Mechanistic understanding of aerosol emissions from a brazing operation

Welding operations produce gaseous and aerosol by-products that can have ad verse health effects. A laboratory furnace study was conducted to aid under standing of the chemical and aerosol behavior of a widely used, self-fluxin g brazing alloy (89% Cu, 6% Ag, 5% P) that is also used with a supplemental fluxing compound to prevent oxidation at the molten metal surface. The res ults indicate that the aerosols generated by the alloy are transient (produ ced over a short duration of time) and are associated with mass transfer of phosphorus species from the molten metal surface to the surrounding gas. I n contrast, when the alloy was used in conjunction with the supplemental fl uxing compound, a relatively nontransient, submicron-size aerosol was gener ated that was several orders of magnitude higher in concentration. Thermodynamic equilibrium analysis suggests that fluoride (a major constitu ent in the fluxing compound) played a significant role in reacting with the brazing alloy metals to form gas phase metal fluoride compounds that had h igh vapor pressures when compared with their elemental or oxide forms. As t hese metal-fluoride vapors cooled, submicron-size particles were formed mai nly through nucleation and condensation growth processes. In addition, the equilibrium results revealed the potential formation of severe pulmonary ir ritants (HF and BF3) from heating the supplemental fluxing compound. These results demonstrated the importance of fluxing compounds in the formation o f brazing fumes, and suggest that fluxing compounds could be selected that serve their metallurgical intention and suppress the formation of aerosols.