Hi Ed,

The alloyed Tungsten electrodes (Zirconium oxide, Thorium oxide, Cerium oxide, Lanthanum oxide) contain these oxides which have higher melting points than pure Tungsten. Thus, they have a greater current carrying capacity resulting in longer electrode life, easier arc initiation and stability, and greater resisitance to atmospheric contamination (both of the electrode and weld pool). Pure tungsten electrodes are generally only preferable for AC welding of Aluminum (electrodes with Thoria additions have been known to split when used with AC). However, I personally like to use an alloyed electrode at all times, unless cost is a factor.
Hope this helps!

Joe17

Thorium is radioactive. The introduction of thorium into the electrode increases its current carrying capacity

Go to Diamond Ground Products website. There is a good explanation on the function of oxides in tungsten electrodes. Basically the oxides have lower electron work functions, so it is easier to "boil" off electrons and thus arc stability is better and starting is easier.

http://www.diamondground.com/

Actually the thoria makes the electode last longer by retarding grain growth at the high temperatures seen in arc welding. The grains in pure tungsten want to grow to seek their lowest free energy state. Growth is faster at high temperatures. At high enough temperature and long enough time, the grain size would grow to the diameter of the tungsten and flake off. Having the thoria scattered in between the grains acts as a barrier that the growing grain must overcome before it can get bigger. That takes more energy, so growth is retarded. In reality, if you're like me you wind up dipping it in the puddle long before it has enough time to start flaking off. :)

Marty Sims