Ozone and the breathing zone - health and safety using gas shielded processes

TWI Bulletin, January/February 1995

Paul Anderson is a Senior Research Welding Engineer in the Arc Welding Section of the Arc, Laser and Sheet Processes Department at TWI. After obtaining BEng (Hons) in Materials Technology at Coventry Polytechnic, he spent a year with British Steel, at the Welsh Laboratories, Port Talbot. He joined TWI in 1990 and has been actively involved with the development of the gas-shielded arc welding processes. His recent activities include the development of shielding gases for the TIG welding of duplex stainless steels and the assembly of a prototype top face penetration control system.

Richard Wiktorowicz is currently employed as a Principal Cylinder Applications Engineer, within the European Technology Group at Air Products plc. After obtaining a BSc(Tech) Hons in Materials Technology from Sheffield University in 1982, he spent 4 years working for a Refractories company, employed within the Research and Development Department.

He joined Air Products in 1986 and has been involved in several different research and development programmes, ranging from the fluorination of plastic containers through to his current activity of managing gas-related projects within the welding and cutting section.

Although mankind breathes air containing ozone in small concentrations, exposure to higher concentrations can adversely affect health. Paul Anderson and Richard Wiktorowicz explain.

During arc welding, ozone is produced as a result of the photolysis of molecular oxygen by the ultraviolet radiation emitted from the arc. The Occupational Exposure Standard (OES) for exposure to ozone is 0.1 ppm for an eight hour time weighted average (TWA) reference period, and 0.3 ppm for a ten minute TWA reference period. ^[1]   The OES represents a concentration at which there is thought to be minimal risk to health. Several studies have demonstrated that, under certain conditions, gas shielded arc welding may give rise to ozone levels which exceed these limits. The objectives of this article are to outline how ozone forms during gas-shielded arc welding and to summarise the key factors affecting the level of ozone measured in the working environment.

Formation and decomposition of ozone in arc welding

Ozone (O ₃) is formed when ultraviolet radiation in the wavelengths 130-242nm interacts with molecular oxygen. In the range 130-175nm, absorption of the ultraviolet radiation readily occurs in the air layer nearest the arc. Approximately half of all the ozone generated is formed within a 10-15cm radius from the arc, in the proximate zone. ^[2] This ozone enters the rising plume of fumes and gases and is dissipated in the area about the welding torch. In draught-free rooms, the plume typically has a diameter of 200-300mm for TIG welding, and 300-600mm for MIG welding.

Draughts can divert the direction of the plume significantly. Although more penetrating, ultraviolet radiation of wavelengths between 175-242nm is less efficient at ozone production. Consequently, relatively low ozone concentrations can also be formed in areas remote from the arc.

The ozone level in the working environment will also be affected by the rate of decomposition of ozone. The factors affecting the rate of ozone decomposition include heat, interaction with the surface of suitable catalytic particles, such as fume, and the presence of chemical species which react with ozone, such as hydrogen or nitric oxide, which may be present as additions to the shielding gas.

Nitric oxide is also formed when air comes into contact with the very hot gas emanating from the arc and weld pool, and with the hot surface of the workpiece. The rate of formation of nitric oxide is initially slower than the rate of ozone production, since it is partly dependent upon the surface of the workpiece attaining a high temperature. The ozone-forming radiation, and thereby the ozone production, reaches a peak within a fraction of a second of arc ignition. This results in a variation in the level of ozone with time, Fig.1.

Measurement of ozone concentration

There are significant differences between the ozone levels reported by various workers. In the main, this is due to differences in the measurement position. The reported measurements of ozone fall into four categories:

• Breathing zone. This measurement of the ozone concentration is taken using a probe positioned behind the welder's face mask. This provides the most accurate indication of the welder's exposure to ozone. Breathing zone measurements should be taken from a point opposite the right or left cheek of the welder and at a distance of approximately 25mm from the cheek. ^[3]  
• 'Proximate' zone. This involves measurement of the ozone concentration at a fixed distance and angle from the arc, typically 150-600mm at 30-60°. However, there can be a significant variation between measured values at positions within this range. ^[2]
• 'Remote' zone. This involves measurement of the ozone concentration at a point outside the region of ozone formation.
• 'Total' ozone emission. This involves the measurement of all the ozone generated within a fixed radius of the working area, typically by monitoring the atmosphere of an enclosed booth.

The lowest ozone measurements are typically recorded in the breathing and remote zones, and are generally an order of magnitude lower than the values observed in the proximate zone. This is due to the welder's headshield. The total ozone emission will give the highest value for the level of ozone, as it includes the ozone in the plume of hot gases rising from the weld. When comparing reported ozone levels from different sources, the measurement position must, therefore, be considered. The OES refers to the exposure of welding personnel to ozone. In order to compare the ozone exposure of the welder with the OES, breathing zone measurements must be used.

There are two techniques which are widely used for ozone measurement. ^[4]   Detector tubes, also known as Draeger tubes, can be used to provide a rapid, inexpensive indication of whether the ozone concentration in the working environment exceeds the OES. Chemiluminescence offers very accurate measurements, and can be used in both laboratory and production environments. This technique is the one most frequently applied in published work, because it is ozone specific.

