High-Resolution Microscopy of Substrate/Coating Interfacial Regions of a Non-Chromate Organic Coating System on AA2024-T3 after Blister Formation

Abstract

The corrosion of a coated metal system was investigated through the application of novel sample preparation and high-resolution microscopy techniques. An AA 2024-T3 substrate with a trivalent Cr process (TCP) conversion coating, a Pr-based primer, no topcoat and a machine-engraved scribe was tested. Preliminary electrochemical impedance spectroscopy experiments were completed to determine water uptake properties of the coating system. Samples were exposed in ASTM B117 neutral salt-spray until there was a visible blister under the coating, which took 13 days. Thin foil samples of the substrate/coating interface were prepared from exposed locations: (1) inside the scribe, (2) at the end of the blister, (3) near the end of the blister and (4) away from the blister. In addition, a thin foil sample was prepared from (5) an unexposed sample away from the scribe of the same sample type. High-resolution imaging and chemical analysis were performed on each thin foil, which seems to be the first time for such analysis under a coating. There were no detectable inhibitors (Pr or Cr) in the scribe. 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. Finally, there was no detectable corrosion under an intact coating after exposure. However, the caveat for all of this work is that each thin foil represented an extremely localized interfacial region.