Investigation on the corrosion properties of Al-Epoxy/Zn based bilayer hybrid coatings on cold rolled steel

A B S T R A C T This study investigates the long-term corrosion behavior of a bilayer aluminum-epoxy (Al-EP) and zinc rich inorganic coating system designed to integrate barrier and sacrificial protection mechanisms. An Al-EP/Zn coating with a well-defined bilayer architecture was fabricated using a dip-spin deposition technique to achieve uniform and reproducible layers, as confirmed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The corrosion performance was evaluated under extended exposure conditions, with electrochemical and environmental testing conducted for periods up to 150 days, exceeding the duration commonly reported in the literature for similar coating systems. Electrochemical impedance spectroscopy (EIS), open-circuit potential monitoring, and salt spray testing revealed a time-dependent evolution of corrosion protection, in which early-stage cathodic protection provided by the Zn-rich layer progressively transitions toward barrier-dominated behavior due to corrosion product formation. Inductively coupled plasma mass spectrometry (ICP-MS) confirmed controlled Zn 2+ release during immersion, while X-ray diffraction (XRD) identified the formation of ZnO and simonkolleite (Zn 5 (OH) 8 Cl 2 ·H 2 O), which contribute to pore blocking and enhanced barrier resistance over prolonged exposure. Secondary ion mass spectrometry (SIMS) depth profiling further demonstrated electrolyte penetration pathways and layer-specific depletion behavior, enabling correlation between microstructural evolution and long-term electrochemical response.