Microstructural changes in zinc aluminium alloy galvanising as a function of processing parameters and their influence on corrosion

The effect of cooling rate and substrate gauge upon the microstructure and corrosion resistance of Galfan (Zn-4.5 wt.%Al) coated steels is presented. The coatings, applied to steel of gauges 0.47 mm (light gauge) and 0.67 mm (heavy gauge) on a coil coating line, were subjected to three different cooling rates by increasing output from 55% to 100% of the total power from a high powered cooling rig. The increase in cooling rate did not significantly alter the volume fraction of the primary zinc, this remaining at ∼20%. However, the size and number of the primary zinc dendrites were altered. The fast cooled samples contained small but numerous (∼3000 mm⁻² in the heavy gauge and ∼2850 mm⁻² in the light gauge) dendrites as opposed to the slow cooled samples where there were fewer (∼1850 mm⁻² for the heavy gauge and ∼1500 mm⁻² in the light gauge) dendrites of greater size. Characterisation of the surface revealed a reduction in eutectic cell size (∼1.8 mm to ∼0.8 mm on the heavy gauge and ∼2.1 mm to ∼1.2 mm on the light gauge) with increasing cooling rate. This leads to an increased unit length of depressed boundary between the eutectic cells. The eutectic microstructure is also finer (with reduced inter-lamella spacing) in the fast cooled samples again reflecting the more rapid nucleation of the coating.The scanning vibrating electrode technique (SVET) has been used to quantify the effects of these microstructural changes upon the surface and cut edge corrosion performance. There is an increase in corrosion activity on the surface of the fast cooled samples (metal loss 150 μg to 260 μg on the heavy gauge and 50 μg to 80 μg on the light gauge) primarily due to the increased length of depressed boundaries. Applying the same analysis to the cut edge, a decrease in corrosion occurs upon the faster cooled specimens. Metal loss calculations show a decrease (140 μg to 75 μg on the heavy gauge and 190 μg to 115 μg on the light gauge) as the cooling rate is increased. The higher intensity long lived anodes at the cut edge in the slower cooling rate samples are directly related to the increase in zinc dendrite size within the coating as nucleation rates are reduced.