Research On Preparation And Corrosion Resistance Of Micro-arc Oxidation Coating For 6061 Aluminum Alloy

6061 aluminum alloy has obvious advantages as marine structural materials because of its high specific strength,low density,good weldability and easy to process and shape.However,due to its active chemical properties,it has poor corrosion resistance in the harsh marine environment.Therefore,it is very important to improve corrosion resistance of aluminum alloy by surface treatment technology,which is of great significance for prolonging its service life.Micro-arc oxidation(MAO)technology has been widely used in surface modification of valve metals such as magnesium,aluminum,titanium and so on.In recent years,it has attracted much attention in the field of improving corrosion resistance of aluminum alloys.According to the corrosion resistance of aluminum alloy MAO coatings,the process parameters,the concentration of single additive in electrolyte and the formula of compound additive were optimized one by one.The phase composition,microstructure and composition of MAO coatings were characterized by X-ray diffraction,scanning electron microscope and energy spectrum.The corrosion resistance of MAO coatings was studied by drop test and electrochemical test.The research results showed that the process parameters and electrolyte changes would affect the process of MAO coatings formation,thickness,morphology and structure compactness,thus affecting the resistance corrosion of MAO coatings.MAO coatings were mainly composed of Al2O3,and the surface is a typical porous "volcano jet" morphology.There were many small particles sintering and pie accumulation around the hole.In the basic sodium silicate electrolyte,with the increase of the input voltage,the thickness of MAO coating increased,the number of discharge holes increased first and then decreased,the diameter size of discharge holes increased,the structure compactness decreased and the corrosion resistance increased first and then decreased.When the input voltage was 450V,the corrosion resistance of the MAO coating was the best.Meanwhile,with the increase of the oxidation time,the thickness of MAO coating increased,the number of discharge holes decreased,and the diameter size of discharge holes enlarged.There were more and more "worm-like"gullies on the surface,and the structure compactness decreased,so the corrosion resistance increased first and then decreased.When the oxidation time was 15min,the corrosion resistance of the MAO coating was the best.The optimum process parameters were as follows:input voltage 450 V,oxidation time 15 min.Using optimized process parameters,when adding a single additive Na6(PO3)6 into the basic sodium silicate electrolyte,with the increase of its concentration,the MAO coating became thicker,the number of discharge holes did not change obviously,the pore size was slightly larger,the structure compactness decreased due to the appearance of pits and holes,so the corrosion resistance increased first and then decreased.When the concentration of Na6(PO3)6 was 0.75g/L,the corrosion resistance of the MAO coating was the best.When adding a single additive H3BO3 into the base sodium silicate electrolyte,with the increase of the concentration,the thickness of the MAO coating decreased,the number of discharge holes and the pore size coating reduced,and the MAO coating structure became uniform and compact.The gully in the film gradually disappeared,so the corrosion resistance of the MAO coating increased first and then decreased.The MAO coating with the best corrosion resistance was obtained when the concentration of H3BO3 was 1.5g/L.Through the mixed experiment of Na6(PO3)6 and.H3BO3 binary additives,the MAO coating was modified by Na6(PO3)6 and H3BO3 together.The thickness,microstructure and structure of the MAO coating still showed regular changes.When the concentration of Na6(PO3)6 and H3BO3 was 0.75g/L and 1.5g/L respectively,the equivalent resistance Rp of porous layer and the equivalent resistance Rb of barrier layers reached the maximum.Compared with MAO coating obtained in the basic sodium silicate electrolyte with the same process parameters,the value of Rp increased by one order of magnitude and the value Rb increased by two orders of magnitude.So the corrosion resistance of the MAO coating was greatly improved.Compared with the polarization curve of 6061 aluminum alloy matrix and its micro-arc oxidation sample,the self-corrosion potential of the samples after micro-arc oxidation was positively shifted to about 180mV,the self-corrosion current Icorr decreases by three orders of magnitude,and the polarization resistance RP increases by three orders of magnitude.The results showed that the MAO coating had strong corrosion resistance and could protect the substrate well.In the immersion test of corrosion medium,the corrosion tendency increased obviously with the dissolution and fall off of the coating at the early period of immersion.In the later period of immersion,the discharge channels were blocked by the partially dissolved coatings and the precipitated corrosion product,which could effectively blocked the corrosion medium into the coatings.So the increase of corrosion tendency was inhibited.