EIS study of environmentally friendly coil coating performances

The corrosion protection of metal structures by the application of organic coatings is obtained using complex paint cycles. The metal pretreatment as well as primer, intermediate and top coat are the usual layers composing the protection system. Although chromate pretreatment processes and chromate containing primers are widely used, they require highly toxic chromic acid solutions, with consequent effluent disposal and ecological problems. This fact pushed many research laboratories to develop new chromium-free pretreatments and primer inhibitors. Electrochemical impedance spectroscopy (EIS) proved to be a very useful technique to study the protection properties of the organic coating because of the possibility of evaluating the corrosion process on the metal substrate and to measure the electric and dielectric characteristics of the coating. In this study, new chromium-free pretreatments and new primers containing ecological inhibitors were applied to steel substrates coated using an Al-Zn alloy. The results were compared with those obtained using traditional protection systems. The characterization of the complete system (pretreatment and primer) in sodium sulfate solution clearly showed the different water uptake behavior of the two primers, which highlights the better barrier properties of the one containing phosphate. However the corrosion protection of the primer containing chromates is also due to the inhibitive action of the chromates. Hence such a type of primer better withstands the presence of defects in the coating. Moreover the presence of environmentally friendly pretreatments do not worsen the corrosion protection which appears comparable to that observed when chromatation is employed.