Tungsten Electrodes

Investing thousands of dollars and hundreds of hours of time into learning TIG welding may be futile if you don't understand the point 

of the process.  That is, the point where the electrons either jump from the torch electrode to the weldment (normal electrode negative), or where they enter the electrode from the weldment (cleaning cycle in aluminum welding, or electrode positive half of the AC wave).  This critical component determines the arc geometry and stability, which affects the molten metal pool, heat affected zone, filler metal deposition, penetration, and more.

Sure, you can get by with only a cursory understanding of tungsten electrodes, such as their diameter, how to sharpen them, and how to set the stick-out from the torch cup.  What about the electrode itself?  What is happening inside and outside that sharpened tip when you pass huge amounts of electrons through it to initiate and sustain an arc of ionized argon?  What happens to it when it heats to 5000 degrees?

Many of the answers to these questions may be found in the following booklet: DGP Tungsten Electrode Guidebook.  Although you should read and study this resource, let us extract some of the important points.

• Oxides
  • Rare earth minerals enhance arc starting and lower the electrode temperature.
  • Finer grain structure allows oxides to migrate to the tip where they create these benefits.  Electrodes with finer grain structure are better, and may be more expensive.  Shop for premium quality if you want the best performance.  Here is one source that offers name-brand electrodes: Arc-Zone.
  • Grain size may increase with heat and time in the electrode tip, eroding the quality of the arc.  A high quality electrode will resist this tendency longer than a lower quality electrode.  You may get longer effective use from a higher quality electrode, assuming of course that you don't contaminate the tip by accidentally dipping it in the molten metal pool.
  • Oxide distribution should be as uniform as possible throughout the electrode, and oxide grain size should be as small as possible to enable easy migration to the tip during welding.
  • Oxides that migrate to the tip will leave a film of rare earth metal on the surface, and this metal requires less voltage to separate the electrons from the metal and form the arc due to what is called a lower "work function."  This results in lower temperature and prevents grain size from growing and inhibiting the migration of oxides toward the tip.
  • Rates of oxide migration and evaporation must be balanced to provide a steady supply of rare earth metal on the tungsten electrode tip.  These rates must also be optimal for long life of the electrode, preventing premature exhaustion of the oxides.  Clearly, the design and manufacture of high quality electrodes is a demanding task.
• Types of oxide
  • Thoriated electrodes contain thorium oxide.  Thorium is a radioactive element and some may be concerned about its health hazards, especially if welding fumes are not exhausted from the workplace.  See the Fume extraction lesson for suggestions on making your workplace safer.
    • This oxide provides one of the lowest work functions and thus runs cooler and tolerates higher amperage.
    • They preserve the sharpened tip longer than pure tungsten or zirconiated electrodes, but not as well as other oxides.
    • They are not good for AC welding, as they do not form a balled tip which is desirable for that process.
  • Ceriated electrode contain cerium oxide.  It is non-radioactive.
    •  This oxide has good arc starting characteristics due to its low work function, and maintains the arc exceptionally well due to the deposited cerium metal on the electrode tip, which has the lowest work function of all.  This allows lower voltages and currents to maintain the arc, making this electrode desirable for welding thin wall tubing and small parts.
    • Paradoxically, cerium oxide has the highest work function of the three common oxides, thoria, ceria, or lanthana, which makes its arc starting capability somewhat less than thoria or lanthana.
    • Cerium vaporizes or burns off slower than thorium, so these electrodes should have a longer life and maintain a finer grain structure under optimal conditions.  However, the oxides migrate faster so at higher amperage oxide grains grow faster than in thoriated electrodes, thus reducing electrode life.
    • Ceriated electrodes are best for short welding cycles at lower amperage.  Thoriated and lanthanated electrodes are better for higher amperage and longer welding cycles.
    • These electrodes may split with AC welding and are best for DC current only.
  • Lantanated electrodes are non-radioactive and contain lanthanum trioxide.  These electrodes are the most popular in Europe and Japan.
    • This oxide has the lowest work function of the three common oxides, meaning it has the easiest arc starting ability.
    • The deposited lanthanum metal work function is between that of thorium or cerium, meaning it provides lower operating temperatures at the tip than thorium, but not as low as cerium.
    • These electrodes exhibit a much longer life than thoriated electrodes if they are not overloaded with current.
    • They tolerate pulse welding better because they resist thermal shock to a greater degree.  This also makes them preferable for short welding cycles, such as spot welding or short runs.
    • This is a good "all around" electrode, as it performs well in both AC and DC applications.
  • Zirconiated electrodes contain zirconium oxide.  This is non-radioactive.
    • These electrodes fall in between pure tungsten and thoriated electrodes in their welding characteristics.
    • The "ball up" easily to provide a more stable arc for AC welding and are thus preferred in this mode.
    • They resist contamination in the AC welding process.
    • They have better arc starting and current carrying capacity than pure tungsten, but perform poorly in other respects compared to ceriated or lanthanated electrodes.
  • Mixed oxides combine different proportions various oxides in one electrode and may result in a good balance of desirable properties.
    • Careful design balances oxide migration and evaporation rates with a low work function of the deposited alloy on the electrode tip.
    • These electrodes start and restart well.
    • In longer runs of 15 minutes or more these electrodes provide particularly excellent service life.