Comparative study between galvanised steel and three duplex systems submitted to a weathering cyclic test

Two types of samples, uncoated galvanised steel (UGS) and galvanised steel coated with chlorinated rubber paint (DS1), alkyd paint (DS2) and water-borne acrylic paint (DS3) were tested in a weathering cyclic chamber. The behaviour evolution of these systems was tracked using electrochemical impedance spectroscopy. For the duplex system, regardless of the type of paint, high impedance values were obtained, even at long exposure time. The UGS samples exhibited lower impedance values. Such behaviours could be interpreted in terms of different nature of the corrosion products generated on zinc surface. Using scanning electron microscopy, energy dispersive X-ray and X-ray diffraction techniques the identification of those compounds was possible. The main corrosion product generated on UGS was hydrozincite, Zn₅(CO₃)₂(OH)₆, in the case of the duplex systems, the corrosion product formed was zincite, ZnO. Such difference suggests that the zinc oxide formed under the coating is the responsible of the better duplex system behaviour and so, it leads a better passivation of zinc surface than the obtained with hydrozincite, which is the main corrosion product formed on UGS. In that sense, the coating acts as a selective membrane, avoiding the CO₂ uptake and so, hydrozincite formation is inhibited. This behaviour was observed even for coatings with poor barrier properties, such as water-borne acrylic one.