Characteristics: Gray or silvery, ductile metal; does not tarnish or oxidize at room temperature. Reacts with oxygen and halogens only when heated. Less corrosion-resistant than tantalum at high temperature. Not attacked by nitric acid up to 100 °C, but vigorously attacked by mixture of nitric and hydrofluoric acids. Hot, concentrated hydrochloric, sulfuric, and phosphoric acids attack it, but hot, concentrated nitric acid does not. Unaffected at room temperature by most acids and by aqua regia. It is attacked by alkaline solutions to some extent at all temperatures.