Electrochemical synthesis and corrosion protection of poly(3-aminophenylboronic acid- co-pyrrole) on mild steel

Abstract

Synthesis of poly(3-aminophenylboronic acid-co-pyrrole) (p(APBA-co-Py)) is carried out potentiodynamically on a pre-passivated mild steel (MS) surface in an oxalic acid solution containing 3-aminophenylboronic acid (APBA) and pyrrole (Py) monomers. The monomer feed ratio was determined using electrochemical impedance spectroscopy (EIS) and adhesion tests. The p(APBA-co-Py) coating is characterized by electrochemically and spectroscopically comparing with poly(3-aminophenylboronic acid) (p(APBA) and polypyrrole (p(Py) homopolymers. SERS, FTIR, XPS, scanning electron microscopy-wavelength dispersive X-ray and- energy dispersive X-ray spectroscopy results indicate the presence of both APBA and Py segments in the p(APBA-co-Py) backbone. The protective properties of the coating are investigated by Tafel and EIS measurements in a 0.50 M HCl solution. The corrosion resistance of p(APBA-co-Py)-coated MS (66.8 Ω cm²) is higher than that of p(APBA)- and p(Py)-coated, passivated, and uncoated MS. The p(APBA-co-Py) coating embodies the advantageous features of both homopolymers. Py units in p(APBA-co-Py) chains improve the protective property while APBA units carrying the -B(OH)₂ group develop the adhesive property of the layer. EIS results show that the p(APBA-co-Py) coating, due to its homogeneous and compact distribution and the formation of a stable interface, enhanced corrosion resistance of MS by 87.4% for 10 hours in HCl corrosive medium.

Fig. 1. (A) First cycles obtained during electrosynthesis of (a) p(APBA) in 0.1 M APBA (b) p(APBA-co-Py) in 0.1 M APBA and 0.075 M Py (c) p(Py) in 0.075 M Py on the pre-passivated MS electrode, v = 20 mV s⁻¹ (c_{oxalic acid} = 0.1 M as electrolyte, curves were shifted for clarity). (B) Multisweep cyclic voltammogram recorded during electrosynthesis of p(APBA-co-Py) on the pre-passivated MS electrode in the polymerization solution containing 0.10 M oxalic acid, 0.10 M APBA, and 0.075 M Py, v = 20 mV s⁻¹. (C) Cyclic voltammograms obtained in monomer-free 0.1 M oxalic acid solution for (a) p(APBA), (b) p(APBA-co-Py), and (c) p(Py)-coated MS electrodes, v = 20 mV s⁻¹ (curves were shifted for clarity).

Fig. 2. Nyquist plots obtained at E_OCP in 0.50 M HCl for the p(APBA-co-Py)-coated electrode prepared in the polymerization solution containing 0.10 M oxalic acid, 0.10 M APBA, and (a) 0.025 M, (b) 0.050 M, (c) 0.075 M, (d) 0.10 M Py (inset: equivalent circuit model).

Fig. 3. Schematic illustration of the synthesized p(APBA-co-Py) on the pre-passivated MS surface.

Fig. 4. (A). SERS spectra and (B) Deconvoluted FTIR spectra of (a) p(APBA), (b) p(APBA-co-Py), and (c) p(Py) films synthesized on the pre-passivated MS surface, and (d) APBA monomer.

Fig. 5. (A) Full XPS spectra and (B) Deconvoluted C 1s, B 1s, and N 1s spectra of (a) p(APBA), (b) p(APBA-co-Py), and (c) p(Py) films synthesized on the pre-passivated MS surface.

Fig. 6. WDX spectra of (a) p(APBA), (b) p(APBA-co-Py), and (c) p(Py) films synthesized on the pre-passivated MS surface.

Fig. 7. SEM images at 2 μm and 20 μm scales, and elemental mappings of (A) p(APBA) (B) p(APBA-co-Py) and (C) p(Py) coatings deposited on the pre-passivated MS surface.

Fig. 8. Tafel polarization curves recorded in 0.5 M HCl for (a) p(APBA), (b) p(APBA-co-PPy), and (c) p(Py) coatings on the pre-passivated MS electrodes.

Fig. 9. (A). Nyquist plots recorded at E_OCP in 0.50 M HCl for (a) bare and (b) passivated MS electrodes, and (c) p(APBA), (d) p(APBA-co-Py), and (e) p(Py) coatings on the pre-passivated MS electrodes. (inset: equivalent circuit models; solid lines and points represent fitted and experimental data, respectively). (B) Nyquist plots recorded during immersion time of 72 hours of p(APBA-co-Py)-coated electrode in 0.50 M HCl at E_OCP.