INFLUENCE OF YTTRIUM ON THE HIGH-TEMPERATURE CORROSION OF CHROMIUM AND FE-28CR IN CO-CO-2-N-2(-SO2) ATMOSPHERES

The influence of yttrium additions on the corrosion behaviour of Fe-28 Cr and pure chromium in CO-CO2-N-2, and in CO-CO2-SO2-N-2 after pre-ox idation, was investigated at 900 degrees C using gas mixtures such tha t Cr2O3 was always the thermodynamically stable phase. Yttrium was eit her alloyed with Fe-28Cr (1 wt.%), or applied as a thin Y2O3 coating o n both metals by spray pyrolysis. The reaction rate of Fe-28Cr in both gas mixtures was marginally reduced by alloying with 1 wt.% yttrium. When a Y2O3 coating was applied, the reaction rate of both Fe-28Cr and pure chromium in the CO-CO2-N-2 atmosphere was markedly reduced, and the growth mechanism of Cr2O3 was altered. Whereas uncoated chromium w as corroded by carbon and nitrogen underneath the Cr2O3 scale, this in ner corrosion was entirely suppressed by the Y2O3 coating. Subsequent sulfidation rates in CO-CO2-SO2-N-2 depended heavily on the duration o f pre-treatment in CO-CO2-N-2. It was found that the Y2O3-coating appe ared to be more effective on pure chromium than on Fe-28Cr, possibly b ecause a less complete Cr2O3 layer of smaller thickness was formed in the latter case. (C) 1998 Published by Elsevier Science Ltd. All right s reserved.