CaP Corrosion-resistant Coatings on AZ31 Magnesium Alloy Regulated by Alkaline-thermal Pretreatment Solutions
FU Hailuo1,*, ZHU Chenghao1, WEI Dali2, LIU Shuyang1, JIAO Yihan1, ZHOU Rui1, LIN Qing1, WANG Jianquan1
1 School of Materials Engineering, Jinling Institute of Technology, Nanjing 211169, China 2 School of Materials Science and Engineering, Southeast University, Nanjing 211189, China
Abstract: To enhance the corrosion resistance of magnesium alloys, AZ31 magnesium alloy was used as the test substrate, and a two-step method was employed to prepare a calcium-phosphate coating with a micro-nano structure on its surface. First, hydrothermal reactions were conducted using Na2CO3, NaOH, and NaHCO3 as alkali hydrothermal solutions to obtain the pretreatment coatings. Subsequently, the pretreated magne-sium alloy was placed in a mixed solution of EDTA-2NaCa and KH2PO4 for another hydrothermal reaction to prepare the CaP coating. The study focuses on the influence of rules and principles of alkaline solutions in the first-step alkali heat treatment on the CaP film layer. The results show that after the two-step treatment, a CaP film layer can be formed on the surface of AZ31 magnesium alloy. However, the composition, microstructure, and corrosion resistance of the film vary significantly due to the influence of the composition and pH value of the alkaline heat solutions. The pretreatment with NaOH forms a dense Mg(OH)2 pretreatment film, which promotes the formation of a subsequent dense hydroxyapatite (HA) layer, resulting in the best corrosion resistance. The corrosion potential (Ecorr) shifts positively by ~1.5 V compared to that of AZ31 magnesium alloy, and the corrosion current density (Icorr) decreases by 5 orders of magnitude. The pretreatment with Na2CO3 gene-rates a porous film containing Mg(OH)2 and MgCO3. Although the subsequent CaP film layer is mainly composed of HA, its compactness decreases, and its corrosion resistance is inferior. The pretreatment with NaHCO3 produces a loose film with a complex composition and structure, causing the subsequent CaP film to contain more magnesium calcium phosphate. The crystals are irregular, with large pores, and the corrosion resistance is the worst.
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2026, Vol. 40 
