Electrochemical Dealloying Synthesis Of Surface-porous Magnesium-Aluminium Alloys

Micro/nano porous metal materials are widely used in aerospace,transportation,building engineering,mechanical engineering,electrochemical engineering,environmental protection engineering and other fields for its many advantages,such as large specific surface area,small specific gravity,low thermal conductivity,good energy absorption and high heat transfer and heat dissipation capacity.At present,dealloying is the most important method for the preparation of such materials.However,the porous metal materials prepared by this method are mainly inert metals such as nano–porous Au and nano–porous Ag,the alloy systems studied consist mainly of active elements and inert elements.To solve this problem,this paper takes Mg–Al alloy which is only composed of active elements Mg and Al as the research object,using 0.6 M NaCl neutral aqueous solution as electrolyte.The electrochemical properties were tested and the electrochemical dealloying was studied under different electrochemical parameters.The following main conclusions were obtained:?1?In 0.6 M NaCl neutral aqueous solution,the critical potential of pure Mg,as–cast Mg_70.5Al_29.5,annealed Mg_70.5Al_29.5,as–cast Mg₈₀Al₂₀,annealed Mg₈₀Al₂₀,as-cast M₁₇Al₁₂ and pure Al are–1.77 V,–1.36 V,–1.40 V,–1.32 V,–1.47 V,–1.12 V and–0.75 V,respectively.The critical difference between pure Mg and alloys?metal?is greater than 450 mV,which satisfies the potential difference between multiphase alloys in the process of dealloying.?2?Single–sized honeycomb pore with an average size of 0.48±0.15?m can be fabricated by electrochemical dealloying as–cast Mg_70.5Al_29.5 alloy at–1.3 V and–1.0 V for 5400 s and 900 s respectively;The bimodal porous structure with a large pore about 47.57±11.43?m and a small pore about 265.60±78.68 nm can be fabricated by dealloying as–cast Mg₈₀Al₂₀ alloy at–1.6 V,–1.2 V and–1.0V.?3?In the process of dealloying,?–Mg phase as anode of micro–galvanic is selective removed to form pores and Mg₁₇Al₁₂ phase as cathode is protected and remained as skeleton structure.It is mainly caused by the structural stability of the Mg₁₇Al₁₂2 cell?4?The results of effect of dealloying time on porous structure show that the formation of porous structure is divided into three stages.In the early stage,the growth of the thickness of the porous layer and the increase of the pore size are simultaneously carried out and the rate of growth is similar.In the middle stage,the thickness of porous layer increases,and there is no obvious change in pore size at this stage.In the later stage,the thickness of the porous layer is basically unchanged,and the pore size increases continuously.It is important to note that in the later stage,as the electrolyte is stored in the pore channel for a long time,with the prolongation of the alloying time,the porous layer begins to form the micro crack which leads to the destruction of the porous layer structure.?5?After annealing at 380?for 48 h,the eutectic structure disappeared and the Mg_70.5Al_29.5 alloy and Mg₈₀Al₂₀ alloy were composed of?–Mg phase and Mg₁₇Al₁₂ phase.The results of dealloying on them showed that the pores of Mg_70.5Al_29.5 alloy increased,the average size is 5.84±2.01?m,and the bimodal porous structure of Mg₈₀Al₂₀ alloy disappeared and the single-sized porous structure with the average ligament width of 7.11±1.96?m,and the ligament spacing with 9.73±2.71?m is fabricated.It indicates that the porous structure is mainly determined by the morphology and the heredity of the structure.