Abstract
The compatibility between nanocontainers and coating formulations is perhaps the last frontier in the quest for functional coatings. Sensing coatings for early metallic corrosion detection is an urgently needed technology by aeronautical companies to mitigate the costs of corrosion through continuous monitoring. In this work, we revisit phenolphthalein encapsulated silica nanocapsules, which were incorporated into a water-based lacquer, resulting in a novel corrosion sensing coating for aluminum alloy 2024 with improved functionality and standard performance. The ability of the coatings to detect corrosion by color change was investigated by immersion and salt-spray tests. During these tests, it was clearly demonstrated that encapsulation of the active compound is essential to obtain a functional coating, since the shell of the silica nanocapsules minimizes the detrimental interaction of the active compound with the coating formulation. The compatibility between nanostructured additives and coatings is almost never taken into consideration in the literature. Herein this aspect evidences the positive effects of active agent encapsulation, which is explored in terms of reactivity, viscoelastic properties, curing, thermal stability, release and leaching studies, hardness, mechanical properties and corrosion resistance. Computer simulations based on the density functional theory and periodic structural models were performed to unveil the interaction mode of phenolphthalein with the metallic surface.
Highlights
It is critical for the aeronautical industry that the generation of smart coatings allows the early detection and continuous monitoring of corrosion
Coated AA2024 plates were immersed in a 5% NaCl solution in order to accelerate corrosion and verify if the coatings were able to detect the associated local change of pH [9,10]
It is possible to verify that the coating containing SiNC-PhPh starts to indicate the presence of corrosion, by the appearance of pink color, only after 8 days of immersion and before the corrosion process is even visible to the naked eye
Summary
It is critical for the aeronautical industry that the generation of smart coatings allows the early detection and continuous monitoring of corrosion. The drawbacks of directly adding active organic compounds to coating formulations are clearly shown in the present study, such as the ‘deactivation’ of the sensing compound in some formulations, lower responsive ability to surrounding conditions, and damaging interaction with the coating formulation resulting in a lower protection of the metal by barrier effect. These findings reinforce the importance of nanocontainers to achieve functional coatings
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