Welding Ta is an art and if going to be used for Corrosion Prevention please take special care in developing good techniques. Use good testing also after welding othewise poor welding will result in leaks.
You don't just get a welding procedure and start welding Tantalum. Ta is much more sensitive to contamination from many sources, all of which can either cause brittle welds and/or hurt the corrosion resistance. It takes years of experience to get it done right the first time. And you can not always tell by the color or look of the weld if you have detrimental effects of contamination. I recommend you go to one of several very good shops that weld Ta on a regular basis and know how to do it right Vendors include Tricor (Houston, Ohio), Astro Cosmos (Ohio, California), Ti-Fabricator, Ellett (Port Coquitlem, Canada), Special Metals (Conroe TX), TITAN (California)
On thread330-261852 there is a link provided by marshall and at that website the pertinent info is as follows:
Commercially pure tantalum is soft and ductile and does not seem to have a ductile-brittle transition. There are several alloys of tantalum commercially available. Even though the material is easier to weld, it should be well cleaned and for best results should be welded in the inert gas chamber. The gas tungsten arc welding process is recommended. Some tantalum products are produced by powder metallurgy technology and this may result in porosity in the weld. The arc cast product does not have porosity. Filler wire is normally not used when welding tantalum and for best results direct current electrode negative is used. High frequency should be used for initiating the arc. Helium is recommended for welding tantalum to provide for maximum penetration since joints are designed to avoid using filler metal.
This should give you a general idea of what is required in case you want to do some experimenting but you need to heed the advice of HightyHo and MikeMet if you intend to actually use the weldment especially if you are welding it for someone else.
Tantalum has a body centered cubic structure; as such, it has a ductile to brittle transition temperature but it is extremely low (cryogenic). Cleanliness is paramount. Contamination from impurities in the shielding/purging, inert gas or from other sources (weld fixtures, machining, etc.) must be minimized. Welding will be much like that for Niobium, Molybdenum, Titanium and Zirconium. High welding residual stress will occur and you should contemplate using an annealing post weld heat treatment depending on service or additional metalworking after welding.
