An Investigation of the Effects of Magnesium Levels on the Kinetics and Mechanism of Cut Edge Corrosion in Organically Coated Zinc Aluminium Alloy Galvanised Steels

Abstract

SUMMARYThe scanning vibrating electrode technique (SVET) has been used to study the effect of variation in magnesium levels (0·01-0·05%) on the kinetics and mechanism of cut-edge corrosion in zinc aluminium alloy, galvanised steel samples over-coated with asymmetric thicknesses of organic coatings, immersed in 5% aqueous sodium chloride. Experimental samples were prepared on 0·7 mm steel substrates with a hot dip bath composition of near eutectic 4·2% aluminium/ca. 95·8% zinc and varying trace levels of magnesium ranging from 0·01 to 0·05%. Organic coating asymmetry was induced by over-coating the metallic coated substrates with a 200 μm PVC based coating on one side and a 15 μm polyester coating on the other. SVET data have shown that with magnesium levels of <0·03% organic coating asymmetry caused localisation of anodic activity proximal to the thicker (PVC) organic layer and cathodic activity localised primarily on the steel/galvanising layer proximal to the thinner (polyester) organic coating. This has been attributed to a form of differential aeration corrosion associated with reduced oxygen access to the zinc layer proximal to the thicker organic coating. However, as the magnesium content of the metallic coating was increased (to 0·04% and 0·05%) the location of anodic and cathodic activity was found to become independent of organic coating geometry. The increasingly intense focal anodic attack at the exposed cut edges, at these higher magnesium levels, was found to be directly related to the formation of significant areas of sub-surface, dendritic, pro-eutectic zinc phases. This highly heterogeneous coating metallurgy overrides the effects of differential aeration corrosion driven by organic coating asymmetry that is the dominant mechanism at lower magnesium levels.