A Novel Organic Conversion Coating based on N-Benzoyl-N-Phenylhydroxylamine Chemistry for the Corrosion Protection of AA2024-T3

This work demonstrates a novel organic conversion coating treatment on an AA2024-T3 substrate. The process is based on simple immersion in aqueous solutions containing an organic molecule, N-Benzoyl-N-phenylhydroxylamine (BPHA), after a hydrothermal oxide has been formed on the alloy surface. Potentiodynamic polarization results indicate that BPHA conversion coating outperforms coatings formed in the same concentration of other corrosion inhibitors, including NaVO3, Na2SiO3, CeCl3, and benzotriazole. BPHA coating also displays strong and persistent corrosion inhibition under ASTM B117 salt spray exposure conditions and static immersion testing. The cross-section of BPHA-coated substrate is prepared by focus ion beam-scanning (FIB), and analyzed by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) coupled with energy dispersive spectroscopy (EDS), which show a thick surface film with a three-layer structure. Raman spectroscopy and XPS are used to examine the surface chemistry of BPHA-coated samples, both of which suggest the formation of a BPHA surface film, possibly stabilized by the keto-to-iminol conversion that made the BPHA molecule strongly nucleophilic. The formation of BPHA-metal oxide complex seems to be closely related to the excellent corrosion resistance of BPHA coating.

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