Chromate Content Bias versus Overspray Particle Size in Three Aircraft Primer Paints

The United States Air Force relies on the corrosion inhibiting properties of chromate-containing primer paints to protect the aluminum skin of its aircraft. Hexavalent chromium (Cr6+) the ingredient responsible for the corrosion inhibiting characteristics of these primers is a known human carcinogen. The concentration of Cr6+ in different particle sizes of paint overspray is important to understand health implications to painters as well as filtration efficiency. Previous research indicates disproportionately less Cr6+ content in smaller particles collected in the overspray of solvent-based epoxy-polyamide paint primers (MIL-P-2377G). This research explores the possibility of a particle size bias in the Cr6+ content of three commonly used aircraft primers: solvent-based epoxy-polyamide, water-based epoxy-polyamide (MIL-PRF-85582C), and solvent-based polyurethane (TT-P-2760A). The mass ratio of air flow to paint flow (A/P ratio) was varied during initial atomization. Seven-stage cascade impactors collected overspray particles into distinct bins with particle size cutoff diameters ranging from 0.7 mm to 34.1 mm. The mass of the dry paint collected in each bin was determined and analyzed for Cr6+ with an Atomic Absorption Spectrometer. In all three primers, smaller particles contained disproportionately less Cr6+ per mass of dry paint than larger particles. Particles with an aerodynamic diameter under 7 mm contained less Cr6+ per mass of dry paint as the particles became smaller. Particles less than 2.6 mm have a mean Cr6+ content of approximately one-third of the expected value. The range of A/P ratios tested in this study had no effect on the Cr6+ content in the overspray