Niobium, as with other reactive metals, owes its corrosion resistance to a passive, tenaciously adherent oxide film. The corrosion properties of niobium are similar to those of its sister element tantalum, however, it is less resistant to more aggressive media. Unlike the other reactive metals, niobium will tolerate the presence of small amounts of fluoride ion. Niobium and its sister tantalum can become embrittled with hydrogen in alkaline service, however, it is one of the few materials that are resistant to aqua regia. The oxidation rate of niobium in air increases with temperature and becomes quite rapid above 500°C. Therefore, when niobium is used at higher temperatures, coatings must be employed to minimize detrimental oxidation. Applications include missiles and aerospace because of strength at elevated temperatures and the chemical industry due to corrosion resistance. It is also resistant to many liquid metals.