Corrosion is a major concern for the long-term durability and structural integrity of steel components of highway bridges when unprotected. The application of protective coatings has been widely used for corrosion mitigation of atmospherically exposed structural steel. At present, the zinc-rich primer (ZRP) based three-coat system is widely used in the United States. The metallic zinc pigment ideally would provide corrosion resistance by sacrificial as well as barrier protection. The life of these coating systems is at best only half of the bridge design life. Furthermore, premature degradation may occur if there are flaws in the system due to improper application. Different additives were considered to improve the performance of ZRP coating system and recently carbon nano-particles gathered attention due to their beneficial characteristics.
The protection mechanisms of zinc-rich coatings have been well studied but the durability of zinc-rich coatings containing carbon nano-particles and their corrosion protection has not been well elucidated for bridge application. In the work presented in this dissertation, a zinc-rich epoxy coating containing carbon nano-particles (NPE-ZRP) have been investigated for possible highway steel bridge application. Coating durability, robustness, and repair considerations in aggressive environments relevant to highway bridges were investigated. The research considered material characterization and exposure to various environments such as inland, beach, salt-fog, immersion and alternate wet/dry exposure to identify the degradation mechanism as well as the durability of NPE-ZRP coating system. After exposure, the extent of coating degradation was evaluated by visual inspection, coating thickness, adhesion measurement, optical and electron microscopy and X-ray diffraction.
The NPE-ZRP coating initially provides barrier performance. The porous nature of the epoxy matrix allows electrolyte penetration from the exposure environment which facilitates the activation of the zinc pigments (cathodic protection) and the associated formation of zinc oxide further enhanced barrier protection. Comparatively, improved barrier performance was observed for the NPE-ZRP coating system even with fewer coating layers as well as less thickness than the conventional ZRP coating. Similar galvanic protection as conventional ZRP with less zinc content was observed. Comparatively faster corrosion rates of NPE-ZRP also portray enhanced continuity through carbon nano-particles. Higher pull-off strength was observed for NPE-ZRP coating apparently due to carbon nano-particles in the epoxy matrix which enhanced the cohesive bond and the adhesive strength by interlocking within the pore spaces of the steel substrate. Pre-exposure to high humidity before the coating application didnโt affect the coating durability but salt contamination and remnant coating layer can hinder the bond of the NPE-ZRP primer with the steel substrate resulted in reduced bond strength. Most importantly, NPE-ZRP coating always showed zinc consumption from the bulk primer layer whereas ZRP showed along with the steel/primer interface. Eventually, NPE-ZRP maintained good bond strength whereas the primer in ZRP loses bond strength at the steel/primer interface
