Effect of Nano-Encapsulated Corrosion Inhibitors on Self-Healing Corrosion Protection of Steel Sheets

A nano-encapsulation system of organic corrosion inhibitors has been designed, which is capable of improving the self-healing corrosion protection without degrading the barrier protection of the coating layers. Core-shell nanoparticles loaded with six types of amine corrosion inhibitors such as ethanolamine, 5-amino-1-pentanol, diethanolamine, triethanolamine, propylamine and dipropylamine, were synthesized by multi-stage emulsion polymerization. Amines were successfully encapsulated into nanocapsules by reacting with carboxylic acid in core and shell polymer during alkalization swelling process. Water-soluble amines were effectively introduced in nanocapsules, and the basicity was the most predominant parameter for amine encapsulation. In alkaline condition, the encapsulated amines were generally well released and the release of the encapsulated amines was pH dependent regardless of the exposure time. In addition, the molecular structure and aqueous solubility were critical factors in the release behaviors. Linear amines and highly water-soluble amines were more easily released to the environments from inside capsules than branched and water-insoluble ones. The nanocapsules were incorporated into the coating resin and were coated on cold-rolled steel sheets to investigate corrosion protection efficiencies. The corrosion inhibitive efficiencies of the nanocapsule-containing coating layers were evaluated by semi-immersion tests and electrochemical tests, electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). The organic coatings with inhibitor-loading nanocapsules showed better corrosion performance than the organic coating with inhibitor-free nanocapsules. Among six amines used in this study, 5-amino-1-pentanol, diethanolamine and triethanolamine exhibited self-healing anti-corrosion performance with recovering coating resistance. The coating resistance of the coating film gradually decreased and then increased via the self-healing corrosion protection of amines release from the nanocapsules. On the other hand, ethanolamine, propylamine and dipropylamine exhibited a rapid drop in the coating resistance, and the resistance continued to decrease without self-recovery. The inhibition efficiencies of encapsulated amines were affected by both intrinsic inhibition activities of amine and encapsulation/release behaviors.