An organic phosphonic acid doped polyaniline/zirconia/epoxy composite coating for metal protection in the marine environment

In this paper, a new type organic-inorganic composite coating is successfully prepared for the metal protection in the marine environment, in which 2-hydroxyphosphonoacetic acid (HPA) connects polyaniline (PANI) and ZrO2 through electrostatic force and Lewis acid-base adsorption, and the carboxyl group in HPA reacts with epoxy (EP) to form PANI-HPA-ZrO2/EP composite coating. The obtained composite coating is uniform, compact and has good adhesion with the substrate. The results of electrochemical tests show that the corrosion current density of the composite coating is five orders of magnitude lower than that of epoxy coating and the coating resistance remains two orders of magnitude larger than that of epoxy coating during the soaking in 3.5 wt% NaCl solution for 30 days. Furthermore, the corrosion protection mechanism is also discussed. In the composite coating, ZrO2 acts as a labyrinth effect, making the corrosive medium reach the tinplate in a more circuitous path. The superior redox ability of PANI and the chelation of PA ions with iron ions further inhibits the corrosion of metals, giving the epoxy coating excellent anti-corrosion properties.