| Micro arc oxidation (MAO) is a surface treatment process for fabricating ceramiccoatings on the surface of valve metals. The MAO coatings are mainly of oxides of thesubstrate materials, which can protect the substrates from corrosion and improve the hardness,wear resistance and electrical insulation properties as well. Micropores are usually inevitablebecause of the sparking discharge that dominates the growth of MAO coatings. Thesemicropores will become a passage through which the corrosive agent penetrates the MAOcoating and reaches the substrate. Therefore, these micropore defects become possiblepreferential sites of corrosion initiation of metal substrate. The improvement of anti-corrosionproperty of MAO coating is limited by its inherent porous characteristic. In order to fully useits high comprehensive properties, sealing post-treatment of MAO coatings is very necessarywhen the coatings are applied in severe corrosive environment. In this study, two sealingtechnologies have been established to improve the protective performance of MAO coatingsprepared on Al alloy6061.Stearic acid and silane coupling agent were employed respectively to carry out sealingtreatments in this work. The morphology of the MAO coatings, as prepared and sealed, wasobserved using SEM and the morphology variation was analyzed. The microstructure of thesealing layers was characterized by infrared spectroscopy. The wetting property of thecoatings was evaluated by a measurement of contacting angles. Potentiodynamic polarizationcurves and neutral salt spray test were carried to measure the protecting performances ofMAO coatings, as prepared and sealed, and the effects of sealing treatments were evaluated.EIS method was applied to analyze the corrosion mechanism of the unsealed and sealed MAOcoatings during immersion in corrosive agent.Results:The duration of MAO processes was found to influence the characteristics as well ascorrosion resistance of MAO coatings intensively. The thickness and surface roughness ofMAO coatings increased as the MAO proceeded and the defects including the micro poresand micro cracks grows larger. The performance for corrosion protecting of the MAOcoatings improved persistently when MAO time was less than60minutes and thenmaintained at a certain level when the MAO process further proceeded. A protecting effect could be observed on the MAO coatings, but the presence of the micropore defects restrictsthe protecting performances at a relatively low level.Sealing treatment used stearic acid was found effective to seal micropores andmicro-cracks in the MAO coating, and then the corrosion resistance of MAO coating wasimproved. MAO coatings prepared under different oxidation time presented similar corrosionresistance after sealing treatment by stearic acid during the short-term immersion test in3.5%NaCl solution. For immersion testing lasting for50h or longer, thicker MAO coatingsexhibited better corrosion resistance.When silanization treatment was applied to MAO coating, not only the residualdischarge micropores and microcracks were sealed, a complete silane layer formed on thesurface of MAO coatings. The improvement in the microstructure greatly enhanced theprotecting performances of the MAO coatings to the substrate. The sealing effect wasobserved better when short duration of MAO was performed, since the smaller micro-pores aswell as fewer micro-cracks are favorable for integral silane film to form. Despite the positiveeffect of silanization treatment, it was found that the MAO coatings dominated the protectingperformances when the immersion time was performed for longer than800hours in3.5%NaCl solution.Comparatively, silanization treatment promoted the protecting performances of MAOcoating from corrosion more effectively than stearic acid sealing treatment. MAO coatingswith thickness of8microns treated by silanization can restrain well after3000hours exposureto the neutral salt spray, indicating excellent corrosion resistance and good sealing effect ofsilanization. |
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