Research On High-temperature Oxidation Resistance And Corrosion Resistant Of Several Ni-based Metals

Steel connecting bar, once used as inert anode connector in aluminum electrolysis industry, is faced with a lot of problems, such as poor oxidation resistance at high-temperature, low adhesive attraction of oxide film, easy to crack at connecting interface of anode. This paper is aimed to obtain the materials used as connecting bar which should have stable structure, better oxidation resistance and corrosion resistance. Powder Metallurgy (PM) and casting are chosen to prepare specimens of connecting bar. The oxidation process and mechanism of several Ni-based alloys were investigated. The corrosion resistance of alloy was studied under the condition of electrolysis. Above all main conclusions can be drawn as followed:1) The density of the PM Ni-20Cu-10Fe-5Al increased little with the increase of forming pressure. It reached the maximum value when sintering temperature reaches 1300℃, but the specimen was overburnt when the temperature was too high, with the porosity increasing. It's determined that the proper sintering temperature is 1300℃, the relative density of the alloy is 93.5%, the grain size is 10-20μm.2) Continuous oxide film formed on the surface of the alloy when the content of Cu and Al was optimizationed. Ni-10Cu-10Fe-7Al showed good high-temperature oxidation resistance, the oxide film also had good heat shock resistance. The oxide film is clearly divided into several layers: the out layer is compound oxide which rich of Ni; the middle layer composed of Fe/Ni-riched mixture oxide and island structure of spinel; the inner layer is continuous and dense Al2O3. Diffusion of elements is inhibited for the presence of continuous Al2O3 and island structure of spinel, which decreases the oxidation rate. Selective oxidation occures in the oxidation process, which makes the middle oxide layer poor of Cu while the matrix near the oxide film poor of Al.3) The weight of annealed casting Ni-10Cu-10Fe-7Al increased by 2.7mg/cm2 after oxide 24h at 960℃. The oxide film increased little with time, whose thickness was approximately 10μm, which showed that the steady film oxide had effectively impedimented the diffusion of elements, so the high-temperature resistance was enhanced. The casting specimens have higher density and better structural homogeneity compared with PM specimens, while the oxide film is much thinner, the interface is more regular and smooth, the structure of the layer is much steadier.4) Thickness of the oxide film increased much with time when NiFeCrSn specimen oxidized at 960℃. The thickness of oxide film was 40-60μm when specimen oxidized for 2d, there was much pore in the oxide film when specimen oxidized for 20d, and internal oxidation happened. The oxide film is made of NiO and spinel of NiFe2O4. The content of NiO increases with time increasing.5) Oxidized NiFeCrSn was used as anode during electrolysis. It showed differences of corrosion resistance among acting surface of electrolysis, dipping part of melt electrolyte and corrosion part of electrolyte atmosphere, and dipping part of melt electrolyte shows the poorest corrosion resistance. While internal oxidation happens in matrix, the Sn-rich outer layer can reproduce.