Improving the functionality and performance of AA2024 corrosion sensing coatings with nanocontainers

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.

Nanonetwork Composite Coating for Sensing and Corrosion Inhibition

“…Therefore, PNL molecule can be used as sensor for detecting early stage of corrosion. [ 37–40 ] Solutions of PVF with various amounts of PNL ( Table 1 ) were electrospun on steel substrates. Afterward, the obtained substrates were further covered by spin‐coating with P(MMA‐ co ‐BA) solutions with two different concentrations ( Figure ).…”

Incorporation of silica nanocontainers and its impact on a waterborne polyurethane coating

“…An alternative solution to achieve a good dispersion of the NCs into the PU matrix is to use the containers as powders and add them into the formulation the same way as pigments. This is done either using specific tools such as extruders or ball mills or simpler mechanical or magnetic stirring, potentially assisted with ultrasonication . Shi et al reported the presence of silicon‐richer areas within the epoxy resin they used even after a 30 min mixing step.…”

“…Recent developments showed very good results and enhancement of the release control using layered double hydroxides, zeolites, hollow fibers, particles, capsules or more unusual ones such as algae exoskeletons . Each one of these systems has shown protective properties when filled with inhibitors, despite sometimes a decrease in passive protection .…”

Hexacyanoferrate‐Intercalated Layered Double Hydroxides as Nanoadditives for the Detection of Early‐Stage Corrosion of Steel: The Revival of Prussian blue

“…In fact, Frankel and Zhang used organic pH indicators such as phenolphthalein or coumarin to detect corrosion at the surface of an Al alloy . In spite of the direct addition of pH indicator being a simple route to impart corrosion detection ability to a protective coating, investigation in the field of anti‐corrosion coatings in the last 20 years has shown that often the direct addition of active compounds to coating formulations leads to unwanted reactions between the added compounds and other coating components, contributing to coating deterioration and/or loss of efficiency of the additive . To overcome this challenge, the concept of nanocontainers, inert structures with ability to store and release active compounds in a controlled way when local conditions are verified, was introduced.…”

“…For the purposes of corrosion sensing, Li and Calle, as well as Maia et al used polymeric particles to control the release of pH indicators and, at the same time, enhance the compatibility of the additive with the coating , . In other works by Tedim and colleagues, mesoporous silica particles loaded with the pH indicators were used instead , . In all the reported works, indication was achieved by color changes due to local alkalinization near the surface as a result of the cathodic half‐reaction of the corrosion process (Figure a).…”