Study On The Role Of Cathodic Polarization In Plasma Electrolytic Oxidation Of An Al-Cu-Li Alloy

Plasma electrolytic oxidation（PEO）is an advanced metal surface treatment technology to prepare hard ceramic coatings on the surfaces of aluminum,magnesium,titanium,zirconium and their alloys.Under the action of high anode current,the micro plasma discharge on the electrode surface promotes the formation of crystallne oxides,so as to improve the comprehensive properties of the coating.The application of cathode current can effectively reduce the number of pores and defects and produce thicker and denser coatings.In particular,when the cathode current density is greater than the anode current density,soft sparks may occur,forming a dense,thick and hard film on the surface of aluminum alloy.These facts prove the importance of cathodic polarization in PEO mechanism;however,its mechanism has not been well revealed.Finding out the mechanism of cathodic polarization and soft sparking is critical for plasma electrolytic oxidation.In this study,PEO of an Al-Cu-Li alloy has been carried out in 5 g l^-1 Na₂SiO₃·9H₂O+1 g l^-1KOH silicate electrolyte by systematically varying the cathodic to anodic current density ratio R（R=j_c/j_a）from 0 to 3.3.The effects of cathodic polarization and soft spark are studied to explore the film formation and discharge mechanism of the film under the soft spark mode.The experiment shows that increasing the initial stage of cathodic polarization can improves the coating growth efficiency until an optimum soft sparking regime is reached at R=1.2,after that the efficiency decreases and becomes thinner,blisters or even falls off to the coatings occur.The optimum soft sparking regime is using the cathodic to anodic current density ratio of～1.2,with coatings thicker than 100μm being formed after treating 60 minutes.The soft sparking coatings formed under R=1.2 are featured by a white outer layer enriched with the electrolyte compositions of Si,Na and K and a uniform and compactα-Al₂O₃ inner layer.Cathodic polarization under higher R values lowered the cathodic resistance of the PEO system and the impedance of the coatings,this may be caused by intercalation of[H^·]ox hydrogen species in the oxide caused by the cathode process.The results presented in this study indicate that there are opposing forces that govern the coating growth with cathodic polarization.The improved efficiency is attributed to the accelerated mass transfer caused by cathodic hydrogen evolution,hydrogen bubbles accelerate the stirring of electrolyte above the coatings.However,cathodic polarization plays a charge extraction effect,preventing the access of anionic coating-forming species,such as O^2-and OH^-.In addition,when R is larger,the stress generated by excessive hydrogen evolution is the other factor detrimental to coating formation.The spark softening is due to the fact that the improved coating conductivity cannot support the high localized anodic current densities that are requ ired for the conventional plasma discharges.Hence,anodic current is more uniformly distributed and the intensity and population density of the plasma discharges are significantly reduced and increased,respectively,leading to the soft sparking mode of coating growth.Reciprocally,controlled potential tests indicate that anodic polarization suppresses the subsequent cathodic hydrogen evolution.Therefore,even if there is a high cathodic potential in the PEO process,the oxide coating will not be destroyed.An important finding of this study is that the anode voltage drop,which is one of the soft spark features thought in the past literature,it is not certain to occur when the optimal cathode and anode current ratio R=1.2 in this paper.In those cases where the voltage does not drop,the coating grows even thicker.This phenomenon may be due to the accumulation of coating thickness compensating for the reduction of coating resistance caused by the embedding of[H^·]ox,and the fact that the edge of the specimen has not been severely oxidized also contributes to the increase of the anode voltage.Hence,the sharp drop in the anode potential of the PEO process can not be used as an important indicator to determine whether a soft spark has occ urred.The corrosion tests of samples with different PEO treatment times of R=0,R=0.7and R=1.2 show that PEO treatment can effectively improve the corrosion resistance of the coating.The corrosion current densities of coatings under R=0 and R=0.7 are decreases by 1-2 orders of magnitude than Al-Cu-Li matrix.Under the condition of optimal soft spark R=1.2,the corrosion current density is lower by one order of magnitude,which enters the soft spark state,and the corrosion resistance of the coating slightly decreases.In the soft spark state,the decrease of corrosion resistance of the coating may be caused by the insertion of more hydrogen complexes.The resistance of the coating decreases,the conductivity increases and the corrosion resistance decreases.In the dry sliding friction test with 5 N load,the coating layer of R=0 3600 s is worn through for 150 s;the friction coefficient of R=0.7 coating continues to increase by0.644,and it is not worn;R=1.2,the wear marks are smooth,and the presence of a dense inner layer makes the coating not abraded through,and the friction coefficient remains high and stable at about 1.42.