Effects of magnesium levels on the kinetics and mechanism of corrosion in coated zinc aluminium alloy galvanised steels

The scanning vibrating electrode technique (SUET) has been used to study the effect of variation in magnesium levels (0.01-0.05%) in near eutectic 4.2% aluminium / ca. 95.8% zinc galvanising. The mechanism of corrosion, in 5% aerated aqueous sodium chloride electrolyte, on the surface of zinc aluminium alloy galvanised steels prior to organic coating and on the exposed cut edges in OCS is markedly different. Increasing the magnesium levels decreases the intensity and localisation of surface corrosive attack in the absence of organic over layers. Surface corrosion initiates at the edges of eutectic nodules. In samples with low magnesium levels there is a higher density of surface zinc rich phases and small cracks at these locations rendering the samples more susceptible towards corrosive attack. In the cut edge corrosion mechanism of organically coated zinc aluminium alloy samples there is an increasingly intense and localised attack as the magnesium levels are increased. This is related to generation of higher concentrations of pro-eutectic zinc rich sub surface phases.