California Air Resources Board

Background

Emissions from stainless steel welding are a source of concern because of the potential health impacts from hexavalent chromium.  In 2001, the California Air Resources Board (CARB) conracted with UC Davis to investigate and improve welding emission estimates.  The study was conducted to expand and update hexavalent chromium emission factors for stainless steel welding and included four welding processes:  gas metal arc welding, shielded metal arc welding (SMAW), flux core arc welding (FCAW), and pulsed gas-metal arc welding.

Experimental Methods

An enclosure was built so that a welder could stand inside and conduct welding during the sampling period. A filtration system was built to capture the welding fume and standard stack sampling methods were used to measure both hexavalent chromium and particulate matter (total PM and PM2.5). Sixty-six tests were performed for several different welding processes using both mild and stainless steel welding electrodes to measure both hexavalent chromium and particulate matter (total PM and PM2.5). 

Results and Discussion

Emission Factors

The emission factors for hexavalent chromium from stainless steel electrodes were determined and compared to existing U.S. EPA data. The study’s results are of comparable magnitude to the U.S. EPA emission factors and those reported by an industry group under similar "average" conditions, typically within a factor of two. Tests run without shielding gas for SMAW and FCAW produced an order of magnitude greater hexavalent chromium emission per unit of electrode consumed.

Findings

Two key findings of the study show: 1)  filters are sufficient to capture chromium emissions during testing and 2), using a shielding gas during flux core arc welding (FCAW) reduces hexavalent chromium emissions by more than 90% and emission factors can be used to quantify emissions from FCAW.  

Recommendation

Because of the large number of variables, that potentially impact hexavalent chromium emissions and the seeming impracticality of determining values of those variables for an emissions inventory, study authors recommend use of the average emission factors reported under test conditions that produce good weld quality.

Further studies

Recent studies completed after the experimental phase of this project had ended (Dennis et al,2002) have illustrated that substantial reduction of hexavalent chromium formation can be achieved by choice of shield gas composition and welding electrode additives. If the emissions inventory continues to demonstrate that hexavalent chromium from mild steel welding operations are a significant source to the ambient air because of its uncontrolled nature, further research into reducing hexavalent chromium during the welding process by utilizing changes to shield gas and electrode composition may be warranted.