Abstract
Vanadates induced or accelerated corrosion is known to occur in fossil energy conversion systems such as direct combustion of coal or especially heavy fuels, oil fired stationary turbines or fluidized bed combustors at rather high oxygen partial pressures. Under such oxidizing conditions, vanadate salts melt at low temperatures (around 600°C) accelerating hot corrosion by fluxing of the protective scales. Vanadates are usually introduced in the systems due to the burning of poor quality fuels. If the fuel is heavily loaded with heavy metals or sulfur, such as oil sands or refinery residues it can still be effectively used for the production of syngas via the partial oxidation. In boilers of the partial oxidation process vanadates condensate under reducing conditions and induce highly accelerated corrosion as well. The boilers or heat exchangers are usually manufactured from low Cr containing ferritic steel (UNS K11597) or ferritic-martensitic steels and are exposed to reductive atmospheres with high CO, H2O and H2S contents at temperatures between 450 and 600°C in which vanadates containing salts are present. The resulting metal wastage of more than 5.3 mm.year-1, produces holes through the heat exchanger after less than two years. The mechanism of corrosion as a combination of the substrate sulfidation and acceleration of the metal loss by dissolution of the formed scales in the salts is discussed.
The resistance of a wide range of alloys against such vanadate deposits is investigated under reducing conditions typical for the partial oxidation process.
