Observations on the effect of low sulfur activity on the oxidation of chromium-depleted zones in a stainless steel

Coupon specimens of 20% Cr-25% Ni-Nb-stabilized steel have been oxidized in an atmosphere of CO ₂-2%CO-670 vppb COS at 0.1 MPa pressure for periods up to 500 hr at 1123 K. Standard specimens, annealed at 1203 K prior to testing, showed an enhancement of iron in the surface scale and a much increased propensity to spall compared with control tests in a sulfur-free atmosphere. The main purpose of the work was to examine the effect of sulfur on the ease of formation of a healing layer under duplex attack produced by depleting specimens in chromium by prior vacuum annealing. It is shown that although a chromium-rich layer had formed, extensive breakdown occurred in the sulphidizing atmosphere leading to continued internal oxidation. Sulfur was found to partition at the base of this attack and to be associated with a large concentration of nickel. Spaliation was also enhanced in the depleted specimens, the favored site being the interface between the spinel and outer iron-rich oxide of the duplex structure. Partitioning of both sulfur and carbon was observed at this interface in those regions of the specimen showing healing layer breakdown.