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Abstract
Coupon specimens of a niobium-stabilized 20Cr–25Ni stainless steel were annealed in a dynamic vacuum at 1273K to produce preferential evaporation of chromium. Two sets of specimens, one coarse grain with a surface-chromium concentration of ∼ 14% and the other having a bimodal grain size distribution and surface concentration of ∼ 16%, were subsequently oxidized at 1123 K in a CO2-CO gas mixture. The coarse-grain samples produced uniformly duplex non-protective oxides while those of mixed grain size exhibited a non-homogeneous reaction. Regions of coarse grains on the latter again developed a duplex pitting-type attack but fine grains were protected by a thin chromia-rich layer. The reasons for this difference are discussed. It is argued that the protective film was not produced as a result of a significantly greater supply of chromium ions but was due to easier nucleation on the fine-grain substrate.