Study Of Corrosion Resistant MAO Coating On 6061 Al Alloy With Low Energy Consumption And Coating Growth Mechanism

Micro-arc Oxidation technology have many advantages such as simple process,superior performance and small environmental pollution.It presents a great potential in the field of the surface modification of aluminum,magnesium,titanium and other alloys.However,MAO must be performed at high voltage and high current density.Therefore,the high energy consumption has become the main factor limiting MAO technology large-scale application in industry.Researching and developing low energy consumption MAO technology has become an urgent demand of industry.In this paper,MAO coating with excellent corrosion resistant was prepared on 6061 Al alloy.A feasible low energy consumption MAO process was obtained by optimizing the MAO basic electrolyte and process parameters,MAO electrolyte additives and sample surface pretreatment technology.In this paper,the orthogonal experiment designing method was used to optimize the Na3PO4-Na2SiO3 composite basic electrolyte.The optimum composition of MAO electrolyte determined by thickness and corrosion resistance of the coating is 30g/L Na3PO4,10g/L Na2SiO3,2g/L NaOH.Base on the optimum electrolyte,the process parameters of MAO process are optimized by orthogonal test method.The optimum MAO process parameters are current density 10A/dm2,frequency 800Hz,duty cycle 40%.The structure,properties and coating forming mechanism of MAO coating were studied by using potassium permanganate(KMnO4)as the additive of MAO electrolyte.The results show that the addition of appropriate KMnO4(2.5g/L)improves the growth rate of the coating,reduces the working voltage of MAO,and thus reduces the energy consumption in the process of MAO(1.78kw·h/(m2·μm)).The manganese in the electrolyte is involved in the growth of MAO coating and formed the manganese oxides in the coating.The addition of KMnO4 changes the MAO coating forming mechanism and the micro-arc discharge tends to occur in the interface region between the inner layer and outer layer of MAO coating(i.e.the type D discharge),which prevents the formation of cavity structure and improves the density of MAO coating,and then enhances the corrosion resistance of the coating.Anodic oxidation pretreatment was used to modify the surface state of the samples.The effects of anodic oxidation pretreatment on the thickness,microstructure,composition and properties of the MAO coating were studied.The anodic oxidation coating replaces the high impedance film in the early stage of MAO process,which increases the breakdown voltage and working voltage of MAO.Moreover,the anodic oxidation coating is transformed into MAO coating during the MAO process,thus increasing the growth rate of the coating and reduce the energy consumption in the process of MAO.The proper anodic oxidation pretreatment can improve the corrosion resistance of the coating.The optimum anodizing pretreatment process in 150g/L sulfuric acid solution is as follows:DC voltage 20V,treatment time 10min,the energy consumption is 1.52 kw·h/(m2·μm).