Many recent investigations of residual fuel oil-ash corrosion have utilized simulated ashes based upon mixtures of Na2SO4 and V2O5. Results of these various studies have not always been easily correlated. It was reasoned that a knowledge of the phase equilibrium relationships in the system Na2O-SO3-V2O5 might provide a reliable basis for the comparison and evaluation of such studies. Accordingly, a compatibility diagram was deduced for the system. Volatilization experiments and phase identification by microscopic and x-ray methods confirmed the essential validity of the diagram. When used as a background for the consideration of previous and current oil-ash corrosion research, the diagram was found to be consistent with most of the recorded findings.
It has been found that Na2SO4 and V2O5 are not compatible with each other in either the crystalline or molten state. They react upon heating with evolution of SO3 to form NaVO3, as well as several complex vanadates, but not Na3VO4 or Na4V2O7. The complex vanadates cause the most severe attack of any of the possible corrodents. Vanadium pentoxide, previously credited with a major role in oil-ash attack, emerges as a relatively unimportant corrodent, since V2O5 as such is absent from all but a few fuel-oil ashes. The only simple sodium vanadate which might function in oil-ash corrosion is NaVO3. There are indications that despite its 74.6 percent content of V2O5, its attack is usually mild. Sulfur trioxide, which has received little attention in high temperature corrosion studies, has given evidence of potentially severe corrosion. In ashes of high sodium-vanadium ratio, some sodium sulfate persists without decomposition, and in such cases may participate in the attack. 3.5.9.
