CORROSION OF NICKEL-BASE ALLOYS BY SUPERCRITICAL CO2

Nickel-base alloys were exposed to flowing supercritical CO2 (P = 20MPa) at temperatures of 700 to 1000 degrees C for up to 1000 h. For comparison, 316L stainless steel was similarly exposed at 650 degrees C. To simulate likely service conditions, tubular samples of each alloy were internally pressurised by flowing CO2, inducing hoop stresses up to 35 MPa in the tube walls. Materials tested were Haynes alloys 188, 230 and 282, plus HR120 and HR160. These alloys developed chromia scales and, to different extents, an internal oxidation zone. In addition, chromium-rich carbides precipitated within the alloys. Air aging experiments enabled a distinction between carburisation reactions and carbide precipitation as a result of alloy equilibration. The stainless steel was much less resistant to CO2 attack, rapidly entering breakaway corrosion, developing an external iron-rich oxide scale and internal carburisation. Results are discussed with reference to alloy chromium diffusion and carbon permeation of oxide scales.