Characterization of the scale on oxidized Fe–Ni–Cr alloys using grazing emission X-ray fluorescence

Abstract

Grazing emission X-ray fluorescence (GEXRF, or refracted X-ray fluorescence—RXF) has been used to characterize the oxide scale (predominately chromia) which formed on an alloy of 55Fe–25Cr–20Ni (wt%) after oxidation for 4 h at 750°C in O 2. Angle dependent X-ray emission spectra I( θ) were acquired for three elements, Cr, Fe and Ni. The measured data were fit by adjusting scale parameters in the calculated spectra such that I( θ) data were simultaneously fit for all three elements. It is shown that the calculated spectra are sensitive to scale thickness, to the Cr depletion zone that develops in the substrate at the scale–metal interface, and to the concentration of Fe and Ni atoms dissolved in the scale. The very demanding requirements imposed by simultaneously fitting all three measured I( θ) curves provide a satisfactory determination of these scale parameters. The GEXRF measurements showed that the scale was about 0.5 μm thick, with an underlying Cr depletion zone extending about 4 μm (midpoint) into the substrate. The average Cr concentration in this depletion zone was reduced from the preoxidized value of 25 to 22.2 wt%. The scale contained about 2.4 at% of Fe and less than 0.5 at% Ni. These measurements demonstrate that GEXRF can, in a single nondestructive measurement, provide key information needed to characterize a thermally grown chromia scale.