Niobium is a fifth-period element of the VB group of the periodic table, with the element symbol "Nb", atomic number 41, a relative atomic mass of 92.91, a density of 8.57g/cm3, and a melting point of 2468℃. The electronic structure of niobium determines that the most stable valence state of niobium is 5. Niobium exhibits 3, 4, and 2 equivalent states.
Niobium is stable at room temperature, and most gases below 100℃ cannot corrode it. The main defect of niobium metal is that its stability disappears with increasing temperature, and the presence of impurities will completely change the properties of the metal. When the temperature exceeds 200℃, niobium can easily oxidize in the air. Above 350℃, an oxide film will form on the surface and gradually thicken with time. Above 400℃, the oxidation rate of niobium accelerates and begins to absorb oxygen. At 900℃, the oxidation rate of niobium is 0.05mm per year.
Niobium begins to react with hydrogen above 250°C. Initially, hydrogen enters the lattice gap of niobium metal to form a solid solution, which reduces the plasticity of niobium and makes it brittle. Finally, it exceeds the solid solution limit to form a hard and brittle hydride.
Niobium and nitrogen do not react at all below 350°C. At 600°C, it begins to absorb nitrogen to form a solid solution.
Above 300°C, niobium reacts rapidly with water vapor to release hydrogen. The reaction formula is as follows
Nb+H2O=Nb2O5+H2
Under high-temperature conditions, niobium can react with most other gases, such as CO, CO2, SO2, NH3, etc.
Niobium is chemically inert in most aqueous solutions at room temperature. It mainly forms a thin and dense passivation oxide film on the surface of metal niobium, which has good corrosion resistance. When the temperature exceeds 150°C, the porous oxide film generated by the reaction will fall off periodically, resulting in accelerated corrosion.
Except for hydrofluoric acid, niobium will not be corroded by other inorganic acids below 100°C, or the erosion rate is very slow.
Niobium has poor stability in alkaline solutions. Since niobium forms hydrated salt in an alkaline solution, niobium dissolves.
Niobium has good high-temperature strength, excellent low-temperature ductility, and processing performance. Niobium is mainly used in the steel industry, nuclear industry, alloys, compounds, and other fields. Nearly 90% of niobium is used in micro-alloyed steel, stainless steel, and heat-resistant steel. Niobium is one of the important alloying elements in steel. Adding a small amount of niobium to steel can greatly improve the strength of steel, improve mechanical properties and welding properties, and improve corrosion resistance. Niobium is added in the form of ferroniobium, mainly as a grain refiner and dispersion hardener for steel. Adding a small amount of niobium to cast iron can promote graphitization, reduce casting cracks, improve the wear resistance of castings, and significantly improve the strength and toughness of cast iron.
Niobium has been used as an alloying element in steel for a long time. Research shows that the excellent effects of niobium come from two aspects: first, niobium can make metal grains finer and the structure uniform; second, niobium can form fine and stable hard phase particles, which are evenly distributed in the matrix of the alloy to play a role in dispersion strengthening and deformation resistance. So far, some of the most important applications of niobium in cast iron mainly include two aspects, one is the automotive industry, such as cylinder heads, piston rings and brakes, etc.; the other is the metallurgical industry, such as high-temperature wear-resistant materials such as rollers.
Niobium steel is generally divided into three categories: ① High-strength low-alloy steel (HSLA), generally containing 0.02%-0.05% niobium. This type of niobium steel has the largest output, accounting for more than 80% of the niobium used in the steel industry ② Low-alloy steel, generally containing 0.02%-1.0% niobium. ③ High-alloy steel, generally containing 0.4%-3.0% niobium. The largest use of niobium steel is pipelines for transporting natural gas and oil.
Niobium iron is the largest niobium intermediate product. As a master alloy, it is widely used in the production of alloy steel and superalloys. The main method used in industrial production is the aluminothermic reduction method. Aluminothermic reduction is a self-heating reaction process that does not require external heating, has a short production process, simple equipment, and easy operation. The aluminothermic reduction reaction is carried out at a temperature at which both the metallic aluminum and the reaction slag are melted. To increase the yield of niobium, the reducing agent must be excessive. To reduce the melting point of the slag, lime 30% by weight of niobium oxide needs to be added to form calcium aluminate. It is generally believed that the heat released by the aluminothermic reaction is sufficient to make the reaction reach 2200-2400℃ so that the niobium in the niobium concentrate is completely reduced.
