pH-Sensitive halloysite nanocontainers encapsulated by silicate-mediated tannic acid networks for the smart epoxy coatings

An important issue regarding halloysite nanotubes as inhibitor containers is the stimuli-responsive performance. A novel approach to constructing pH-sensitive halloysite nanocontainers is developed in this study. The silicate-mediated tannic acid complexes (Si-TA) are deposited on the etched halloysite nanotubes (EHNTs) loading Na 2 MoO 4 , endowing the Si-TA/Mo@EHNTs nanocontainers with the high pH-sensitivity. Both the release rate and ratio of the inhibitor increase with increasing pH. The sensitivity of the nanocontainers to the high pH value arises from the rapid hydrolysis of Si-TA encapsulation layers in the basic media. It is evidenced that gallic acid (GA), generated from the hydrolysis of the Si-TA complexes, can further dimerize into ellagic acid (EA) in the basic media. The smart epoxy coatings with the stimuli-responsive performance are prepared by incorporating the Si-TA/Mo@EHNTs nanocontainer, and exhibit better corrosion resistance compared with the epoxy coatings without the nanocontainers as well as directly incorporated with Na 2 MoO 4 . The elevated pH in the cathodic region of corrosion cell trigger the release of MoO 4 2- ions from the nanocontainers, inhibiting the corrosion of the exposed substrates effectively. Furthermore, the EA-Fe(III) depositions could be formed by the combination of the EA compounds from the damage of the encapsulation layers with the Fe 3+ ions, acting as the barrier on the exposed substrates to improve the corrosion resistance of the coating.