Some TIG electrodes in use in the early 1970's were marked Thoriated Tungsten and Non-Thoriated Tungsten. What were the specific purposes or properties for these TIG welding electrodes?
According to TWI-Global
TIG welding electrodes usually contain small quantities of other metallic oxides which can offer the following benefits:-
- facilitate arc starting
- increase arc stability
- improve current-carrying capacity of the rod.
- reduce the risk of weld contamination
- increase electrode life
Oxides used are primarily those of zirconium, thorium, lanthanum, yttrium or cerium. Additions are usually of order 1%-4%. All these oxides greatly improve arc initiation, especially when direct current (DC) welding is employed. Thorium oxide (thoria) has been used for many years having been found effective in terms of long life and thermal efficiency. Zirconium oxide (zirconia) has been commonly used for alternating current (AC) TIG welding, normally for welding aluminium.
Because thorium is radioactive there are hazards associated with using it. The main cause of concern with thorium are the alpha particles it emits. Alpha particles cannot penetrate the skin, but if they enter the lungs or digestive track they can be carcinogenic.
The issue with using thorium in electrodes is not so much about radiation exposure during welding, but the potential to inhale or ingest thorium laced when the tips of electrodes are ground to have a conical point. The conical point is required to maintain maximum arc stability during welding. Due to electrode erosion during welding, the tip needs to be frequently ground.
During the grinding process thorium laced dust is produced. If not enough care is taken during the grinding of the electrodes (via dust masks & ventilation) there is a risk to the person grinding the electrodes in breathing in the thorium, and alpha particles, in the dust. This then increases the posibility of the person doing the grinding of developing cancer. The risk of cancer is low, but it appears to be enough of an issue for the Danish Welding Institute to recommend the phasing out of thoriated electrodes because non-radioactive alternatives are available. The linked article goes into more detail.
According to Welding Tips & Tricks pure tungsten electrodes are inferior to thoriated electrodes becaue they "quiver and dance" when the current is increased, leading to difficulty in producing a good weld. The site recommend lanthanated electrodes for those concerned about radiation exposure associated with thoriated electrodes.
Similar warning about the inhalation or ingestion of thorium laced dust can be found on these sites:
Diamond Ground Products with recommendations from the American Welding Society & the Welding Institute
Miller Welds gives a brief comparison between pure tungsten and thoriated electrodes:
Tungsten electrodes have the highest consumption rate of all electrodes. They provide excellent arc stability for AC balanced wave welding and good stability for sine wave welding. They are generally not used for DC welding because the arc are not as strong as those produced by tungsten alloy electrodes.
Thoriated electrodes are apparently the most commonly used electrodes. They are preferred for longevity and ease of use.
Thorium increases the electron emission qualities of the electrode, which improves arc starts and allows for a higher current carrying capacity. This electrode operates far below its melting temperature, which results in a considerably lower rate of consumption and eliminates arc wandering for greater stability. It also features a lower level of weld contamination than other electrodes.
Unlike pure tungsten, these electrodes are only for speciality type AC welding (thin gauge aluminium or materials less than .060-in.), but they are exceptional for DC electrode negative or straight polarity on carbon and stainless steel, nickel and titanium applications.