Preparation And Corrosion Behavior Of MAO/PTFE Composite Coating On AZ31B Magnesium Alloy Substrates

Micro-arc oxidation is a promising surface treatment method for improving the corrosion resistance of magnesium alloys,but the micro-arc oxidation film prepared in the basic alkaline silicate electrolyte has inevitable self-defects,there are numerous micron-sized holes and a small number of microcracks on the MAO surface for the the escaping of the solid melt and gas in the micro-arc discharge channel.Corrosive ions,such as Cl^-,enter the interior of the film along the holes and corrode the substrate,and thus become a bottleneck restricting the corrosion resistance of the micro-arc oxidation film.The addition of nano-particle additives to the electrolyte can decrease the porosity of MAO film by one-step reaction.In addition,the thickness also slightly increases,as well as the corrosion resistance and wear resistance of the film.In this paper,Al₂O₃,MgO and ZrO₂ nanoparticles were added to the basic electrolyte,and their effects on the surface morphology,cross-sectional morphology,phase composition,corrosion resistance and wear resistance were investigated.The results show that under the preparation parameters of this paper,the final voltage after the reaction of 1500 s can be up to 410 V,but the thickness of the films are about 7μm,and the thickness of the MAO films prepared during the most research reports are greater than 20μm.The thin layer of MAO films prepared in this paper have better corrosion resistance than most studies reported.The optimal doping concentration of Al₂O₃ nanoparticle is 7 g/L,the corrosion current density is reduced to 4.35×10^-9 A/cm²,and the minimum friction coefficient is0.225,because Al₂O₃ has higher hardness and strength,and the film becomes denser.The optimal doping concentration of MgO nanoparticle is 4 g/L,the corrosion current density is reduced to 4.28×10^-9 A/cm²,and the minimum friction coefficient is0.228,respectively.Unstable MgO combines with H₂O to form Mg（OH）₂,filling the corroded channels and slowing corrosion.The ZrO₂ nanoparticle is different from the above two types,and the corrosion current density is reduced to 4.47×10^-9 A/cm² only when the concentration is 1 g/L.The best incorporation concentration for the wear resistance is 7 g/L,the friction coefficient drops to 0.210.Because there is a conversion of m-ZrO₂ to t-ZrO₂.An inner layer of MAO film was prepared by adding 4 g/L MgO to the base electrode solution.The outer polytetrafluoroethylene（PTFE）film was prepared by radio frequency magnetron sputtering.The thickness of the film affects the corrosion resistance of the composite film.When the sputtering time is 90 min,the MAO film thickness is 7.23μm,the PTFE film thickness on the glass slide is 3.77μm,and the composite film thickness is 10.4μm,which is slightly less than the sum of the double film thickness.This is because the roughness of the MAO film is larger and the combination of the two films is"serrated",and the corrosion current density was reduced by 5 orders of magnitude compared to AZ31B magnesium alloy,and it was5.75×10^-10 A/cm ²,and no visible pitting after 336 hours of salt spray test.