Effect of powder painting procedures on the filiform corrosion of aluminium profiles

In this work, the filiform corrosion behaviour of powder painted aluminium profiles was studied, and the coating barrier properties together with adhesion to the substrate were analysed. Samples coated with a traditional painting procedure (one powder layer application followed by curing), and other samples coated using a special cycle to obtain wood grain effect were compared using accelerated filiform corrosion tests. Moreover, in order to better understand the degradation mechanisms of painted metal substrates, thermal stresses were applied to accelerate the natural weathering. The effects of the thermal aging were analysed by electrochemical impedance spectroscopy. Wood grain effect coatings showed to better resist, with respect to a traditional coating, to filiform corrosion when testing was performed following the DIN 65472 standard. On the contrary, electrochemical impedance measurements showed better barrier properties against water uptake for the traditional coating after thermal ageing. Different barrier properties were found to depend on the microstructure of the two coatings after curing. The traditional one has a homogeneous microstructure, with completely fused powder grains and very fine size pigments. On the contrary, wood grain effect coating possesses a "defective" microstructure with a widespread distribution of partially fused powders grains determining a dense network of grain boundaries, moreover coarser size pigments are present. This microstructure decreases the coating barrier properties, but also can determine a loss of plasticity and the coating under mechanical stress can crack modifying the filiform corrosion behaviour.