超高频脉冲电流作用下纳米Al_(2)O_(3)颗粒对Mg-Gd-Y-Zr合金微弧氧化涂层的影响

Effect of Al_(2)O_(3) Nanoparticles on Microarc Oxidation Coatings Formed on Mg-Gd-Y-Zr Alloy under the Action of Ultra-high Frequency Pulse Current

目的进一步提高Mg-Gd-Y-Zr合金微弧氧化涂层的耐腐蚀性能。方法采用超高频微弧氧化技术在含有Al_(2)O_(3)纳米颗粒的溶液中制备了微弧氧化涂层。利用扫描电子显微镜(FESEM)、能谱仪(EDS)和X射线衍射仪(XRD)对微弧氧化涂层的表面形貌、截面形貌、成分和晶体结构进行分析。利用极化曲线和电化学阻抗谱(EIS)测试了涂层的耐腐蚀性能。结果频率由0.5 kHz提升至20 kHz后,涂层表面放电孔洞面积由0.07~24.4μm^(2)降低至0.08~6.3μm^(2),涂层的孔隙率由6.47%减小至3.35%。Al_(2)O_(3)纳米颗粒的添加使超高频涂层表面形成大量自封闭孔洞结构,进而进一步降低了涂层表面的孔径面积(0.1~4.63μm^(2))和孔隙率(0.97%)。极化试验表明,提高频率至20 kHz,涂层的自腐蚀电流密度由4.7×10^(-6)A/cm^(2)降低至4.7×10^(-7)A/cm^(2),添加Al_(2)O_(3)纳米颗粒,涂层的自腐蚀电流密度进一步降低至1.7×10^(-7)A/cm^(2),表明其耐蚀性能显著提高。阻抗谱显示,20 kHz-Al涂层具有最大的阻抗,说明该工艺可有效提高微弧氧化涂层的耐蚀性能。结论超高频可有效降低放电孔洞尺寸,提高微弧氧化涂层的致密性,改善涂层的耐腐蚀性能。超高频与Al_(2)O_(3)纳米粒子的协同作用使涂层表面形成自封闭孔洞结构,进一步提高微弧氧化涂层的致密性和耐腐蚀性能。

Microarc oxidation(MAO)is a kind of technology that causes partial plasma discharge by applying high voltage to melt metal oxides and form ceramic coatings on light metal(aluminum,magnesium and titanium)and their alloys with the thickness of tens of microns to hundreds of microns,which have the advantages of wear resistance,corrosion resistance,electrical insulation,etc.Due to the inherent plasma discharge phenomenon in the MAO discharge process,the discharge gas and molten metal oxide will be ejected from the inside of the coating.During the ejecting process,defects such as discharge channels and micro-cracks are formed under the rapid quenching effect in the aqueous solution.This defect structure has a high specific surface area and can serve as a penetration channel for the corrosive medium,affecting the overall corrosion resistance of the MAO coating.Therefore,avoiding or sealing these discharge channels is crucial for improving the corrosion resistance of coatings and enhancing the large-scale application of light alloys.In order to further improve the corrosion resistance of Mg-Gd-Y-Zr alloy,this manuscript prepared microarc oxidation coating by ultra-high frequency microarc oxidation technology in the solution containing Al_(2)O_(3) nanoparticles.In the MAO process,Na_(3)PO_(4)(5 g/L),Na_(2)SiO_(3)(5 g/L),KOH(1 g/L),KF(3 g/L)were used as the main solution.The particle size of Al_(2)O_(3) nanoparticles was 20 nm,and the concentration was 8 g/L.The Al_(2)O_(3) nanoparticles were dispersed by sodium dodecyl sulfate(SDS)and then added to the main solution.The experiment was conducted at frequencies of 0.5 kHz and 20 kHz for 600 s at a current density of 4 A/dm^(2).The surface morphology,cross-sectional morphology,composition and crystal structure of the microarc oxidation coating were tested by field-emission scanning electron microscopy(FESEM),energy dispersive spectroscopy(EDS)and X-ray diffraction(XRD).The corrosion resistance of the coating was tested by polarization curve and electrochemical impedance spectroscopy(EIS).The experimental results show that when the frequency increases from 0.5 kHz to 20 kHz,the area of discharge micropores on the coating surface decreased from 0.07-24.4μm^(2) to 0.07^(-6).3μm^(2),the porosity of the coating decreased from 6.47%to 3.35%.The addition of Al_(2)O_(3) nanoparticles further reduced the micropore area of the coating surface(0.1-4.63μm^(2))and porosity(0.97%)by forming a large number of self-sealing pore structures on the ultra-high frequency coating surface.The polarization experiment shows that the corrosion current compactness of the coating reduced from 4.7×10^(-6) A/cm^(2)(0.5 kHz)to 4.7×10^(-7) A/cm^(2)(20 kHz),the corrosion current of the coating is further reduced to 1.7×10^(-7) A/cm^(2) after adding Al_(2)O_(3) nanoparticles,indicating that its corrosion resistance is significantly improved.The EIS shows that the 20 kHz-Al coating had the largest modulus which indicates that this process can effectively improve the corrosion resistance of the microarc oxidation coating.Ultra-high frequency can effectively reduce the size of discharge micropores and porosity,improve the corrosion resistance of the microarc oxidation coating.Due to the synergistic effect of ultra-high frequency and Al_(2)O_(3) nanoparticles,a self sealing microporous structure is formed on the surface of the coating,which further improves the compactness and corrosion resistance of the microarc oxidation coating,providing a new idea for the design and preparation of high-performance microarc oxidation coatings.