High temperature corrosion of nickel and dilute nickel-based alloys in (SO 2O 2)/SO 3 mixtures

Abstract

In this paper, surveys are given of the reaction behaviour of high purity nickel, commercial quality nickel and dilute nickel-based alloys containing alloying additions of silicon, manganese and/or iron in (SO 2O 2)/SO 3 at 700°C. High purity nickel reacts rapidly in gas mixtures with sufficiently high SO 3 pressures to form NiSO 4 on the scale surface (the sulphate mechanism). The scale then primarily consists of NiO plus Ni 3± x S 2. The sulfide forms continuous channels through the scale; these serve as diffusion paths for rapid outward diffusion of nickel and thereby sustain the rapid reaction. The dilute nickel alloys and the commercial quality nickel (in which the main impurities are silicon, manganese and iron) exhibit greatly improved corrosion resistance in O 2-rich gas mixtures (e.g. O 2-4%SO 2); the sulphate is still formed on the scale surfaces and the scales also contain sulphides. It is concluded that the positive effects of impurities and alloying additions are that they partially segregate at the surface of the NiO grains and thereby affect the interfacial properties of the oxide grains and the sulphide. The ability of the sulphide to wet the oxide grains is reduced, the sulphide no longer forms continuous channels and the reaction rates are decreased. The wetting of different oxides (NiO, CoO, feldspar and Cr 2O 3) by Ni 3S 2 has been studied, and the results are in qualitative accord with the proposed mechanism.