Electronic effects on protective mechanism of electropolymerized coatings based on N-substituted aniline derivatives for mild steel in saline solution

Abstract

Polyaniline (PANI), poly(N-methylaniline) (PNMA) and poly(N-formylanilide) (PNFA) coatings were fabricated over mild steel via chronoamperometric strategy in oxalate electrolyte. Scanning electron microscope (SEM), attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) evidenced the polymerization of monomers. Anticorrosive effects of PANI, PNMA and PNFA coatings for steel substrate were evaluated during long-term immersion in 3.5% NaCl solution by potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and frequency modulation (EFM). Morphological variation for coated specimens before and after immersion in NaCl solution was also monitored by SEM and atomic force microscope (AFM). Differentiated protection capability were observed for three coatings following the sequence of PNFA > PNMA > PANI. Through electrochemical and interfacial analyses, electroactivity and physical barrier were ascertained as the critical factors, especially the former one, in the long-term protection capacity. Electroactivity was distinguished by the elevated apparent current density from non-destructive EFM measurements. Furthermore, electron-donation and -withdrawal effects of N-substituents played an essential role in coating electroactivity. In detail, N-substituents improved the anodic protection and physical barrier of PNMA and PNFA coatings. Particularly, formyl with electron-withdrawal effect reinforced the anodic protection and thus the superior the anticorrosion efficiency of PNFA for underlying steel.