Double Shroud Delivery of Silica Precursor for Reducing Hexavalent Chromium in Welding Fume

Abstract

The welding process yields a high concentration of nanoparticles loaded with hexavalent chromium (Cr⁶⁺), a known human carcinogen. Previous studies have demonstrated that using tetramethylsilane (TMS) as a shielding gas additive can significantly reduce the Cr⁶⁺ concentration in welding fume particles. In this study, a novel insulated double shroud torch (IDST) was developed to further improve the reduction of airborne Cr⁶⁺ concentration by separating the flows of the primary shielding gas and the TMS carrier gas. Welding fumes were collected from a welding chamber in the laboratory and from a fixed location near the welding arc in a welding facility. The Cr⁶⁺ content was analyzed with ion chromatography and X-ray photoelectron spectroscopy (XPS). Results from the chamber sampling demonstrated that the addition of 3.2∼5.1% of TMS carrier gas to the primary shielding gas resulted in more than a 90% reduction of airborne Cr⁶⁺ under all shielding gas flow rates. The XPS result confirmed complete elimination of Cr⁶⁺ inside the amorphous silica shell. Adding 100∼1000 ppm of nitric oxide or carbon monoxide to the shielding gas could also reduce Cr⁶⁺ concentrations up to 57% and 35%, respectively; however, these reducing agents created potential hazards from the release of unreacted agents. Results of the field test showed that the addition of 1.6% of TMS carrier gas to the primary shielding gas reduced Cr⁶⁺ concentration to the limitation of detection (1.1 μg/m³). In a worst-case scenario, if TMS vapor leaked into the environment without decomposition and ventilation, the estimated TMS concentration in the condition of field sampling would be a maximum 5.7 ppm, still well below its flammability limit (1%). Based on a previously developed cost model, the use of TMS increases the general cost by 3.8%. No visual deterioration of weld quality caused by TMS was found, although further mechanical testing is necessary.

Keywords:

• hexavalent chromium
• silica precursor
• welding fume

ACKNOWLEDGMENTS

The authors appreciate Tooele Army Depot (TEAD) providing the field test platform, and Major Analytical Instrumentation Center (MAIC) at the University of Florida providing access to the XPS. Financial support of this study was provided by the U.S. Department of Defense through the Environmental Security Technology Certification Program (ESTCP) under Grant No. WP-0903. The authors would like to thank Dr. David Phillips at Ohio State University for assisting with the cost assessment. Jianying Guan acknowledges the University Scholarship Program at University of Florida for supporting undergraduate students involved in the study.