Corrosion Aspects of the Vanadium Problem In Gas Turbines

The effect of V2O5 and mixtures of V2O5 and Na2SO4 on turbine fouling and corrosion is discussed. Laboratory tests have shown that none of the commercially available alloys is immune from attack when these mixtures are molten, that is, at temperatures above 650 degrees C. Nickel base heat resisting alloys of the Nimonic type offer higher resistance to attack than the austenitic steels and some degree of protection is afforded by electro-deposited coatings of chromium. Steels and heat resisting alloys are not attacked to any significant extent at temperatures below the melting point of the ash, but copper base alloys suffer considerable corrosion at temperatures as low at 500 degrees C. A detailed investigation of the effect of additives on the corrosion rate of V2O5-Na2SO4 ashes indicates that corrosion can be considerably reduced and that such a method of approach is promising. Suitable additions may be made by a mixture of oil or water soluble materials with the fuel, by suspension in the fuel, or by separate injection into the combustion chamber with the object of reducing corrosion and deposition. Laboratory corrosion tests, using ash additives such as ZnO, Al2O3 and kieselguhr, have shown that the rate of attack can be greatly accelerated if the additive is present in certain critical proportions, but when this is exceeded, corrosion is considerably reduced. With MgO, however, no such increase in attack was noted. Rig tests have confirmed some of the beneficial effect of additives.