| Magnesium and its alloys are known as green engineering materials in the 21st century due to their low density,high specific strength and stiffness,good damping and shock absorption,good thermal conductivity,good electromagnetic shielding effect,excellent machining performance,stable part size and easy recycling.However,their low mechanical properties and corrosion resistance seriously limit their wide engineering application and industrialization.In recent years,the severe plastic deformation technology represented by equal channel angular extrusion（ECAP）has been fully developed in the field of metal material processing.This technology can refine the overall structure of bulk material to submicron or even nanometer level by introducing a large amount of strain without changing the cross-section shape,which can simultaneously improve the strength and toughness of the material.However,at present,the research on the corrosion and protection of ultrafine grained magnesium is not sufficient,especially the research on the surface modification of ultrafine grained magnesium is rare.In this work,coarse-grained pure magnesium is used as raw material,and then ultrafine-grained pure magnesium is obtained by equal channel angular extrusion technology.Both the coarse-grained and ultrafine-grained pure magnesium samples were modified by one-step hydrothermal method in stearic acid ethanol solution and zinc nitrate+stearic acid ethanol solution at different reaction time and temperature to form hydrophobic coatings.Then the morphology and composition of the prepared samples were analyzed by scanning electron microscopy and Fourier infrared spectroscopy.The hydrophobicity of the samples was tested by contact angle tester.The corrosion resistance of hydrothermally treated samples was tested in3.5wt.%NaCl solution by open circuit potential,electrochemical impedance spectroscopy and potentiodynamic polarization curve.The concrete researches are as follows:1.For coarse-grained pure magnesium matrix,the grain size is large and irregular,and the surface is hydrophilic.Its impedance in 3.5wt.%NaCl solution is only 717.6 ohm·cm2,and the corrosion current density is 3.2×10-5 A·cm-2,which implies its poor corrosion resistance.After being treated with stearic acid ethanol solution under different hydrothermal conditions,hydrophobic coatings were successfully obtained on the surface of coarse-grained pure magnesium.Most of the coatings exhibit irregular micro/nano hierarchic structures,which can capture some air pockets to make the coatings have hydrophobicity.The results of electrochemical test show that the corrosion resistance of the hydrophobic coating treated in stearic acid ethanol solution does not follow the simple rule that better corrosion resistance will be found for samples treated with the extension of reaction time or the increase of reaction temperature,but there exists a critical point of reaction temperature and reaction time.After hydrothermal treatment with stearic acid ethanol solution at 130℃for 2 hours,a hydrophobic coating with a contact angle of 116.39°can be formed on the surface of coarse-grained pure magnesium.The AC impedance of this coating in 3.5 wt.%NaCl solution can reach19465 ohm·cm2;and its corrosion current density is as low as 1.6×10-6 A·cm-2,which is only one twentieth of the matrix,showing excellent corrosion resistance.When coarse-grained pure magnesium is hydrothermal treated in zinc nitrate stearic acid ethanol solution,thanks to the formation of dense and regular granular surface morphology,the contact angle of the coating is generally higher than that of the coating treated in stearic acid ethanol solution under the same conditions.The highest value can reach 154.849°,showing superhydrophobicity.However,the results of electrochemical test show that the corrosion resistance of the coatings treated in zinc nitrate stearic acid ethanol solution is generally less than that of the samples treated in stearic acid ethanol solution under the same conditions.Based on the results of coarse-grained pure magnesium treated in two different hydrothermal solutions,it can be considered that the contact angle of the surface treated in the same solution has a certain positive correlation with the corrosion resistance,but for the surfaces treated in different solutions,there is no obvious simple relationship between the contact angle and the corrosion resistance.The contact angle of the samples treated in zinc nitrate stearic acid ethanol solution is generally larger,but the corrosion resistance is lower than that of the samples treated with stearic acid ethanol solution.2.For ultrafine grained pure magnesium,compared with non-extruded pure magnesium,equal channel angular extrusion changes their microstructures.The grains of ECAP-LD ultrafine grained pure magnesium samples cut along the extrusion direction are elongated and broken along the extrusion direction,showing an obvious streamline along the extrusion direction,and the average distance between grains is 5.92 microns;The ECAP-TD ultrafine grained pure magnesium samples cut along the transvers direction show a uniform structure similar to fine equiaxed crystals.In general,after 16 passes of ECAP processing,the grain size of the initial pure magnesium has been greatly reduced,from>100 microns for the initial coarse-grained state to about 5 microns.The corrosion resistance of ultrafine grained pure magnesium matrix in 3.5wt.%NaCl solution is higher than that of coarse-grained pure magnesium.After being treated in stearic acid ethanol solution under different hydrothermal conditions,hydrophobic coatings were successfully obtained on the surfaces of two kinds of ultrafine grained pure magnesium samples.Comparatively speaking,thanks to the uniform equiaxed structure,the hydrophobic coating formed on the surface of ECAP-TD sample has uniform and fine flower like or granular micro/nano hierarchic structure,which can capture a large number of air pockets and result in good hydrophobicity.The coating morphology of ECAP-LD sample is less regular than that of ECAP-TD sample,and most of them are distributed unevenly in mastoid structure.The electrochemical test results show that there are also critical points of reaction temperature and reaction time for ultrafine grained samples.Below the critical point,with the increase of reaction temperature or time,the corrosion resistance of ultrafine grained samples increases correspondingly due to the increase of coating thickness,showing higher impedance and lower corrosion current density.However,when it is higher than the critical point,the further increase of reaction temperature or the extension of reaction time will reduce the corrosion resistance of the sample.The reason is that the surface activity of the sample is too high at high temperature or under a long reaction time,resulting in a large number of cracks in the generated coating,which weakens the protective effect of the coating.The impedance of ECAP-TD samples treated with stearic acid ethanol solution at 130℃for 2 hours is as high as 33863 ohm·cm2 and its corrosion current density is 5.0×10-6 A·cm-2.The long-term study of electrochemical impedance spectrum shows that the impedance of ultrafine grained samples decreases significantly with the extension of immersion time in the first day.However,with the further extension of immersion time,the impedance of the ECAPed samples keep stable in a certain range.After 5 days of immersion,the impedance of ECAP-TD sample is still 5431 ohm·cm2,which is higher than the initial impedance of the corresponding matrix,indicating that the sample has a good long-term corrosion resistance.The regularity of ECAP-LD samples is similar to that of ECAP-TD samples,but their properties are mostly slightly lower than those of ECAP-TD samples treated under the same conditions,indicating that the anisotropy after ECAP processing will have a certain impact on the properties of surface coatings for samples cut in different directions. |
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