Zinc runoff from organically coated galvanised architectural steel

Abstract

The metallic runoff from the exposed cut-edges of galvanised steel substrates (zinc and an experimental zinc/aluminium alloy) over coated with organic coatings have been quantified over a 30-month period of external exposure. The organic coatings comprised a 200 μm PVC plastisol coating on one side and a 15 μm polyester coating on the other with compatible chromate based pre-treatments and primers. A novel method of increasing cut-edge length per unit area has been developed in which samples are punched with a regular array of holes. This increases the available cut-edge length per unit area by three orders of magnitude over a standard roofing panel. The only measurable leachates detected (>0.5 ppb (part per billion)) over the exposure period have been zinc and iron. All the samples showed significant reduction in zinc runoff upon prolonged exposure with zinc leaching concentrations over a three-month sampling period typically falling from a maximum 1–2 ppm (part per million) upon initial exposure to 0.05–0.1 ppm after 12 months exposure in the harshest industrial and marine sites. Metal ion runoff has been lowest in rural weathering locations (typically 0.5 ppm in a three-month exposure period upon initial exposure falling to <0.05 ppm over the same time period after 12 months exposure). In general at the more severe marine and industrial sites the greatest metal runoff levels were detected from organically coated substrates made from experimental metallic coatings with a 4.95% aluminium, 0.05% magnesium and 95% zinc metallic coating with less runoff leaching from 99.85% zinc coatings. In the former substrate this is due to the presence of subsurface pro-eutectic zinc rich dendrites that are exposed at the cut-edge but are not present in the corrosion resistant aluminium rich surface of the material. In these cases zinc leaching is independent of whether the samples are over-coated with organic layers.