Study On Effect Of Ceramic Anodizing Pretreatment On MAO Of Aluminum Alloy In Weak Acid Electrolyte

The ceramic coating produced by the micro-arc oxidation(MAO)of aluminum alloy has excellent hardness and thickness,and has good properties such as friction resistance,corrosion resistance,and thermal shock resistance and so on.At present,most of MAO processes were carried out in alkaline electrolyte.MAO process in the weak-acid electrolyte situation has rarely been studied.Considering that compared with alkaline media,weakly acidic media can greatly reduce pollution,so the study of MAO process in weak-acid situation is of great significance for the industrial production of MAO.Due to the weak corrosiveness of the weak acid situation to the oxide film,in the middle of the MAO process,the generated oxide film blocks the contact between the electrolyte and the substrate,so the properties of the MAO coating formed in the weak acid situation is poor.To overcome this disadvantage,pretreatment before MAO process was considered to improve the performance of MAO coating obtained in weak-acid electrolyte.it is well known that the hardness of the aluminum alloy oxide film is related to the porosity and phase composition of the oxide film.Therefore,the ceramic anodizing process is used as a pretreatment before MAO in weak-acid situation due to its low porosity and the electrolyte of it is also safe and non-polluting.On the one hand,the low porosity of ceramic anodizing is used,on the other hand,the amorphous Al₂O₃in the composition of the ceramic anodizing film is transformed into a thermally stable state?-Al₂O₃by the transient high temperature caused by the arc discharge during the MAO process to increase the hardness of the MAO coating in weak-acid situation.In this paper,an improved MAO coating was prepared by using ceramic anodizing as the pretreatment method.The effect of ceramic anodizing pretreatment on the performance of MAO coating in weak-acid situation,and the Growth mechanism of MAO coating based on ceramic anodizing pretreatment film were studied by using SEM,XRD,Vickers hardness tester,electrochemical experiments and other means.And by adjusting the concentration of the electrolyte,the effect of the ceramic anodizing pretreatment film on the performance of the MAO coating under different electrolyte concentration conditions was studied.The results show that after the pretreatment of ceramic anodizing,during the MAO process,electrical breakdown occurs in the weaker areas of the pre-anodized film to form discharge channels.The weak-acid electrolyte is in contact with the aluminum alloy substrate through the discharge channel.Under the action of high temperature and high pressure generated by the arc discharge in the channel,the electrolyte reacts with the aluminum alloy substrate to form a MAO coating.At the same time,the pre-anodizing ceramic oxide film is melted,crystallized and converted into MAO coating under the action of high temperature and high pressure,which increases the thickness of the MAO coating,and in this process,the amorphous Al₂O₃ in the ceramic oxide film is converted into thermally stable?-Al₂O₃ under the action of high temperature,so that the hardness of the MAO coating is significantly improved.Due to the increase in the content of?-Al₂O₃ in the MAO coating after ceramic anodizing pretreatment,the corrosion resistance of MAO coating with pre-anodized is significantly improved compared to the MAO coating without pre-anodized,but due to the residual pre-treatment ceramic oxide film in the MAO coating,and the overall roughness of the film layer is also improved compared to MAO coating without pretreatment.By adjusting the electrolyte concentration and comparing the voltage-time curves under different electrolyte concentration conditions,it is known that the loss of the pre-formed film and the formation of the MAO coating proceed simultaneously,under the condition of low electrolyte concentration,the formation rate of the MAO coating is greater than the consumption rate of the pre-formed film,and the voltage shows a steady increase.As the electrolyte concentration increases,the citrate concentration rises,due to the citrate chelates metal aluminum ions,it reduces the formation of Al₂O₃,thereby reducing the formation rate of the MAO coating,which makes the voltage fluctuate in some intervals.When the sodium citrate concentration increased from 40g/L to 60g/L,the thickness,hardness and roughness of the MAO coating with pre-anodized did not change much.