HIGH-TEMPERATURE OXIDATION OF A 20/25 STAINLESS-STEEL IN HIGH-PRESSURE CARBON-DIOXIDE

Kinetics of high-temperature oxidation, and of oxide spallation, are reported for a 20Cr-25Ni Nb-stabilized stainless steel, exposed in carbon dioxide at 40 bar pressure in the temperature range 1115-1240 K. Gross weight-gain kinetics were parabolic with respect to time, with an activation energy of 370±16 kJ/mol. Oxidation rates are shown to be consistent with diffusion control in the inner chromia layer of a two-layer oxide. Intergranular oxidation of silicon was observed beneath the oxide scale. Oxide spallation occurred at all temperatures and increased linearly with gross weight gain. At 1240 K 50% of the oxide which formed spalled at the first weighing. Thereafter, the fraction spalling decreased to about 10%. The difference is attributed to a change in the oxide composition from an M2O3 structure to M3O4, which requires greater compressive stresses to induce spallation during cooling. © 1990 Plenum Publishing Corporation.