COATING PROTECTION OF SEVERAL STEELS AFTER THEIR EXPOSURE AT HIGH-TEMPERATURE TO OXYGEN AND SULFUR MIXTURES

Exposure to oxidation and sulfidation between 300 and 700 degrees C ar e used to study the corrosion behaviour of ASTM A335 P-5 and P-22 stee ls, used in boiling water pipelines in industrial burners. Several sur face modification methods have been applied in order to enhance the ox idation/sulfidation resistance. Emphasis was placed on thermal stabili ty of the coating formed, comparing with the cases where the coating i s necessary. ESCA, SEM, analytical electron microscopy, and X-ray diff raction were used to characterize the surface microstructure. Accordin g to the results obtained, the authors suggest a mechanism of the oxid ized layers formed as a product of the degradation processes.