Studies On Machanism Of Nucleation And Corrosion Of Electrodepositon Of Aluminum On Magnesium Alloy In Ionic Liquid

Magnesium and its alloys are being widely used in transportation, machinery manufacturing, 3C, etc. areas, for they have many excellent physical and mechanical properties, such as small density, high specific strength, good thermal conductivity, good damping, electromagnetic shielding and some other excellent performances. However, the application of magnesium and its alloys are limited due to the activity electrochemical of magnesium which makes the corrosion resistance of magnesium and its alloys be poor.In this paper, the surface of magnesium alloys is protected by the way of electrodepositing aluminum in ionic liquid at room temperature, which, compared with other normal ways of improving the corrosion resistance of magnesium alloys, has advantages of environmental protection, high productive efficiency and ease of recycling.The ionic liquid used in this experiment is the solution whose molar ratio of AlCl₃-EMIC is 2:1. The effects of different pretreatments on the appearance and adhesions between the AZ91D magnesium alloy and aluminum coatings electrodepositon from the ionic liquid are studied. The adhesions are estimated by score-tearing test.The surface morphology, binding of matrix/coating are analyzed by SEM ,EDS and XPS tests. The results show that : the oxidation film of matrix cannot be completely removed by pretreatment of mechanical polishing, aluminum layers and matrix combine not good; Mg(OH)_2-xF_x film, which is good for gaining dense aluminum layers but cannot improve the adhesion between the alunimun coating and matrix, comes out by the acid pickling of HF; the appearance , density and the adhesion with matrix of aluminum layers can be improved by the acid pickling in dilute phosphate.The acid's mass ration is 10%, and the pickling time is 20s. The dilute phosphate erosion can remove the oxidation film on magnesium alloys and form porous phosphate film which plays an important role in preventing the surface of magnesium alloys be oxidized again.The effect of current density and electrodeposition time on the appearance and corrosion of aluminum coatings electrodeposited in the ionic liquid are researched by SEM and neutral salt spray tests. The results show that when in a lower current density, aluminum deposits in porous phosphate film at the first time which is good for adhesion. The aluminum coating, acquired on AZ91D magnesium by firstly pickled in the 10%wt phosphate for 20s ,then conducted at 20 mA/cm² for 2500s, have a silver-white appearance and uniform grain sizes. Testing in the neutral salt spray test for more than 300 h, it will not come out the corrosion spots and the corrosion grade is up to 9.The behaviors of aluminum ions on the different electrodes during deposited process were studied by cyclic voltammetry test and chronoamperometry test. It shows that the reduction of aluminum ions is quasi-reversible and controlled by diffusion. In the AlCl₃-EMIC ionic liquid of which the molar ratio is 2:1, the electric double layer charging appears in the fommer time of the aluminum ions deposition, and both three-dimensional progressive nucleation and three-dimensional instantaneous nucleation are found in the latter time.Polarization test and AC impedance test were carried out in the NaCl solution of which the ratio of weight were 3.5%. The result of polarization curves test shows that the polarization resistance of aluminum layer increases an order of magnitude than that of magnesium matrix while the map of AC impedance test shows that the magnesium matrix dissolves fastly in the solution, and adsorption products appeare which are protected by electrodeposition coating on the aluminum.