Effect of nanocrystallization on the corrosion resistance of K38G superalloy in CO + CO2 atmospheres

The corrosion resistance of the cast superalloy K38G and a sputtered nanocrystalline coating of the same material was investigated in pure CO in the temperature range, 850–1000°C and in CO-20 vol.% CO₂ at 900°C. The cast K38G alloy formed Cr₂O₃ and TiO₂ scales, and a zone of internal Al₂O₃ precipitation. Weight-gain kinetics followed the parabolic rate law under all conditions investigated. The sputtered K38G nanocrystalline coating, however, formed a single-phase Al₂O₃ scale and no internal-oxidation zone. The parabolic rate constants for nanocrystalline coating oxidation were about one order of magnitude smaller than those of the cast alloy. The changes in reaction morphology and rate are attributed to the more rapid grain-boundary diffusion of aluminum in the nanocrystalline material.