Scanning transmission electron microscopy analysis of substrate/coating interfacial regions on AA2024-T3 after blister formation

The corrosion of AA2024-T3 with a trivalent Cr process (TCP) conversion coating and Pr-based primer was investigated through the application of novel sample preparation and high-magnification scanning transmission electron microscopy (STEM) techniques. Electrochemical impedance spectroscopy was utilized to determine water uptake properties of the coating system. A method to extract thin foil samples of substrate/coating interfaces was demonstrated using a focused ion beam approach. Thin foils were prepared from locations around a blister that formed at a scribe edge after a 13-day exposure in ASTM B117. Imaging and chemical analysis were performed on each thin foil using STEM and energy dispersive spectroscopy. An unexposed region of the same sample type was also studied. Active corrosion inhibitors (Pr and Cr) were not detected in the scribe after exposure. Mg was enriched on the substrate surface and in the TCP region of exposed and unexposed samples, likely an artifact of the coating preparation process. Additionally, chloride was found deeper in the primer and TCP regions at locations closer to the blister. Furthermore, there was no detectable corrosion under an intact coating after exposure for this coating system, with the caveat that each thin foil represented an extremely localized interfacial region.