WELDING RESEARCH shift of 8 hours, work week of 40 hours, and working lifetime of 40 years. Chromium compounds whose formation in the plume outside the shield gas is possible, given the components in the base metal and the wire, include the chromates. Table 2 lists chromates (calcium chromate, water soluble and unspecified insoluble chromates, lead chromate, strontium chromate, and zinc chromate) for which TLVs exist. Chromates can form by reaction between ingredients in the base metal and substances in the surrounding environment. Zinc chromate can form from reaction between chromium, zinc, and oxygen. Chromates can also form by reaction involving substances in the surrounding environment. There are no specific TLVs for these substances. Substances in the surrounding environment possibly involved in formation of chromates in the welding 88-s WELDING JOURNAL / MARCH 2016, VOL. 95 plume include calcium, sodium, and potassium (water-soluble chromates), lead, and strontium. Sodium, potassium, calcium, lead, and strontium are not identified as being present in the aluminum alloys generally used in shipbuilding applications. In addition, manufacturers may add additional trace elements such as beryllium, cadmium, lead, and nickel not always indicated in product specification sheets (Refs. 4–7). The ability of chromates containing these elements to form in the welding plume in the absence of these elements in the aluminum alloys is highly unlikely. Paint dust aerosolized during abrasive blasting is a potential source of these compounds. Whether these chromates actually are present is not known. Chromates containing these elements usually are encountered in other industrial processes such as paint application and removal, and water treatment, where these substances are present as ingredients in chemical products. WorkSafeBC, the regulator having jurisdiction over the shipyard in which this work occurred, designates Cr(VI) compounds as substances to which employers must maintain exposure as low as reasonably achievable (ALARA) below the TLV as an additional precaution (Ref. 11). In this regard, the U.S. Occupational Safety and Health Administration (OSHA), part of the Department of Labor, lowered the permissible exposure limit for exposure to hexavalent chromium in 2006 to 0.005 mg/m3 (Ref. 12). Information in Table 1 and Table 2 provides the basis for a concern regarding the form in which chromium can occur. Occurrence in a form that has a high exposure limit and is not carcinogenic obviously raises the comfort level about chromium. On the other hand, occurrence in a form that has a low exposure limit and is carcinogenic raises considerable concern about worker protection. Articles in the industrial hygiene literature on chromium in welding plumes and grinding dust provide some additional guidance. Of course, the focus of the articles is related to stainless steels, where the chromium content ranges from 16 to 20% of the alloy vs. up to 0.40% in aluminum alloys (Ref. 13). The proportion of Cr(VI) compounds in the plume from argon-shielded welding of stainless steels is 2 to 4% of total chromium (Refs. 14, 15). Chromium VI compounds are absent in grinding dust from stainless steel alloys (Ref. 16). Experimental Procedure Laboratory studies of welding plumes typically utilize an enclosed, conically shaped collector containing several sampling probes that is positioned over the welding electrode. This equipment is not normally available in the real-world environment of a production facility. One way to overcome this problem is to use a robotic welding machine during welds on long joints. These machines also offer the ability to locate multiple sampling cassettes at the height of the breathing zone of a welder engaged in work on horizontal surfaces. These units offer the additional benefit of sampling during realworld operation. The containment provided by the hood and associated curtains provides the opportunity to collect large quantities of material in a short time. This avoids artifacts due to potential aging of the material after collection. Collecting sufficient welding plume in a short period of time is essential to preserving the relative occurrence of the oxidation states and chemical form of chromium as present in the welding plume. The height of the hood is about the same as the distance of the welding shield from the arc, so that the age of the plume will be about the same as experienced by welders. Fig. 3 — Underside of the collector hood showing the cluster of sample cassettes.
Welding Journal | March 2016
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