Influence of Zinc Content and Chloride Concentration on the Corrosion Protection Performance of Zinc-Rich Epoxy Coatings Containing Carbon Nanotubes on Carbon Steel in Simulated Concrete Pore Environments

The influence of zinc content on the mechanisms of corrosion protection of zinc-rich epoxy primers containing carbon nanotubes (CNT-ZRPs) on carbon steel under exposure to simulated concrete pore solutions was investigated. The barrier and cathodic protection control mechanisms were characterized by electrochemical techniques, such as open-circuit potential and electrochemical impedance spectroscopy, accelerated tests in a salt spray chamber, and high-resolution techniques, such as scanning electron microscopy coupled with energy dispersive x-ray spectroscopy and x-ray diffraction. Based on the zinc content, three mechanisms of corrosion protection were identified. The CNT-ZRP with 60 wt% Zn exhibited good barrier protection during the entire immersion period as a result of the highly cross-linked character of the epoxy binder. In contrast, the CNT-ZRP with 70 wt% Zn afforded short-term sacrificial protection to the metallic substrate, followed by intermediate barrier protection. Furthermore, it was found that the presence of CNTs in the coating system with 70 wt% Zn enhanced the electrical contact between the zinc particles and the carbon steel surface, which is required to guarantee an efficient galvanic protection process. In addition, CNTs increased the barrier properties of the coating, suggesting that CNTs blocked micropores and defects in the material hindering the diffusion of the electrolyte throughout the coating. Finally, an extended galvanic protection was provided for the CNT-ZRP with 80 wt% Zn. Insoluble zinc corrosion products were found inside the material and at the coating surface, as a result of the galvanic protection process and a self corrosion process of the zinc particles. The influence of chloride concentration on the corrosion degradation mechanisms of these coating systems was also investigated. It was found that concrete pore environments with low chloride concentration promoted the passivation of the carbon steel surface and the formation of solid zinc corrosion products. In contrast, the simulated concrete pore solution with high chloride concentration induced the breakdown of the passive layer, blister formation, and dissolution of zinc corrosion products previously formed during the sacrificial protection process or the self-corrosion process.