Preparation And Charaterization Of Multilayer On AZ91D Magnesium Alloys

Many excellent properties of magnesium alloy make it the most promising structural materials in non-ferrous metal materials. However, the magnesium alloy has a standard potential negative characteristic, deciding to its poor corrosion resistance, which seriously limits the applications of magnesium alloys. In order to improve the properties of magnesium alloy, in this paper, combining with the technology of plasma electrolytic oxidation (PEO) and plasma electrolytic saturation (PES), multilayers were prepared on AZ91D magnesium alloys. Firstly the surface morphology,phase composition,element distribution and corrosion resistance of the multilayers obtained were studied, and then the effect of the changes of parameters such as voltage, time, duty cycle and others of PES or PEO on surface morphologies and corrosion resistance were inspected.Modified layer with certain thickness was obtained on magnesium alloy after treatment by PES in solution of CH3NO, and the corrosion resistance of Mg substrate was different amplitude improved. Multilayers with porous appearance characteristics was obtained on magnesium alloy after treatment by PES and PEO. As the effect of PES process, multilayers'surface also has uneven characteristics appearance, such as"gravel","tumor block". The content of the phase composition, except for Mg₂SiO₄, MgO that is common in PEO coatings, there are also some high-quality phases were formed as C or N, such as Si₃N₄, SiC, AlN in multilayer. Besides, Ion implanted layer contained C and N was formed near to the base coatings and substrate. The corrosion resistance of magnesium alloys has been significantly improved as new morphological features and phase composition of multilayer under certain conditions.In the preparation of multilayer, with the voltage increased in the PES process, the thickness of multilayer was about 20.5μm without significant changing, but the trend of corrosion potential is gradually increasing. According to the corrosion current density and polarization resistance of multilayers, when the voltage is 310V, the corrosion resistance of the sample is the best; With the extension of the time in PES process, the thickness and porosity of multilayer show overall rising trend. And the corrosion potential shows first increased and then decreased trends. As effected of the thickness and structure of multilayer, the corrosion resistance is the best when the corresponding sample treated for 30⁶0min; with the increased of duty cycle in the PES process, the thickness of multilayer keeping about 22μm, the corrosion current density was an overall decline trend, and the polarization resistance overall upward trend. The corrosion potential showed first increase then decreases. When duty cycle controlled in 40⁵0%, the corrosion resistance of corresponding sample is better. The changes of parameters in the PES process have no obvious effect to the mainly phase deposited in multilayer.With the increasing of voltage in the PEO process, the aperture and thickness of multilayer obtained gradually enhanced, but the changing of corrosion resistance with the voltage of PEO, different testing methods showed different rules. In the tafel polarization curve tests, with the voltage increased, the corrosion resistance of multilayers showed first increased and then decreased trend, when the voltage of PEO was 400V, corrosion resistance was the best. While in the acid drip experiments, the corrosion resistance of multilayer was overall improving as voltage rising. With the extension of time in the PEO process, the thickness of the multilayer obtained enhanced gradually, and the surface morphology of multilayer was approaching the average, the corrosion resistance of the corresponding samples overall rising trend. When voltage of 400V, processing time for 30⁶0min of PEO, the result show that corrosion resistance of the samples processed was better. Similarly, with the changes of parameters in the PEO process, the mainly phase deposited in multilayer did not have apparent effect.