Tantalum

Tantalum is a refractory metal with an extremely dense oxide layer that makes it resistant to almost all acids. The problem when welding: as soon as it gets hot, it eagerly absorbs oxygen, nitrogen, and hydrogen from the surroundings and becomes brittle as a result. So you can only work with absolutely clean shielding gas or in a vacuum, preferably with electron beam or laser. In the heat-affected zone, the grain can become coarse, which kills toughness. Without the right equipment and experience, keep your hands off it – any contamination is a predetermined breaking point later.