Hybrid Zn₃(PO₄)₂/BTA@SiO₂ inhibitive system for synergistic enhancement of active protection in waterborne epoxy coating

Zinc phosphate (ZnP) commercial corrosion inhibitor tends to show low protective efficiency in waterborne coatings due to hydrophilic nature of waterborne resins, possibility of ZnP leaching out, and the creation of entry channels for corrosive substances. Herein, the substitution of a fraction of ZnP with pH-responsive chitosan-capped benzotriazole-loaded mesoporous silica microcontainers (CTS@BTA@MSP) is shown to synergistically improve the performance of waterborne epoxy coating by enhancing the active inhibition properties. Two coating systems were prepared with active pigment (ZnP + CTS@BTA@MSP) concentration of 0.5 vol% and 1 vol% relative to the primary pigment barium sulphate, at a ratio of 1:1 (Z1S1), 1:2 (Z1S2) and 2:1 (Z2S1). Poor performance was observed at 1 vol% compared with their 0.5 vol% counterparts, with the use of ZnP alone displaying the poorest performance. Electrochemical impedance spectroscopy (EIS) of intact coating after 20 days of immersion showed impedance at lowest frequency (|Z| 0.01Hz ) for REF sample at (3.13 ± 0.10) x 10 6 Ω cm 2 , compared with the best performing Z1S2 0.5vol% coating which was (2.03 ± 0.09) x 10 7 Ω cm 2 . The performance of Z1S2 0.5vol% was corroborated by salt spray tests. Moreover, EIS and SEM/EDX assessments of scribed coating at pH = 3, 7 and 11 showed an increase in the self-healing properties of the Z1S2 0.5vol% coating, especially in pH 11 condition. Further FTIR examination revealed a complex inhibitive film on steel mainly comprising an adsorbed BTA film, phosphate film, chelates from BTA complexes with Zn and Fe ions, and zinc oxides/hydroxides. The obtained results demonstrate that 0.5 vol% addition of ZnP and CTS@BTA@MSP at a ratio of 1:2 synergistically improves the self-healing/active protection of the coating. With its exceptional performance at pH 11, the marine environment with neutral and alkaline pH will benefit from such hybrid coating. • Protection with zinc phosphate alone is limited in waterborne epoxy coatings. • Sol-gel process for one-pot encapsulation of mesoporous SiO 2 with benzotriazole demonstrated. • Improved active protection by combining zinc phosphate with BTA-loaded SiO 2 microcontainers. • Better protection of scribed coatings in pH 11 over higher BTA release of microcontainers in pH 3 was revealed and explained.