Biomass corrosion behavior of steels and coatings in contact with KCl/K2SO4 at 550 °C under an oxy-fuel combustion atmosphere: A screening laboratory test

Increasing the operating temperature in biomass-fueled power plants is a very difficult task as the corrosion rate generated by alkaline deposits, sulphur and/or chlorine present in biomass, will dramatically rise with increasing temperature, and a higher content of water vapor and SO2 characteristic of oxy-combustion will likely further increase it. Due to these aggressive conditions, it is very important to develop laboratory testing facilities to evaluate the viability of using new materials and to provide information allowing to design new ones, in particular if the operating parameters change. In this work, laboratory testing was carried out to evaluate the effects of salt deposits, as well as the effect of different substrates and coating compositions on the corrosion mechanism. Exposure was performed at 550 °C for >600 h in a flowing model biomass oxy-combustion atmosphere containing 60%CO2, 30%H2O, 8%O2, 2%N2 (v.%), 400 vppm HCl and 2 vppm SO2. The samples were covered with KCl + K2SO4 prior to exposure. The behavior of three steels (P92, T22 and SANICRO 28) and several coatings in the above described biomass model atmosphere is compared. The coatings include a slurry applied diffusion aluminide, a Cr aluminide as well as Fe50Cr, Ni20Cr and Ni5Al deposited by HVOF thermal spray. The observed corrosion rate tendency based on mass variations, do not match that observed by microstructural and phase composition analysis (FESEM and XRD) of the exposed samples due in some cases to spallation and in others to corrosion products stuck to the samples. The ranking of the studied materials and coatings based on oxide scales thickness spallation and coating degradation is as follow (from higher to lower degradation): T22 > Ni5A l > P92 > SANICRO28 > Cr Aluminide > Ni20Cr > Fe50Cr > Aluminide. Both composition and morphology played an important role in the corrosion resistance of the materials.