Growth of permanganate conversion coating on 2024-Al alloy

The growth of permanganate conversion coating on aluminum 2024-T3 alloy has been studied by characterizing, with scanning Auger microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy, the coatings formed by immersion of the alloy in the coating bath (containing KMnO4 and Na2B4O7, pH 9.1) for different periods of time and at different temperatures. At room temperature, during the first 1–5 min of immersion, MnO2 deposits are formed only on the second-phase intermetallic particles (of Al–Cu–Mg and Al–Cu–Fe–Mn types), but the coating starts to develop on the Al matrix surface after 5–10 min. The coating slows down and stops after about 150 min, with a thinner deposit over the alloy matrix. The process is accelerated at higher temperatures, for example at 68 °C it self-limits after about 3 min. The electrochemical growth process appears to follow that established for the chromate conversion coatings, although XPS does not detect significant MnO4− incorporation into the permanganate coatings.