Duplex-layered manganese phosphate conversion coating on AZ31 Mg alloy and its initial formation mechanism |
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| Affiliation: | 1. ENCOMAT, University of Vigo, E.E.I., Campus Universitario, 36310 Vigo, Spain;2. AIMEN Technological Centre, O Porriño, Spain;1. School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266510, China;2. Environmental Corrosion Center, Institute of Metals Research, Chinese Academy of Sciences, Shenyang 110016, China;3. School of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400050, China;1. Department of Mechanical Engineering, PO Box 755905, University of Alaska Fairbanks, Fairbanks, AK 99775, USA;2. College of Mechanical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China;1. Corrosion and protection Laboratory, Key Laboratory of Superlight Materials and Surface Technology (Harbin Engineering University), Ministry of Education, Nantong ST 145, Harbin, 150001, China;2. Corrosion and Protection Division, Shenyang National Laboratory for Materials Science, Northeastern University, 3-11 Wenhua RD, Shenyang, 110819, China;3. Corrosion and Protection Division, Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang, 110016, China;4. School of Engineering, RMIT University, Carlton, VIC, 3053, Australia |
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| Abstract: | A duplex-layered phosphate conversion coating was obtained on AZ31 Mg alloy by substituting NaF bath with a citric bath. The morphology, composition and corrosion resistance of the coating were investigated using SEM, EDS, SPM and electrochemical methods. A three-stage mechanism for initial formation of the coating was proposed: Dissolution of the loose oxide film and deposition of Mg3(PO4)2 and AlPO4, formation of a composite intermediate layer of Mg3(PO4)2, AlPO4 and Mg(OH)2, and deposition of manganese phosphate nuclei followed by growth and lamination of the nuclei. The nuclei preferentially deposit at the Al–Mn phase surface and near the grain boundary. |
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| Keywords: | A Magnesium A Acid solution B SEM B Polarization C Passive films |
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