Unraveling the corrosion protection mechanisms of sustainable inhibitors in epoxy coatings: Bridging solution-phase behavior and coating performance

• Eco-friendly pigments' inhibition in epoxy matrix was systematically evaluated. • Improved barrier properties and corrosion resistance observed in static conditions. • Hydrodynamic flow reduced the anti-corrosion performance of pigmented coatings. • Stability of protective films determined the inhibitory effectiveness of pigments. • Stable protective films provided high resistance against undercoating corrosion. The corrosion inhibition behaviours of three eco-friendly inhibitors under static and dynamic conditions were investigated to establish a correlation between their performance in aqueous solution and their effectiveness in coating applications. Calcium borosilicate (CBS), zinc calcium strontium aluminum orthophosphate silicate hydrate (ZPS), and strontium phosphosilicate (SPS) pigments were characterized as mixed-type inhibitors, predominantly functioning through physical adsorption mechanisms. Assessment of their performance was conducted using electrochemical impedance spectroscopy, potentiodynamic polarization, and scanning electron microscopy, combined with energy dispersive spectroscopy analysis. Electrochemical measurements revealed that SPS and ZPS offered superior inhibition performance compared to CBS, supported by surface analysis. It was shown that CBS was ineffective in forming a protective layer on mild steel in NaCl solution, whereas ZPS and SPS developed protective films with notable anti-corrosion properties, achieving inhibition efficiencies of approximately 55% and 75%, respectively. The superior ZPS performance was attributed to the formation of a thin zinc- and phosphorous-rich oxide layer, although this layer exhibits instability under hydrodynamic flow conditions. In contrast, SPS forms a compact and robust strontium, phosphorus, and silicon-rich oxide film, maintaining stability in static and dynamic environments. In inhibitor-embedded epoxy coatings, CBS slightly improved barrier properties but demonstrated limited corrosion inhibition under dynamic conditions due to the instability of its protective layer. Conversely, ZPS and SPS significantly mitigated corrosion-induced delamination of the coating in stagnant conditions, with the SPS-pigmented coating demonstrating superior performance under dynamic conditions. Additionally, no sign of initiation of localized undercoating corrosion was observed in the SPS-pigmented coatings.