The use of thoriated tungsten electrodes

Introduction

Tungsten electrodes are used when arc welding with the Tungsten Inert gas (TIG) process or when plasma welding. In both processes the electrode, arc and weld pool are protected from atmospheric contamination by an inert gas. A tungsten electrode is used because it can withstand very high temperatures with minimal melting or erosion. Electrodes are made by powder metallurgy and are formed to size after sintering.

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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.

Hazards

Thorium (Th) is slightly radioactive with a long half life and emits mainly alpha (α) particles, but occasionally some beta (β) and gamma (γ) radiation is emitted. Alpha particles cannot penetrate skin or even paper. However, they are harmful if released inside the digestive tract, or inside the lungs, where they act as a carcinogen.

Thorium oxide is, therefore, a low level radioactive material which may give rise to both a small external radiation hazard and an internal hazard from ingestion or inhalation. The external hazard estimated for a welder holding an electrode for a whole year is a very small fraction of the maximum permissible radiation dose and it is concluded that the external radiation hazard is likely to be negligible.

There is almost no release of radioactive material during arcing. However, to achieve maximum arc stability the electrode tip is ground to a conical point before use. This shape is maintained during use by regular regrinding. During the grinding process, particles of tungsten may be produced with thoria on the surface. It is these dust particles that create the major hazard, as they may be inhaled, and the thoria may release alpha particles from the surface.

In a review of air sampling measurements carried out during grinding it was concluded that during grinding air concentrations could approach or even exceed concentrations at which it would be necessary to consider designating the area as a controlled area as defined by the Ionising Radiation Regulations 1999.

However, the risk of cancer in TIG welders due to thoria exposure is very low, since the exposure times to individuals are invariably small. The Danish Welding Institute estimates that of 1200 full-time TIG welders, a cancer incidence of 0-3 may occur during a thirty year working life. While this figure is considered acceptable, the Danish Welding Institute has recommended that thoriated tungsten be phased out in Denmark since non-radioactive alternatives are available.

Safe working conditions

Storage

It is recommended that thoriated electrodes are stored in steel boxes, clearly labelled with the radiation trefoil. When stored in closed boxes, there is no significant hazard in handling and storage.

Small numbers (1 day's supply) of electrodes can be handled by welders safely without any special precautions.

Preparation/Grinding

Grinding creates the greatest hazard as the exposed tungsten/thoria area is greatly increased and fine particles of potentially radioactive dust are released into the atmosphere.

It is recommended that a dedicated grindstone with local dust extraction is used, and a simple filter mask is worn unless the number of electrodes used is very small (less than about 20 per year). If the grinding wheel is not fitted with a protective viewing screen, eye protection should be worn. The air extract from the grindstone should be arranged so that the particles are deposited into a substantial disposable bag.

A safe method of collecting and handling the dust from the collection unit must be used to minimise release to the atmosphere (for example, it could be placed in a sealed paper/plastic bag.)

The area round the grinding wheels should be cleaned daily with a vacuum cleaner to remove dust particles. If a high efficiency vacuum cleaner is not available, then the material should be damped down to minimise dust.

Workers should be encouraged to wash their hands before using the toilet facilities, and before taking work breaks, and for this reason the washing facilities should be close to the work areas.

Welding

Some vaporisation of tungsten does occur during welding but it is a very small amount and the corresponding level of radioactivity is extremely low. No special precautions are necessary.

However, as with grinding thoriated tungsten electrodes, wearing mouth, nose and eye protection during welding would further reduce any risk of contamination.

Alternatives to thoriated tungsten

Lanthanum, cerium, yttrium and zirconium oxides can all be used with tungsten. While they are all marginally radioactive, the risk is even lower than with thoria, so no specific precautions are needed. The general consensus of users is that ceriated or lanthanated tungstens are acceptable alternatives to thoriated tungstens, particularly with a DC current, while zirconiated tungsten is preferred for AC current welding. There are very minor differences in the arc voltages required for equal currents between the various alternatives. Consideration should be given to justifying the use of thoriated tungsten electrodes in preference to other suitable alternatives.

Official guidelines

The International Institute of Welding Health and Safety Commission VIII states that there are no specific hazards regarding storage, handling or welding but dust extraction equipment should be used on the grindstone and respiratory protection should be worn by the operator during grinding. This is covered by the Health and Safety Executive Guidance note HS(6) 53 The selection, use and maintenance of respiratory protective equipment.

In the UK the use of thoriated tungsten electrodes is subject to the Ionising Radiations Regulations 1999. Exposure must be kept as low as is reasonably practicable, and the guidelines in the previous sections are designed to achieve this.

Local rules and adequate supervision are required. The employer must appoint a Radiation Protection Supervisor (RPS), who should be responsible for

The implementation and monitoring of all health and safety procedures for the storage, grinding, use and disposal of thoriated tungsten electrodes and all by-products.
Keeping records of the amount of thoriated tungsten in store, and the numbers of electrodes issued to each welder.
Giving instruction in the correct use and grinding procedures to the welders. These procedures should be in the form of written instructions, in addition to verbal presentations. The welders must know the name to the RPS.
Monitoring to check that the welders are carrying out the grinding procedures correctly, and that the dust extraction system is working effectively.