| A superhydrophobic phosphate/fatty-acid salt compound coating was fabricated via a phosphate chemical conversion treatment of AZ91D magnesium alloy and subsequent electrodeposition. The surface morphology of the resultant surfaces was characterized by scanning electron microscopy(SEM) and atomic force microscope(AFM). The cross section morphology of the resultant surfaces was characterized by SEM. The composition of the fatty acid salt layer was studied by energy dispersive spectrometer(EDS) and fourier transform infrared spectrometer(FTIR spectrometer). The corrosion resistance was studied by electrochemical workstation and immersion test. The superhydrophobic property of the compound coating was characterized by water contact angle(AC), slide angle, advancing angle and receding angle. The stability on superhydrophobic property of the compound coatings was characterized by measuring the contact angles of solution with different pH value. Finally, we briefly introduce the extension experiment, which is combined with the method of electrodeposition and MEMS, preparing a composite coating layer on the surface of AZ91D magnesium alloy. The layer has superhydrophobic properties in certain areas.The superhydrophobic compound coating consists of two layers. The first one is a phosphate conversion layer which is uniform in texture and partially contains intrinsic microcracks. With these microcracks, the phosphate conversion layer can not protect the magnesium alloys from being corroded even though itself has corrosion resistance to a certain extent. The second layer is a superhydrophobic fatty acid salt layer with lager numbers of microspheres which are covered by numerous nanostrips or nanotubers. The superhydrophobic fatty acid salt layer preferentially grows in the microcrack regions in the phosphate conversion layer, then it covers whole phosphate conversion layer surface finally.We synthetically analyzed the results of EDS and FTIR spectrometer. The second layer, named as superhydrophobic fatty acid salt layer, is mainly composed of cerium fatty acid salt. We compared the polarization curves of magnesium alloy sample covered with compound coating,magnesium alloy sample covered with phosphate conversion layer and bare magnesium alloy sample, and we found the corrosion resistance of magnesium alloy sample covered with phosphate conversion layer is better than bare magnesium alloy sample’s, but the corrosion resistance of magnesium alloy sample covered with compound coating is the best. The superhydrophobic phosphate/fatty-acid salt compound coating has excellent superhydrophobic property. The water contact angle is up to 160 degrees, and the water slide angle approximates to 2 degrees, and the contact angle hysteresis approximates to 5 degrees. The superhydrophobic compound coating has good stability in 3.5% sodium chloride solution, and the stability remains good to hydrochloric acid solution with pH value from 2 to 6. The superhydrophobic compound coating is less stable in NaOH solution with pH ranging from 8 to 13, but the superhydrophobic property can still retain for a certain time.The fatty acid salt layer can fill effectively the partial microholes and microcracks on the phosphate conversion layer, and the phosphate conversion layer can strengthen the fatty acid salt layer just as a kind of skeleton to the latter one. There will be a air film on the surface of magnesium alloy sample covered by superhydrophobic phosphate/fatty-acid salt compound coating. The air film can separate the sample from electrolyte. Even if the superhydrophobic surface is broken, the remaining fatty acid salt has filled the partly intrinsic microcracks,and the new compound coating can still separate the sample from the electrolyte. So, we can effectively enhance the corrosion resistance of magnesium alloys via covering this superhydrophobic phosphate/fatty-acid salt compound coating on the surface of the magnesium alloy. |
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