Evaluation of accelerated aging procedures of painted galvanised steels by EIS

Coil-coated sheets are usually cut and shaped or deep drawn to obtain the final profile of the manufacture. Mechanical deformation can affect the barrier properties of an organic coating and the adhesion to the substrate, thus accelerating degradation by natural weathering. UV radiations too can deeply affect the barrier properties of organic coatings and contribute along with rain, snow, thermal and hygrometric cycles and other factors to a fast decaying of the anticorrosive properties of painted systems. In this work some coil coating products for outdoor exposure, were subjected to the combined action of UV irradiation and mechanical deformation in order to simulate some of the stress occurring in their actual use. After mechanical deformation and UV irradiation, samples were exposed to a proper thermal cycle in order to accelerate water uptake and to discriminate in very short time among different formulations. Electrochemical impedance spectroscopy (EIS) was used to monitor the evolution of the systems during the accelerated aging. Qc and Rp (i.e. the constant phase element describing the dielectric properties and the ionic resistance of the coating) were selected to describe the protective system evolution. Adhesive and barrier properties were monitored and discussed during the combined thermal-mechanical and photo-oxidative aging. (c) 2005 Elsevier Ltd. All rights reserved.