HIGH-TEMPERATURE CORROSION OF PURE CHROMIUM IN CO-CO2-SO2-N-2 ATMOSPHERES

The corrosion of pure chromium has been investigated at 1173 K in CO-C O2-SO2-N-2 with oxygen pressures of 1 x 10(-19) and 1 x 10(-18) atm an d sulfur pressures in the range 3 x 10(-9) to 3 x 10(-7) atm. The equi librium gas compositions were such that Cr2O3 was stable with respect to the sulfide, carbide and nitride of chromium in all cases. Scale gr owth both above and below the original metal surface resulted. The ext ernal layer consisted of a mixture of oxide and sulfide, the volume fr action of the sulfide phase decreasing with decreasing P(S-2), until n o sulfide was observed in the lowest P(S-2) gas. High corrosion rates in high P(S-2) gases were due to high volume fractions of sulfide phas e in the external product layer, which showed an aligned fibrous struc ture and provided fast diffusion paths for metal transport. Inner corr osion was caused by carbon penetration leading to carbide formation. T his process was suppressed in low P(S-2) gases and the expected nitrid ation never occurred. These effects are ascribed to site-blocking by a dsorbed sulfur on oxide grain boundaries.