Resistance of metallic substrates protected by an organic coating containing aluminum powder

The corrosion behavior of an epoxy primer containing aluminum powder (10 vol.%) applied on carbon steel and on galvanized steel was examined by electrochemical impedance spectroscopy (EIS). The data show that this coating is more protective when applied onto carbon steel substrates, and that on galvanized steel thicker coatings allow to achieve similar protection levels as those obtained for carbon steel. These effects are probably due to aluminum pigments providing a cathodic protection of the substrate, and to the resulting products precipitating inside the pores of the polymeric coating. Three stages can be distinguished during exposure of the coated specimens. Upon immersion of the coated samples in the test solution, a pre-saturated stage is observed. After a certain period of immersion, which strongly depends on the thickness of the applied coating, a saturation stage is reached in which an effective protection of the metallic substrate against corrosion is achieved. Finally, at sufficiently long exposure times, swelling through the coating eventually leads to the detachment of the coating.