Study of Chromium in Gas Metal Arc Welding Fume

T. W. Eagar, Sc. D., P. Sreekanthan, N. T. Jenkins Massachusetts Institute of Technology

and

G.G. Krishna Murthy, Ph. D., J. M. Antonini, Ph.D., J. D. Brain, Ph.D. Harvard School of Public Health

ASM-AWS Trends in Welding Research Conference Calloway, GA, June 1998

Conference Session: Welding Consumables

Keywords: chromium, fume, toxicity, shielding gas, flux, industrial health effects Abstract may not be similar to the chemistry of Cr in electroplating baths, where it is a known carcinogen. Some toxicologists take the overly simplistic approach that it is the mere presence of a chemical element in any form that damages the body. Recall Recent OSHA mandates have greatly reduced the permissible that Na and Cl are poisons, whereas NaCl is essential to life. exposure levels (PEL) to hexavalent chromium (Cr VI), a known Oxidation is a critical factor in evaluating the activities of human carcinogen. Because the amount of Cr VI in welding chromium compounds. Chromium exists in myriad forms and fumes is uncertain, a study was undertaken to understand the oxidation states, of which trivalent chromium (Cr III) and influence of shielding and fluxes on the composition and hexavalent chromium (Cr VI) are of interest. Cr VI is more oxidation states of various metals in welding fume, in particular toxic than Cr III -- its strong oxidative nature may be the that of gas metal arc welding (GMAW). Both thermodynamic underlying basis for its genotoxicity (ref. 1). calculations and experiments show the presence of minor Oxidation also determines the type of reactive oxygen amounts (less than 0.3 wt% of fume) of Cr VI in the fume in species present on the surface of fume particles, the amount of the absence of fluxes. This amount appears to be a function of which varies with time. In turn, this influences the resulting the shielding gas used. Because the oxidation state of level of lung injury that can be caused by welding fume. chromium can change through interaction with the environment, To understand the chemical form of fume, the evolution of studies were performed which showed that fresh fume produced welding fume under different shielding gas conditions during a greater response in animal lungs than fume which had been gas metal arc welding (GMAW) of stainless steels was observed. aged for 24 hours in air. In the high temperature encountered in GMAW, the reactive components of the shielding gas dissociate and react with the Introduction molten metal alloy, with the result that oxygen dissolves into the metal and oxidizes certain alloying elements. Welding fume is an unwelcome byproduct of the welding Subsequently, tests were performed to evaluate the degree of process, leading to losses in worker productivity and other lung inflammation in rats due to the presence of reactive oxygen inconveniences. Chromium, which is present in welding fume species in fresh versus aged fume. of stainless steel, is now recognized, in certain chemical states and concentrations as a carcinogen. Although there is no Theoretical Considerations epidemiological data that shows that welders have greater incidences of cancer than the general population, OSHA and This study develops a predictive capability for chrome other organizations are proposing lowering the PELs content in welding fume, given the shielding gas composition. (Permissible Exposure Levels) of Cr by 10 to 100 fold. In the This can be obtained from known initial conditions through a worst case, this would require welders to wear space-suit like consideration of a thermodynamic model of the reactions protective equipment, and in the best case, it will severely retard occurring in GMAW. Our concern is the molten metal - productivity and greatly increase costs. While this might be shielding gas reactions that take place (ref. 2). considered a social problem rather than a scientific one, there is This situation can be modeled using the tool of thermo- a host of scientific questions remaining to be addressed chemical analysis. Thermo-chemical analysis allows us concerning the risk of Cr in welding fume. The chemical form and its pathway into the human body must be understood scientifically if we are to establish meaningful standards for the workplace environment. While it is known that Cr exists in welding fume, the exact chemical form of Cr in the fume is not understood. It may or

