation times in these tests.