Microstructure And Corrosion Resistance Of Micro Arc Oxidation Coatings On Aluminium And Aluminium Alloy

Aluminum and its alloys were widely used in the engineering field owing to their low density,high specific strength and good plasticity.However,their softness,poor corrosion and wear resistance restricted their further application to some extent.Therefore,it is especially importment for aluminum and its alloys to be modified by surface strengthening treatment.Among many technologies for surface strengthening modification of aluminum and its alloys,micro-arc oxidation（MAO）showed good application foreground due to its various advantages such as easy operation,simple process,environmental protection,corrosion-and wear resistance,biological and optical properties.Nevertheless,the complex reaction processes and variable technological parameters of the MAO technology led to variability in the coating evolution,inconsistency in the cognition of growth mechanism and diversity in the corrosion resistance.In this dissertation,both AA1060 Al and AA6061 Al alloy were used as the substrate materials.The dissertation dealt with the effects of the variations in the technical parameter,including the surface condition of a substrate,electrolyte composition,electrical parameter and external conditon of the MAO process.The dissertation studied the effects of the variations in the technical parameters on the microstructure evolution and corrosion resistance of the MAO coatings on Al and its alloys,which combined with the observation analysis of the MAO coating obtained by electrochemical detachment method.The relationship between the corrosion resistance and microstructure of the MAO coatings was also analyzed.Further,the effects of the variations in the technical parameters on the effect mechanism of discharge characteristics and the growth process of the MAO coatings were discussed.The three aspects mentioned above could provide certain theoretical support for the praticial application of MAO technology.Hence,the exploration of the microstructure evolution rule,growth mechanism and corrosion resistance of the MAO ceramic coatings on Al and its alloys during MAO might provide more theoretical and scientific data support for basic development of the MAO technology and optimal regulation of the property of the relevant MAO coatings on Al and its alloys.The obtained experimental results of the effect of a pre-treated high-temperature film on MAO of 6061 Al alloy showed that the formation of the typical bulging and pit structures on the surface of the resultant MAO coatings（specimens with treatment time of2 min,4 min and 8min）was tightly associated with different sequences of the plasma discharges in the Mg-rih and Al-rich areas.The formation of the crack on the surface of the resultant MAO coating was associated with different sequences of the plasma discharges at Mg-rich and Al-rich areas,weak binding properties due to different sequences of the products,the difference in the physical state during the confluence of two products and the different chemical compositions of two products.The obtained experimental results of the effect of adding ZnO particles on MAO of1060 Al showed that ZnO particles were inertly incorporated in the resultant coating.The Zn element in the obtained ZnO-incorporated coating was predominantly resident in the nodules on the coating surface observed in the surface morphology,and that it was mainly distributed in the surface layer of the coating observed in the cross-sectional morphology.Compared with the ZnO-free coating,a larger population of smaller-sized complex pancake structures were created in the ZnO-incorporated coating,especially complex sealed-hole pancakes,which may be triggered by B-type plasma discharges with reduced intensity but enhanced density and lower sintering temperature for the resultant molten materials during MAO affected by the incorporation of ZnO particles.The corrosion resistance of the ZnO-incorporated coating was strengthened.The R_pvalue of the ZnO-incorporated specimen was approximately 1.5 times larger than that of the ZnO-free specimen and the R_invalue of the inner coating in the ZnO-incorporated specimen was approximately 1.7 times than that for ZnO-free specimen.The obtained experiment results of the effect of various positive-negative voltage ratios on MAO of 1060 Al showed that the use of the negative voltage facilitated the increase in the thickness of the MAO coating,while the use of the positive voltage facilitated the improvement in the coating compactness.Compared with the specimens processed under the conditions of 10 min+0 min and 0 min+10 min through the two-step treatment regime,the specimens processed under the conditions of 3 min+7 min,7 min+3min and 5 min+5 min specimens through the two-step regime possessed better corrosion resistance,which had an order of magnitude higher value of the polarization resistance R_p,an order of magnitude lower value of the donor concentration N_qand a slightly higher value of the inner coating resistance R_in.The obtained experimental results of the effect of introducing ultrasound on MAO of1060 Al showed that the introduction of ultrasound caused more pancake structures with smaller dimensions in stagesⅡ-Ⅳand improved the corrosion resistance of MAO coatings prepared in stagesⅡ-Ⅳto a certain extent.Along with the increase of the ultrasound powers,the coating thicknesses increased and their surface arithmetic mean height S_adecreased for the coatings concerned.Also,the areas of the nodule structures decreased and the Si elements in the coatings changed from the surface distribution to the homogeneous distribution along with the increase of the ultrasound powers.The specimen treated under the ultrasonic power of 108 W exhibited the best corrosion resistance and its value of polarization resistance R_pwas approximately 2.3,1.5,2.1 times greater than those of the specimens under the ultrasonic powers of 0 W,72W and 144 W.