to evaluate the oxidizing potentials of reactive gas mixtures, the Table 1. Equilibrium in the Cr-O system thermo-chemical stability of condensed metal and oxide phases, Equilibrium over Cr(s,l) and the equilibrium pressures of volatile species over the Cr(s,l) ⇐⇒ Cr(g) condensed phase (ref. 3). These are a function of temperature Cr(s,l) + 0.5 O2(g) ⇐⇒ CrO(g) and oxygen content of the gas mixtures. Cr(s,l) + O2(g) ⇐⇒ CrO2(g) With an understanding of the thermodynamics, various Cr(s,l) + 1.5 O2(g) ⇐⇒ CrO3(s) quantities such as final molar fractions for different shielding Cr(s,l) + 1.5 O2(g) ⇐⇒ CrO3(g) gas mixtures can be determined and applied to fume composition calculations. To keep the model simple, flexible, 2 Cr(s,l) + 1.5 O2(g) ⇐⇒ Cr2O3(s,l) and accurate, thermodynamic equilibrium for the gas-metal reactions will be assumed in the following computations. Equilibrium over Cr2O3(s,l) The basic equations for deriving and using thermo-chemical 2 Cr(g) + 1.5 O2(g) ⇐⇒ Cr2O3(s,l) data is the standard Gibbs free energy equation: 2 CrO(g) + 0.5 O2(g) ⇐⇒ Cr2O3(s,l) 2 CrO2 (g) ⇐⇒ Cr2O3(s,l) + 0.5 O2(g)

ΔG°= −RT ln KP (1). 2 CrO3(g) ⇐⇒ Cr2O3(s,l) + 1.5 O2(g)

For a pure metal A, the free energy of evaporation, ΔGe°, is Fume formation involves the evolution of the gaseous given by: species listed in the table. For general oxidation of Cr, the reactions can be written as x Cr + y O2 ↔ CrxO2y. For this system, Eq. 1 can be rewritten as ΔGe °= −RT ln PA ° (2).

PCrxO2 y − ΔG° If A is not pure, but is in solution, the free energy change on ln = (6). y x RT evaporation is given by ΔGe°−ΔG , where ΔG is the partial molar PO2aCr free energy of mixing of A for the reaction We see that the vapor pressure of the chromium oxides is a function of temperature and also the partial pressure of oxygen, Asolution ⇔ Agas (3). which is given by the oxygen content in the shielding gas (typically argon-based). For a pure argon shielding gas, there is When metal A evaporates from an alloy, the total free energy change is then given by

Temperature (C) 2700 2400 1500 P A 0 ΔGtot °= RT ln (4). P ° A -0.5

The term P A PA ° is defined as the thermodynamic activity, -1

where we can use the expression: )

-1.5y

x -2r ln P A = ln PA °+ ln a A (5). C

(

-2.5g

If the composition of the metal is known, the activity for an l element can be obtained from Hultgren’s tabulated values for -3 binary melts (ref. 4). These numbers are strictly for binary melts, however, in the absence of strong interactions between -3.5 atoms, the data can be used to approximate a more complex alloy (ref. 5). Using this, it is possible to calculate the vapor -4 pressures for alloying elements in any typical alloy (ref. 2). So far, we have only dealt with elemental vaporization from -4.5 the alloys. To model fume formation in its entirety, oxidation- -5 enhanced vaporization also needs to be considered. Table 1 3.7 4.2 4.7 5.2 5.7 6.2 6.7 shows the chemical equations for the Cr-O volatile species 10, 000 / T above the condensed phases Cr(s,l) and Cr2O3(s,l). Volatile species are formed by direct evaporation and by the addition or Cr (g) CrO2 (g) removal of oxygen from Cr (s,l) and Cr2O3 (s,l) respectively. CrO (g) CrO3 (g)

Figure 1. Chrome Species Vapor Pressures from 99% Argon, 1% Oxygen Shielding Gas

only Cr(g) in the fume during formation. With an addition of quickly and effectively with the aid of prepackaged 1% oxygen in the shielding gas mixture, PO2=0.01. Using this thermodynamic software (ref. 2). value in Eq. 6, activities (vapor pressures for gases) for the The vapor pressures for the chrome system are now obtained chromium species are plotted as a function of temperature, as recognizing that the chrome reactions are actually occurring in a shown in Figure 1, where it is possible to identify the dominant multi-component system. Figure 2 shows the equilibrium specie for any given temperature - e.g. at 2400°C, Cr(g) is the activities (vapor pressures for gases) as functions of temperature dominant form. For CrO2(g) and CrO3(g), an increase in for an argon-based shielding gases with 1% oxygen. temperature is accompanied by an decrease in vapor pressure, Cr(g) is the dominant form in the chrome system. Cr VI, in whereas for Cr(g) and CrO(g), there is a increase in vapor the form of CrO3(g) has the lowest activity among the gaseous pressure with temperature. species. Cr(g) is representative of elemental vaporization. The oxides represent the various valences of chrome in fume. That is, the presence of CrO is represented by Cr II, Cr2O3 shows Cr III, 84 CrO2 is Cr IV, and finally CrO3 is Cr VI. The specific amounts of these compounds depends on the oxygen partial pressure as seen in Eq. 6 which suggests that the valence of chromium 72 varies with the oxygen potential of the shielding gas. The treatment so far has considered chromium and oxygen as 60 the only components of the system. However, the welding

