Zinc rich primers (ZRPs) have been widely applied in metallic structures especially for iron, due to its sacrificial cathodic protection and barrier protection. The main factor that influences its anticorrosion performance is the electrical connection between zinc particles, considered as percolation. The objective of this work is to improve the anticorrosion performance of a commercial ZRP by using an intrinsically conductive polymer (ICP). Of the commonly available ICPs, polyaniline (PAni) was chosen due to its high stability and simple synthesis using its monomer aniline. 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. Considering conductive state ES, the effects of different dopants have been investigated on corrosion prevention behavior of PAni containing ZRPs using multi-tool techniques, including camphor-10-sulfonic acid (CSA), phenylphosphonic acid (H2PP), hydrochloride acid (HCl) and phosphoric acid (H3PO4). In addition, to improve the protective performances of ZRP combined with PAni doped with CSA, higher contents of conductive pigment and multiwall carbon nanotube (MWCNT) were considered. Also, the zinc oxide nanoparticles were added into ZRP with PAni doped by H2PP acid, to check the combination effect of PAni and ZnO nano powder in coating performance. The corresponding corrosion mechanisms were proposed, based on the coating properties and results of an immersion test and open circuit potential, electrochemical, and localized electrochemical tests.
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.
Considering the effects of different dopants, the decomposition temperatures of the synthesized conductive powders follow the following order: PAni-H2PO4 ¿ PAni-Cl ¿ PAni-CS ¿ PAni-HPP. This indicated that the decomposition temperature of the benzene backbone is increased using inorganic dopants H3PO4 and HCl, but decreased when doping by CSA and H2PP organic acids. The addition of PAni ES increased the coating adhesion with the following efficiency order: sulphonate acid ¿ phosphonic acid ¿ hydrochloride acid. The resistivity of different coatings follow the order of ZRP ¿ ZRP-PAni-H3PO4 ¿ ZRP-PAni-H2PP ¿ ZRP-PAni-CS ¿ ZRP-PAni-Cl, obtained from the four pin method. Combined with the EIS results tested immediately after being immersed in sodium chloride solution, PAni containing ZRPs were found to be more conductive than the commercial ZRP, indicating that the small amount of PAni ES improved the percolation condition of ZRP. Especially, the addition of PAni doped by HCl increased the conductivity dramatically. The FTIR results verified the existence of conductive pigment in the coated samples, and the analysis of IR absorption bonds indicated that the dopants influence the electrical conductivity, due to the highly p-conjugated structures of the PAni.
Anti-corrosion performances of PAni containing ZRPs have been investigated by long-term non-destructive electrochemical tests in 3.5wt% NaCl solution. The results showed the modified ZRP improve both CP and barrier properties. Normally, three main periods can be recognized: the activation of zinc particles, the competition period between zinc activation and increasing resistance of zinc oxide products, and the stable period. The earlier period is a charge transfer control, and the later stage was controlled by a diffusion process. The modified ZRP with organic acids present a similar performance trend as a function of time, while the inorganic acid dopants doped PAni containing ZRP have similar anticorrosion behavior. The ZRP combined with PAni-Cl produces the best improvement both at the CP and barrier periods. SVET was successfully applied to investigate the interface galvanic couple of zinc rich primer and the steel substrate. For the galvanic couple of the commercial ZRP and steel, the ZRP failed to provide sacrificial protection; this was most likely due to the isolated zinc particles in the surface. The PAni containing ZRP exhibited the ability to sacrificially protect the exposed steel in a defect. And different interfacial dynamic mechanisms have been proposed.
In conclusion, the commercial ZRP has a poor Zn-to-Zn connection condition, which decreases the CP efficiency. The activation and consumption of zinc particles were fast. In addition, the formed zinc oxide products were not compact in the coating interface, which easily dropped off during 60 days of immersion. During the second period of 60 days, the coating formed a layer of stable oxide products to prevent further corrosion. Zinc particles failed to provide sacrificial protection due to the poor electrical connection at galvanic couple of the commercial ZRP and steel. As for the modified ZRP with PAni-CS pigment, the coating behaved with a similar behavior trend as a function of time, indicating similar mechanisms as the commercial ZRP. But it slowed the activation speed of zinc particles and improved the CP effect. At the same time, the formed corrosion products were more compact than that for the commercial ZRP, further improving better barrier protection. At the scratched coating surface, this modified ZRP provided sacrificial cathodic protection to the steel substrate. But non-uniform corrosion current was produced, indicating that the addition of PAni-CS pigment did not significantly improve the connection condition of zinc particles. The primer ZRP-PAni-HPP exhibited similar mechanism as ZRP-PAni-CS. This coating also provided a better CP, effect and the formed corrosion products were able to prevent ZRP from further corrosion. At the coating defects, it provided similar behavior as ZRP-PAni-CS but with better protection. After longer period, this coating seems to provide different behavior as PAni-CS containing ZRP, which may be due to the activation of PAni conductive pigment in the interface. In regards to ZRP-PAni-H2PO4, different behaviors were obtained compared with the previous three coatings. The addition of PAni-H2PO4 seems to not successfully improve coating conductivity, but the activation of zinc particles was much slower in this coating and also stable, indicating a better CP property. In addition, the formed corrosion products in the coating interface provided barrier protection. Considering the interface mechanisms at simulated galvanic couple of modified ZRP and steel, the release of dopant acid and formation of complex passive corrosion products was proposed. The modified ZRP with PAni-Cl present similar electrochemical behavior as ZRP-PAni-H2PO4. Besides, the addition of PAni-Cl improved coating conductivity significantly. Most of important, it provided the best performance, compared with other ZRPs.
Different contents of conductive pigment PAni-CS were added to ZRP, 0.1wt%, 0.2wt% and 0.3wt%, to check if the higher content of conductive pigment improves the anticorrosion performance. The higher addition of PAni-CS improved the percolation of ZRPs, verified by the coating conductivity. However, it may also induce a higher porosity, and this improvement may not be beneficial. At coating defects, the higher content of PAni-CS presents better performances. The addition of MWCNT into PAni-CS provided better coating resistance quickly after the activation of zinc particles. During the diffusion process control, ¿ of ZRP-PAni-CS-MWCNT was much larger than the only PAni-CS containing ZRP. It was concluded that the combination of MWCNT and PAni-CS improved the ZRP performances both at the early activation period and later diffusion control periods, which was also verified by SVET test. For PAni-HPP containing ZRP, the attempt of improving coating performances by ZnO nano particles was also investigated. The addition of ZnO nano particles did not improve the electrical connection of Zn-to-Zn particles. The details of EIS analysis showed that the addition of ZnO made the consumption of zinc particles more stable and slower, which would be helpful to increase the effective CP period. At scratched interface, after longer immersion, the combination of PAni and ZnO was proposed.
A transmission line model (TLM) was designed according to the coating physical properties, which provided an approach to further understand the anticorrosion mechanisms. For modified ZRP by conductive pigments, the Zn-to-Zn contact resistance was analyzed and combined with normal EIS test. The fitting results by TLM verified the improved zinc electrical contact condition by PAni, and the anticorrosion mechanisms proposed. Therefore, TLM would provide a novel approach in searching for proper conductive pigments to improve ZRP performances.