How aluminium additions improve the performance of zinc‐rich organic coatings

Abstract

Novel sacrificial zinc‐rich organic coatings, with varying additions of aluminium, were prepared and tested for anticorrosion performance. Electrochemical measurements (potential vs. time and electrochemical impedance spectroscopy) were carried out to investigate cathodic protection and barrier performance while neutral salt spray and immersion experiments tested long‐term performance. Analytical scanning electron microscopy and X‐ray diffraction were used to characterize coatings before and after testing. Formulations containing aluminium significantly outperformed the standard 100% zinc‐rich coating with the greatest improvement occurring at 10%–15% aluminium by volume in the dry film. This improvement was caused by the dispersal of aluminium between zinc particles, which improved packing and enabled greater efficiency in zinc consumption resulting in extended galvanic protection times for steel substrates. The expected zinc corrosion product (basic zinc chloride, simonkolleite) was present within the coating as well as a Zn–Al layered doubled hydroxide. The latter's presence demonstrates that dissolution of aluminium contributed to the longevity of the galvanic action. The new Zn–Al formulations are extremely promising alternatives to standard zinc‐rich epoxy coatings, significantly reducing zinc loading and increasing the sacrificial lifetime.