VII-1.
The use of an inert gas (such as argon) to blanket the welding arc environment in order to prevent intrusion of oxygen and hydrogen provides a practical way of welding aluminium, magnesium and other reactive metals [VII–1]. Carbon and tungsten are the two materials used to form thermionic cathodes for welding. Pure tungsten, however, when operated in the cathode spot mode, allows the tip to melt, forming an enlarged sphere, which in turn allows the arc column to wander (loss of arc stability). Thoriated tungsten electrodes are used in the welding industry. Thorium, in the form of thorium oxide, is added to tungsten welding electrodes which are used in tungsten inert gas arc welding. The addition of thorium improves arc starting and stability, reduces weld contamination and gives longer electrode life. However, thorium is a naturally occurring radioactive element and as such may pose a radiation hazard [VII–2]. When an electrode becomes molten, material is sputtered off in relatively large quantities on arc starting and in lesser quantities during welding. The electrode is slowly consumed. Welders may inadvertently be exposed to radiation during welding and grinding operations with thoriated tungsten electrodes. Inhalation of dust particles during grinding is the main concern. The radiation hazard from grinding can be greatly reduced by wearing a dust mask and any other personal protective equipment suitable for such operations, the use of an effective exhaust system and, whenever possible, the use of preground thoriated tungsten electrodes. Thoriated tungsten electrodes used in tungsten inert gas arc welding normally contain thorium at 0.5%, 1%, 2% and 4%. Their annual production is estimated at 4–5 million electrodes per year in the United States of America [VII–3].