| Magnesium(Mg)alloy shows unique advantages and broad application prospects in the fields of aerospace,automobile manufacturing,electronics and electrical appliances and other high-tech equipment because of its light weight,high strength and excellent mechanical properties.However,Mg has high activity and extremely negative potential,which leads to galvanic corrosion process when it contacts with other metal connectors,resulting in serious damage of Mg alloys,Surface alloying and chemical conversion process are regarded as the effective method to enhance the corrosion resistance of Mg alloys and improve the galvanic corrosion.Among them,surface alloying can improve the passivity and corrosion resistance of Mg surface without changing the mechanical properties of the matrix,which is the most promising method to realize industrial application.However,it will cause difficulties in the process of thermal diffusion and uneven thickness of the alloyed layer because the grains of Mg materials are usually coarse,which needs to be solved urgently in the practical application.On the other hand,Layered Double Hydroxides(LDHs)have been widely used in Mg alloys due to their excellent barrier properties and ion exchange capacity.However,LDH surface film is usually very thin,and it is difficult to resist the harsh mechanical environment such as scratching and wear,so the chemical conversion process of LDH is difficult to be used alone to realize the surface protection of Mg alloys.In this study,aiming at the galvanic corrosion of Mg alloys,combined with the methods of surface alloying and LDH chemical conversion film,the structural evolution,growth conditions and formation mechanism of binary Mg-Al LDH film and Zn-Al LDH film were explored;The surface alloyed layer was used to provide metal source for in-situ growth of LDH film to realize combination of alloyed surface and LDH conversion film.The mechanism of composition and structure of the alloyed layer and alloyed-LDH composite materials on multifunctional characteristic of the superhydrophobicity,corrosion resistance and self-repair properties was revealed.The main research contents are as follows:(1)The surface burnishing was introduced to the surface alloying process of Mg substrate.The grain of pure Mg was refined by the surface burnishing,which provided a fast diffusion channel for the thermal diffusion process and improved the diffusion efficiency of the alloying process.The refined grains provided fast "diffusion channel"for thermal diffusion,and the as-prepared alloyed layer with a thickness of up to 5.8 mm(3 mm alloy layer +2.8 mm transition layer)as realized by A1 alloying process.The burnished&Al-alloyed layer was mainly composed of Al3Mg2 phase andβ-Mg17Al12 phase,and the transition layer is mainly composed of β-MgnAl12 phase andα-Mg phase,The high Al content makes burnished&Al-alloyed surface had positive corrosion potential and very low corrosion current.And its breakdown potential greater than-1.0 V.It suggests that the galvanic corrosion of the alloyed layer in contact with other metals may be eliminated.In addition,the burnished&Al-alloyed sample also exhibited a good protective effect on long-term corrosion resistance and corrosion resistance after scratches.(2)In order to evaluate the feasibility of in-situ conversion from alloy phase to LDH surface film,the effect of Al content on the growth process of LDH surface film on Mg-Al alloys was investigated,as well as the relationship between hydrothermal experimental conditions and corrosion resistance were investigated.When the content of A1 in Mg alloy is more than 20 at.%,a more compact and uniform LDH conversion film is formed on the surface,and the corrosion resistance is also improved with the increase of A1 content.The in-situ conversion from Mg-Al alloy phase to Mg-Al LDH film was realized without introducing any metallic or chemical additives.The transformation mechanism is as follows:the Mg component in the alloy phase was firstly converted into Mg(OH)2 surface film under neutral condition,Al(OH)4-was released from the alloy phase and finally Al(OH)4-was exchanged with the Mg2+of Mg(OH)2 and was realized to form Mg-Al LDH film.(3)The co-sputtered Zn-Al transition layer was introduced into the in-situ conversion process of Zn-Al LDH film,and the galvanic simulation experiment based on easily-regulated Zn/Al sputtering film was designed.The traditional research on complex hydrothermal components was transferred to the investigation of the structure evolution of Zn/Al couple surface,which can overcome the difficulty of real-time monitoring of multiphase hydrothermal solution.The conversion mechanism from Zn-Al transition layer to Zn-Al LDH film including the electrochemical process,the transformation of Al(OH)4-and ZnO and the combination process to form Zn-Al LDH film were revealed.According to the evolution role of the structure of conversion film during the hydrothermal process,the structural conversion mechanism from "selective dissolution" to "solution infiltration" was proposed.Based on the "sacrificial transition layer" and "regeneration LDH film" conversion process,the growth process of Zn-Al LDH films with controllable thickness on substrate materials with different shapes,sizes and materials was realized.(4)According to the optimized conditions for the formation of Mg-Al LDH film and the suitable composition of A1 alloyed surface(β-Mg17Al12 and Al3Mg2 alloy phase),a uniform and dense A1 alloyed-LDH composite surface was prepared by in-situ converting the alloy phase via hydrothermal treatment.The Al alloyed-LDH sample showed strong protective effect due to the excellent barrier ability of LDH surface film and the basic protection of the alloyed layer.After 90 days of long-term corrosion,the impedance at low frequency is still higher than 105 Ω·cm2.More importantly,the alloyed metal surface at the bottom would exposed once the LDH surface film was destroyed,the alloy phase could dissolve metal ions and reconstruct the LDH film,showing good self-repairing characteristics.It is believed that the preparation route can provide more possibilities for the application of Mg alloys in extreme environments.(5)Furthermore,the technical route of in-situ growth of LDH surface film on alloyed surface was extended to the preparation of ternary LDH surface film.The ternary Mg-Zn-Al LDH film was grown on the Zn-Al alloyed AZ31 surface via directly hydrothermally treated the Zn-Al alloyed layer.Further modification with lauric acid(LA)could change the LDH film from hydrophilicity to super-hydrophobicity(WCA of 156.1°).The synergistic effect of the physical barrier property of superhydrophobic surface,the intrinsic "intelligent" protection of LDH film and the high passivity of the alloyed layer at the bottom make the superhydrophobic Zn-Al alloyed-LDH composite surface have long-term corrosion resistance.The in-situ growth mechanism and multi-stage micro-nano structure of the composite surface exhibited excellent bonding strength and mechanical wear resistance;It can still maintain structural stability and environmental durability in acid,alkali and salt environment.It provides candidate materials for the practical application of Mg alloys in harsh environment and special working conditions. |
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