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Thermodynamic equilibrium calculations as an occupational assessment tool: welding alloy examples.

Zimmer-AT; Biswas-P
American Industrial Hygiene Conference and Exposition, May 9-15, 1998, Atlanta, Georgia. Fairfax, VA: American Industrial Hygiene Association, 1998 May; :72
Census data indicates that over 700,000 workers in the United States are involved in using over 80 different types of welding and applied processes. These processes produce a wide variety of fumes composed of a complex array of metals, metal oxides, and other chemical species volatilized from either the base metal, the welding electrode, or the flux material. Although a body of industrial hygiene sample data is available to characterize gas and fume emissions for certain weldmg operations, limited data exists for many welding processes including soldering and brazing. The purpose of this study was to illustrate the utility of thermodynamic equilibrium (TEA) as a qualitative assessment tool for the alloys and fluxing agents encountered during welding operations. TEA is based on a method that directly minimizes Gibbs free energy, subject to constraints on the total number of moles of each element in the system. Equilibrium values (relative abundance and phase) of each chemical species was determined by using a computer code known as STANJAN. This program iteratively solves a nonlinear matrix involving "I" elements and"n" chemical species. Two metal alloys, representative of medium (brazing) and highˇtemperature (arc welding) operations, were selected for study. One of the major findings of this study is the importance of the fluxing agents used in conjunction with the metal alloys. The use of fluoride as a fluxing agent strongly influenced the behavior of the metals found in each alloy in that gas phase metal fluoride species were formed at temperatures significantly lower than would have occurred had the fluoride not been present in the system. Upon cooling to ambient temperatures, these species will markedly increase the fume generation through aerosol dynamic processes such as nucleation, coagulation. and condensation. Experimental data from other studies tends to .support the TEA results.
Exposure-assessment; Thermodynamics; Welding; Industrial-processes; Industrial-exposures; Qualitative-analysis; Flux-density; Molecular-structure; Weight-measurement; Metal-compounds; Metal-fumes; Metal-oxides; Soldering-alloys; Arc-welding; Analytical-methods; Computer-software; Chemical-properties; Chemical-structure; Fluorides; Temperature-effects; Gases; Fumes
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American Industrial Hygiene Conference and Exposition, May 9-15, 1998, Atlanta, Georgia