Development of Permanganate-Based Coatings on Aluminum Alloy 2024-T3

Abstract

Abstract Permanganate coatings show promise as replacements for chromate conversion coatings on aluminum alloys used in the aerospace industry. The development of permanganate conversion coatings on AA2024-T3 (UNS A92024) from acid solution was followed for up to 240 s using scanning electron microscopy (SEM), x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and Rutherford backscat-tering spectroscopy (RBS). The permanganate coatings have a manganese dioxide (MnO2)-rich exterior surface covering a mixed Al-Mn oxide with a composition close to MnO2·Al2O3. RBS and AES depth profiles indicate that Mn is distributed throughout the oxide over the metal matrix, while XPS confirmed that Mn(IV) and Al(III) were both present in the coating. The onset of coating formation from permanganate solutions is rapid (<30 s) with the maximum thickness (50 nm to 70 nm) being reached after approximately 60 s. The limit on coating thickness may be due to the passivation of boehmite (AlOOH) by MnO2. In...