| HENG Zhidan,MA Ying,OU Kaiqi,LIANG Zhilong,LI Zhengqiang,AN Xiaoli,WANG Sheng.Effect of the Relationship between the Ratio of OH−/F− and SiO32− on Micro-arc Oxidation Coatings on Aluminum Alloys[J],53(18):100-115 |
| Effect of the Relationship between the Ratio of OH−/F− and SiO32− on Micro-arc Oxidation Coatings on Aluminum Alloys |
| Received:April 08, 2024 Revised:May 24, 2024 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2024.18.008 |
| KeyWord:A356 aluminum alloy micro-arc oxidation electrolyte NaOH/KF ratio microstructure anti-corrosion |
| Author | Institution |
| HENG Zhidan |
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou , China |
| MA Ying |
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou , China |
| OU Kaiqi |
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou , China |
| LIANG Zhilong |
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou , China |
| LI Zhengqiang |
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou , China |
| AN Xiaoli |
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou , China |
| WANG Sheng |
State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou , China |
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| Abstract: |
| In this study, the micro-arc oxidation coatings are fabricated on A356 aluminum alloy substrates in silicate solutions by varying the ratio of NaOH concentration to KF concentration of the electrolytes. The work aims to explore the mechanism of the effect of the relationship between the ratio of NaOH/KF concentration and Na2SiO3 concentration on the corrosion resistance of the micro-arc oxidation coatings, and find out the desired matching among the electrolytes applied in micro-arc oxidation treatment solution. Eddy current thickness meter, scanning electron microscope (SEM), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD) were employed to determine and characterize the thickness, surface morphology, cross-sectional morphology, elemental composition, phase composition of the coatings, respectively. The corrosion behavior and electrochemical resistance of the coatings were assessed by electrochemical impedance spectroscopy (EIS) and dynamic potential polarization curves with an electrochemical workstation. In addition, the anti-corrosion ability of the coatings in acidic situation was checked with spot test. The results showed that both NaOH and KF were important components in silicate electrolytes for the forming process of micro-arc oxidation coatings. NaOH promoted the formation of a sufficient passivation film on the initial surface of the A356 aluminum alloy substrate and dramatically improved the conductivity of the solution, which in turn created the precondition for the breakdown of the micro-arc oxidation reaction. KF could enhance the discharge effect after the electric breakdown on the substrate surface during the micro-arc oxidation treatment, initiate more discharge sparks, improve the growth rate of the coatings and promote the deposition rate of the coating-forming substances. When the ratios of NaOH/KF in the electrolyte were 0∶1, 1∶2 and 2∶1, respectively, the coatings with good appearance could not be obtained. When the ratio of NaOH/KF was adjusted to 1∶1, the micro-arc oxidation coating with complete coverage, uniform continuity and smoothness was achieved on the substrate. However, due to the shortage of Na2SiO3 as the primary coating-forming agent in the electrolyte, the thickness of obtained coating increased slightly compared with that in the situation when the NaOH/KF ratio was 1:0, but both the size of micropores on the coating surface and the number of large micropores (>3 μm) also increased, which weakened the corrosion resistance of the coatings to some extent. Then, when the NaOH/KF ratio was kept at 1∶1, doubling of the concentration of Na2SiO3 as the primary coating-forming agent promoted the movement of discharge sparks on the substrate surface. At this time, the surface defects of the obtained coating were reduced, the coating thickness was increased, the coating compactness was improved, and the contents of the phases with good anti-corrosion ability, such as α-Al2O3, γ-Al2O3, Mullite, and AlF3 obtained in the coatings were obviously increased, which contributed to improving the anti-corrosion ability of the coatings in the acid corrosive medium and the neutral NaCl corrosive medium. Therefore, during the micro-arc oxidation process on A356 aluminum alloy, when the concentration of Na2SiO3 in the silicate electrolyte is higher than the concentrations of NaOH and KF, and the concentrations of NaOH and KF are similar, the interaction between SiO32−, OH−, and F− in the micro-arc oxidation reaction exhibits a pronounced effect on increasing the coating thickness, decreasing the defects on the coating surface, and promoting the deposition rate of anti-corrosion phases in the coatings. Consequently, this significantly enhances the corrosion resistance of the coating. |
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