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 react ion kinetics were measured discontinuously by interrupting the exposur e 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. Un der 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 sur face oxides were identified as Cr2O3 and NiO. It is proposed that, und er erosion-corrosion, the erosive stream prevents the formation of a c ontinuous layer of chromia by removing the oxide faster than it can sp read later ally. The specimen is said to be in a state of erosion-main tained transient oxidation. This mechanism implies that it would be di fficult for protective scales to form in the presence of erosion and t he oxidation behavior of an alloy cannot be used as a guide to its res istance to erosion-corrosion.