Sulfate-Vanadate-Induced Corrosion of Different Alloys

Corrosion of boilers and heat exchangers is accelerated in the presence of vanadium, sodium, and sulfur from low-grade fuels. Several iron- and nickel-based alloys were immersed in 60 mol% V2O5-40Na(2)SO(4) salt for 1000 h in order to investigate their degradation behavior at 600 A degrees C in air. Materials performance was analyzed by means of substrate recession rate and metallographic characterization. Their corrosion mechanism is characterized by the formation of a sulfide/oxide layer adjacent to the metal, the dissolution of scale oxides in the molten deposit, and their precipitation near the outer surface of the deposit. High Ni- and Cr-containing alloys show the lowest metal loss rates. Al addition was detrimental due to low-melting eutectic AlVO4-V2O5 formation. Fe-Cr-based alloys showed the highest metal loss rates. In such alloys, high Cr additions (above 20%) did not improve the performance due to the negative synergetic effect by simultaneous dissolution of Fe2O3 and Cr2O3. The predominant salt composition at the corrosion front varied from vanadate rich to sulfate rich during the exposure. This change in the attacking salt makes it difficult to find a protective material for mixed sulfate-vanadate-induced corrosion.