镁合金表面纳米二氧化铈/磷酸盐复合转化膜的制备及其防护性能

添加纳米颗粒可改善金属表面膜层的性能,但目前添加纳米颗粒改善镁合金表面磷化膜性能的报道较少。通过向磷化处理液中添加纳米二氧化铈(nano-CeO_2)颗粒在镁合金表面制备了一层纳米二氧化铈/磷酸盐复合转化膜,采用X射线衍射仪(XRD)、X射线光电子能谱(XPS)、电化学阻抗谱(EIS)和极化曲线等手段研究了添加nano-CeO_2颗粒对膜层成分和防护性能的影响,讨论了nano-CeO_2颗粒的作用机制。结果表明:复合转化膜的相成分为Zn_3(PO_4)_2·4H_2O、Zn_2Mg(PO_4)_2和CeO_2,在单组分磷化膜成分的基础上多出了CeO_2相。在硼酸缓冲溶液中,单组分磷化膜的膜层电阻(R_c)和低频阻抗值(R_(0.01 Hz))分别为561.74 kΩ·cm~2和938.11 kΩ·cm~2,而复合转化膜的R_c和R_(0.01 Hz)分别为2 428.98 kΩ·cm~2和3 985.61 kΩ·cm~2;与此同时,覆盖复合转化膜镁合金的腐蚀电流密度为4.05×10~(-7)A/cm~2,而覆盖单组分磷化膜镁合金的为8.38×10~(-6)A/cm~2,R_c和R_(0.01 Hz)的增大以及J_(corr)的减小说明复合转化膜的防护作用明显优于单组分磷化膜的防护作用。nano-CeO_2颗粒的作用机制主要归因于两个方面:第一,nano-CeO_2颗粒在处理液中的添加有利于磷酸盐晶核的形成;第二,nano-CeO_2颗粒作为一种不溶性固体粒子在膜层中的存在可以强化膜层的物理屏蔽效应。

Preparation and Corrosion Resistance of Nano-CeO2/Phosphate Composite Conversion Coatings on AZ91D Magnesium Alloy

A nano-CeO2/phosphate composite conversion coating was prepared on magnesium alloy AZ91 D by adding nano-CeO2 particles into treatment bath.The effects of nano-CeO2 on phase composition and corrosion resistance of coating were studied by X-ray diffraction (XRD),X-ray photoelectron spectroscopy (XPS),electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization,and the action mechanism of nano-CeO2 was discussed.Results showed that the the composite coating was mainly composed of Zn3 (PO4)2 · 4H2O,Zn2Mg(PO4)2 and CeO2.Compared with the composition of monocomponent phosphate coating,only CeO2 was detected in the composite coating.In borate buffer solution,the corrosion resistance and the low frequency impedance of the monocomponent coating were 561.74 kΩ · cm2 and 938.11 kΩ · cm2,while those of composite coating were 2 428.98 kΩ · cm2 and 3 985.61 kΩ · cm2.Meanwhile,the corrosion current density of composite coating on Mg alloy was 4.05× 10-7 A/em2,higher than 8.38× 10-6 A/cm2 of monocomponent coating.The increase of coating resistance and the decrease of corrosion current density by the addition of nano-CeO2 particles indicated the corrosion resistance of the nano-CeO2/ phosphate composite coating was better than that of the monocomponent phosphate coating.Besides,the action mechanism of nano-CeO2 was mainly contributed to two aspects:firstly,nano-CeO2 particles in solution could promote the formation of phosphate nucleation;secondly,as insoluble solid particles,nano-CeO2 particles in the coating could strengthen the physical barrier function of coating.