Study on the Structure and Corrosion Resistance of Sealing Micro-Arc Oxidation Coatings of Three Types of Siloxanes in Hydrolytic Solutions at Different pH Values
To further enhance the corrosion protection capability of micro-arc oxidation (MAO) coatings on magnesium alloys,the effects of three different pH values of siloxane hydrolysis solutions on the surface structure and corrosion resistance of MAO/siloxane composite coatings on surface of AZ31B magnesium alloy were investigated. MAO coatings were prepared in a silicate electrolyte system using a constant pressure method. Subsequently,the pH values of the siloxane hydrolysis solutions were adjusted,and siloxane coatings were formed on the MAO coat-ings by immersion. The micro-morphology,porosity,thickness,roughness,hydrophobicity and corrosion resistance of the coatings were studied using scanning electron microscopy (SEM),Image-J image analysis,thickness gauge,surface roughness tester,contact angle goniometer and electrochemical workstation. Results showed that compared to micro-arc oxidation coatings,the MAO/siloxane coatings significantly repaired the micropores and defects on the surface of the micro-arc oxidation coatings. The MAO/siloxane coatings also imparted hydrophobic proper-ties,effectively preventing or slowing down the penetration of corrosive media and enhancing the corrosion resistance of the coatings. Among the three siloxanes compared,the composite coating prepared from KH-570 hydrolysate with a pH of 6 exhibited the lowest porosity,a water con-tact angle of (126.5±2.3)°,and a self-corrosion current density of 3.647×10-8 A/cm2,demonstrating the best corrosion resistance.
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