Chromate Content versus Particle Size for Aircraft Paints

Many industries rely on the corrosion inhibiting properties of chromate-containing primer paints to protect metal from oxidation. However, chromate contains hexavalent chromium (Cr6+), a known human carcinogen. The concentration of Cr6+ as a function of paint particle size has important implications to worker health and environmental release from paint facilities. This research examines Cr6+ content as a function of particle size for three types of aircraft primer paints: solvent-based epoxy-polyamide, water-based epoxy-polyamide, and solvent-based polyurethane. Cascade impactors were used to collect and separate paint particles based on their aerodynamic diameter, from 0.7 to 34.1 μm. The mass of the dry paint collected at each stage was determined and an atomic absorption spectrometer was used to analyze for Cr6+ content. For all three paints, particles less than 7.0 μm contained disproportionately less Cr6+ per mass of dry paint than larger particles, and the Cr6+concentration decreased substantially as particle size decreased. The smallest particles, 0.7 to 1.0 μm, contained approximately 10% of the Cr6+ content, per mass of dry paint, compared to particles larger than 7.0 μm. The paint gun settings of air to paint ratio was found to have no influence on the Cr6+ bias.