TRANSIENT OXIDATION IN FE-CR-NI ALLOYS - A RAMAN-SCATTERING STUDY

Using Raman scattering we have investigated the oxidation, in air, of the Fe-Cr-Ni stainless steels Fe-25Cr-20Ni, Fe-25Cr-20Ni-3Zr, and Fe-2 4Cr-3Zr (wt.%) as a function of temperature in the range 300 to 1000 d egrees C. The Raman technique is very sensitive to, and provides a cle ar identification of, the oxides Fe2O3 and Cr2O3. However, the techniq ue is insensitive to NiO, FeO, and does not give a clear identificatio n of spinels. The Fe-Cr-Ni alloys form chromia scales at temperatures greater than similar to 800 degrees C. At lower oxidation temperatures , transient phases are observed With a 1-h heat treatment at 300 degre es C, we observe the formation of an unidentified scale; we speculate that it is either amorphous or consists of disordered spinel(s). Near 400 degrees C we begin to observe hematite (Fe2O3). The intensity of t he Fe2O3 signal increases with temperature to similar to 600 degrees C and then decreases, being largely replaced by the signal from Cr2O3 T he thickness of the Cr2O3 scale increases with temperature up to simil ar to 1000 degrees C above which spallation becomes apparent. Spinel p hases also apparently persist in the scale to 1000 degrees C.