Corrosion protection mechanism in zinc-rich epoxy coatings: Role of carbon black and zinc morphology

Zinc-rich epoxy (ZRE) coatings are widely used for the heavy-duty corrosion protection of steel structures in marine and industrial environments. The protective performance of ZRE coatings relies on two mechanisms: an initial cathodic protection provided by sacrificial zinc particles, followed by a barrier protection resulting from formation of zinc corrosion products that fill micropores within the coating and/or deposit on exposed steel when coating damage occurs. To ensure effective cathodic protection, a high zinc loading is required. However, despite the high zinc content, the formation of insulating corrosion products disrupts electrical connectivity between zinc particles, causing most of the zinc to become electrochemically inactive. In addition, the high zinc loading often results in reduced mechanical properties as well as weakened adhesive and cohesive strength. This thesis aims to address these limitations and improve the performance of ZRE coatings. To achieve this objective, two main strategies are proposed: (1) enhancing coating performance by improving the electrical conductivity between zinc particles and the steel substrate through the addition of carbon black as a conductive pigment, and (2) modifying zinc particle morphology and crystallographic orientation to achieve a more electrochemically active zinc.

The influence of a carbon black conductive network on the corrosion protection performance of zinc-filled epoxy coatings with varying zinc contents was investigated. Two formulations containing 3.6 and 4.5 vol.% carbon black were determined as being above the percolation threshold of carbon black in the system. Zinc contents of 55, 66, and 75 wt.% were subsequently incorporated into these coatings. The results showed that coatings with lower zinc contents (55 and 66 wt.%) exhibited severe corrosion, as the high carbon black loading primarily increased coating porosity without connecting sufficient zinc to be electrochemically activated and compensate for this effect. In contrast, increasing the zinc content to 75 wt.% led to a significant improvement in coating performance. SEM cross-sections after the salt spray exposure confirmed that corrosive species did not reach the coating-metal interface in the coating containing 75 wt.% zinc and 3.6 vol.% carbon black, consistent with EIS results showing increased low-frequency impedance due to pore filling by zinc corrosion products. This enhanced zinc activation resulted in a rapid positive shift of the OCP, leading to premature loss of cathodic protection. Overall, these results demonstrate that the formation of a carbon black conductive network is beneficial only when a sufficient amount of zinc is present to be activated, react with the corrosive species, and fill coating pores through the formation of corrosion products. Excessive carbon black addition, however, increases coating porosity and electrical pathways, accelerating zinc depletion and ultimately deteriorating coating performance.

To obtain a comprehensive understanding of the role of carbon black, the influence of carbon black properties on the corrosion protection performance of zinc-rich epoxy coatings was investigated. Three carbon blacks, acetylene black, Ketjenblack EC-300J, and Ketjenblack EC-600JD, were characterized using X-ray photoelectron spectroscopy (XPS), N₂ adsorption/desorption analysis, and conductivity measurements. Ketjenblack EC-600JD exhibited the highest surface area and pore volume, a predominantly mesoporous structure, and a higher concentration of surface functional groups. Its high surface area enabled the formation of a conductive network at significantly lower loadings, minimizing increases in coating porosity. In addition, its greater surface hydrophilicity and mesoporous structure enhanced binder-filler adhesion, resulting in improved corrosion protection performance in both zinc-free and zinc-rich epoxy coatings. These results demonstrate a strong correlation between carbon black properties and the optimal loading required for effective corrosion protection.

Inspired by studies in energy storage and battery research showing that different zinc crystal planes exhibit distinct surface energies and corrosion activities, the effect of zinc morphology and crystallographic orientation on the corrosion protection performance of zinc-rich coatings was investigated. A straightforward high-temperature process was developed in which zinc particles were melted and resolidified at a slow cooling rate of approximately 2 °C min^-1, promoting the exposure of more electrochemically active crystallographic planes. To prevent particle coalescence during thermal treatment, the most effective carbon black identified previously was mixed with the zinc particles prior to heating, acting as a physical barrier. Two zinc-rich coatings were formulated with identical carbon black contents: one containing untreated zinc (ZRC-CB) and the other containing thermally treated zinc (T-ZRC-CB). Corrosion performance evaluation showed that the T-ZRC-CB coating exhibited faster zinc activation and sustained cathodic protection for nearly 70 days, compared to approximately 10 days for the ZRC-CB coating. In addition, rust creep was significantly reduced in the T-ZRC-CB coating (1.3 mm) compared to the ZRC-CB coating (2.6 mm). These results demonstrate that modifying zinc morphology to increase the exposure of active crystallographic planes significantly enhances the corrosion protection performance of zinc-rich epoxy coatings.