Investigating the effects of carbon black conductive network on the anti-corrosion performance of zinc-filled epoxy coatings

The effects of a carbon black (CB) conductive network on the anti-corrosion performance of zinc-filled coatings were investigated using open circuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS), accelerated corrosion tests, and scanning electron microscopy (SEM). A percolated CB network formed when acetylene black content exceeded 2.7 vol.% in the dry film. To examine its effect on corrosion performance, 55, 66, and 75 wt.% zinc were incorporated into two formulations containing 3.6 and 4.5 vol.% CB. At 55 and 66 wt.% zinc, severe corrosion occurred because the CB network increased coating porosity, while the zinc level was insufficient to counteract this effect. In contrast, increasing zinc content to 75 wt.% significantly improved performance. SEM cross sections after 30 days of salt spray exposure showed that no corrosive species reached the coating-metal interface in the 75 wt.% zinc and 3.6 vol.% CB sample. This result was consistent with EIS data, which revealed increased low-frequency impedance after 30 days due to pore sealing by zinc corrosion products. Rust creep tests further confirmed that this coating exhibited the lowest rust creep, even compared to the 85 wt.% zinc-rich reference coating.