Adhesion-Diffusional-Based Corrosion Protection Mechanisms of Polyaniline-Primed Fluoropolymer Coatings

In this study, we developed a series of dual-coating systems composed of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and electrodeposited polyaniline (PANI) as the primary coating and primer, respectively, for stainless steel (SS). Prior to the PVDF-HFP topcoat application, PANI (in either a doped emeraldine salt (ES) or dedoped base (EB) form) was electrodeposited potentiodynamically on either an etched or unetched SS substrate using H₂C₂O₄ as the supporting electrolyte. Tape tests showed improved PVDF-HFP adhesion on EB-PANI-primed substrates than on ES-primed samples. Impedance analysis and potentiodynamic chemistry were used to quantitatively investigate the electrochemical behavior and corrosion kinetics of the coated samples, respectively, in a 3.5 wt % NaCl corrosive solution. Electrochemical results revealed that the PVDF-HFP/EB-PANI dual-coating system exhibited superior corrosion resistance (low-frequency impedance modulus, |Z|_0.1Hz ∼ 10⁶ Ω cm²) compared to either the dual PVDF-HFP/ES-PANI or pristine PVDF-HFP coating (|Z|_0.1Hz ∼ 10⁴ Ω cm²). Based on the correlation between electrochemical data and water uptake, high water diffusivity for ES-PANI-primed PVDF-HFP coating (diffusivity, D ∼ 10.0 × 10^–10 mm² s^–1) was obtained due to the ionic solvation of counterions, while relatively lower diffusivity for the EB-PANI-primed counterpart (D ∼ 5.0 × 10^–10 mm² s^–1) was observed due to the effective impingement of water ingress by the insulating hydrophobic EB-PANI undercoat. Overall, we investigated the impact of the electronic state of an electroconductive polymer primer on the corrosion resistance of a two-coat system with a fluoropolymer topcoat and explored nuanced effects on adhesion and electrolyte diffusion, providing comprehensive insights and laying the foundation for tailored design strategies to enhance corrosion resistance, filling a critical gap in the current understanding and guiding future research in coating science innovation.