Study On The Preparation And Anti-corrosion Performance Of Environmentally Friendly Composite Films On Light Alloy Substrates

In recent years,light alloy materials have been widely used in the field of human life,industrial production and social development.The magnesium alloys and aluminum alloys are the most widely used non-ferrous metals among light alloy materials,however their susceptibility to corrosion has not only causes economic losses and waste of resources,but also leads to the environmental pollution and raises security concerns.Therefore,the study on corrosion protection of light alloy materials has great scientific significance and application value.Generally,the deposition of cover layers on metal surface to achieve the purpose of isolating the metal from the corrosive medium is a traditional corrosion protection method,which includes the chromate passivation film and phosphate conversion film.However,beacause the two technologies lead to serious environmental pollution,their use has been restricted at present.Under this circumstance,it is urgent to develop a new environmentally friendly metal surface treatment technology.A large amount of research has been devoted to the application of layered double hydroxides(LDHs)and organic phosphonic acid compounds in the field of corrosion in recent years.In this paper,the corrosion resistance and corrosion inhibition mechanism of LDHs films modified on the surface of magnesium alloys were first studied.Then,a high-adhesion phytic acid-based colored chemical conversion film was also prepared on the surface of aluminum alloys,and its properties corrosion resistance and adhesion performance were investigated.The main contents of this thesis are as follows:(1)Preparation of layered double hydroxide anticorrosive films on the magnesium alloy.Combining the advantages of co-precipitation method with in-situ growth method,MgAl-NO3 LDH films and ZnAl-NO3 LDH films were prepared on the surface of AZ31B magnesium alloy.Using the characterization methods of surface morphology,elemental analysis,electrochemical impedance spectroscopy(EIS),polarization curve and stability test were used to investigate the different performance of two LDHs films The principle is that the dense laminate structure of the LDH films act as an effective barrier,which can retard the penetration of the electrolyte solution and extend the path of diffusion.On the other hand,it can adsorb Cl-and reduce the concentration of corrosive ions in the solution,due to the unique ion exchange characteristics of LDHs Its adsorption capacity is affected by the crystal size and specific surface area of LDHs.The results show that the surface of magnesium alloy covered with LDH films has excellent corrosion resistance,and the order of corrosion resistance is:MgAl-NO3 LDH films>ZnAl-NO3 LDH films(2)Preparation of MgAl LDH films modified by phenylphosphonic acid(PPA)on the magnesium alloy.LDHs can be used as micro/nano containers and have the function of releasing corrosion inhibitors.In this paper,three methods(in-situ growth method,anion exchange method in aqueous solution,anion exchange method in alcohol solution)are used to introduce small-molecule PPA with simple structure and low phosphorus into LDHs.And the differences in surface morphology,corrosion protection performance,electrochemical impedance behavior and corrosion durability of the LDH films modified by the three methods were analyzed and compared.The results show that the anti-corrosion performance of LDH films modified by PPA has been greatly improved.And the different preparation methods have large differences in impedance behavior and corrosion inhibition mechanism.Comprehensively,the order of the corrosion resistance of the PPA-modified LDH films is:MgAl-PPA LDH films>W-PPA-MgAl LDH films>E-PPA-MgAl LDH films(3)Preparation of phytic acid-based chemical conversion film on aluminum alloy and investigation on performance of electrostatic spray coating.Phytic acid(PA)is a new type of environmentally friendly organic phosphate compound.Based on the good chelation between PA and Zn2+,PA was used as the main film-forming substance and Zn2+ was induced to form a film.The phytic acid-based chemical conversion film was assembled on the surface of 6063 aluminum alloy.The optimal preparation process is:the molar ratio of PA/Zn2+ was 1/3;the addition of NaF was 0.5 g/L.The adhesion performance of the powder coating after electrostatic spraying was evaluated by impact test,boiling water test and bending test.In order to further improve the adhesion performance of the coating,1.0 g/L sodium chlorate was added to the film-forming solution.The results show that the PA-Zn-F chemical conversion film modified by sodium chlorate prepared in this paper has good adhesion properties after electrostatic spraying of the powder:the weight impact height can reach 80 N/cm2,and both boiling water and bending test ratings reach level 0.