A microstructural study of preferential oxidation at the grain boundaries of Ni-Cr alloys

The oxidation behavior of alloy grain boundaries in model Ni-Cr alloys was investigated. Two binary alloys with nominal wt.% compositions of Ni-10Cr and Ni-20Cr were used. Oxidation was performed in air for 50 hr at 1000°C. The grain boundaries intersecting the alloy surface in Ni-10Cr did not exhibit oxidation, whereas the alloy formed a thick (≈60 μm) oxide layer which formed inwardly. The grain boundaries in this alloy showed a "passivating influence" at the adjacent regions and retarded oxide formation. An examination of the Ni-20Cr cross section revealed preferential oxidation to a depth of 65 μm at the alloy grain boundaries intersecting its surface, while the oxide at the surface was a few micrometers thick. It is noted that the extent to which the grain-boundary oxidation differs from the alloy surface oxidation depends on the Cr content of the alloy. It is also considered that the grain-boundary oxidation behavior in different Ni-Cr alloys changes as a function of Cr content.