An investigation of the effects of magnesium levels on the kinetics and mechanism of cut edge corrosion in organically coated zinc aluminum alloy galvanised steels

The scanning vibrating electrode technique (SVET) has been used to study th e effect of variation in magnesium levels (0.01-0.05%) on the kinetics and mechanism of cut-edge corrosion in zinc aluminum alloy. galvanised steel sa mples over-coated with asymmetric thicknesses of organic coatings, immersed in 5% aqueous sodium chloride. Experimental samples were prepared on 0.7 n lm steel substrates with a hot dip bath composition of near eutectic 4.2% a luminium/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 me tallic coated substrates with a 200 mum PFC based coating on one side and a 15 mum polyester coating on the other: SVET data have shown that with magn esium levels of <0.03% organic coating asymmetry caused localisation of ano dic activity proximal to the thicker (PVC) organic layer and cathodic activ ity localised primarily on the steel/galvanising layer proximal to the thin ner (polyester) organic coating This has been attributed to a form of diffe rential aeration corrosion associated with reduced oxygen access to the zin c layer proximal to the thicker organic coating However as the magnesium co ntent of the metallic coating tvas increased (to 0.04% and 0.05%) the locat ion of anodic and cathodic activity was found to become independent of orga nic coating geometry. The increasingly intense focal anodic attack at the e xposed cut edges. at these higher magnesium levels. was found to be directl y related to the formation of significant areas of sub-surface, dendritic, pro-eutectic zinc phases. This highly heterogeneous coating metallurgy over rides the effects of differential aeration corrosion driven by organic coat ing asymmetry that is the dominant mechanism at lower magnesium levels.