Corrosion and creep of Alloy 800 H under simulated coal gasification conditions

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Abstract
When the FeNiCr alloy Alloy 800 is exposed to a mixture of H2S, CO and H2O (as well as to a CO free gas of this composition at 800°C and total pressure of 1.5 bar), internal as well as external corrosion takes place and sulfides, oxides and carbides are formed. Corrosion results in a decrease of creep strength under high load while it remains practically unchanged or is even improved under medium and low load. The unexpected absence of nickel sulfide formation in the above gas mixture is attributed to the comparatively low nickel activity in the alloy so that sulfide formation would occur only at sulphur partial pressures exceeding the pressures used in this investigation. The increase rate of crack propagation under load can be interpreted in terms of the cracking of the oxide surface layer on the metal, the cracks allowing the aggressive gas to freely penetrate to the metal surface. The tests with the CO free gas demonstrate that the strength loss is principally caused by sulfidation and oxidation while carburisation which leeds to a strengthening results in somewhat slower destrengthening rate.