Microstructure and Electrochemistry of Al-Rich Primer
Authors: Wang Shan-Shan , Wang Xi , Palani Siva , Rose Alan , Frankel Gerald S. 
Published: February 2020

The Al-rich primer, developed by NAVAIR is designed to be anodic relative to high strength Al alloys such as AA2XXX and AA7XXX, so that it drives the mixed potential of the Al alloy substrates into a cathodic region below the breakdown potentials of Al alloys, preventing localized corrosion. The Al-rich primer under study is composed of spherical Al-Zn-In pigments and epoxy binder. The spherical pigments are achieved from atomization of Al-Zn-In bulk sacrificial anode used on hulls of boats operating in seawater. The size of spherical pigments in the primer ranges from with size ranging from a few hundreds of nanometers to tens of microns. The average size of the pigments is around 7 µm in diameter. The active Al-Zn-In pigments are further treated with trivalent chromium passivation (TCP) solution for different dwell times to reduce the self-corrosion of the pigments.

In this study, the Al-rich primer coated on TCP-treated AA2024-T3 substrate provided by NAVAIR was investigated. The microstructure of the Al-rich primer was studied using a combination of X-ray microscopy (XRM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. 3D reconstruction microstructure of the Al-rich primer was acquired by XRM, showing that the primer contains Al-Zn-In particles with various sizes, epoxy and a significant volume of voids. The planar and cross-section views of the Al-rich primer were also examined by SEM. Since mechanical polishing damaged the pigments and smeared epoxy, so ion polishing was used to prepare the planar and cross-sectional samples of the Al-rich primer to reduce materials damage and achieve a less-damaged surface. Similar to what was found in XRM results, voids were also found in the primer. In addition, some relatively large Al-Zn-In pigments with size of tens of microns were surrounded by some very fine Al-Zn-In pigments with size of a few hundred of nanometers to form agglomerates. The cross-section view examined by TEM showed that some fine pigments directly contacted large pigments, and others bonded with epoxy. Heavy elements enriched at grain boundaries in the pigments. STEM-EDS mapping will be conducted on the pigments.

To understand the electrochemistry of Al-Zn-In pigments, the microstructure and electrochemistry of Al-Zn-In bulk alloy were studied. Zn enriched along grain boundaries in bulk alloy. Large intermetallic particles with size of tens of microns located at grain boundaries. Some intermetallic particles were enriched with Fe and Si, and some were enriched with In. In situoptical microscopy was used to observe the pitting initiation and growth on the both polished and TCP-treated Al-Zn-In bulk alloy during anodic potentiodynamic polarization in 3.5% NaCl solution. Pits initiated around some intermetallic particles. The composition of these particles will be determined.

When the primer is exposed to bulk solution or salt spray environment, electrolyte saturates the epoxy. Thus, the environment in which the substrate and Al-Zn-In pigments are surrounded is not bulk solution or electrolyte layer on the primer surface, but electrolyte saturated in epoxy. To simulate the environment of the primer, the special coplanar electrodes were designed. Ag/AgCl electrode was used as reference electrode; pure Ni was chosen as counter electrode; working electrode was AA2024 or Al-Zn-In bulk alloy. Three electrodes were mounted in epoxy and then polished. The neat primer without pigments were applied on the polished surface of coplanar electrodes. The coplanar electrodes were immersed in 3.5 wt% NaCl solution. The neat primer was saturated with electrolyte after 24 h of immersion. Potentiodynamic polarization and electrochemical impedance spectroscopy tests were conducted to measure the electrochemistry under the neat primer.

Galvanic corrosion between the AA2024 substrate and Al-Zn-In bulk alloy was also studied. Each alloy was connected with electrical wires, and then both of them were put side by side with around 2 mm separation and then mounted in epoxy. The galvanic panels were polished and then exposed to NaCl solution or salt spray chamber. Galvanic currents between them were measured during exposure. The experiments are ongoing.