An Investigation Of The Formation Mechanism And Corrosion Resistance Of Plasma Electrolytic Oxidation Coated 6061 Aluminum Alloy

The plasma electrolytic oxidation?PEO?coatings were fabricated on 6061aluminum alloy with a home-made WH-1A PEO experimental setup.All the coatings were fabricated on 6061 aluminum alloy at a constant current density of 4.4 A/dm².A series of PEO coatings were detached from aluminum alloy substrate by using an electrochemical method for further researches.The main research contents in clude as follow:The formation mechanism,phase compositons,phase transformation,phase distribution,corrosion resistance and corrosion process of the coatings were investigated as a function of PEO processing time.The results show that:There exists cracks and pores on the outer surface,it will become more obvious over time.With elongating treatment time and increasing the thickness of coating,the elemental composition of C/E interface becomes enriched in Si element.All the C/S interfaces have the aggregation morphology of hemispherical oxides,and the agglomeration bodies become larger over time.The aluminum and oxygen elements remain nearly unchanged for the sample of different treatment time points between the C/S interface and the C/E interface.But the silicon element have a tendency of enrichment from the C/S interface to the C/E interface in the sample T-30.The main phase compositions of PEO coatings are?-Al₂O₃,?-Al₂O₃,mullite and amorphous phases.The amorphous phases in the PEO coatings are mainly alumina and silica.With elongating the treatment time,the content of amorphous phases and?-Al₂O₃ phases decrease,whereas the content of?-Al₂O₃ and mullite are opposite.The amorphous phases are mainly found in the outer layer of the coatings in the sample T-30.There is a compact thin layer at the coating/substrate interfacial region of the PEO samples,and the thickness of this layer maintains at¹?m,does not increase with treating time.There exists cavities on the top of the compact thin layer in the sample T-2 for the first time,and the volume of cavities get bigger with time.The corrosion resistance of coatings is not in direct proportion to the thickness value,the corrosion resistance of coatings is the common function of the thickness and structural feature of coating,so the corrosion resistance of sample T-2 is best.After 40 h immersion treatment in 3.5 wt.%NaCl solution,the outer surface of sample was damaged and the area where near coating/substrate interface was able to detect the corrosion products containing Na and Cl elements.In addition,there are injurious effect on the inner surface morphology,the typical feature hemispherical oxides have been damaged a certain extent.After 720 h immersion treatment in the3.5 wt.%NaCl solution,the corrosion products have been accumulation in the discharge pores,the morphologies of outer surface have been changed.There are already corrosion pits and a mass of corrosion products in the substrate/coating interface.In addition,the typical morphologies of hemispherical oxides virtually disappeared after 720 h corrosion treatment and the the areas that have been corroded become larger.The holistic bulged morphologies that exist in the inner surface have been changed to a great extent.At this stage,the corrosion reactions have tended to complete and the PEO coatings are losing its function of corrosion resistance.