Construction And Corrosion Resistance Of Superhydrophobic Micro-nanostructure On The Surface Of Magnesium Alloy

As the lightest structural materials,magnesium alloy has excellent physical and mechanical properties and has become a hot spot in the industrial manufacturing now.However,the nature of magnesium is very active and easily oxidized to form a loose oxide film on the surface,which causes accelerated corrosion.Low corrosion resistance seriously hinders the wide application of magnesium and its alloys.Therefore,it is of significance to improve the corrosion resistance of magnesium alloys.Superhydrophobic coating can effectively isolate the contact between the corrosive liquid and the substrate to a certain extent,thereby greatly reducing the corrosion probability and rate.The construction of surface roughness and low surface energy material modification are necessary to prepare superhydrophobic coatings.However,most of the methods usually involve the introduction of micro-nanostructures from externally introduced materials,and the adhesion strength to the substrate is poor.Some require special equipment,complicated preparation process,high cost,or use of strong acid and some environmentally unfriendly organic reagents,and causes more pollution by-products.Therefore,the Mg?OH?₂ protective layer directly grown on the surface of the magnesium alloy by hydrothermal treatment,being of a micro-nano-scaled roughness,good adhesion to the substrate.Moreover,a large amount of hydroxyl groups on the surface is favorable for surface modification.Stearic acid?STA?is a long-chain acid,with hydrophobic effect,grafted onto the surface of magnesium alloy via covalent bonds after the dehydration condensation between carboxyl groups of STA and hydroxyl groups on the surface of magnesium alloy to reduce the surface energy of magnesium alloy.Fluorosilane?FAS?is easily hydrolyzed to form fluorine-containing silanol,which can be dehydrated and condensed with the surface hydroxyls on magnesium alloy or other silanol groups to form covalent bonds or Si-O-Si bonds.These strong bonding ensured the strong connection,and C-F groups significantly reduced the surface energy to achieve superhydrophobic effect.In this dissertation,a hydrothermal treatment without any chemical additive is used to directly grow a Mg?OH?₂ protective coating with uniform,good adhesion and micro-nano roughness on the magnesium alloy AZ31B substrate.Then two superhydrophobic coatings were obtained by modification of stearic acid?STA?and 1H,1H,2H,2H-tridecafluorooctyltriethoxysilane?PTES?on Mg?OH?₂ protective coating.These three coatings underwent different characterizations and tests.The results show that 3 h of hydrothermal treatment was most suitable for the construction of superhydrophobic coatings,and the formed Mg?OH?₂ coating had a certain protective effect against corrosion.For the superhydrophobic anti-corrosion coating?SAC?obtained by stearic acid?SAC-STA?,the best superhydrophobic effect with the water contact angle?WCA?of 159°and sliding angle?SA?of 7°was achieved when the STA concentration was 7 mmol/L and the modification time was 4h.In the electrochemical test,the E_corr of the best SAC-STA sample was increased from-1.5220 V?naked AZ31B?to-1.3264 V,and the i_corr was reduced by more than 2orders of magnitude.For the fluorosilane superhydrophobic anti-corrosion coating?SAC-PTES?,the WCA value was as high as 164°.Interestingly,water droplet were difficult to adhere onto its surface with excellent self-cleaning performance,the sample being left for eight months.In the electrochemical test,the E_corr of the SAC-PTES sample was further increased to-1.3248 V,and the i_corr was reduced by more than 4 orders of magnitude.In the immersion test of3.5 wt.%NaCl aqueous solution for 10 days,the superhydrophobic magnesium alloy surface only corroded along X cuts without any extension,showing excellent corrosion resistance and durability.In addition,the corrosion protection mechanisms of two superhydrophobic coatings were discussed and the adhesion of two coatings was also investigated.