On the Role of Zn Pigments Against Natural and Cathodic Overprotection Disbondment of a Multilayer Paint System

The presence of Zn pigments combined with the barrier effect of a multilayer paint system (Zn-rich epoxy/epoxy/polyurethane-coated steel) has mitigated the disbondment process in natural and overprotection cathodic conditions. The evaluation of the protective properties was performed by means of electrochemical impedance spectroscopy measurements. The samples submitted to 1 week of cathodic polarization showed lower impedance values than those exposed to natural disbondment or no disbondment. Impedance and open circuit potential values confirmed the protection of disbonded areas by a cathodic (Zn pigments dissolution) and barrier protection mechanism. More specifically, samples cathodically polarized for 24 h showed good protective properties resulting from a concomitant action of both short cathodic polarization and cathodic protection mechanisms. Negligible values of disbondment radial length after the radial cut test and adhesion measurements confirmed a high resistance of the paint. In addition, optical and scanning electron microscopy, together with energy dispersive x-ray spectroscopy analysis of the damaged areas, showed the presence of Zn corrosion products protecting the defects.