不同复盐电解液中NaVO3对铝合金微弧氧化热控膜层结构和性能影响的对比研究OA北大核心CSTPCD

The work aims to employ two types of double salt electrolytes consisting of Na2SiO3-Na3PO4 and Na3PO4-Na2SiO3 dominated by Na2SiO3 and Na3PO4,respectively,with a consistent concentration gradient of NaVO3 to fabricate the micro-arc oxidation coatings on the surface of A356 aluminum alloy substrates and investigate the effect of the coloring salt NaVO3 on the microstructure of the micro-arc oxidation coatings.Comparisons were made regarding the alterations in blackness,thermal control property,corrosion resistance,and adhesion strength of the coatings,and the merits and drawbacks of the micro-arc oxidation coatings derived from the two types of double salt electrolytes were assessed.The experimental design was executed by a proportional division batch method.Color Reader,eddy current thickness meter,scanning electron microscope (SEM),laser scanning confocal microscope (LSCM),electron microprobe (EPMA),energy dispersive spectrometer (EDS),X-ray diffraction (XRD),emissometer,and solar spectrum reflectometer were employed to characterize the color,thickness,surface morphology,cross-sectional morphology,elemental distribution,surface roughness,phase composition,emittance,and absorptance of the coatings.The corrosion resistance of the coatings was evaluated through electrochemical test,spot test,salt spray test.The adhesion strength of the coatings was examined via thermal shock test.The results revealed that,firstly,the newly synthesized vanadium oxide within the micro-arc oxidation coatings played a crucial role in determining the dark color of the coatings following the addition of the coloring salt NaVO3 in both double salt electrolytes.The color of the coatings initially intensified,before gradually softening,transitioning from light brown to dark brown and to black,and eventually settling at a light black hue.At a concentration of 12.06 g/L of NaVO3,corresponding to the S3 and P3 schemes,the prepared coatings exhibited the darkest color with the blackness value of the S3 and P3 schemes reaching 23.41 and 20.31,respectively.The coatings fabricated in the Na3PO4 dominant electrolyte were comparatively darker.At a concentration of 19.98 g/L of NaVO3,ablation occurred,precluding the formation of a complete coating.This suggested that an excessively high concentration of NaVO3 was unfavorable for the growth of micro-arc oxidation coatings.Secondly,among two types of double salt electrolytes,the coatings obtained from the 3# scheme,characterized by its darkest color,minimal blackness value,and maximum thickness,demonstrated the most superior thermal control properties.Specifically,the absorptance and emittance of the coatings in the S3 scheme were 0.932 and 0.888,respectively,while those in the P3 scheme were 0.934 and 0.841,respectively.Therefore,both the S3 and P3 coatings should be categorized as high-absorptivity and high-emissivity thermal control coatings.Thirdly,upon introduction of coloring salt into two types of double salt electrolytes,self-sealing pores manifested on the surfaces of the micro-arc oxidation coatings,which proved to be highly advantageous in enhancing the corrosion resistance of the coatings.The corrosion resistance of the coatings fabricated in the Na3PO4 dominant electrolyte surpassed that of the coatings fabricated in the Na2SiO3 dominant electrolyte in both acidic and neutral chloride corrosive environments.Compared to the control group without coloring salt,the coatings fabricated with the 3# solution demonstrated the highest corrosion resistance.The corrosion current density was diminished by an order of magnitude,while the linear polarization resistance was augmented by at least an order of magnitude.In addition,the corrosion potential experienced a positive shift of 0.2 V.Among them,the corrosion resistance of the coatings obtained from the P3 scheme exhibited a slightly superior performance compared to those in the S3 scheme.Following a 1365 hours of neutral salt spray test,a negligible quantity of corrosion products manifested on the coating surfaces in the S3 and P3 schemes,with the corrosion products on the P3 surface being even more scarce.Finally,the results of the thermal shock test indicated that the thermal shock resistance of the coatings obtained in the Na2SiO3 dominant electrolyte was superior to that obtained in the Na3PO4 dominant system,and the coatings from the 3# scheme 3 experienced earlier peeling in comparison to those in the 2# scheme in each system.However,even the coating from the less favorable P3 solution could withstand 65 thermal shocks at high temperature of 200 ℃.In summary,vanadium oxide,which makes the coating darker,is deposited in the coatings in both double salt electrolytes after addition of the coloring salt NaVO3.When the concentration of NaVO3 is 12.06 g/L,the thermal control coatings with excellent corrosion resistance,good adhesion strength,high absorptivity,and high emissivity can be obtained in each system.The coatings based on the Na3PO4 dominant system demonstrate a slightly superior performance.