| SHAO Zhong-cai,DAI Shi-hang,WEI Shou-qiang.Study on Zinc-Phosphate Chemical Conversion Coating on Magnesium Alloy[J],47(4):133-139 |
| Study on Zinc-Phosphate Chemical Conversion Coating on Magnesium Alloy |
| Received:August 12, 2017 Revised:April 20, 2018 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2018.04.020 |
| KeyWord:magnesium alloy conversion coating phosphate corrosion resistance temperature electrochemistry |
| Author | Institution |
| SHAO Zhong-cai |
School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang , China |
| DAI Shi-hang |
School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang , China |
| WEI Shou-qiang |
School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang , China |
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| Abstract: |
| The work aims to optimize preparation of zinc-phosphate chemical conversion coating on magnesium alloy. The influence of main salt, temperature and additives on the zinc-phosphate chemical conversion coating on magnesium alloy was studied and the zinc-phosphate chemical conversion coating on magnesium alloy was prepared. The corrosion resistance of the chemical conversion coatings was evaluated by drop test and electrochemical test. The coatings were characterized by laser scanning confocal microscopy, scanning electron microscopy (SEM) and X ray diffraction (XRD). The concentration of Zinc Oxide, phosphoric acid, sodium fluoride and sodium tartrate in the conversion solution had a great influence on the corrosion resistance and thickness of the conversion coating. In a certain range of concentration, the corrosion resistance of the conversion film increased with the concentration. As temperature increased, the corrosion resistance of the conversion coating increased at first and then decreased, and the dropping time reached 50 ℃ at 59 s, the coating was relatively compact and the gap was smaller. In the selected additive, EDTA could obviously improve the corrosion resistance of the coating. The coating was homogeneous and compact, and the corrosion potential of the conversion coating with 0.3 g/L EDTA was 0.05 V higher than that without additives at positive shift and dropping time increased to 68 s. The main components of magnesium alloy zinc-phosphate conversion coating were: Zn3(PO4)2, Zn2Mg(PO4)2, AlPO4 and Al12Mg17. The electrochemical impedance radius of the conversion coating was large, the corrosion current density was low (2.594×106 A/cm2), the corrosion potential was shifted obviously, and the corrosion resistance was better and the roughness of the conversion coating was small (2.396 μm) and smooth. The optimum formula of zinc-phosphate conversion on magnesium alloy is zinc oxide 2 g/L, phosphoric acid 12 g/L, sodium fluoride 1 g/L, sodium tartrate 4 g/L, conversion temperature at 50 ℃ and conversion time of 20 min. The corrosion resistance of the conversion solution can be obviously improved by adding EDTA. |
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