Chemical properties of tantalum and niobium

At room temperature, tantalum does not react with oxygen. When heated to above 600°C in air, a white, very solid refractory oxide Ta2Os is formed on the surface. This oxide is different from the oxides of molybdenum and tungsten. It has a high melting point and is difficult to volatilize. It is quite stable and will not be reduced by hydrogen even at high temperatures. In a vacuum, the evaporation temperature of this oxide is also very high. Niobium, like tantalum, begins to oxidize at 400°C to form Nb2O. Therefore, although the work functions of tantalum (work function 4.12eV) and niobium (work function 3.99eV) are relatively small, their high electron emission rates of pure metals are difficult to obtain applications.

When tantalum powder is heated, it strongly combines with oxygen, halogens and sulfur. Tantalum and nitrogen can combine to form TaN.

Tantalum, carbon, CO and CO2 can form extremely refractory carbides TaO with high thermal radiation coefficient. Therefore, it is quite good to use TaO as an incandescent radiator. However, TaO has low strength (about 2x10N/m2) and poor machinability, which limits its application.

Acids (except hydrofluoric acid) have no effect on tantalum. Alkali only has a certain effect on tantalum in a molten state or a high concentration solution, forming tantalate. Therefore, hydrofluoric acid or molten alkali can be used to corrode tantalum. However, tantalum can be quickly dissolved in a mixture of hydrofluoric acid and nitric acid (4:1).

The chemical properties of niobium are similar to tantalum. It is a stable metal at room temperature and has strong corrosion resistance. It is insoluble in aqua regia and concentrated nitric acid, but quickly dissolves in a mixture of nitric acid and hydrofluoric acid. Niobium reacts slowly with hydrofluoric acid. Niobium can react with alkaline solutions to make niobium brittle. Niobium reacts with molten alkali to form niobate. Niobium is not corroded by sodium and mercury.