Adherence of alumina scale formed on Fe–20Cr–4Al alloys with noble metals (Pd, Pt)

Adherence of oxide scale formed on Fe-20Cr-4AI (standard: 4ppmS), Fe-20Cr-4AI (FZ: <1 ppmS) purified by floating zone melting, and Fe-20Cr-4Al-(0.05, 0.1, 0.3, 0.5)-Pd(Pt) alloys was studied in oxygen for 18 ks at 1373-1673 K, by mass gain measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Spalling of oxide scale on the standard alloy occurred along the alloy grain boundaries after oxidation at 1373 K, and was found to spall over the entire surface after oxidation at 1473 and 1573 K. However, the standard alloy showed good oxide adherence after oxidation at 1673 K. The other alloys showed good oxide adherence after oxidation at any temperature. Mass gain of the FZ and noble metal-containing alloys increased with increasing temperature of oxidation, and showed almost the same values at each temperature. Oxide scale on the standard alloy showed wavy morphology after oxidation at 1373 and 1473 K, and changed to planar morphology after oxidation at 1573 and 1673 K. On the other hand, oxide scale on the other alloys showed almost planar morphology after oxidation at any temperature. Oxide scale formed on all the alloys was only found to be alpha-Al2O3 after oxidation at any temperature.