The evolution and oxidation of carbides in an Alloy 601 exposed to long term high temperature corrosion conditions

Research highlights: → Electron microscopy of metal dusting corrosion in plant exposed alloy 601. → Nanoscale charaterisation of carbide precipitation sequence and oxidation. → Metastable M3C2-x carbide formation. - Abstract: Inconel 601 alloy has been studied after 2 years exposure in a commercial methanol plant. Carbon penetration leads to precipitation of nanometer-scale M23C6 and coarser M7Cr3, tens of microns below and within several microns of the alloy surface respectively, as predicted by thermodynamic calculations. Previously unreported M3C2-x precipitates form between the M23C6 and M7C3 in the C-concentration gradient and sometimes outcrop the surface. Carbides are Cr-rich, the smallest may contain Fe, orientated with respect to the matrix and oxidize when exposed to the corrosion product and synthesis gas. The Cr2O3 formed is not optimal for protection from carbon ingress.