Effect of Secondary Oxidants (CO2 and/or H2O) on Oxidation of Fe-20Cr-(Mn, Si)
Authors: Cui Hongji , Zhang Jianqiang , Young David 
Published: February 2024

Ferritic heat resisting alloys are widely used in industry owing to their combination of economy, reasonable strength, and high temperature corrosion resistance. Alloy design centres on forming and maintaining an external, slow growing protective oxide scale by selective oxidation of the desired metal, commonly chromium. Design methods based on diffusion theory succeed when the service environment is oxygen or clean, dry air. However, they can fail badly when secondary oxidants (like CO2 and water vapour) are present, which is commonly the case in energy production etc. This work investigates the effect of CO2 and H2O on corrosion behaviour of Fe-20Cr, in comparison with that in pure O2. The alloying effect of Si and Mn on oxidation behaviour of Fe-20Cr in different gases is also investigated.

Model alloys Fe-20Cr, Fe-20Cr-1Si, Fe-20Cr-2Mn (all in wt%) were arc melted and annealed. Sample coupons were cut, ground, mechanically polished to 3mm finish, and further electro-polished to remove subsurface deformation zones. They were then exposed to Ar-2O2, Ar-20H2O-2O2, Ar-20CO2-2O2, Ar-CO2-H2O and Ar-CO2-H2O-O2 (all vol%) gas mixtures, at 650°C for 150h. Corrosion products and cross-sections were examined using optical microscopy, X-ray diffraction, Raman Spectroscopy and scanning electron microscopy equipped with energy dispersive X-ray spectrometry.

The alloy behaved protectively in Ar-2O2, forming a very thin Cr2O3 layer on its surface. However, when CO2 or/and H2O was added to oxygen, breakaway oxidation resulted, forming thick multi-layered oxide scales. Detailed analysis revealed that this multi-layered structure contains four layers, where the outer layer was Fe2O3, overlying an Fe3O4 layer and FeCr2O4 spinel innermost layer, and an internal oxidation zone was formed beneath the scale. In the low environment of Ar-20CO2-20H2O, Fe3O4 layer, FeCr2O4 spinel layer and internal oxidation zone formed a three-layered structure. No Fe2O3 was found, which is attributed to the decrease in oxygen partial pressure.

Carburisation of Fe-20Cr also resulted from exposure to the three CO2-bearing gases. The deepest and densest internal carburisation zone formed in Ar-20CO2-2O2. In Ar-20CO2-20H2O-2O2 and Ar-20CO2-20H2O, internally precipitated carbides were sparser, but still obvious.

Silicon addition dramatically improved the oxidation resistance of Fe-20Cr. Weight gains in all gases were sharply decreased compared with those of Fe-20Cr. As with Fe-20Cr, a very thin Cr2O3 layer on the Si-bearing alloy reacted in Ar-2O2,together with a thin underlying silica layer, indicating that the alloy was fully protected. Unlike Fe-20Cr, the Fe-20Cr-1Si alloy was also protected in Ar-20CO2-2O2. However, when water vapour was present, the alloy started to go into breakaway, forming local Fe-rich oxide nodules. In Ar-20H2O-2O2, some small nodules, consisting of external Fe2O3 and inner spinel were distributed on an otherwise thin, protective chromia scale. In Ar-20H2O-20CO2-2O2 and Ar-20H2O-20CO2, an additional Fe3O4 layer was observed in nodules, between Fe2O3 and inner spinel. Thus, for Fe-20Cr-1Si samples, Fe3O4 can only be observed if 20%CO2 and 20%H2O were both present. Additionally, some small internal oxide particles were observed at breakaway nodule areas in all these three gasses. Internal carbides were found only under the big iron oxides nodules grown in Ar-20CO2-20H2O-2O2.

Manganese addition did not much influence the corrosion resistance of Fe-20Cr. In general, oxide morphologies were similar to those of Fe-20Cr. In Ar-2%O2 gas, a thin Mn rich chromium oxide layer was formed over the entire alloy surface, indicating that sample was fully protected. Multi-layered thick oxides developed when CO2 or/and H2O were added. For high gases with the addition of H2O or CO2 (Ar-20H2O-2O2, Ar-20CO2-2O2, and Ar-CO2-H2O-O2), three-layered oxide structures were found to be made up of Fe2O3, Fe3O4, and spinel layers, in sequence from the outer surface to the inner alloy. It is worth noting that the oxide layers were not completely continuous in Ar-20CO2-2O2. About 15% of the cross-section area was protected which indicated that Mn can slightly improve the corrosion resistance in dry CO2 and O2 environments. In lower oxygen partial pressure gas (Ar-CO2-H2O), no Fe2O3 was found, and only Fe3O4 and spinel constituted the oxide scale. The main difference was that the internal oxidation zone became tiny on Fe-20Cr-2Mn samples. Internal carbides were still found under inner oxide in all CO2 bearing gases, most obviously in Ar-20CO2-2O2. However, in all cases the carbide quantities were far less than those in Fe-20Cr, indicating that Mn addition partially inhibited carburisation.

After the thermodynamics of the reaction product phase assemblages are considered, this complex pattern of reaction products is discussed in terms of the permeability of different oxides to carbon- and hydrogen-bearing species, along with interactions between secondary oxidants on oxide surfaces and grain boundaries.