The corrosion of alloy 625 (NiCr22Mo9Nb; 2.4856) in high-temperature, high-pressure aqueous solutions of phosphoric acid and oxygen. Corrosion at sub- and supercritical temperatures

Abstract

Corrosion of alloy 625 (NiCr22Mo9Nb; 2.4856) pressure tubes was investigated in aqueous solutions of phosphoric acid (0.05 — 1.0 mol/kg) and oxygen (0.48 mol/kg) at temperatures up to 500°C and pressures of 24 MPa. Reaction times were up to 185 h. At acid concentrations below 0.1 mol/kg, no corrosion was found caused by the high-temperature solution. Acid concentrations higher than 0.1 mol/kg were extremely corrosive at temperatures between 400 and 490°C and led to leakages at reaction times below 50 h, corresponding to corrosion rates of up to 850 μm/h. The observed leakages were not caused by stress corrosion cracking, but by deep holes with polished surfaces which were found exclusively at the upper side of the horizontally mounted and passed tube reactors. A mechanism will be proposed that can explain as well the unexpected corrosive attack of alloy 625 through higher concentrations of phosphoric acid and the totally different corrosive behaviour of other mineral acids under similar conditions of reaction.