Comparison of the Isothermal Oxidation Behavior of As-Cast Cu–17%Cr and Cu–17%Cr–5%Al. Part II: Scale Microstructures

The isothermal oxidation kinetics of as-cast Cu–17%Cr and Cu–17%Cr–5%Al in air were studied between 773 and 1,173 K under atmospheric pressure. Details of the oxidation kinetics of these alloys were discussed in Part I. This paper analyzes the microstructures of the scale and its composition in an attempt to elucidate the oxidation mechanisms in these alloys. The scales formed on Cu–17%Cr specimens oxidized between 773 and 973 K consisted of external CuO and subsurface Cu₂O layers. The total thickness of these scales varied from about 10 μm at 773 K to about 450 μm at 973 K. In contrast, thin scales formed on Cu–17%Cr–5%Al alloys oxidized between 773 and 1,173 K. The exact nature of these scales could not be determined by X-ray diffraction but energy dispersive spectroscopy analyses were used to construct a scale composition map. Phenomenological oxidation mechanisms are proposed for the two alloys.