HIGH-TEMPERATURE CORROSION OF CR2O3-FORMING ALLOYS IN CO-CO2-N-2 ATMOSPHERES

The corrosion of Fe-28Cr, Ni-28Cr, Co-28Cr, and pure chromium in a num ber of gas atmospheres made up of CO-CO2(-N-2) was studied at 900 degr ees C. In addition, chromium was reacted with H-2-H2O-N-2, and Fe-28Cr was reacted with pure oxygen at 1 atm. Exposure of pure chromium to H -2-H2O-N-2 produced a single-phase of Cr2O3 In a CO-CO2 mixture, a sub layer consisting of Cr2O3 and Cr7C3 was formed underneath an external Cr2O3 layer. Adding nitrogen to the CO-CO2 mixture resulted in the for mation of an additional single-phase layer of Cr2N next to the metal s ubstrate. Oxidizing the binary alloys in CO-CO2-N-2 resulted in a sing le Cr2O3 scale on Fe-28Cr and Ni-28Cr, while oxide precipitation occur red below the outer-oxide scale on Co-28Cr, which is ascribed to the s low alloy interdiffusion and possibly high oxygen solubility of Co-Cr alloys. Oxide growth followed the parabolic law, and the rate constant was virtually independent of oxygen partial pressure for Fe-28Cr, but varied between the different materials, decreasing in the order chrom ium >Fe-28Cr>Ni(Co)-28Cr. The formation of an inner corrosion zone on chromium caused a reduction in external-oxide growth rate. Permeation of carbon and nitrogen through Cr2O3 is thought to be due to molecular diffusion, and it is concluded that the nature of the atmosphere affe cts the permeability of the oxide.