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Dual pH-Sensitive Smart Coatings for Corrosion Inhibition of AA2024-T3

Aluminum alloys (AAs) have been broadly used in numerous applications due to their exceptional mechanical properties. Although AAs also exhibit outstanding resistance to general corrosion, they suffer from localized corrosion, such as pitting and crevice corrosion, and can therefore lead to unforeseen catastrophic failure. pH-sensitive smart coatings have been considered to be a promising strategy of combating the localized corrosion of AAs due to the unique self-healing capabilities. These smart coatings can sense the local pH decreases and increases induced by the anodic and cathodic reactions of AAs, respectively, thereby releasing corrosion mediating agents to the actively corroding sites on demand. However, the majority of smart coatings developed to date can only sense either acidic or alkaline condition, but not both. In this study, smart coatings that can respond to both acidic and alkaline environments are developed. These dual-pH sensitive smart coatings are based on weak polyelectrolyte coacervates consisting of polyacrylic acid and polyethylenimine or chitosan. When corrosion inhibitors such as SrCrO4 or Ce(NO3)3 are dispersed in these coatings and subsequently immersed in aqueous solutions with different pH values, the release rates of corrosion inhibitors are much higher in the acidic and alkaline environments than that of the neutral environment. Deposition of these inhibitor-loaded smart coatings on AA2024-T3 substrates leads to a significant improvement of the corrosion resistance. Furthermore, corrosion inhibitors are also encapsulated into the core-shell particles and nanofibers through the co-axial electrospray and electrospinning techniques, respectively. The shells of the particles and nanofibers are made from the polyelectrolyte coacervates, so they are dual-pH sensitive. These particles and nanofibers are sandwiched in an organic coating matrix and applied on the AA2024-T3 substrates, forming self-healing smart coatings with substantially improved corrosion resistance as evidenced by the electrochemical impedance spectroscopy analysis. In particular, the nanofibers embedded smart coatings can repeatedly heal the intentionally scribed area of the coated AA2024-T3 owing to the transport of corrosion inhibitors from remote areas through the nanochannels created in core-shell nanofibers.