WELDING RESEARCH Chromium Emissions during Welding in an Aluminum Shipbuilding Environment The study assessed emissions using a welding machine and production welding operations Introduction Chromium is element number 24 in the Periodic Table (Ref. 1). Chromium is used as an alloying element in production of metallic alloys, especially stainless steels (Ref. 2). It is also an additive in the aluminum alloys used in fabrication of marine vessels (Refs. 3–7). Benefits of adding Cr to Al alloys include control of grain structure, prevention of grain growth in aluminummagnesium alloys, and prevention of recrystallization in aluminummagnesium silicon or aluminummagnesium zinc alloys during hot working or heat treatment (Refs. 4–7). The aluminum from which the alloys are made is 99.996% pure before addition 86-s WELDING JOURNAL / MARCH 2016, VOL. 95 of other metals. The 5000 series alloys contain at least 90% aluminum by weight. The 6000 series alloys contain at least 93% aluminum by weight. The other elements are added in precisely measured quantities. Table 1 provides the standard composition of alloys of aluminum used in marine applications (Refs. 4–7). Table 1 indicates the concentration of chromium in aluminum alloys used in marine applications ranges from 0.05 to 0.35% in the 5000 series to 0.04 to 0.40% in the 6000 series. Chromium is a minor additive compared to other elements. This paper reports on an investigation of emissions of chromium compounds during structural fabrication welding of aluminum at a shipyard in Vancouver, British Columbia. This shipyard typically uses 5000 and 6000 series aluminum, and performs welding with argon-shielded gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) processes. This paper addresses emissions only during GMAW processes. This work occurred indoors in a partly climate-controlled building. Humidity control and cleanliness are important issues in the aluminium vs. steel shipbuilding environment. The fabrication building in this shipyard contains heating, ventilating, and air-conditioning (HVAC) equipment with filtration for partial recirculation. As a result, the air is clean compared to that in a steel shipyard where “welding smoke” is readily visible in the roof space of the building. Welding indoors in this controlled environment also contrasts with welding on structural steels that occurs outdoors in conditions often deemed inhospitable. Chromium in welding plumes can occur in several possible forms starting with elemental (metallic) Cr(0), and compounds containing chromium in different oxidation states, namely Cr(II), Cr(III), and Cr(VI). Occurrence of a particular oxidation state depends on location in the plume relative to the protective gaseous shield. Elemental (metallic) chromium can result from spatter expelled during the welding BY T. N. MCMANUS AND A. N. HADDAD ABSTRACT Chromium is one of the minor additives contained in aluminum alloys (0.05 to 0.35% in the 5000 series to 0.04 to 0.40% in the 6000 series) used in marine applications. Argonshielded gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) processes are typically employed in a shipyard in Vancouver, British Columbia, in fabrication of ship structures. While metallic chromium Cr(0), Cr(III) oxide, and Cr(VI) compounds can occur in the welding plume, Cr(III) and Cr(VI) compounds are the more likely. This study assessed emissions during welding using an ESAB A2 welding machine and production welding operations. Airborne concentrations of soluble and insoluble forms of Cr(VI) compounds are likely to exceed relevant Threshold Limit Values (TLVs) or, more especially, TLVs + as low as reasonably achievable (ALARA) below the exposure limit (25 and 10 g/m3, respectively) during production welding operations. This study provides evidence that airborne concentrations of hexavalent chromium are very likely to exceed the OSHA regulatory limit of 5 g/m3, necessitating control measures regardless of the exposure limit employed by the regulator in the jurisdiction. KEYWORDS • Aluminum Alloys • ArgonShielded GMAW • Cr(VI) • A2 Welding Machine • Production Welding Processes T. N. McMANUS (nwohs@mdi.ca) is with NorthWest Occupational Health & Safety, North Vancouver, Canada, and with Programa de PósGraduação, University Federal Fluminense, Niterói, Brazil. A. N. HADDAD (assed@poli.ufrj.br) is with Escola Politécnica, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil.
Welding Journal | March 2016
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