The effect of pre-oxidation of chromia and alumina forming alloys on erosion in laboratory simulated fluidized-bed conditions

The effects of pre-oxidation of alloys have been investigated in laboratory simulated fluidized bed erosion-corrosion environments. Two alloys, Incoloy 800H and Fecralloy, which formed chromia and alumina respectively, under steady state oxidation conditions, were tested. The research consisted of preoxidizing the alloys in a range of oxidation environments. including air, oxygen and H-2/H2O. The scales were then examined and analysed to determine the composition and adhesion to the metal. The erosion-corrosion tests were carried out at 500-degrees-C, using 200-mu-m silica as erodent, at velocities of 1.9 m s-1, and for exposure times of up to 168 h. The specimens were analysed subsequently by analytical scanning electron microscopy and X-ray diffraction. The pre-oxidation results showed that. for both alloys, the scales formed in H-2/H2O were more adherent and slower growing than those formed in air or oxygen. The erosion-corrosion tests on pre-oxidized specimens indicated that the scales formed in H-2/H2O were the most resistant to particle impacts. The results also showed that pre-oxidized Incoloy 800H eroded more rapidly than pre-oxidized Fecralloy. As the erosion-corrosion rates of the two non pre-oxidized alloys were dissimilar to each other, it was not possible to establish whether there was a significant difference between the erosion resistance of the pre-formed scales. under these conditions. Possible reasons for the improved performance of the scales formed in low oxygen pressures are discussed. The differences in erosion-corrosion rates of the alloys are also addressed, with emphasis on alloy deformation characteristics and transient oxidation rates.