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.