Corrosion sensing and self-healing composite coatings on magnesium alloy AZ31 enabled by stimuli-responsive microcapsules loaded with phenolphthalein and epoxy resin

Polymer coatings inevitably suffer from micro-damage during film formation and the in-service period, leading to interfacial corrosion of the metal substrate and even catastrophic accidents. Constructing a coating system that integrates early corrosion sensing and efficient damage self-healing capabilities is highly pursued yet difficult to achieve in practice. This work reports a strategy for designing smart anticorrosion coating with autonomous corrosion-detecting and in-situ self-healing functions. The dual-functional composeite coating was designed on magnesium (Mg) alloy AZ31, via embedding urea-formaldehyde microcapsules loaded with phenolphthalein (PhPh)/epoxy resin. By immersing the scribed samples in a 3.5 wt% NaCl solution, the interfacial corrosion reaction at coating damage can be timely sensed and visualized by the apparent pink color in just 7 min. The local alkaline environment produced by Mg corrosion, initiates the chromogenic reaction of PhPh and indicates interfacial corrosion. Meanwhile, the released epoxy resin, as a healing agent can effectively fill the coating defects and thus prevent the intrusion of corrosive medium. Electrochemical impedance spectroscopy shows that the long-term corrosion resistance of the coating was significantly improved after introducing synthesized microcapsules. After 115 d of immersion, the low-frequency impedance of the composite coating with 3 wt% of microcapsules (PUM3) is above 1010 Ω·cm2, whereas the pure polyurea coating (PUA) has dropped to 109 Ω·cm2.This feasible and effective design concept of microcapsule-based intelligent anticorrosive coating provides a new idea for developing environmentally adaptive protective materials with corrosion sensing and self-healing functions.