| Density of magnesium is 1.74 g/cm3, which is 64%, 25%, and 20% lighterthan those of aluminum, zinc and steel respectively. Magnesium alloys are kindof the lightest structural metal alloys, which have incomparable ratio of strengthto mass. Because magnesium alloy has high specific stiffness, high heat andelectrical conductivity, good damping and shock absorption, excellentelectromagnetism shield capability, easy machining, good recovery capability,magnesium and its alloys gain increasing attention in automotive,communication apparatus and aerospace applications. Therefore, it is consideredthat magnesium alloys are raw engineering materials which are the most worthy,exploited and applied metal materials in 21st century.Although magnesium has bright applied future, the commercial applicationsof magnesium alloys have been limited mainly because of its poor corrosionresistance properties which cannot be achieved only by using cost intensivemagnesium alloys. The coating is thinner without satisfied performancesfabricated by conventional methods, which are always introduce poisonousmaterials and pollute environment. The ceramic coatings that grow on originalsurface of substrates are alloyed combination with substrates and it is easy tocontrol the size of pore. Therefore, it is an efficient way to solve worse corrosiveproperties of magnesium by MAO. So a series of dielectric coatings on thesurface of substrates have been fabricated by MAO, the formation process ofceramic coatings has been discussed , technical parameters have been studied inthis dissertation and then the optimized technical parameters have been gained.The series of AZ91 are widely used in commercial application, which havebright future. It is not systematic to study how to improve the corrosive propertyof AZ91 by MAO at home so it is meaningful to make a research on this aspect.In this article, AZ91 that is used to make 4GE bonnet of Hongqi saloon car wasstudied. It will provide theory reference for magnesium bonnet commercialproduction by studying corrosive resistance property, tensile property andinfluences of technical parameters on properties. The mechanism of MAOprocess was also mentioned.1 The mechanism of MAO process discussionIt was obvious that the reaction had different period with the increase ofvoltage , the reaction rate decreased until it came to the balance of dissolvingand formation. During first period, there was amorphous dielectric filmsfabricated around the substrates with high insulating performance . When thevoltage got up to a critical value, the film was broken down in the relativelythinner by voltage. The phenomenon was observed that there were sparklesaround the substrates. During the reaction, pores were produced by current andfilled with materials from outside and sinter of metal substrates in hightemperature immediately through the discharging channel.2 Influences of technical parameters on the fabrication of film2.1 Influence of Current density on the fabrication of filmThe thickness was increase when current density increasing, but the quality ofsurface decreased. It was because the dielectric resistance increased with thethickness increasing. In order to push the reaction ,current density should beadded up to meet the enough energy which could break down the film.2.2 Influences of power frequency on the fabrication of filmThe thickness and surface roughness of ceramic film increased and then wentdown with the increase of frequency .So it will lead to the conflict with thethickness and surface roughness when frequency increases. It has beensuggested that frequency be improved meanwhile ratio of occupancy-vacancywas decreased to get better surface.2.3 Influence of auxiliary salt NaOH on the fabrication of filmThe thickness increased firstly then decreased with the concentration of NaOHincreasing. It was because ionized [OH-] could favor the formation of MgO.Ionization degree had the same situation as the thickness of film. Furthermore itis good for stabilization of solution proper quantity NaOH.3 Influences of different electrolyte system on the fabrication of filmThe optimized technical parameters in aluminate system : NaAlO2 20g/l,NaOH 6g/l, oxidation time 30min,current density 6A/dm2. In this system,current density and oxidation time were the main influence factors. Porousnessrate was the lowest in the three systems. The corrosive resistance property inNaCl was also the best. The content of the electrolyte effected corrosiveproperty greatly. The average tensile strength of treated samples was a bit lowerthan untreated ones.The optimized technical parameters in phosphate system : Na3PO4 10g/l,NaOH 3g/l, oxidation time 20min,current density 3A/dm2. In this system, theconcentration of NaOH was the main influence factor. Plainness of film surfacewas not good, and there were melt grains which piled in pores. Averageporouness rate was two times as that in aluminate system. The corrosiveresistance property in NaCl was also good. The ceramic coating did not affectthe tensile strength greatly.The optimized technical parameters in hexametaphosphate system :(NaPO3)6 2g/l, NaOH 3g/l, oxidation time 10min,current density 6A/dm2. In thissystem, the content of electrolyte was the main influence factor. Averageporouness rate was the largest in the three systems. The corrosive resistanceproperty in NaCl was the worst. The ceramic coating did not affect the tensilestrength greatly.Generally speaking, difference among the properties of MAO ceramic filmin the three kind of electrolytes was great. The results show that micro-appearance and properties are greatly different in different electrolyte systemunder various parameter condition. The MAO coating didn't affect the tensileproperty a lot. No large pieces of peeled ceramic layers are detected on thespecimen surface after tensile test. the corrosive properties of magnesium alloywith MOA coating are improved greatly compared with untreated magnesiumalloy so that corrosion current has decreased by 3 orders of magnitudes. Theproperties of ceramic coating in aluminate system were the best compared withthe other system after optimizing the technical parameters.
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