atmosphere is actually a multi-component system. The presence e

a of several alloying elements with different properties leads to 48t

n numerous reactions occurring simultaneously and / or in e

competition with one another. e 36P

Temperature (C) 2700 2400 1500 24 0

-2 12

-4 0

)

y 1500 1700 1900 2100 2300 2500 2700

x Temperature (C) r -6

(

Cr Fe

-8g

o l Mn Ni -10 Figure 3 Metal Percentages in Vapor for 99% Ar, 1% O2 -12 Shielding Gas It is possible to predict relative amounts of the products. In -14 Figure 3, the amount of vaporized metal as a fraction of the total vapor in the products is given for a shielding gas -16 composition of 99% argon, 1% oxygen. The percentages are 3.7 4.2 4.7 5.2 5.7 6.2 6.7 10, 000 / T plotted as a function of temperature. Table 2 lists the percentages of metals in the vapor during formation for 2400° Cr (g) CrO2 (g) C, the theoretical droplet temperature. The table lists these percentages for various shielding gas compositions. The table CrO (g) CrO3 (g) shows that iron is the dominant metal although there are also significant amounts of chromium and manganese as well. Figure 2 Chrome Species Activities (Vapor Pressures for Gases) Considering only the chrome system, it is possible to for 99% Ar, 1% O2 Shielding Gas identify the amounts of the different species in the vapor. This is shown in Table 3 for 2400° C, though it is as easy to Using the Gibbs Energy Minimization Method, the most calculate these percentages for other temperatures. One notices stable phase compositions, where the Gibbs energy of the that Cr(g) is the dominant form for all gas compositions. system reaches its minimum at a fixed mass balance (a CrO3(g), and hence Cr VI, is found only in very small amounts, constraint minimization problem), constant pressure and about 1.30E-4 percent of total vapor, or 5.00E-4 percent of the temperature, can be found. These calculations can be performed chrome content. As oxidizing potential increases, there is an increase in the CrO3(g) content, although this is very small – as seen from the table, a 900% increase in the oxygen content experimental results. This suggests that other forms of produces only a 1% increase in the CrO3(g) content. chromium have indeed been oxidized to Cr VI following formation. The amount of Cr VI, however is still small when Table 2. Theoretical Predictions of Metal Content in Fume Vapor compared to total fume, or even total Cr in fume. (Weight Percentage of Total Vapor) of GMAW using 308 Stainless Steel Electrodes, Droplet Temperature of 2400°C Table 4. Chrome Analysis Results Shielding Gas Cr Cu Fe Mn Ni Experimental Theoretical Experimental Theoretical Shielding Gas * wt% Cr VI in * wt% Cr VI in 99 Ar 1 O2 24.52 0.0216 52.51 14.89 5.99 wt% Cr VI in wt% Cr VI in total fume total chrome 98 Ar 2 O2 24.33 0.0245 52.75 14.74 6.09 total fume total chrome 95 Ar 5 O2 23.77 0.0257 53.45 14.29 6.41 Ar 0.252 2.05 90 Ar 10 O2 22.80 0.0281 54.56 13.52 7.04 Ar 0.101 0.0 1.13 0.0

95 Ar 5 CO2 26.88 0.0578 58.73 6.26 5.99 98 Ar, 2 O2 0.199 1.53 98 Ar, 2 O2 0.236 1.29E-4 1.89 5.00E-4 Table 3a. Theoretical Calculations for Chrome Species in Fume 95 Ar, 5 O2 0.219 2.00 Vapor (Weight Percentage of Total Vapor) of GMAW, using 308 95 Ar, 5 O2 0.155 1.30E-4 1.52 5.14E-4 Stainless Steel Electrodes, Droplet Temperature of 2400°C 95 Ar, 5 CO2 0.196 1.62 95 Ar, 5 CO 0.144 1.17E-4 1.30 4.12E-4 Shielding Gas Cr(g) CrO(g) CrO2(g) CrO3(g) 2 100 CO2 0.020 0.29 99 Ar 1 O2 19.86 5.76 4.14E-1 1.29E-4 100 CO2 0.152 1.05 98 Ar 2 O2 19.69 5.73 4.13E-1 1.29E-4 95 Ar 5 O 19.19 5.65 4.11E-1 1.30E-4 * NIOSH 7600 analysis of fume at DataChem Labs, Salt Lake City, 2 UT 90 Ar 10 O2 18.35 5.50 4.07E-1 1.31E-4 95 Ar 5 CO2 22.05 5.99 4.02E-1 1.17E-4 These experimental results are compared to literature values. Moreton, et. al, conducted fume emissions tests when welding Table 3b. Theoretical Calculations for Chrome Species in Fume stainless steels (ref. 7). For most metals, especially total Cr and Vapor (Weight Percentage of Total Chrome Vapor as shown in Cr VI, there is substantial agreement between this work and Table 2) of GMAW, using 308 Stainless Steel Electrodes, Droplet theirs. Temperature of 2400°C Subsequently fume, both fresh and aged, of respirable size, Shielding Gas Cr(g) CrO(g) CrO2(g) CrO3(g) was collected and intratracheally fed to male CD/VAF rats.

