Microstructure and Electrochemistry of Al-Rich Primer

Abstract

Al-rich primer (AlRP) is designed to be anodic relative to high strength Al alloys, so that it drives the mixed potential of Al alloy substrates (e.g. AA2024) into the cathodic region below the breakdown potentials of the Al alloys, preventing localized corrosion. The Al-rich primer (AlRP) under study is composed of spherical Al-Zn-In pigments in an epoxy binder. The 2D and 3D microstructure of the AlRP was studied using a combination of X-ray microscopy, plasma focus ion beam/scanning electron microscopy, and transmission electron microscopy techniques. The primer contained Al-Zn-In particles with various sizes ranging from tens of nanometers to microns, epoxy binder and a significant volume of voids. Potentiodynamic polarization testing was performed to measure the electrochemistry of the Al-Zn-In bulk alloy under neat primer in NaCl solution to simulate the environment which Al-Zn-In pigments are exposed to in the real primer. The bulk alloy under neat primer exhibited higher breakdown potential than the bare bulk alloy exposed to NaCl solution. The polarization curves of the bulk alloy under neat primer provide more accurate boundary conditions for modeling of the galvanic interaction between the AlRP and an alloy substrate. Galvanic corrosion between an AA2024 sample and Al-Zn-In bulk alloy was also studied using a zero resistance ammeter. A trivalent chromium process treatment on galvanic couples can protect AA2024 substrate from cathodic corrosion.Acknowledgements: this project is funded by the Office of Naval Research through the Small Business Innovation Research (SBIR) program. We thank Naval Air Systems Command (NAVAIR) for providing Al-rich primer samples. We acknowledge Dr. Mansoureh Norouzi Rad at Zeiss for 3D microstructure reconstruction on Al-rich primer using X-ray microscope, and Thermo Fisher Scientific for 3D microstructure reconstruction on Al-rich primer using Helios Plasma FIB/SEM. We appreciate Dr. Babu Viswanathan at OSU for STEM characterization, and Swagelok Center for Surface Analysis of Materials (SCSAM) at Case Western Reserve University for Ar ion polishing.