The influence of gaseous and solid aggressive spices on the corrosion process of steel is a major subject of interest in biomass combustion studies. The slagging, fouling and high temperature corrosion are the most important problems in biomass-firing units and lead to heat transfer limitation, bed agglomeration and metal surface degradation. The work reports the results of the laboratory-scale corrosion studies of selected steel grades under aggressive species in gaseous and solid phases. Two steel grades: 10CrMo9–10 and X10CrMoVNb9–1 were analysed. Studied steel samples were specially prepared: polished, cleaned, degreased and covered with a synthetic deposit: KCl, KCl+K2SO4 (in an eutectic point) and corn straw ash doped with 0.5 wt. % of KCl to simulate salt condensation process occurring in a real boiler. The corrosion experiments were carried out under oxidizing conditions at 610 °C for 168 hours. To simulate aggressive atmosphere: 1000 ppm or 2000 ppm of HCl and steam (H2O) were added to the dry synthetic air. After the experiments the corrosion coupons were mounted in epoxy resin and cut off in the middle to reveal the cross-sections. The corrosion products were analysed using SEM-EDS (Scanning Electron Microscopy with Energy Dispersive Spectroscopy) technique. The results showed that both steam and HCl has the major impact on steel oxidation at studied temperature. The scale formed in an aggressive environment is porous and do not adhere well to the steel surface.