The Hot Corrosion of CoCrAIY Turbine Blade Coatings by Na SO, and Vaporous NaCI

To investigate whether unsaturated NaC1 vapor can have effects in l~ot corrosion, experiments have been conducted wherein specimens cut from CoCrA1Y-coated turbine blades were sprayed with Na2SO4 (1-2 mg/cm2), and then exposed in a tube furnace at 1625 ~ under an air stream containing 300 ppm NaC1 (1/20 the NaC1 saturation pressure at 1625 ~ with inspection and sulfate recoating every 24 hr. Several chloride effects were evident within 100-200 hr under this test regime. These included: (i) increasing the Na2SO4 vaporization rate; (it) causing formation of a spalling A1203 surface oxide; (iii) removing Cr by the formation of volatile complexes which apparently contain S and Na as well as Cr and C1; (iv) interacting with Ti at the coating-blade interface; and (v) accelerating sulfidation-oxidation attack upon the CoCrA1Y coating. This latter attack occurs when both blade metal and coating are exposed simultaneously to the sulfate, as at a cut edge. It is confined to the CoCrA1Y layer, and consists of a sulfidation front and oxidation fron