Effects Of Silicon On The Oxidation Properties Of Hastelloy N Superalloy At Elevated Temperature

Hastelloy N is a Ni-16Mo-7Cr-4Fe nickel-based superalloy specially developed for molten salt reactors that can meet the required corrosion resistance and strength requirements for molten salt reactor environments.In the molten salt reactor,the Cr in the alloy reacts with the fluoride in the molten salt.Based on the considerations of safety and corrosion resistance,the content of Cr in the alloy is controlled to about 7%.However,the nickel base superalloy can achieve good oxidation resistance only if the Cr content is above 15%.Therefore,the high temperature oxidation resistance of Hastelloy N alloy needs to be further improved.In this paper,Ni-16Mo-7Cr-4Fe-xSi?wt.%?superalloys were prepared by the magnetic-controlling tungsten electrode arc furnace at high vacuum.The isothermal oxidation behavior of Hastelloy N alloys with different silicon contents at 700°C and850°C and the cyclic oxidation behavior at 850°C were investigated by using discontinuous weigh method.Experiments recorded the oxidation kinetic curves by recording the oxidation weight gain data of alloys with different silicon contents.The phase structure,surface morphology and cross sectional morphology of the oxide products were analyzed by scanning electron microscopy,energy dispersive spectroscopy,X-ray diffraction and electron probe.The results showed that Si-addition can significantly improve the oxidation resistance of Hastelloy N alloy.With increasing of Si contents,oxide film thickness decreased,and surface spinel oxidation products were refined.The oxidation rate of alloys decreased with increasing of Si contents in alloys.All alloys mentioned above had reached complete antioxidant levels and no scale spallation were detected at 700?.The oxide film appeared cracking and spalling due to the temperature increase,the oxidation rate gradually decreased with the increase of Si contents,and all alloys reached antioxidant levels after oxidation for 100h at 850?.The oxide film of alloys composition were uniform at 700?.The outer film of alloy mainly consists of NiO,NiFe₂O₄,NiMn₂O₄ and the inner is a mixing layer of Cr₂O₃,MoO₂.It was observed that the surfaces of the alloys exhibited a multi-layer structure after 100h oxidation at 850?.The outer oxide film of Hastelloy N superalloy mainly consists of NiO,NiFe₂O₄ and other oxide composites.The middle layer is confirmed to be Cr₂O₃ and MoO₂.Under two kinds of temperature conditions,when the content of Si reaches 0.5%,the SiO₂layer appears at the interface between the oxide layer and the substrate.Because the continuous dense SiO₂ layer was formed at the interface between the oxide layer and the substrate,the oxidation resistance of the alloy was improved.The cyclic oxidation behavior of Hastelloy N alloy with different silicon contents in air at 850?were investigated.The oxidation weight gain decreased with increasing of Si contents in alloys.During the cyclic oxidation process,alloys with 0.5Si and 1Si contents showed no significant scale peeling.In the later period of oxidation,the alloy with high Si contents tended to be stable,and the oxide scale of the low Si alloy is severely exfoliated and the weight gain of oxidation reduced.The surface oxide layer of Hastelloy N alloy with 0-0.2%Si contents were non-smooth and the non-flaking area were a continuous and dense spinel oxide composed of NiO and NiFe₂O₄.The spalled area were a loose oxide surface,which mainly contains mixed oxidation products of MoO₂ and Mo.The composition of the alloys oxide film were basically the same as that of the isothermal oxidation.