Niobium is also known as Columbite, the name originally used in the USA. Niobium has a bright metallic lustre, in platinum-white. It takes a bluish cast when exposed to air at room temperature for an extended period. It starts to oxidize in air at 200 C (392 F). When processed, it must be placed in a protective atmosphere at moderate temperatures. Niobium is ductile, malleable, and highly resistant to corrosion. Niobium does not naturally occur in the free state; it is usually found with tantalum, although large amounts of Niobium have been found in carbonatites. Extensive mineral deposits are found in Canada, Brazil, Nigeria, Zaire and Russia. Niobium has superconductive properties and it has been useful in making superconductive magnets combined with Zirconium wire, which retains its superconductivity in strong magnetic fields. Niobium, like Tantalum, can act as an electrolytic valve allowing alternating current to pass in only one direction through an electrolytic cell. Niobium is used in arc-welding rods for stabilized grades of stainless steel. It is also used in advanced airframe systems. Other uses for Niobium are as lamp filaments and to make jewellery. It can be coloured by an electrolytic process.