The response of alloys to erosion-corrosion at high temperatures

Pure nickel, Ni-20Cr and Ni-30Cr alloys were exposed to conditions of erosion and corrosion simultaneously at 700 degrees C and 800 degrees C. The exposures were made using normal impact of an air stream loaded with 20-mu m alumina. The alumina particles flowed at the rate of 400 mg per min and the velocities used were 75 m/s and 125 m/s. The reaction kinetics were measured discontinuously by interrupting the exposure and measuring the weight loss. The specimens were examined using SEM-EDAX both on the surface and in cross-section. Under simple oxidation, the alloy specimens developed a thin protective layer of chromia. Under erosion-corrosion conditions, this protective scale was prevented from forming and the alloys were found to undergo aggressive attack at a rate that was the same as that experienced for pure nickel, the surface oxides were identified as Cr2O3 and NiO. It is proposed that, under erosion-corrosion, the erosive stream prevents the formation of a continuous layer of chromia by removing the oxide faster than it can spread later ally. The specimen is said to be in a state of erosion-maintained transient oxidation. This mechanism implies that it would be difficult for protective scales to form in the presence of erosion and the oxidation behavior of an alloy cannot be used as a guide to its resistance to erosion-corrosion.