Formation mechanism of an aluminium-based chemical conversion coating on AZ91D magnesium alloy |
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Authors: | Wei-ping Li Xi-mei Wang Li-qun Zhu Wen Li |
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Institution: | (1) Advanced Materials Laboratory, Department of Materials Science and Engineering, Tsinghua University, Beijing, 100084, China;(2) Atomic-Scale Surface Science Research Center, Yonsei University, Seoul, 120-749, South Korea; |
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Abstract: | An environmentally clean aluminium-based conversion coating on AZ91D magnesium alloy was studied in aluminium nitrate solutions.
The morphology, composition, structure, and formation mechanism of the coating were investigated in detail using scanning
electron microscopy/energy dispersion spectrometry, X-ray diffraction, transmission electron microscopy, and electrochemical
corrosion tests. The results show that the conversion coating is composed of magnesium, aluminium, and oxygen, and shows an
amorphous structure. In the initial stage of coating formation, the grain-like nucleus is composed of Al10O15·xH2O, (Al2O3)5.333, Al2O3, AlO(OH), MgAl2O4, (Mg0.88Al0.12)(Al0.94Mg0.06)2O4, and (Mg0.68Al0.32)(Al0.84Mg0.16)2O4. The conversion coating formed in the 0.01 mol/L aluminium nitrate solution for 15 min can improve the corrosion resistance
of the magnesium alloy greatly. The discussion reveals that the possible formation mechanism for the aluminium-based conversion
coating is the reduction reaction on micro-cathodic sites due to the electrochemically heterogeneous magnesium alloy substrate. |
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