Study On Influence And Mechanism Of Interfacial Bond On Corrosion Resistance Of MAO Coated ZM5 Mg Alloy

Magnesium（Mg）alloy is typically light alloys with a density of 1.7g/cm³.It’s the lightest structural metal in industrial applications,and used extensively in automotive,electronics,and aerospace applications.MAO treatment is an effective way to improve the corrosion resistance of Mg alloys,which are characterized by the presence of pores on the MAO coating.Previous research has focused on reducing the porosity to improve the corrosion resistance of MAO coatings.In contrast,the mechanical properties of the MAO coating,especially the bond strength,are key factors in determining the corrosion resistance of the coating.Therefore,the interfacial bond has an important influence on the corrosion resistance of MAO coatings.However,as far as we know,the relationship between interfacial bonding and corrosion resistance of MAO coatings is rarely reported.Therefore,the thesis systematically investigates the factors of the failure behavior of MAO coatings.The influence of substrate surface condition and interfacial bond on the corrosion resistance of MAO coatings is discussed.And,the acid pretreatment of Mg alloys is carried out to further improve the interfacial bond of MAO coatings.The main contents are as follows:（1）The effect of nanoparticles on the properties of MAO coatings is investigated,and the factors contributing to the failure behavior of MAO coatings are analyzed.The results show that the hardness of MAO coatings containing Si C and Ce O₂ nanoparticles is enhanced by 3.74%and44.59%,respectively.The porosity of Si C-MAO increases by 6.60%,while that of Ce O₂-MAO decreases by 23.90%.During MAO treatment,the nanoparticles preferentially fill the pores and cracks in the MAO coating,enhancing the plasma discharge intensity and improving the growth rate of the MAO coating,resulting in a higher hardness of the MAO coating containing nanoparticles.Si C-MAO has the largest stable current density,resulting in a larger porosity.Therefore,the corrosion medium is easier to enter the coating,resulting in Si C nanoparticles unable to improve the corrosion resistance of the coating.Ce O₂-MAO has a lower stable current density and the nanoparticles play the role of filling the pores with lower porosity.The smaller porosity can inhibit the corrosive medium from entering the coating,resulting in Ce O₂nanoparticles can improve the corrosion resistance of the coating at the same time.In summary,reducing the porosity can improve the corrosion resistance of the coating.In summary,the reduction of the porosity can improve the corrosion resistance of the coating.（2）The effect of two electrolytes on the corrosion resistance of MAO coatings at three voltages is investigated.The factors contributing to the failure behavior of MAO coatings are analyzed.And the relationship among porosity,interfacial bonding,and coating corrosion resistance is systematically discussed.The results show that increasing the voltage can reduce the porosity of MAO coatings and increase the bonding strength of the coatings.The reduced porosity of the coating effectively inhibits the penetration of corrosive media into the coating,thus improving the corrosion resistance of the coating.Increasing the voltage increases the bond strength of the MAO coating,which inhibits corrosion expansion.Thus,it can improve the corrosion resistance of the coating.Moreover,at the same voltage,MAO coatings prepared in phosphate-based electrolytes have smaller porosity and lower bonding strength compared to silicate-based electrolytes.Although the smaller porosity can inhibit the corrosive medium from entering the coating at the early stage of immersion,the bonding strength is weaker and cannot effectively inhibit the corrosion expansion,resulting in the weaker corrosion resistance of the MAO coating prepared in the phosphate-based electrolyte.In summary,reducing the porosity and inhibiting the penetration of corrosive media into the coating improved the corrosion resistance of MAO coatings.Enhanced interfacial bonding inhibited corrosion expansion,thus also improving the corrosion resistance of MAO coatings.（3）The effect of three morphologies on the corrosion resistance of MAO coatings is investigated,and the effect of interfacial bonding on the corrosion resistance of MAO coatings is explored in depth.The results show that surface roughness has little effect on the thickness and chemical composition of MAO coatings.As the surface roughness increased,the porosity of the MAO coatings increased from 6.71%to 9.08%and the bond strength of the coatings increased from 20.97 MPa to 29.40 MPa.The larger porosity results in poorer corrosion resistance at the beginning stage of immersion.However,as the immersion time increased,the grooved has a high bond strength,which effectively inhibits the expansion of corrosion at the interface and slows down the degradation of the MAO coating corrosion resistance.（4）The effects of five acid-etching pretreatments on the performance of MAO coatings are investigated and further improved the interfacial bonding properties of the coatings to provide a theoretical basis for the development of green,efficient pretreatment methods.The results shows that the acid etching pretreatment can reduce the porosity of MAO coatings from 6.82%to 5.89%.Citric acid,phosphoric acid and nitric acid are used to reduce the content of the second phase on the surface of the magnesium alloy,and hydrofluoric acid and phytic acid pretreatment are used to form a thin film on the second phase on the surface of the magnesium alloy.The above two types of acid etching pretreatment results in a uniform state of the substrate surface,reduces the probability of B-discharge,reduces the generation of penetration pores.Thus enhances the interfacial bonding of the coating.Acid-etching concentration,treatment time and treatment temperature all affect the performance of the MAO coatings after the acid-etching pretreatments.