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

• Published in: Corrosion Science
• Main Authors: Lin, Jun-Kai, Jeng, Kai-Li, Uan, Jun-Yen
• Other Authors: National Chung Hsing University,Department of Materials Engineering, 國立中興大學材料工程學系, Miao-zhen Luo
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier Ltd. 2011
• Subjects: A. Magnesium; A. Acid solutions; B. Polarization; B. IR spectroscopy; B. Raman spectroscopy; C. Amorphous structures; Carbonic acid
• Online Access: http://hdl.handle.net/11455/43182; https://doi.org/10.1016/j.corsci.2011.07.035

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.