Investigations Of Microstructure Evolution And Corrosion Resistance Of Plasma Electrolytic Oxidation Coatings On AA1060 Aluminum

In the present work,the plasma electrolytic oxidation?PEO?treatment of AA1060aluminum was carried out.The effects of NaOH concentrations on the growth of PEO coatings were studied in various concentrations of NaOH in a Na₅P₃O₁₀ solution,and emphatic investigation was performed on the coatings fabricated in the electrolyte containing 1 g/L NaOH,focusing on the microstructure evolution of these coatings.The influence of Na₂SiO₃·9H₂O concentrations on coating characteristics were discussed.The results revealed that the concentration of NaOH determined the types of anodic films and the activity of plasma electrolytic oxidation.Porous-type anodic film with uniform pore diameter were fabricated at the anodic stage of PEO process in the electrolyte containing 1 g/L NaOH.Subsequently,detachments and cracks took place on the anodic films,and these weak regions occurring dielectric breakdown preferentially resulting from the concentration of electric current.Phosphorus element existed as amorphous phase in the middle of the two interfaces.The oxide particles on the anodic film coating/substrate?C/S?interface underwent profile variations from hemisphere to polygon,and the diameter was decreasing over time.The diameter of the semi-spherical oxides generated by plasma discharges on the PEO coating C/S interface was first increasing with time and then remained constant values.The results also revealed that the formations of pores of anodic films and plasma discharge of PEO coatings were both easy to occur around the existing pores and the existing discharge channels.A pancake-like structure corresponded to a discharge channel,a cavity,and a barrier layer protrusion whose area was similar to that of the corresponding pancake structure.The difference in corrosion resistance of the substrate from which the native oxide film was removed,the substrate covering the native oxide film,and the samples covering the PEO coatings were compared.The results demonstrated that the sample treated with PEO for 10 min showed the lowest corrosion current density and the highest inner barrier resistance,and the corrosion resistance was the best.Various PEO coatings were fabricated in the electrolytes with different concentrations of Na₂SiO₃·9H₂O.The results showed that adding Na₂SiO₃·9H₂O in the electrolyte could change the types of anodic film before the occurrence of plasma discharge.The anodic film continued to grow in the PEO process after being broken down for a long duration.Characteristics of PEO coatings with a similar thickness acquired in various electrolytes were also evaluated.XRD results indicated that these coatings were mainly composed of?-Al₂O₃ and?-Al₂O₃,and the?-Al₂O₃ proportion in the coatings decreased with increasing Na₂SiO₃·9H₂O concentration.The electrochemical data showed that the corrosion resistance of these PEO coatings decreased by adding Na₂SiO₃·9H₂O in the electrolyte.