Study On Construction And Corrosion Resistance Of Hydrophobic Composite Film On Magnesium Alloy Surface

Magnesium alloy is the lightest metal structural material,with specific strength,high specific rigidity,good thermal conductivity and electromagnetic shielding performance.Compared with aluminum alloy and steel,it has a lower modulus of elasticity.Under the same stress conditions,it can consume more deformation work,has noise reduction and vibration reduction functions,and can withstand greater shock and vibration loads.It is the "green engineering material of the 21st century".However,magnesium has high electrochemical activity,which is prone to electrochemical corrosion,loss of original mechanical strength,and reduced service life.Improving the corrosion resistance of magnesium alloys is of great significance to the further promotion of their large-scale use.As a layered two-dimensional material,layered double hydroxide(LDH)can capture chloride ions in the environment by its own anion exchange;ZIF-8 as a zeolite imidazole ester framework material has the characteristics of zeolite molecular sieves Excellent thermal stability and good chemical stability,and rely on its own porous characteristics to trap air to form an air film,which slows down the contact area with the external corrosive environment;the construction of LDH and ZIF-8 composite film layer.In turn,it can achieve a significant corrosion protection effect on the base material.In this paper,by changing the added urea content during the hydrothermal process of AZ91D magnesium alloy,a protective film with controllable microstructure such as Mg-A1 LDH morphology,size,crystallinity,porosity,cracks,etc.was grown on the surface of the magnesium matrix in situ..Simultaneously.The LDH film grown by adding nitrate was prepared.Through X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),Fourier infrared spectroscopy(FTIR),the morphology and structure of the film and its growth model were analyzed,and the polarization curve test and electrochemical impedance spectroscopy test were used.The electrochemical properties of the film.In addition,the core-shell structure composite powder of ZIF-8 coated Zn-Al LDH micro-particles(Zn-Al LDH@ZIF-8)was prepared by the co-precipitation method,and it was coated on the original by polyvinylidene fluoride(PVDF).The surface of the Mg-Al LDH film is hydrophobically modified by stearic acid to form a multi-level hydrophobic composite protective film.The microscopic morphology and anti-corrosion properties of the composite film are studied.The main conclusions are as follows:By changing the urea content,the pH value of the hydrothermal environment can be adjusted(9.4,10.4,11.2,11.4),and Mg-Al LDH films with different tissue morphologies can be obtained.As the PH value increases from 9.4 to 11.2,the half-height widths of the LDH characteristic peaks of(003)and(006)crystal planes decrease,indicating that the crystallinity of LDH gradually improves,while the thickness of the LDH film and the thickness of the LDH lamella gradually increase.The film porosity and cracks are reduced.At PH 11.2,the thickness of LDH is 118 nm and the porosity of the film is 48.17%.However,when the pH exceeds 11.2,a large number of short and deep micro-cracks appear on the surface of the film.Selecting PH11.2 for additive growth of LDH film has better crystallinity than LDH film prepared by in-situ PH11.2,but the crack width of LDH film reaches 1μm.Through the analysis of the external additive growth model,a large amount of aluminum ions are hydrolyzed in the initial stage,causing the pH in the hydrothermal environment to decrease to acid 3.5.The violent overflow of hydrogen leads to the formation of a large number of micro-cracks in the film.The in-situ growth only relies on a small amount of the magnesium substrate itself.The hydrolysis of Mg17Al12 provides Al3+ required for LDH nucleation and growth,with mild reaction conditions and fewer cracks.Through the polarization curve test,the corrosion current density(icorr)of the LDH film grown on PH 11.2 is increased from 3.475×10-7 A cm-2 to 1.428x10-7 A cm-2 compared with the magnesium alloy AZ9 1D.The self-corrosion potential(Ecorr)The magnesium alloy itself moves forward from-1.598 V to-1.572 V.The icorr of the externally grown LDH film is 2.704×10-7 A cm-2,and the Ecorr is-1.552 V.Other LDH films grown at different pH values Compared with the magnesium alloy,the layer icorr has decreased,indicating that the in-situ LDH film layer has made a contribution to the corrosion resistance of the magnesium alloy.In the short-term 3.5wt%sodium chloride solution immersion test,the impedance spectra of different samples were compared,PHI 1.2 showed the best charge transfer resistance,and in the 3.5wt%sodium chloride solution long-term immersion test,the electric double layer The capacitance value keeps fluctuating in the range of 1 for a long time,which has an inhibitory effect on the expansion of the corroded area,and exhibits excellent corrosion resistance.However,after 7 hours,the increase in charge transfer resistance means the formation of corrosion products.Therefore,only building the Mg-Al LDH film on the magnesium alloy substrate cannot achieve long-term effective corrosion protection of the magnesium alloy.The Zn-Al LDH powder with a size of about 1 micron was prepared by the co-precipitation method,and used as a zinc source and template for the growth of ZIF-8 to obtain a core-shell powder of ZIF-8 coated with Zn-Al LDH with a size of about 150 nm.The LDH@ZIF-8 core-shell composite powder was spin-coated on the surface of the first layer of PHI 1.2 in-situ grown LDH film using PVDF as a binder.By comparing XRD,FTIR,SEM,TEM analysis,the morphology and structure of the composite film were observed and analyzed.When only PVDF is spin-coated,the icorr increases from 1.428x 10-7 A cm-2 to 1.039×10-8 A cm-2 due to the hole-filling effect of PVDF.After the introduction of LDH@ZIF-8 composite powder,due to the hydrophobic effect of the ZIF-8 porous material,the hydrophobic angle was increased to 83.2°,and the icorr was further increased to 6.546×10-9 A cm-2;LDH@ZIF-8 was modified by stearic acid.The contact angle reaches 105.6°,which meets the requirements of hydrophobic coating(CA>90°),and the icorr reaches 1.205×10-10 A cm-2.After being immersed in 3.5%NaCl solution for a long time,in the first 13 days,relying on the anion exchange between the hydrophobicity of ZIF-8 and LDH,the excellent anti-corrosion performance is maintained.