Construction and study on corrosion protective property of polydopamine-based 3-layer organic coatings on aluminum substrate

Metal aluminum (Al) is being widely used in industry and can be easily corroded in severe conditions. Recently, dopamine has been demonstrated to be an effective adhesive molecular that can be attached to virtually all material surfaces to form stable polydopamine coating (PDAc), exhibiting promising prospects in many fields. In this work, to improve the anti-corrosion performance of Al sheet, a novel polydopamine-based 3-layer organic coating has been constructed by a multi-step self-assembly technique for the first time. In brief, a self-assembled monolayer of (3-mercaptopropyl)trimethoxysilane (coded as MPTS-SAM) was firstly prepared for enhancing the adhesion between the PDAc and the Al substrate. Subsequently, a PDAc was assembled onto the Al substrate by a simple immersion into the dopamine hydrochloride solution at a pH of 8.5. At last, a layer of tetradecanoyl chloride was grafted onto the PDAc surface through amidation, which could apparently slow down the water diffusion to the interface of PDAc and Al for its prominent hydrophobicity. Corrosion resistance of the synthesized coating in NaCl solution was evaluated by potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) measurement, respectively. As results show, PDAc on bare Al can protect the substrate from corrosion to a certain extent. Significantly, after being mediated by MPTS-SAM underlayer and covered by tetradecanoyl chloride outer layer, the anti-corrosion capability of the coating has been improved markedly.