Vanadium-Containing Oil Ash Corrosion of Boilers Under Oxidizing and Syngas Atmospheres

Corrosion in oil-fired boilers is accelerated in the presence of vanadium, sodium, and sulfur from low grade fuels. Several iron-based alloys and a nickel-based alloy were studied in order to verify the effect of different Cr and Al contents under conditions typical for the partial oxidation process. Materials performance was analyzed by means of substrate recession rate and metallographic characterization. Samples were exposed to H2S-containing syngas atmosphere in order to simulate boiler operating conditions without ash deposition. The prevailing corrosion mechanism under this reducing atmosphere was sulfidation. In order to simulate boiler conditions with oil ash deposits, samples were immersed in 60 mol% V2O5-40 Na2SO4 salt in the same syngas atmosphere. The prevailing corrosion mechanism in this case was sulfidation under a reducing atmosphere and dissolution of iron or chromium from the substrate. The nickel-based substrate showed the best performance under these experimental conditions. In order to simulate air inlet due to shutdowns, samples were immersed in the same salt during exposure in air. The prevailing corrosion mechanism was still sulfidation in the oxidizing atmosphere accelerated by the presence of vanadate salt. This corrosion mechanism is characterized by the formation of a sulfide/oxide layer adjacent to the metal, the dissolution of oxides in the molten deposit, and their precipitation near the outer surface of the deposit.