Influence of galvanised surface state on the duplex systems behaviour

The influence of galvanised surface conditions on the duplex systems behaviour is analysed. A paint system was applied over new and weathered galvanised steel samples. A comparative study was performed by adhesion tests and electrochemical impedance spectroscopy (EIS) technique. Previously, the roughness profiles of galvanised surfaces were evaluated. The obtained results point out that a certain weathering degree is necessary for optimum adherence between paint and metallic surface and for good corrosion protection. Nevertheless, when the weathering degree is too high, the protection efficiency decreases. This behaviour can be explained in terms of galvanised surface roughness. The smooth surface of fresh galvanised steel makes difficult the establishment of adhesive bonds at coating-metal interface, leading to poor behaviour, at initial stages. If the weathering degree is too high, the surface roughness can hinder optimum coverage by the organic coating and the passive layer remains partially bare; as a consequence, the protection features are lower than those obtained for an average weathering degree where good adhesion and complete coverage are reached.