Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. II. Influence of sulphur

The oxidation of presulphidized Ni-Cr alloys has been studied by taking into account the influence of the two distinct oxidation mechanisms described in part I of this article. Sulphur enters the Cr₂O₃ scale (in Ni-34Cr alloys) mainly as S^2– species, which at high temperatures increases the V_Cr content, and hence the oxidation kinetics. Sulphur is randomly distributed in the scale, except at the inner oxide-alloy interface, where intergranular microsulphides are analyzed in the oxide-scale zone. In the case of NiO, NiCr₂O₄, Cr₂O₃ oxide multilayers (in a Ni-20Cr alloy), sulphur in the S^2– state is distributed in the oxide layers or at Si-precipitate interfaces. Such a distribution leads to crack formation, especially during cooling.