Degradation examinations of outer surface of low-emission steam boiler evaporator tubes are presented in the paper. Metallographic evaluation of scale morphology as well as determination of chemical composition of micro sites and distribution of elements on cross-sections is reported. Water-wall tubes were exposed in steam boiler for up to two years period. Common tubes produced from 16Mo3 steel suffered sulphide and molten salts corrosions. Thermal sprayed tubes with the single layer of 70%Ni 30%Cr alloy demonstrate better corrosion resistance, but become susceptible to sulphide corrosion due to presence of internal voids and cracks. Double layer Al2O3/Ni-30Cr coatings withstood corrosion attack for more than one year of operation, and then failed due to internal heterogeneity and crack generation.