99 Ar 1 O2 76.27 22.14 1.59 4.96E-4 Both short- and long-lived reactive oxygen species were present 98 Ar 2 O2 76.21 22.19 1.60 5.00E-4 on the surface of freshly generated fumes. It was found that freshly generated welding fume induced greater lung 95 Ar 5 O2 76.01 22.36 1.63 5.14E-4 inflammation than aged fume, most likely due to the higher 90 Ar 10 O2 75.66 22.66 1.68 5.40E-4 95 Ar 5 CO 77.54 21.05 1.42 4.12E-4 concentration of reactive-oxygen-species in fresh fume surfaces 2 (ref. 8). Oxidation is a critical factor in evaluating the potential to cause injury. This model has only predicted percentages during fume formation. However, upon formation these particles are exposed to air and are subject to further oxidation, which will change the Conclusions composition of the fume. 1. Given the welding electrode and shielding gas compositions, it is possible to predict the welding fume Experiments composition during formation as generated by vaporization from the GMAW droplets of stainless steel, through GMAW fume samples were collected for elemental analysis thermodynamic calculations. and chrome analysis. Subsequently, surface analysis was 2. These calculations were applied to the study of chromium performed. in welding fume generated by GMAW of SS. Fume samples were also analyzed for hexavalent chrome in 3. The chromium content was found to vary with the shielding addition to the total chromium content. NIOSH method 7600 gas composition. was used for this purpose, though there is some uncertainty 4. Cr(g) was the dominant form of the chromium in fume for about this procedure (ref. 6). During the steps following all the shielding gas compositions. digestion of the filters, one of the reagents could potentially 5. Calculations showed the presence of Cr VI in the form of oxidize the chrome species, thus all the Cr could be converted to CrO3(g) in welding fume, although its vapor pressure was 2 Cr VI. to 4 orders of magnitude below Cr(g). The results from this analysis are presented in Table 4. Cr 6. CrO3 in the gas is much less than Cr(g), but Cr(g) may later VI was present in the fume in amounts ranging from 0.02 to oxidize in air. 0.25 percent by weight of total fume, which correspond to 0.29 7. The amount of CrO3(g) was also shown to vary with the to 2.05 percent of the chrome content in fume. There is a oxidizing potential of the shielding gas, but by less than difference in Cr VI content versus Cr(g) of three orders of 1% for a 900% increase in oxygen content. magnitude between theoretical predictions during formation and 8. The study proved that the CrO3(g) that evolves from the droplet by vaporization is very small, but in the future it must be understood if it is later converted to a harmful form. 9. Studies at the Harvard School of Public Health, Boston, MA, were conducted to study the effect of GMAW of SS on the lungs of rats. Freshly generated welding fume induced greater lung inflammation in rats than aged fume. 10. The theoretical method established in this work can be applied to study of other metal vapors, including manganese, in fume generated by GMAW of SS. 11. These concepts can be extended to FCAW and other joining processes that generate a considerable amount of fume.

Acknowledgments

The authors wish to thank the Office of Naval Research for sponsoring this study under grant no. N00014-91-J-1415, monitored by G. Yoder. They also would like to thank their respective academic institutions for providing the facilities and environment for this work: Dr. T.W. Eagar heads the Department of Materials Science and Engineering at MIT, where P. Sreekanthan (now in the software industry) and N. T. Jenkins pursue their graduate studies and research. Dr. J. D. Brain heads the Department of Environmental Health at the Harvard School of Public Health, where Dr. J. M. Antonini (now with NIOSH) and Dr. G. G. Krishna Murthy are fellow researchers.

References

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