Unraveling the effect of dopants of electropolymerized polyaniline coating on controlling the oxidation of austenitic stainless steel in contaminated phosphoric acid medium

Abstract

Phosphoric acid solutions with aggressive impurities consisting of chloride and fluoride ions as well as dissolved oxygen can severely damage exposed metallic materials. Herein, two doped polyaniline coatings are electrochemically deposited onto 904 L austenitic stainless steel (904 L ASS) to serve as a post-treatment for this alloy, aiming to minimize localized corrosion during its application in phosphoric acid industries. The two PANI films doped with H₃PO₄ (PANI-PO₄) and KNO₃ (PANI-NO₃) are formed in the oxidation state of leucoemeraldine and emeraldine salts, respectively, and provide excellent corrosion protection of 904 L ASS in phosphoric acid solution contaminated with fluoride, chloride and sulfate. The surface analyses are performed using various methods such as EIS, potentiodynamic polarization, ATR-FTIR and UV–vis spectroscopy, SEM, and AFM. The electrochemical results demonstrate that both deposited polymers offer high anodic protection efficiency for the 904 L ASS surface, with 94.1 % and 95.7 % for PANI-PO₄ and PANI-NO₃, respectively. Besides their common role as a physical barrier, this research attempts to emphasize the anodic protection efficacy of doped PANI films against local corrosion of 904 L ASS by facilitating the oxidation of the underlying substrate and strengthening the passive layer at the metal solution interface. The surface analyses reveal that chloride ions are embedded within both PANI coating chains. This causes minimum broad local corrosion instead of deeper pits, indicating that both coatings enhance the corrosion resistance of 904 L ASS by facilitating the self-repair of the passive film. Consequently, PANI coatings offer a feasible solution to address corrosion issues faced in the phosphoric industry.