Study On The Plasma Electrolytic Oxidation Of AZ31 Magnesium Alloy In Tungstate-containing Electrolyte And The Black Coating Formation Mechanism

Magnesium alloys have many excellent properties such as low density,high strength to weight ratio and good castability.However,due to their poor wear and corrosion resistance,they are usually supposed to undergo some surface treatment so as to prolong the serving duration of related products.Plasma electrolytic oxidation(PEO)is a newly developed surface treatment technology,which could be used to fabricate ceramic coatings on the surface of valve metals like Mg,A1 and Ti,and PEO has been intensively utilized to improve the wear and corrosion resistance as well as biocompatibility of those valve metals.Adding Na2WO4 to the electrolyte could facilitate the fabrication of W-containing PEO coatings,which have great application potential in many fields like photocatalysis and semiconducting materials.Despite the fact that there are mountains of study regarding PEO of magnesium,no consensus understanding has been achieved among researchers about the coating formation mechanism of PEO on magnesium.The process of PEO is influenced by a variety of factors,but the role current frequency plays still remains controversial.In the present study,PEO of AZ31 magnesium alloy has been carried out in the alumin ate-tungstate electrolytes.By changing factors like composition of electrolyte,frequency and current regimes,the discharging process of PEO and phase composition and microstructure as well as properties of the fabricated coatings have been investigated.Plus,PEO coating formation mechanism on magnesium alloy has also been explored.SEM,XRD,XPS,photovoltaic conversion and real-time image methods are used in the present study.In addition,sealing treatment has also been carried out on the as-fabricated PEO coatings in cerate and phosphates sealing reagents;microstructure and electrochemical properties of sealed PEO coatings have been investigated.The main contents are as follows:(1)The thickness and roughness of PEO coatings increased with increase of Na2WO4 concentration in the electrolyte.PEO coating changed from white to black in color after adding Na2WO4 to the electrolyte,WO3 and W18O49 were formed in the coating.The PEO coatings showed a typical pancake structure,W species enriched at the interface between the coating and metal substrate while the content of A1 species decreased along the depth of coating.Corrosion resistance of PEO coatings were improved after adding Na2WO4 to the electrolyte.Coating fabricated in electrolyte with 10 g·L-1 Na2WO4 showed the best corrosion resistance with the Ecorr being-1.467 V and jcorr being 8.22 ×10-7A·cm-2.In the present study,PEO coating was formed by penetrating discharges;sequential anodizing revealed that at the early stage,coating grew both inwards and outwards.At the later stage of PEO,the anodic current density within the discharge channels could reach up to 104 A cm-2,which was high enough to melt a relatively large area beneath the pancake structure,resulting in the inward grew of PEO coating,meanwhile,water in the discharge channels was decomposed by tremendous heat and thus generating anomalous gas.(2)From 100 to 2000 Hz,under bipolar regime,the intensity of discharges,volume of gas collected,thickness and roughness of PEO coatings as well as content of W species in the coatings increased with increase of current frequency.It took around 2 ms for the discharge channels to be cooled down.At 100 Hz,the interval between discharges was long enough for the cooling of discharge channels,therefore the sparks were uniformly distributed on the sample surface with a small size.At 2000 Hz,however,the discharge channels were not fully cooled due to the short interval between pulses,and discharges were therefore more liable to concentrate in the molten zone,resulting in less number of sparks with bigger size on the sample surface.Sparks were observed throughout the whole PEO process at 2000 Hz while no sparks existed after the termination of current pulse at 100 Hz.The differences in discharging processes gave rise to a more dense PEO coating with less micro defects at 100 Hz,while coating obtained at 2000 Hz showed more micro pores and cracks.At unipolar regime,as a result of no H2 was released at the anodic region,lacking of extra stirring effect of H2 in the electrolyte leads to a higher atomic ratio of W/Al in the coating,hence a darker coating was obtained(3)Sealing treatment was carried out for the PEO coatings in Ce(NO3)3-cintaining and Al(H2PO4)3-containing sealing reagents,new phases like AIPO4,Al(PO3)3 and Al(H2PO4)3 appeared in the sealed coatings while no Ce-containing phase was detected.Pancake structures disappeared after sealing treatment,and the size of micro pores decreased.Potentiodynamic polarization test showed that Ecorr shifted positively after sealing,and jcorr was lower than the unsealed coating.Electrochemical impedance spectroscopy(EIS)test revealed that the Rcorr climbed up from 9.7 kQ cm2 to 82.4 k? cm2 after sealing,no pitting corrosion was observed during the EIS test,which means the corrosion resistance of the magnesium substrate was further improved.In conclusion,based on the most recent development of PEO on magnesium alloys,in this thesis,with the aim of improving the surface properties of magnesium alloy,effects of Na2WO4,frequency and negative current on PEO of AZ31 magnesium alloy were studied.Apart from that,coating formation mechanism of PEO was also explored;sealing treatment was carried out on the as-fabricated coatings.Some progress has been made in the preparation and modification of PEO coatings.