Effect of pre-oxidation treatment on high-temperature oxidation behavior of MP35Nalloy

Under high temperature and low oxygen pressure conditions, it is a reducing atmosphere for the matrix element Co, and oxidation will not occur, which can effectively protect the matrix element. However, it is an oxidizing atmosphere for active elements (Al, Cr, Si) with greater affinity for oxygen. These active elements will preferentially oxidize to form a dense oxide film to protect the matrix element. The addition of alloying elements such as Al, Cr, and Si to preferentially form oxides with the matrix elements is the essence of the alloy’s resistance to high-temperature oxidation. Under low oxygen pressure conditions, the oxide film formed on the surface of the alloy is uniform and dense. At the same time, the basic principle of selective oxidation is used to appropriately reduce the content of alloying elements required to form the oxide film. The oxide film formed after selective oxidation of alloy elements under low oxygen pressure is denser and smoother than the oxide film formed in the air. Therefore, the morphology of MP35Nalloy after pre-oxidation under different conditions was studied in depth, and the alloy composition and process were explored and optimized to obtain a pre-oxidation film with good oxidation resistance, providing a theoretical basis for in-depth exploration of the selective oxidation of cobalt-nickel-based high-temperature alloys.

Conclusion

• After pre-oxidation at 1000 ℃ and 10–17 atm oxygen pressure for 5 h, the surface oxide of MP35Nalloy without Si is M2O3 (M: Al and Cr), mainly Cr2O3; with the increase of Si content, the Al content in the corundum-type oxide M2O3 on the MP35Nalloy surface gradually increases, and the internal oxide is Al2O3, but the degree of internal oxidation is relatively light, and Al2O3 is distributed in a dendritic manner; when the Si content (mass fraction) is 3%, the Al content in the corundum-type oxide M2O3 increases sharply, and a nearly continuous band-shaped Al2O3 oxide film is formed inside. Therefore, the addition of Si element promotes the external oxidation of Al.

• At 1000 ℃ and 10–25 atm oxygen pressure, the M in the corundum-type oxide M2O3 on the alloy surface of Co-Ni-20Cr-Mo-4Al-1Si is mainly Al, and a continuous Al2O3 film is formed on the surface. At an oxygen pressure of 10–20 atm, the outer oxide film after 5 h of pre-oxidation is Cr2O3, and the inner oxide is Al2O3; at an oxygen pressure of 10–17 atm, the alloy oxide layer does not change much, and a large amount of spinel phase oxide is generated on the surface in the air atmosphere. From the cross-sectional morphology, it can be seen that the outer oxide film is also spinel.

• After Co-Ni-20Cr-Mo-4Al-1Si was pre-oxidized at 1000 ℃ and 10–17 atm for 1, 5, and 10 h, the outer oxide film on the alloy surface was Cr2O3, and the inner oxide was Al2O3. As time goes by, the coarse Cr2O3 particles on the surface are transformed into a dense Cr2O3 oxide film. The needle-shaped oxides in the early stage of Al2O3 oxidation gradually become continuous and the oxide layer becomes thicker. When the pre-oxidation time is extended to 20 h, the outer Cr2O3 film is completely peeled off, and a continuous outer oxide Al2O3 film is formed on the alloy surface.

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