Effect of nanosilica content on the corrosion inhibition of composite coatings of a filled epoxy resin grafted with a hydrophobic fluoroalkylsilane: a dual critical concentrations interpretation

3,3,3-Trifluoropropylmethyldimethoxysilane (TMDMS) was successfully grafted to molecules of an epoxy resin (EPR) of bisphenol-A origin with an epoxy value of 0.440, i.e. EP44, by transesterification (i.e. alcoholysis) of the TMDMS methoxyls with the EP44 hydroxyls under anhydrous dibutyltin-dilaurate catalysis. Meanwhile, a nanosilica powder was modified with 1,1,1,3,3,3-hexamethyldisilazane (HMDS) via electrophilic substitution of the nanosilica hydroxyls by the HMDS trimethylsilyls to significantly decrease its surface hydroxyls for controlled aggregation. Six coatings, onto an NaOH solution-treated tinplate substrate, of EP44, of the TMDMS-grafted EP44, and of the composites of the TMDMS-grafted EP44 filled with 0.5, 1, 3 and 5 wt% of the HMDS-modified nanosilica were then step-cured mildly with an amino-terminated polyamide from a solution. Owing to the introduction of superhydrophobic trifluoropropyls possibly plus a reduction in the hydrophilic hydroxyl concentration present, the hydrophobicity, evaluated by the water contact-angle, of the EP44 coating increased considerably upon its TMDMS grafting, which then changed little with further addition of the nanosilica. Electrochemical impedance spectroscopy data and simulations revealed that the anticorrosive performance of the TMDMS-grafted EP44 coating, upon immersion into an NaCl solution, was significantly improved compared with the EP44 coating, primarily due to its remarkably enhanced hydrophobic barrier to the water-mediated corrosives (water, NaCl, oxygen, other molecules and ions, etc.). However, the anticorrosion behaviour of the composite coatings was dominated by a corrosion inhibition (i.e. physical barrier) mechanism by the nanosilica filling the pores (free volumes, voids, cracks, etc.) susceptible to the corrosives. As the nanosilica content steadily was raised from 0 to 5 wt%, the corrosion inhibition of the composite coatings first intensified, probably thanks to an enhancement of the filling rate of the pores, and subsequently weakened, presumably due to an increase in the porosity from increased size exclusion of aggregated nanoparticles, and finally improved again possibly owing to a densification of the outside-of-pore barriers of greatly aggregated, size excluded particles, which constituted dual critical concentrations (DCCs) of the nanosilica at 0-0.5 and similar to 3 wt%, respectively, that gave rise to a maximum followed by a minimum in the corrosion inhibition. To our knowledge, this has been the first work reporting this unique DCCs behaviour for EPR/nanosilica coatings, which, dictated by a hydrophobic EPR matrix of small porosity as well as a modified nanosilica of controlled aggregation, may universally be extended to the corrosion inhibition of other polymer/nanoceramic coatings.