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Pure nickel, Ni-20Cr and Ni-30Cr alloys wereexposed to conditions of erosion and corrosionsimultaneously at 700°C and 800°C. The exposureswere made using normal impact of an air stream loadedwith 20-μm alumina. The alumina particles flowed at therate of 400 mg per min and the velocities used were 75m/s and 125 m/s. The reaction kinetics were measureddiscontinuously by interrupting the exposure and measuring the weight loss. The specimens wereexamined using SEM-EDAX both on the surface and incross-section. Under simple oxidation, the alloyspecimens developed a thin protective layer of chromia. Under erosion-corrosion conditions, thisprotective scale was prevented from forming and thealloys were found to undergo aggressive attack at a ratethat was the same as that experienced for pure nickel, the surface oxides were identified asCr2O3 and NiO. It is proposedthat, under erosion corrosion, the erosive streamprevents the formation of a continuous layer of chromiaby removing the oxide faster than it can spread laterally. The specimen issaid to be in a state of erosion-maintained transientoxidation. This mechanism implies that it would bedifficult for protective scales to form in the presence of erosion and the oxidation behavior of analloy cannot be used as a guide to its resistance toerosion-corrosion. |
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