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-CO2-SO2-N-2 with oxygen pressures of 1 x 10(-19) and 1 x 10(-18) atm and sulfur pressures in the range 3 x 10(-9) to 3 x 10(-7) atm. The equilibrium gas compositions were such that Cr2O3 was stable with respect to the sulfide, carbide and nitride of chromium in all cases. Scale growth both above and below the original metal surface resulted. The external layer consisted of a mixture of oxide and sulfide, the volume fraction of the sulfide phase decreasing with decreasing P(S-2), until no 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 phase in the external product layer, which showed an aligned fibrous structure and provided fast diffusion paths for metal transport. Inner corrosion was caused by carbon penetration leading to carbide formation. This process was suppressed in low P(S-2) gases and the expected nitridation never occurred. These effects are ascribed to site-blocking by adsorbed sulfur on oxide grain boundaries.