Effect Of Oxygen Partial Pressure On The Formation Of Ni-Cr-Fe Based Alloys Surface Oxide Scale

As carburization resistance and anti-coking materials, Ni-Cr-Fe-based alloys are widely used in petrochemical and refinery processing units. Some special oxidation resistance materials were needed for the low oxygen partial pressure environment. The high temperature oxidation resistances of the selected alloys were investigated under the wet hydrogen atmosphere. And the oxidation kinetics of composite oxide film was also studied. Besides, the formation model of composite oxide film was established by the thermomechanical calculation of oxides. Furthermore, the oxidation mechanism of composite oxide film was analyzed. It was found that the formation and growth kinetics of oxide scale are rather sensitive to the oxygen partial pressure of the system.Both as cast Ni-Cr-Fe based alloy and Inconel 625 alloy were used as the testing samples. The samples were oxidized in the low oxygen partial pressure environment at 1050℃. Especially, the as cast Ni-Cr-Fe alloy was oxidized in the wet hydrogen atmosphere which had different contents of water vapour. The effects of different oxygen partial pressure on the oxidation rate were investigated, as well as the composition of oxide scale composition of oxide scale and microstructure of oxide scale.Camparative tests were performed between the as cast Ni-Cr-Fe alloy and Inconel 625 alloy in different circumstances atmosphere including wet hydrogen and pure hydrogen environment. Influence of oxidation atmosphere and the flow rate of reactive gas on the formation of the oxide scale were also discussed. Additionally, the oxidation mechanism of alloys were analysed and compared by the cyclic oxidation experiments of two alloys in the wet hydrogen environment. Weight gain, scanning electron microscopy (SEM/EDS), X-ray diffraction analysis (XRD), metallographic observations and other testing methods were used in this study. Besides, the oxidation mechanism of the Ni-Cr-Fe based alloys were probed by analyzing the influences of the oxygen partial pressure and the atmosphere.Thermodynamic calculation was introduced to anlyze the formation of the composite oxide film at 1050℃, oxidation kinetics of alloys and the model of composite oxide film formation were also established.According to Gibbs free energy of oxide formation, Fe, Ni are not oxidized, meanwhile, Cr, Mn, Si and other elements are oxidized competitively under the oxygen partial pressure of experimental environment. From outmost surface to inner substrate, the oxide scales are composed of Mn-Cr spinel oxide, Cr₂O₃, and amorphous SiO₂ respectively. These oxide scales bearing flat structure, compact texture, uniform and fine oxide particles show excellent oxidation resistance properties. The thickness and morphology of oxide scales are rather sensitive to the oxygen partial pressure of the system. Moderate oxygen partial pressure (15.9×10^-18atm) had helped to significantly promote the formation of spinel on the surface. At lower oxygen partial pressure, thick and porous-structured oxide scale is much more beneficial on the alloy surface. In contrast, at higher oxygen partial pressure, a thin and dense oxide scale is inclined to form.It demonstrates that the new as cast Ni-Cr-Fe alloy and Inconel 625 have the same excellent oxidation resistance in weak oxidizing atmosphere by comparing the atmosphere and cyclic oxidation experiments. Besides, the oxidation kinetics of two alloys accords with the parabolic law, and the oxidation mechanism is also consistent with Wagner thick-film theory. The Ni-Cr-Fe alloy ions diffuse mainly through grain boundaries or defects. Oxide ridges with network and porous structure are formed on the surface. However, they drop off with the increase of oxidation time. On the other hand, Inconel 625 ions diffuse mainly through bulk diffusion which based lattice diffusion, and thin and dense oxide scale on the surface. The oxidation kinetics of composite oxide film was generalized through the high temperature oxidation tests of Ni-Cr-Fe based alloys. Moreover, the formation model of composite oxide film was established by the thermodynamic calculation of metal oxide. And the anti-oxidation mechanism of composite oxide film is analyzed. These provide a fundamental understanding of the scale formation properties of Ni-Cr-Fe based alloys oxidized in weak oxidizing atmosphere.