Hafnium produces a protective film of oxide or nitride upon contact with air and thus has high corrosion resistance. Hafnium is fairly resistant to acids and is best dissolved in hydrofluoric acid, in which procedure the formation of anionic fluoro complexes is important in stabilizing the solution. At normal temperatures hafnium is not particularly reactive but becomes quite reactive with a variety of nonmetals at elevated temperatures. It forms alloys with iron, niobium, tantalum, titanium, and other transition metals. Because of its high neutroncapture cross section and its excellent mechanical properties, hafnium is used for fabricating nuclearcontrol rods.