Preparation and evaluation of corrosion resistance of a self-healing alkyd coating based on microcapsules containing Tung oil

A commercial coating was transformed into a self-healing coating by inserting urea-formaldehyde microcapsules (MCs) containing tung oil (TO), to improve coated metal corrosion resistance. The microcapsules were characterized regarding their morphology and chemical composition, and the presence of tung oil as core material was also proven by FTIR analysis. MCs were then incorporated into the matrix of a commercial mono-component alkyd coating, being added in the same proportion as the corrosion inhibitor (zinc phosphate) present in the original commercial coating. Corrosion resistance was evaluated by the comparison between samples coated with an original commercial coating, commercial coating without zinc phosphate and a commercial coating containing MC., employing a corrosive saline medium of NaCl 3.5 % (w/w), where were carried out the electrochemical analysis of open circuit potential (OCP) and electrochemical impedance spectroscopy (EIS). For this, the stimulus-responsive capacity of microcapsules was proven through mechanical disruption. It was observed that the additive coating presented a satisfactory performance for application in 1020 carbon steel specimens, keeping its properties practically unchanged. The self-healing ability of the coating has been proven and the corrosion protection performance of the coating has been assessed through electrochemical techniques, and the results of the OCP and EIS tests showed that the microcapsules in the coating matrix caused a beneficial increase in barrier properties. Additionally, MC additive coating protected the metallic substrate when it suffers a mechanical defect through the release of the oil and its active protection conditioned to the self-healing effect. Thus, when comparing the types of coatings tested, samples containing MCs of tung oil proved to be a more promising additive than the original additive. Also, a self-responsive stimulus mechanism for this smart coating was proposed based on the blistering caused by the presence of corrosion products.