- PURE TANTALUM (Ta 99.9%min) Tantalum seamless tube(ASTM B521-98 UNS Grade R05200) OD 2.0 up to 38 mm x Wall Thickness 0.2 up to 5 mm x Length up to 6meters max. Tantalum welded tube(ASTM B521-98 UNS Grade R05200) OD 10 up to 38 mm x Wall Thickness 0.2 up to 5 mm x Length up to 6 meters max.