Preparation And Corrosion Resistence Of CeO₂/MAO Composite Coating On AZ31B Magnesium Alloy

Because of excellent physical and mechanical properties,magnesium and its alloys are widely used in automobile,electronic industries and aerospace.However,low corrosion resistance and high chemical activity of magnesium alloys significantly restrict its further application.Numerous studies have been carried out to overcome these drawbacks by coating protective layers.Microarc oxidation（MAO）was based on the in-situ growth method which can provide a ceramic-like-coatings and enhance corrosion performance.However,high voltage in the process of microarc oxidation often leads to micropores or cracks that provide the channel for aggressive medium to the substrate,hence,decreases its corrosion resistance significantly.Cerium-based oxide coating is known as corrosion inhibitors for magnesium and aluminum alloys has attracted more and more scholars’attention.Currently,few studies about the post-treatment of CeO₂ coating deposited by magnetron sputtering have been reported.In this study,a CeO₂/MAO composite coating on AZ31B magnesium alloy was prepared by microarc oxidation and magnetron sputtering composite technology.The effects of electrical parameters included duty cycle,oxidation time,frequency and current density on the surface morphology and electrochemical performance of the ceramic film during the micro-arc oxidation process were investigated.And magnetron sputtering power and sputtering time of the composite coatings were also studied.The result shows that the MAO coating has the best corrosion resistance when the micro-arc oxidation duty cycle is 40%,the oxidation time is 20 minutes,the frequency is 1000 Hz,and the current density is 0.2 A/cm².The MAO coating is mainly composed of MgO and Mg₂SiO₄ with an inner dense and an outer porous layer.After the salt spray test,the MAO coated sample surface spread some pinholes as well as droplets which gradually increase until 336 h.On that basis,the CeO₂/MAO duplex coatings were prepared by microarc oxidation and magnetron sputtering composite technology.After sputter CeO₂ coating,micropores decrease and few pores can be found.CeO₂ coating achieved continuous and uniform coverage of MAO coating,which made up for the lack of micro-arc oxidation process.The composite coating showed the lowest corrosion density of2.27×10^-99 A/cm² and lowest charge transfer resistance of 8.1×10⁷Ω·cm² when the sputtering power and time was 90 W and 6 h,respectively.Only some small corrosion pits are detected until 336h.