Cross-sectional characterization of the conversion layer formed on AA2024-T3 by a lithium-leaching coating

A. Kosari, P. Visser, F. Tichelaar, S. Eswara, J. N. Audinot, T. Wirtz, H. Zandbergen, H. Terryn, J. M.C. Mol

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Abstract

This work focuses on the cross-sectional characterization of the protective conversion layer formed on AA2024-T3 by lithium-leaching from a polyurethane coating in a corrosive environment. The layer shows a multi-layered arrangement comprising nanoscopic local phases. Transmission electron microscopy (TEM) and complementary high-resolution secondary ion mass spectroscopy (SIMS) were employed to observe the cross-sections of the entire layer formed at different locations of a 1-mm-wide scribe, in terms of morphology, structure and chemical composition. The conversion layer was comprised of two ubiquitous sublayers; a thin dense layer (i.e. 150 nm) adjacent the alloy substrate and a porous layer. The former represents an amorphous lithium-containing pseudoboehmite phase, Li-pseudoboehmite, whereas the latter is composed of amorphous and crystalline products; an outer columnar layer merely seen on the peripheral region is also crystalline. Through a sandwich structure and the d(003) basal spacing, the crystalline phases were identified as Li-Al layered double hydroxide. Although lithium was found uniformly spread within different regions, the local phases with no/low concentration of lithium were revealed with energy filtered TEM and confirmed with SIMS analysis.

Original languageEnglish
Article number145665
Number of pages8
JournalApplied Surface Science
Volume512
DOIs
Publication statusPublished - 2020

Keywords

  • AA2024-T3
  • Energy Filtered TEM
  • High Resolution TEM
  • Layered double hydroxide
  • Leachable lithium carbonate
  • SIMS

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