Chemical Properties of Tantalum and Niobium
① Tantalum and niobium are stable in air at room temperature. Signs of oxidation can only be seen when heated to 200 to 300°C. Above 500°C, metal tantalum and niobium both accelerate oxidation and generate oxides Ta2O5 and Nb2O5.
② Tantalum and niobium also have a very important characteristic, that is, they have the ability to absorb hydrogen, oxygen, nitrogen and other gases. These trace impurities have a significant impact on the mechanical and electrical properties of metal tantalum and niobium. At low temperatures, they absorb hydrogen slowly, and when the temperature reaches about 500°C (for tantalum) and 360°C (for niobium), their hydrogen absorption rate is maximum. At this time, not only adsorption occurs, but also compounds, namely hydrides (NbH, TaH), are generated. After these two metals absorb hydrogen, they become brittle. However, after being heated to above 600°C in a vacuum, all the hydrogen in the metal is released again, and all the original mechanical properties can be restored. Tantalum and niobium absorb nitrogen at 600°C and generate TaN and NbN at higher temperatures. Their melting points are 2300°C and 3080°C respectively.
③ Tantalum and niobium, together with boron and silicon, can produce refractory solid borides and silicides: NbB2 (melting point 2900°C), TbB2 (melting point 3000°C), TaSi2 (melting point 2400°C).
④ Carbon and carbon-containing gases (CH4, CO) interact with tantalum and niobium at high temperatures (1200~1400℃) to form solid refractory carbides: Tac (melting point 3880℃) and NbC (melting point 3500℃).
⑤ Hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and some organic acids of any concentration cannot corrode tantalum and niobium at room temperature or below 100~150℃. In hot hydrochloric acid and hot sulfuric acid, tantalum has better corrosion resistance than niobium. Both tantalum and niobium are soluble in hydrofluoric acid.
⑥ Tantalum and niobium have poor stability in alkaline solutions, and hot alkali solutions can obviously corrode tantalum and niobium. They are rapidly oxidized in molten alkali or soda to form the sodium salts of niobic acid and tantalic acid.