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

Abstract 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 Al 3+ . The leaching step evidently caused crystallization from an amorphous precursor layer to a crystalline coating. Nuclear magnetic resonance analysis indicated that Al 3+ , which was originally randomly coordinated with surrounding OH − in the precursor, coordinated octahedrally with OH − groups in the crystalline conversion coating.