Sulfate-induced high-temperature corrosion of nickel

Abstract

Corrosion of high-purity nickel with deposits of sodium sulfate has been studied at 900°C in O₂+4.2 % SO₂ at gas pressures ranging from 700 to 0.5 Torr. Corrosion rates in the gas mixture are faster than in oxygen or sulfur dioxide alone. At the higher pressures the initial reaction is parabolic, but subsides after extended reaction. The parabolic reaction is concluded to comprise a diffusion-controlled transport of oxygen atoms and sulfur dioxide molecules through the molten sulfate layer with dissolved nickel oxide. Ni-S liquid solution is formed at the metal surface during the reaction. At reduced gas pressures the initial reaction is nonparabolic (linear and subsequent gradually increasing rate) prior to the parabolic stage. After extended reaction at sufficiently low partial pressure of SO₂ or reaction in 1 atm O₂ significant amounts of Ni-S liquid solution cannot be detected at the metal surface below the sulfate melt. The reaction mechanism is illustrated and discussed in terms of the stability diagrams of the Na-O-S and Ni-O-S systems.