Enhanced corrosion protection of low-carbon steel using electrochemically synthesized polyaniline and polypyrrole bilayer coatings

This study investigates the strategic sequencing and electrochemical growth mechanics of polyaniline (PANI) and polypyrrole (PPY) bilayer coatings electrodeposited on low-carbon steel (LCS) to optimize chromate-free, active corrosion protection for marine environments. Addressing a critical knowledge gap regarding how interfacial layer sequencing dictates defect density and long-term ion transport channels, homopolymer and bilayer (PANI/PPY and PPY/PANI) coatings were synthesized via a highly controllable galvanostatic route in an aqueous oxalic acid medium at a constant current density of 0.66 m/A cm − 2 . In situ potential–time (E–t) transients revealed distinct induction periods of 270 s for PANI and PPY on the bare substrate, driven by the initial nucleation and precipitation of a passive iron oxalate interphase. Conversely, both bilayer configurations exhibited an immediate elimination of this induction period during the second-layer deposition within the one-step galvanostatic route. This transition indicates that the underlying primary polymer acts as an effective electronic and physical matrix that suppresses ongoing substrate dissolution, thereby facilitating continuous, uninterrupted vertical polymer growth. Fourier-transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) confirmed successful bilayer formation, revealing distinct, sequence-dependent morphological variations. The corrosion protection performance was evaluated in an aqueous 3% NaCl solution using potentiodynamic polarization. The bilayer architectures demonstrated significantly enhanced, reproducible protection efficiencies compared to their homopolymer counterparts, with the PANI/PPY configuration offering superior performance, yielding a protection efficiency of 92.5%. This enhanced resistance is fundamentally governed by a cooperative mechanism: the inner PANI layer provides robust interfacial passivation and localized sacrificial redox activity, while the outer PPY layer establishes a dense, low-defect physical barrier that significantly restricts the mass transport of aggressive chloride ions (Cl − ) to the steel interface. Unlike conventional composite configurations, this work systematically elucidates how the precise galvanostatic deposition sequence (PANI/PPY versus PPY/PANI) dictates initial iron oxalate passivation kinetics and downstream barrier properties, offering a tailored, chromate-free anti-corrosion architecture for marine applications.