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 coatings 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 properties， 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 contact angle of (126.5±2.3)°， and a self-corrosion current density of 3.647×10-8 A/cm2， demonstrating the best corrosion resistance.