Panels coated by hot dipping with zinc (HDG), Zn-5Al (Galfan) and Zn-1.5Al-1.5Mg coatings at different thicknesses were phosphated and painted on an industrial line. Crevice panels with non-painted bare parts modelling conditions in hem flanges, reference panels with open surfaces and formed non-painted panels were exposed to a cyclic accelerated automotive test. Zn-Al-Mg coatings with the thickness of 10 mm provided similar or even better protection than HDG and Galfan at 20 mm in both confined and open configurations. In comparison to 10-mm HDG, the Zn-Al-Mg coating delayed red rust appearance in crevices by a factor of 2 and the maximal depth of corrosion in the steel substrate was by 42% lower. Confined areas were more corroded than open surfaces. For HDG, the time to red rust appearance dropped by 50-75%, corrosion attack in steel was from 3.5 to 7 times deeper and mass gain was about 2.3 times higher in crevices than on open surfaces. Corrosion of Zn-Al-Mg may be more affected by local environmental conditions created by the crevice configuration than for HDG. Red rust appearance on formed panels of 20-mm Galfan, 7-, 10-and 14-mm ZnAl-Mg was delayed to 10-mm HDG by a factor of 2.8, 3.5, 3.8 and >4.5, respectively. No adverse effect of forming was noticed. The results indicate that 2-to 3-fold reduction of the coating thickness for Zn-Al-Mg alloy coatings in comparison to traditional HDG may be possible without compromising the corrosion performance.

During hot dip galvanising, the metal strip passes into the zinc bath at a temperature of ~460°C. Iron from the strip dissolves into the bath during this operation, leading to problems such as dross formation and pickup. The model described in the present paper has been created in an attempt to quantify and understand the amount of iron dissolving from the strip as it passes through the zinc bath. This is a mathematical model based on well de ned rst principles of nucleation, growth and diffusion. The effect of process parameters on the amount of iron dissolving from the strip has been analysed, showing possible actions that may help to reduce the amount of dissolution.MST/5833

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