Cathodic delamination of coil coatings produced with different Zn-based intermediate metallic layers

The work aims at investigating the effect of the nature of the intermediate zinc-containing layer in coil coatings has on their corrosion resistance against cathodic delamination. Over a 3 months period, cathodic disbonding has been measured for three types of coil-coated steel whose difference was in the zinc-based metallic intermediate coating, namely galvanised, galfan and aluzinc. The test specimens were exposed in 3.5 wt.% NaCl aquaeus solution and polarised to either −1050 mV SCE with aluminium anodes or to −1300 mV SCE with magnesium anodes. For the sake of comparison, immersed specimens at their spontaneous open circuit potential were also studied. The corrosion performance was determined on the basis of coating degradation and interface delamination of the tested samples. It was demonstrated that the nature of the zinc-based metallic layer had a large effect on cathodic disbonding. Under all the conditions explored, coil coatings containing an aluzinc (Zn-55Al) or a galfan (Zn-5Al) intermediate layer show higher resistance against disbonding that those with galvanised (Zn).