Effects Of Distance Between Anode And Cathode On Properties And Load Model Of Micro-arc Oxidation Coatings On Magnesium Alloys

Micro-arc oxidation is a surface treatment technology capable of growing ceramic coatings in situ on the surface of light metals.Because the coating has the advantages of high hardness,good wear resistance and corrosion resistance,and strong bonding with the substrate,it has become one of the important means for surface protection of light metals,and has broad application prospects.In this paper,the magnesium alloy of AZ31B is used as the substrate material,and a micro-arc oxidation coatings with different anode-cathode distances are prepared in a silicate electrolyte system by using a self-developed high-power micro-arc oxidation pulse power source.Using eddy current thickness gauge,SEM,EDS and XRD to detect the thickness,microstructure,element and phase composition of coatings.Image-J software was used to analyze the micropores and porosity of the coating surface.Combining the above factors with electrochemical experiments to reveal the corrosion resistance of the coating.Based on the characteristics of the micro-arc oxidation process and the waveform,an equivalent load circuit was established,and the value of the equivalent load was calculated and verified by simulation.Finally,the influence of the anode-cathode distance on the equivalent load value and the relationship between the load value and the coating properties were analyzed.When the anode-cathode distance is 10mm,the surface current of the sample is too large due to the too short distance,and the microstructure on the surface of the coating is severely damaged.Most of the micropores are melted into large holes or disappear,and the surface is mostly"cauliflower-like",Micropore statistical significance.From a distance of 10mm to 400mm,the total number of micropores decreased first and then increased,but the porosity decreased monotonously from25.78%to 19.06%,which was caused by the reduction of larger micropores and the increase of pinholes.From the porosity analysis alone,it can be known that low porosity can reduce the passage of corrosive media into the substrate,which is beneficial to improve corrosion resistance.But in the whole process,the thickness of the coating decreases first,then increases and then decreases,while the compactness improves first and then worsens.In the range of 200mm-400mm,the coating thickness has a great difference,so the effect of coating thickness on corrosion resistance is greater than porosity.Combining the above factors with the potentiodynamic polarization curve,polarization resistance and corrosion current density of electrochemistry statistics,it is concluded that the corrosion resistance of the coating shows the trend of getting better first and then getting worse.That is to say,the corrosion resistance of the coating is the worst at 10 mm and the best at 200 mm with a difference.Their corrosion current density differs by three orders of magnitude.After analyzing the micro-arc oxidation process and waveform characteristics,an equivalent load circuit model was established in parallel with resistor R1 and capacitor C,and then in series with resistor R₁ and inductor L.Among them,a voltage regulator D was connected in series with R₁ branch.The waveform data was read with Matlab software,and the equivalent load value in the model was calculated quantitatively.Subsequently,the model was established in Multisim software,and the calculation results were substituted for simulation.The simulation waveform was compared with the actual waveform.The two are consistent,so it can be proved that the model can be applied to micro-arc oxidation load analysis.In addition,the changes of anode-cathode distance can affect equivalent load characteristics.As the distance increases,the equivalent capacitance decreases first and then increases,which affects the arc discharge process to a certain extent.Therefore,the influence of capacitors and inductors must be considered comprehensively when selecting process parameters and developing power supplies.The equivalent resistance R₁ corresponds to the film resistance.Combining the coating performance and load analysis,it is found that the value of the equivalent resistance R₁ has a corresponding relationship with the corrosion resistance of the coating.This can provide a basis for rapid assessment of process suitability and field assessment of coating corrosion resistance.