The oxidation of nickel-chromium alloys in atmospheres containing sulphur dioxide

The simultaneous attack of oxygen and sulphur on a range of NiCr alloys containing 2–30%Cr has been investigated between 500 and 790°C in Ar-10% SO₂ atmospheres. Reaction kinetics were followed using an automatic recording balance and scale structures were examined by metallography, scanning electron microscopy and microprobe analysis.Compared with pure Ni, the addition of up to 5%Cr at 500°C, or 15%Cr at 700°C, has little effect on kinetics. At higher Cr levels (8–10% at 550°C or 15–20% at 700°C) initial protective behaviour is followed by a period of rapid attack. Even with Cr contents as high as 30% the protection shown is not as good as in the absence of sulphur from the atmosphere. The failure to form protective oxides is thought to be due primarily to the penetration of initially formed Cr₂O₃ scales by sulphur, possibly by solution and diffusion, leading to the formation of duplex (oxide + sulphide) scales which grow rapidly by nickel ion migration.