Welding fumes from stainless steel gas metal arc processes contain multiple manganese chemical species

Abstract

Fumes from a group of gas metal arc welding (GMAW) processes used on stainless steel were generated using three different metal transfer modes and four different shield gases. The objective was to identify and measure manganese (Mn) species in the fumes, and identify processes that are minimal generators of Mn species. The robotic welding system was operated in short-circuit (SC) mode (Ar/CO₂ and He/Ar), axial spray (AXS) mode (Ar/O₂ and Ar/CO₂), and pulsed axial-spray (PAXS) mode (Ar/O₂). The fumes were analyzed for Mn by a sequential extraction process followed by inductively coupled plasma–atomic emission spectroscopy (ICP-AES) analysis, and by X-ray diffraction (XRD). Total elemental Mn, iron (Fe), chromium (Cr) and nickel (Ni) were separately measured after aqua regia digestion and ICP-AES analysis. Soluble Mn²⁺, Fe²⁺, Fe³⁺, and Ni²⁺ in a simple biological buffer (phosphate-buffered saline) were determined at pH 7.2 and 5.0 after 2 h incubation at 37 °C by ion chromatography. Results indicate that Mn was present in soluble form, acid-soluble form, and acid-soluble form after reduction by hydroxylamine, which represents soluble Mn⁰ and Mn²⁺ compounds, other Mn²⁺ compounds, and (Mn³⁺ and Mn⁴⁺) compounds, respectively. The dominant fraction was the acid-soluble Mn²⁺ fraction, but results varied with the process and shield gas. Soluble Mn mass percent in the fume ranged from 0.2 to 0.9%, acid-soluble Mn²⁺ compounds ranged from 2.6 to 9.3%, and acid plus reducing agent-soluble (Mn³⁺ and Mn⁴⁺) compounds ranged from 0.6 to 5.1%. Total Mn composition ranged from 7 to 15%. XRD results showed fumes had a crystalline content of 90–99% Fe₃O₄, and showed evidence of multiple Mn oxides, but overlaps and weak signals limited identification. Small amounts of the Mn²⁺ in the fume (<0.01 to ∼1% or <0.1 to ∼10 μg ml⁻¹) and Ni²⁺ (<0.01 to ∼0.2% or <0.1 to ∼2 μg ml⁻¹) ions were found in biological buffer media, but amounts were highly dependent on pH and the welding process. Mn generation rates for the fractions were tabulated, and the influence of ozone is discussed. The conclusions are that exposures to welding fumes include multiple Mn species, both soluble and insoluble, and that exposures to Mn species vary with specific processes and shield gases.