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