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

Grazing emission X-ray fluorescence (GEXRF. or refracted X-ray fluorescence - RXF) has been used to characterize the oxide scale (predominately chromi a) which formed on an alloy of 55Fe-25Cr-20Ni (wt%) after oxidation for 4 h at 750 degreesC in O-2 Angle dependent X-ray emission spectra I(theta) wer e acquired for three elements, Cr, Fe and Ni. The measured data were fit by adjusting scale parameters in the calculated spectra such that I(theta) da ta were simultaneously fit for all three elements. It is shown that the cal culated spectra are sensitive to scale thickness. to the Cr depletion zone that develops in the substrate at the scale-metal interface, and to the con centration of Fe and Ni atoms dissolved in the scale. The very demanding re quirements imposed by simultaneously fitting all three measured I(theta) cu rves provide a satisfactory determination of these scale parameters. The GE XRF measurements showed that the scale was about 0.5 mum thick, with an und erlying Cr depletion zone extending about 4 mum (midpoint) into the substra te. The average Cr concentration in this depletion zone was reduced from th e preoxidized value of 25 to 22.2wt%. The scale contained about 2.4at% of F e and less than 0.5at% Ni. These measurements demonstrate that GEXRF can, i n a single nondestructive measurement. provide key information needed to ch aracterize a thermally grown chromia scale.