Bio-inspired designing of FeCO3–polystyrene composite corrosion products for enhanced corrosion control

FeCO₃ corrosion product layers on carbon steel offer intrinsic protection in CO₂-rich pipeline environments, yet their susceptibility to localised damage and mechanical removal limits long-term reliability. Inspired by biomineralisation, we engineer FeCO₃–polystyrene (PS) composite layers in situ on X65 carbon steel by introducing surface-functionalised PS microspheres during FeCO₃ growth in CO₂-saturated brine (pH 6.6, 60°C). Carboxyl- (PS–COOH) and amine-functionalised (PS–NH₂) microspheres at 1–100 ppm were systematically compared using electrochemical measurements (LPR, potentiodynamic polarisation), XRD, FTIR, SEM, FIB-SEM cross-sectioning, and white-light interferometry. PS–NH₂ promotes cooperative occlusion, preserving rhombohedral FeCO₃ symmetry with minor lattice strain and producing dense, fine-grained, uniform films — achieving 98.8% inhibition efficiency at 48 h. PS–COOH induces lattice distortion and spherulitic domain formation through a growth-capping mechanism, yielding 97.8% inhibition efficiency. Both composites substantially outperform unmodified FeCO₃ (93.3%) and markedly suppress localised pitting, as confirmed by interferometry. The convergent XRD–SEM–FIB–profilometry dataset demonstrates that functional group identity governs occlusion mode, film microstructure, and barrier performance. This bio-inspired design strategy offers a scalable route to engineer hybrid mineral–polymer layers with tunable microstructure for corrosion protection in CO₂-rich industrial environments.