| Magnesium alloy is a magnesium-based light structure material with remarkable performance.It has various excellent properties such as light mass,small density,resistance to vibration and magnetism,as well as strong plasticity and easy processing,which enables it to be applied in multiple fields such as aerospace,machinery manufacturing,and vehicle engineering etc.However,its relatively high activeness makes it easy to form loose thin magnesium oxide coating,and its equilibrium potential is so low that it is prone to lead to galvanic corrosion;its resistance to corrosion is also poor;all those shortcomings have limited its application.To improve corrosion resistance of magnesium alloy,chemical etching was firstly performed to enhance its surface roughness;on the basis of which,vanadium/zirconate conversion coating was prepared;hydrophobization was then performed for the coating using stearic acid ethanol solution to obtain hydrophobic coating with excellent corrosion resistance.In this paper,single factor experiment was regarded as the reference condition;orthogonal experiments were then carried out for optimization.During chemical etching contact angle was used as a criterion to obtain the optimal etching effect,which was seen when using 0.10mol/LHF etching solution at temperature 15?for a duration of90s.Copper plating was performed under such etching conditions and the best effect was seen when temperature was 10?,current density 0.25A/dm2 and electroplating time 15min.After etching and copper plating treatment,the magnesium alloy obtained certain roughness.Hydrophobization was then performed and the contact angle could reach 141.9°,showing that the preliminary goal of hydrophobization was already obtained.However,the coating surface after copper plating was relatively poor needing improvement.To improve corrosion resistance,copper plating coating was replaced by vanadium/zirconate conversion coating.By comparing salt-spray resistance duration and corrosion current value,the final formula is chosen as sodium metavanadate 4g/L,hexafluorozirconic acid 4g/L,nickel nitrate 1g/L,and sodium molybdate 4g/L.The optimal conversion effect was seen with duration 45min and temperature 55?.The conversion coating obtained under such condition is superior to that by copper plating in quality,as shown by an enhanced corrosion resistance and salt-spray resistance duration of 8.5h.XRD analysis showed that the conversion coating mainly comprises of Zr?OH?4,V2O5,VO2,and MgF2.SEM and EDS indicated that the conversion coating consists of two layers–the bottom layer is cracked shading mainly in V element,and the top layer is clustered compound mainly in Zr element.At last,hydrophobization was performed for the magnesium alloy with vanadium/zirconate conversion coating using electrochemical process.Taking contact angle and salt-spray resistance duration as representation means,the optimal treatment effect was obtained when temperature was25?,current density 7mA/dm2,and duration 10min.The optimal hydrophobization solution comprises of 0.015mol/L stearic acid ethanol solution,200mL/L water,and3.5g/L cerous nitrate.Through SEM it is observed that when rare earth element is added,the coating shape is changed–the micro/nano convex structure has changed to lamellar structure and finally formed nano-scale lamellar structure,thus presenting a coating possessing both hydrophobicity and corrosion resistance.The AZ91D magnesium alloy corrosion-resistant hydrophobic coating's contact angle could reach 153.5°and its salt-spray resistance duration could reach 105h.The minimum corrosion current of the coating could reach 7.382×10-7A,two orders of magnitudes lower compared to that of the magnesium alloy base which is 9.778×10-5A,indicating that the prepared hydrophobic coating has sound corrosion resistance. |
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