Crystallization of a chemical conversion layer that forms on AZ91D magnesium alloy in carbonic acid

This investigation studied the crystallization of a chemical conversion layer that formed on AZ91D Mg alloy in carbonic acid. The layer was an amorphous precursor to a crystalline Mg–Al layered double hydrotalcite, which improved the corrosion resistance of the alloy. The precursor must be treated with a strongly alkaline solution, causing the leaching out of amphoteric Al3+. The leaching step evidently caused crystallization from an amorphous precursor layer to a crystalline coating. Nuclear magnetic resonance analysis indicated that Al3+, which was originally randomly coordinated with surrounding OH− in the precursor, coordinated octahedrally with OH− groups in the crystalline conversion coating.

Figure optionsDownload as PowerPoint slideHighlights
► An amorphous precursor layer formed on Mg–Al–Zn alloy in carbonic acid.
► The amorphous layer has high Al3+ concentration.
► Leaching Al3+ out of precursor layer was the first step of crystallization.
► Al3+ leaching only occurred in a strongly alkaline solution.
► The formula of crystalline conversion coating is Mg4.38Zn0.22Al2(OH)13.192CO3·mH2O.