Factors affecting ozone concentration

• The composition of the workpiece material and the filler wire.
• The welding process and, during MIG welding, the metal transfer mode.
• The composition of the shielding gas.

Factors of secondary importance have been reviewed in detail elsewhere. These include:

• In TIG welding: the welding current, arc length and electrode shape. ^[2]
• The frequency of welding. ^[2]
• The shielding gas flow rate and the size of the shielding gas shroud. ^[2]
• The use of a barrier to the ultraviolet emissions. ^[5]  

Composition of the workpiece material and filler wire

The Table lists ozone concentrations in the welder's breathing zone measured in the laboratory for the MIG and TIG welding of various materials. For each welding process, the highest ozone concentrations are typically generated for aluminium.

Laboratory measurements of ozone concentrations in the welder's breathing zone

It is thought that the differences between the materials are due to the presence of metal vapour in the arc. This vapour may have a significant effect upon the emission spectra, increasing the amount of ozone forming radiation.

In practice, the workpiece material is determined by the application and cannot normally be changed. However, the selection of consumable wire composition will influence the level of ozone generated in some applications. For example, the welding of pure aluminium or Al-5%Si by the MIG process generates relatively high levels of ozone. The use of a consumable wire containing up to 5% magnesium can lead to a significant reduction in the ozone concentration measured at a fixed distance from the arc, Fig 2.

Welding process

In general, MIG/MAG welding generates higher levels of ozone than TIG welding. This is thought to be because ozone generation in MIG/MAG welding is critically influenced by the transfer of metal across the arc. Ozone levels exceeding the OES may be generated under certain conditions during the MIG/MAG welding of aluminium, stainless steel and mild steel. The most important factor affecting the ozone level in a given MIG/MAG welding application is the metal transfer mode. The highest ozone levels are associated with the spray metal transfer mode. In contrast, the dip or 'short circuit' metal transfer mode offers lower ozone levels. This is the result of a reduction in the ozone-forming radiation, which may be due to decreased metal vapour passed across the arc. Figure 3 compares the ozone concentration in the proximate zone for the MIG welding of aluminium in the spray and dip metal transfer modes with argon shielding gas. ^[4]

Shielding gas

The composition of the shielding gas can have a significant effect upon both the rate of ozone formation and decomposition.

Reducing ozone formation

The intensity of ozone producing radiation can be reduced by modifying the composition and, therefore, the emission spectra, of the shielding gas. The highest intensity of ozone-forming radiation is emitted from the arc plasma formed with nitrogen. This is ten times stronger than the radiation emitted by a pure argon arc plasma, and several hundred times stronger than the radiation emitted by a pure helium arc plasma. ^[4] For inert gases, mainly used for TIG and plasma welding and for the MIG welding of non-ferrous metals, the replacement of a proportion of argon in the shielding gas with helium can lead to a significant reduction in the ozone level, Fig.4. Pure helium appears to lead to an increase in the ozone level during the TIG welding of stainless steel.

For active gas mixtures containing CO ₂ and/or O ₂, mainly used for the MIG welding of ferrous metals, as the proportion of CO ₂ in the shielding gas increases, there is a reduction in the ozone emission rate, Fig.5.

Nitrogen-bearing shielding gases are finding increasing use for the TIG and MIG welding of duplex and superduplex stainless steel. When welding these metals using pure Ar shielding and backing gas, nitrogen is lost from the weld metal. This can result in a significant reduction in corrosion performance. This nitrogen can be replaced by the use of shielding gases containing levels of nitrogen tailored for the material. However, the addition of nitrogen to argon shielding gas leads to a significant increase in the ozone level. It is therefore beneficial to use a shielding gas mixture containing a proportion of helium, Fig.6. In addition to reducing the ozone level below the OES, the helium addition can also allow a significant increase in productivity, which further limits the exposure of the welder as the job is completed in less time.

Increasing ozone decomposition

Additions can be made to the shielding gas to increase the rate at which the ozone decomposes by chemical reaction. Mixtures containing hydrogen, used for the TIG welding of austenitic stainless steels and nickel alloys, change the emission spectra of the arc and react with ozone, to form water and oxygen. The addition of hydrogen to the shielding gas significantly reduces the ozone level in the working environment, Fig.7.

It has been claimed that the addition of a small proportion of nitric oxide (NO) to the shielding gas reduces the ozone concentration in the working environment. ^[6] Ozone reacts with NO to form nitrogen dioxide (NO ₂) and oxygen. The effect of NO additions appear to be greatest in the plume of hot gases rising from the weld: this explains the differences in reported ozone concentrations between workers carrying out measurements of the total ozone emission (for example, ^[6] ) and breathing or proximate zone measurements, where preferred practice is for the welder to avoid the plume (for example, ^[8] ).

Summary

During arc welding, ozone is produced as a result of the photolysis of molecular oxygen by the ultraviolet radiation emitted from the arc. Under certain conditions, gas shielded arc welding may give rise to ozone levels which exceed the OES. The composition of the base metal and filler wire and the welding process used will have a significant influence on the ozone levels, but it may not be possible to vary these factors. Typically, there is more flexibility in the choice of the shielding gas composition. The addition of helium, CO ₂ or hydrogen to argon-based shielding gas mixtures may result in a significant reduction in the ozone concentration in the welder's breathing zone.

References