Abstract
The objective of this work is to improve the anticorrosion performance of a commercial zinc rich primer (ZRP) by using an intrinsically conductive polymer polyaniline (PAni). Different states of PAni including nonconductive emeraldine base (EB) and conductive emeraldine salt (ES) were added to ZRP, to study the effects of the different oxidation states on anticorrosive performance. EB is electrically neutral while doped (protonated), and the resulting ES form is highly electrically conductive. The synthesized PAni additives and coated panels were characterized by X-ray diffraction pattern (XRD), Fourier transform infrared (FTIR), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA) and other normal coating evaluation techniques. The anticorrosion performances were studied in 3.5 wt. % NaCl solution by electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP). The results show that the addition of a small amount of conductive PAni to ZRP slowed the activation process of zinc particles and further improved the cathodic protection effect. It also provided better barrier performance compared with the commercial ZRP. The nonconductive PAni EB accelerated the activation of zinc particles, and the formed zinc oxide products were compact and provided better barrier performance than the commercial ZRP.
Introduction
Zinc rich primers (ZRPs) have been widely applied in metallic structures especially for iron. The protection mechanisms are considered to be: sacrificial cathodic protection (CP) at the earlier stage and barrier protection at later stages [1]. One of the main concerns of ZRP is the CP efficiency, which is related to the contact condition of zinc particles, referred to as percolation [1]. There are different ways to improve the percolation conditions: increase in PVC [2, 3]; improve coating application procedure [4]; and add other conductive pigments like carbon nanotubes (CNTs) [5] and intrinsically conductive polymers (ICPs) [6, 7]. ICPs can undergo oxidation-reduction reactions by gaining or losing electrons from the surrounding environment [8], which have already been considered in the development of protective coatings. These are three kinds of common ICPs [8]: polyaniline, polypyrrole, and polythiophene. Polyaniline (PAni) is considered in this work, due to its excellent environmental stability, controllable electrical conductivity, and interesting redox properties and non-toxicity [9]. PAni can be polymerized from the cheap aniline monomer and can present three idealized oxidation states: leucoemeraldine (C6H4NH) n, emeraldine ([C6H4NH] 2 [C6H4N] 2)n including emerraldine base (EB) and emeraldine salt (ES), and (per)-nigraniline (C6H4N) n. Among these oxidation states, the EB form can be changed to ES if doped or protonated with the imine nitrogens protonated by an acid [10]; this works in the opposite way as well. Therefore, EB is considered as the most useful form of PAni [8]. This attracts lots of studies of different forms of PAni on the corrosion behavior of various ferrous and non-ferrous metals.
