Niobium and tantalum are often associated, but the content of niobium in the crust is about 2.4 × 10 -3%, 10 times that of tantalum, mainly in the form of niobium iron ore. Generally, the corrosion resistance of niobium is higher than that of titanium and zirconium and slightly lower than that of tantalum. Because the price of niobium is lower than that of tantalum, niobium can be used to replace more expensive tantalum in some corrosive media. At the same time, the relative density of niobium is only about 1/2 of tantalum. Under the same component size, the amount of niobium is only about 1/2 of tantalum, which can reduce the cost.
Like tantalum, niobium becomes a passive corrosion-resistant metal by forming a dense oxide film on its surface, so the corrosion resistance of niobium is close to that of tantalum. Niobium is mainly used in some strong acid media with strong reducibility at low temperature, but it is not corrosion resistant in hydrofluoric acid, hot concentrated sulfuric acid, sodium hydroxide, potassium hydroxide and other media, and the corrosion rate is also high in hot concentrated hydrochloric acid and hot concentrated phosphoric acid. Care should be taken when applying niobium in these media.
The boiling point of niobium is; 4927 ℃, melting point: 2468 ℃, belonging to refractory metal. Niobium can be formed at 350~400 ℃, 950~1000 ℃ and fully annealed at about 1200 ℃. Niobium begins to oxidize at 230 ℃ in the air, and strongly oxidizes at 300 ℃. When the temperature is higher than 400 ℃, the oxide film is destroyed and falls off, greatly accelerating the oxidation rate. Niobium begins nitriding at 600 ℃ in air. Niobium will absorb hydrogen at 250 ℃ ~950 ℃ in hydrogen containing medium. Therefore, the welding and heat treatment of niobium should be carried out in vacuum or under the protection of inert gas, that is, the thermal process above 300 ℃ should be carried out under the protection of vacuum or inert gas or high-temperature coating. When niobium equipment and containers are exposed to the atmosphere, the application temperature should generally not exceed 230 ℃, and the temperature can be appropriately increased only when it is ensured that it is not in contact with the atmosphere. Niobium should be protected by high-purity inert gas in the same way as tantalum when welding, and it should also be placed under inert gas protection when the temperature is above 230 ℃. It is better to stop supplying inert gas after the niobium weldment is cooled to 200 ℃.
Niobium materials have been used in pressure vessels, but there is no formal standard for niobium vessels. Pure niobium is mainly used in pressure vessels. Adding about 1% zirconium can improve the strength, but the corrosion resistance and plasticity are slightly reduced.