Assessment of Evaporator Tubes Corrosion in Low-Emission Steam Boilers

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