Effect Of EDTA-2Na On The Structure,corrosion Resistance And Growth Mechanism Of Micro-arc Oxidation Coating For Aluminum

Aluminum alloy is widely used in daily life and engineering applications because of its low density and high strength,but its corrosion resistance is poor,so it should be strengthened by surface treatment.Among many surface treatment methods of aluminum alloy,micro-arc oxidation has been widely used because of its simple operation and low pollution.In this master thesis,the influence of additive disodium ethylenediaminetetraacetate(referred to as EDTA-2Na)on the aluminum alloy AA1060 of micro-arc oxidation(MAO)was studied by comparing the phase,surface morphology,current curve and electrochemical impedance spectroscopy results under the constant voltage mode.By comparing the coating morphology and Mott-Schottky curve at different time points and different angles,the influence of EDTA-2Na on the growth process of the micro-arc oxidation coating was also studied.The growth model of micro-arc oxidation under the mechanism of "soft spark" was proposed.The results of studying the effect of the additive EDTA-2Na on the structure and corrosion resistance of the micro-arc oxidation coating show that the addition of EDTA-2Na can reduce the light and acoustic emission and the current,and the whole process present "soft spark" state.SEM and EDS results show that after 5 g/L EDTA-2Na treatment,there are more discharge channels under the surface of the sample,and the silicon content of the inner layer of the coating increases,indicating that the discharge mode has changed.The polarization test shows that with the increase of EDTA-2Na content,the corrosion resistance of the coatings gradually increases.However,EIS measurement shows that when the additive concentration is 3 g/L,the outer layer of the prepared coating has greater corrosion resistance,which means that compared to the inner layer,the outer layer plays a more critical role in the corrosion resistance of the sample.Research on the effect of the additive EDTA-2Na on the evolution of the micro-arc oxide coating structure shows that the additive EDTA-2Na can weaken the intensity of the B-type discharge,leaving more obvious discharge channels on the surface of the coating and the cavity caused by B type discharge was covered.With the extension of the micro-arc oxidation time,the Si content at the C/S interface of the coating prepared in the additive containing 5 g/L EDTA-2Na gradually exceeds the C/S interface of the coating prepared in the electrolyte without additives.The increase of Si content indicates that the D-type discharge method has gradually replaced the B-type discharge and become the dominant discharge method under the "soft spark" mechanism.It is also the increase in the probability of the D-type discharge that makes the outer cavity of the coating filled up.The outer layer become more compact.The weakening of the spark brightness on the surface of the coating under the "soft spark" mechanism with EDTA-2Na can be attributed to the weakening of the intensity and number of the B-type discharge and the increase in the number of invisible D-type discharges.