Investigation into the effects of metallic coating thickness on the corrosion properties of Zn–Al alloy galvanising coatings

Galvalloy (4.5% Al 95.5% Zn) coatings were produced on a continuous coil coating line at Corus Colors’ Shotton works with varying metallic coating thickness from 7.8 μm (120 g m−2) to 48 μm (325 g m−2) controlled using air knives. An overall decrease in aluminium content from 5.1% to 4.5 wt% and a primary zinc volume fraction increase from 16.2% to 32.8% occurred as the coating thickness is decreased. This reflects greater nucleation in thinner coatings and some removal of Al enriched molten phases. The scanning vibrating electrode technique (SVET) was used to show that increasing the coating thickness from 7.8 to 48 μm resulted in a decrease in the level of zinc removed during corrosion from a 20 mm exposed cut edge from 530 to 220 μg in the 24 h exposure to 5% NaCl solution. The same trend was also observed when external zinc runoff measurements were made at the Port Talbot weathering site. The increasing corrosion observed at lower coating weights results from greater undercutting of these coatings that are further away from a eutectic composition and an increasing tendency for crevice driven corrosion brought about through primary zinc dendrite interconnectivity.