Silicon Effects on the Corrosion of Ferritic and Austenitic Chromia Forming Alloys in Wet and Dry CO2

Model alloys of Fe-9Cr, Fe-20Cr and Fe-20Ni-20Cr (wt.%) with Si (0.1, 0.2 and 0.5%) were reacted isothermally in dry Ar-20CO₂ gas at 818 ^oC.  Silicon-free Fe-9Cr formed thick scales of FeO and FeCr₂O₄, Fe-20Cr formed thin scales of Cr₂O₃ alone, and Fe-20Ni-20Cr formed Fe₃O₄, FeCr₂O₄ and FeNi₃. Silicon additions suppressed formation of iron oxides on Fe-9Cr and Fe-20Cr-20Ni, and dramatically slowed oxidation of all alloys. Scales on 0.2Si-alloys consisted of an outer layer of Cr₂O₃ and an inner layer of amorphous SiO₂. Internal precipitation of chromium-rich carbides took place in all undoped alloys, but was suppressed in Si-bearing alloys. Wagner’s theory of alloy depletion is shown to provide a successful description of these phenomena.

Addition of water vapour to the gas makes it much more corrosive.  All silicon-free alloys underwent breakaway corrosion, developing thick iron-rich oxide scales and internal carbide precipitates.  Silicon additions to Fe-9Cr had no detectable effect on reaction in wet gas, but were beneficial in the higher chromium alloys.  Silicon additions substantially improved both oxidation and carburisation resistance in wet CO2, by forming a layer of silica beneath the chromia scale.

In both wet and dry gases, the silica layer acted as a barrier to carbon entry, and slowed outward metal diffusion, thus slowing scaling rates.  Silicon-bearing austenitic alloys underwent scale spallation on cooling from reaction in wet gas, but not after reaction in dry CO₂.  No spallation was observed for ferritic alloys after reaction in either wet or dry CO₂. The contributions of thermal and growth stresses to spallation, and of growth stresses to the onset of breakaway are discussed.