Encapsulation of Amine Corrosion Inhibitors into Nanocapsules for Self-Healing Protection of Steel

Abstract

Organic coating has been widely applied on metal substrates for metal protection in industrial fields. However, the corrosion resistance of organic coating layer is catastrophically decreased when coating layer is damaged due to external impact or corrosive species. In order to provide long-term protection effect and to prolong the maintenance intervals, corrosion inhibitors are incorporated in the coating. Direct addition of corrosion inhibitors in coating layer can provide additional active corrosion protection of steel. However, it can cause coating delamination and uncontrollable release of corrosion inhibitors. Encapsulation of corrosion inhibitors into nanocontainers is one of the promising methods to achieve self-healing corrosion protection due to its facile fabrication and wide applicability. In this dissertation, the inhibition efficiency of amines was evaluated to understand inhibition mechanisms, and then encapsulation/release behaviors and self-healing corrosion protection of encapsulated corrosion inhibitors were investigated. The inhibition efficiency of various amines was evaluated by electrochemical tests such as potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) measurements, and surface properties were analyzed by contact angle measurements and Fourier transform infrared (FTIR) spectroscopy. It was revealed that the amines with long carbon chain and less hydroxyl groups showed better corrosion protection in early stage of corrosion than the amines with short carbon chain and additional hydroxyl groups. In addition, the inhibitive efficiency of amines was dependent on adsorption of amines on the steel surface and stability of passive film. Nanocapsules as carriers for corrosion inhibitors were designed by modifying the conventional synthesis method of hollow core-shell particles. Polymeric nanocapsules loaded with six types of amine corrosion inhibitors were synthesized by multi-stage emulsion polymerization. Amine corrosion inhibitors were encapsulated by acid-base interaction between carboxylic acids in core polymer and amines. Synthesized nanocapsules were measured around 350 nm, and then the size of nanocapsules was increased to 400-500 nm after neutralization. Encapsulation efficiency of amines was improved when nanoparticles were simultaneously neutralized with amines. Depending on the basicity and water solubility of amines, different amounts of releasable corrosion inhibitors were encapsulated into the polymer capsules. The amines with strong basicity were more effectively encapsulated into nanocapsules than the amines with weak basicity since stronger bases could be easily protonated to react with acid in core polymer than weaker ones. Encapsulated organic amines were generally well released under alkaline conditions, and linear amines were more easily released from inside capsules than branched ones. The nanocapsules loaded with corrosion inhibitors were incorporated into the coating resin and were coated on cold-rolled steel to investigate corrosion protection performance. The corrosion inhibitive efficiencies of the nanocapsule-containing coating layers were evaluated by semi-immersion test and electrochemical tests such as EIS and scanning vibrating electrode technique (SVET). The specimens coated with nanocapsules loaded with corrosion inhibitors showed better corrosion resistance than those coated with inhibitor-free nanocapsules. It was revealed that the intrinsic properties of the amines as well as their encapsulation/release behaviors determined the barrier property and self-healing protection capability of the coating layer.