A problem of high-temperature corrosion of the structural materials in mineral combustion products of natural fuel is considered. The values of Gibbs free energy are calculated for reactions of Armco-iron, cobalt, nickel NV-2 and vanadium VEL-2 in fused polymeric potassium phosphate and sodium tetraborate in the presence of V2O5, MoO3, WO3 and B2O3 as well as KVO3, K2CrO4, K2MoO4, K2WO4, NaV6O15, NaV3O7.8 and NaVO3. Determination of average corrosion rate by gravimetric method makes it possible to find the effective activation energy of oxidation reactions.