Niobium is a very soft (HV 40), malleable metal that is air-stable up to approx. 250°C, forms a thin protective oxide film and has a body-centered cubic crystal structure. Niobium reacts with oxygen, nitrogen, hydrogen and carbon at temperatures over 250°C, so any processing above that temperature must be performed in a vacuum or under inert gas. It is highly resistant to corrosion by all acids, except for fuming sulfuric acid (oleum) and hydrofluoric acid, but will react with alkaline solutions. It occurs with valences of -3, -1, 0, +2, +3, +4 and +5 in compounds, although pentavalent niobium is most common. Niobium is superconductive at temperatures below 9.46 K. Compounds such as NbTi and Nb3Sn are also valuable superconductors.