Oxidation Behaviour Of GH3535 Superalloy At Elevated Temperature

Nickel-based superalloy has been widely used in the chemical, oil, nuclear industry and other fields due to high temperature strength, good toughness, excellent corrosion and oxidation resistance. In the Strategic Priority Research Program of the Chinese Academy of Sciences named as “Thorium Molten Salt Reactor System”started at 2011,Mo-Cr-Ni alloys has been selected as the key structural material in thorium-based molten salt reactor. In the molten salt reactor, structural materials have to suffer the neutron irradiation and the long-term molten fluoride salt environment. Thus, in addition to adequate mechanical properties, oxidation and corrosion resistance must also be considered to ensure reactor safety. GH3535 alloy is developed specifically for thorium molten salt reactor independently by China, which is still in its infancy development stage. The merits of its antioxidant properties in high temperature environment will directly affect the safety and operation lifetime of the molten salt reactor, so it is necessary to study the oxidation resistance of GH3535 alloy.In this paper, isothermal oxidation and cyclic oxidation experiments of GH3535 alloy were carried out at different temperatures. Scanning electron microscope(SEM),X-ray diffraction analysis(XRD), electron probe microanalysis(EPMA), X-ray synchrotron radiation fluorescence technology(XRF)and other modern analytical testing methods were used, respectively. After the samples oxidized, oxidation kinetics analysis, composition of the oxide film, surface morphology and cross section morphology were studied to reveal the mechanism of high-temperature oxidation of GH3535 alloy.Experimental results has shown that isothermal oxidation weight gain results at700℃ for2000 hours were 1.642mg/cm2. Oxidation kinetics follow the cube law. After870℃ and 980℃ isothermal oxidation, crack and spallation of oxide film occourred due to the temperature increases, and the obvious weight gain were 0.73mg/cm2and2.00mg/cm2, respectively. Oxide film thickness of GH3535 alloy oxidized at 700℃ for2000 h is 4-6μm, which is composed of two layers. The outer layer are Ni O and Ni Fe2O4,and the inner one is mainly Cr2O3, Mo O2 and Ni Cr2O4 oxide with no obvious internal oxidation phenomena. At 870℃ isothermal oxidation after 100 h, the oxide film thickness is about 10μm, and continuous degree of oxide film got worse. At 980℃oxidation after 100 h, the outer oxide film started peeling, revealing the inner oxide film,accompanied by the appearance of internal oxidation holes. With temperature further increased, Cr, Mo formed volatile substances which led to the decrease of oxidation resistance of the alloy. Therefore, temperature has the most important effect on oxide scale of GH3535.Cyclic oxidation test were carried out on GH3535 alloy at 650℃ and 700℃ for1000 cycles. Alloy surface spalling or holes did not occur during the experiment.Oxidation kinetics still follow cube law. After cyclic oxidation at 650℃, fine particles formed and surface oxide spinel structure of the oxide film is not formed. However,oxidation results at 700℃ has shown the formation of dense oxide film with spinel structure in the surface. Therefore, GH3535 alloy has shown good oxidation resistance at 650℃ and 700℃. It is concluded that GH3535 alloy is a promising material for molten salt reactor.