Synthesis Of Oxide Films On AZ91Mg Alloys And Their Corrosion-resistance

Magnesium and its alloy are the lightest metal materials，they have attractedgreat attention due to their low density, high strength-to-weight ratio, good damping,strong thermal conductivity, good electrical conductivity, electromagnetic shielding,processed and casting easy, and good recycling properties. They have been usedwidely in many fields, such as in automotive, aerospace electronics,3C anddegradable biomaterials. But the high chemical reactivity results in a poor corrosionresistance, which limited applications gravely. In this paper, sol-gel method, liquiddeposition method and hydrothermal in-situ method are employed to coat MgFe₂O₄、NiO and Mg（OH）₂films on AZ91Mg alloy, respectively. The film thickness,morphology, structure of the phases, binding force and corrosion resistance werestudied by X-ray diffraction, SEM, EDX, potentiodynamic polarization andlectrochemical impedance.MgFe₂O₄films were prepared on the surface of AZ91magnesium alloy by thesol-gel method. The different MgFe₂O₄films prepared by orthogonal design, in whichthe number of sol-gel coating layer, temperature, constant time, molar ratio ofn(Mg²⁺)/n(Fe³⁺) were the factors. The results showed that the morphologies and filmthickness are decided by the layers of film, the binding force are decided by sintertemperature. The optimization condition was affirmed and verified by the tests. Theresults indicated that:（1） the order of the degree of various factors impact oncorrosion current density is: the number of coating layer> constant temperature>n(Mg²⁺)/n(Fe³⁺)> constant time; The parameters of optimization condition: coatingnumber is1, constant temperature is400℃, n(Mg²⁺)/n(Fe³⁺)=0.7/2, constant time is5h; It was found that the corrosion current density of the optimization sampledecreased to about1magnitude order, the corrosion potential shifted690mV toforward direction. It can be conclude that the corrosion resistance of AZ91magnesium alloy was greatly improved.Dense nano-NiO film was prepared on the surface of AZ91magnesium alloy byliquid phase deposition method with urea and Ni （NO3）₂6H2O as raw materials. Theeffect of sinter temperature, deposition time and constant temperature time on morphology and corrosion resistance of NiO film was researched. The resultsindicated that the film thickness increased with deposition time prolonging until5h.And the increasing sinter temperature the corrosion resistance first increased and thendecreased, the sample treated at400℃showsthe best corrosion property. The longconstant temperature time increase the corrosion property, and the3h-4h are almostsame. The corrosion of NiO films synthesized under different condition aredetermined mostly by film thickness, while the thickness are decided by NiO contentand binding force. The parameters of optimization condition: deposited for5h,sintered at400℃for3h. The corrosion current density of the optimization NiO filmdecreased to about1magnitude order, the corrosion potential shifted706mV toforward direction. The corrosion resistance of AZ91magnesium alloy was greatlyimproved by NiO film.aerosol OT was used as surfactant to prepared Mg（OH）₂film via hydrothermalin-situ method. The effect of pH, time, temperature on the film composing,morphology, film thickness. It was found that the obtained films have excellentbanding force with AZ91substrate. The corrosion properties are determined by pH,film thickness and density, the corrosion sequence is pH=10>9>12>11. underdifferent treated time the corrosion sequence is3h>4h>6h>8h, under differenttreated temperature the corrosion sequence is120℃>110℃>140℃>160℃. Theparameters of optimization condition pH10、120℃for3h. The corrosion currentdensity of the optimization Mg（OH）₂film decreased to about1magnitude order, thecorrosion potential shifted720mV to forward direction. MgO film was obtainedthough sintered the optimization Mg（OH）₂film at450℃for5h. The corrosioncurrent density of the optimization NiO film decreased to about1magnitude order,the corrosion potential shifted486mV to forward direction, which means that MgOfilm also shows good corrosion resistance.