Fabrication And Corrosion Resistance Of Superhydrophobic Coatings On The Surface Of Magnesium Alloy

Magnesium（Mg）and its alloys are vital engineering materials due to their light weight,high stiffness and good machinability.However,high surface reactivity and poor corrosion resistance limit their applications and reduce their economic value.Hence,surface modification and the construction of protective coatings are necessary to improve corrosion resistance and extend the service life of Mg-based materials.Though physical barrier coatings have been widely used for corrosion protection of Mg alloys,they cannot satisfy the long-term service of Mg alloys in some complex service environments,especially in moist environments or in corrosive media containing chloride.In this case,it is necessary to prepare a superhydrophobic surface on Mg alloys to ensure their water-repellent properties.The superhydrophobic surfaces can reduce the contact area between the surface and the corrosive media,thereby improving the corrosion resistance of Mg alloys and the stability of the anti-corrosion coatings.In addition,the superhydrophobic surfaces have some special functions and can further broaden the industrial application range of Mg alloys.In this paper,we used AZ31,one of the most widely used Mg alloys,as the substrate and constructed different superhydrophobic protective coatings with hierarchical macro/nanostructures on its surface.We systematically investigated its wettability,adhesion force and corrosion protection performance in a 3.5 wt%Na Cl solution.Moreover,we proposed the possible mechanisms underlying the formation and corrosion resistance of the superhydrophobic e-DTMS coating.The main results are described as follows.（1）The superhydrophobic dodecyltrimethoxysilane（e-DTMS）coating with hierarchical macro/nanostructures was successfully constructed on the surface of AZ31 by one-step electrodeposition in the dodecyltrimethoxysilane（DTMS）-containing precursor solution.The morphology and corrosion resistance were strongly influenced by the applied potential and deposition time.The e-DTMS coating prepared at-1.9 V for 400 s had an excellent self-cleaning performance with a contact angle of 158°±2.3°,and it also had excellent stability.The e-DTMS surface significantly reduced the contact area between the alloy and water.The corrosion current density(2.11×10^-7 A·cm^-2)of the coated AZ31 decreased by about two orders of magnitude compared with 7.34×10^-5A·cm^-2 of the uncoated AZ31.The results demonstrated that the e-DTMS coating remarkably enhanced the anti-corrosion performance of AZ31 alloys.Furthermore,the e-DTMS coating could still maintain its high anticorrosion performance after 250 h of immersion in the 3.5 wt%Na Cl solution.（2）To enhance corrosion resistance,a hydroxyl（-OH）-rich Mg（OH）₂ bottom layer was first fabricated on the surface of AZ31 by the hydrothermal method.Then,the superhydrophobic e-DTMS coating was fabricated by electrodeposition to eliminate the defects of the Mg（OH）₂ bottom layer.The Mg（OH）₂/DTMS composite coating was composed of walnut-like hierarchical micro-nano structures,and the Mg（OH）₂ bottom layer could increase its superhydrophobicity and corrosion resistance.Moreover,we performed a potentiodynamic polarization test and found that the corrosion current density of the composite coating was 1.777×10^-8 A·cm^-2,which decreased by about three orders of magnitude compared with 7.34×10^-5 A·cm^-2 of uncoated AZ31.The results suggest that the Mg（OH）₂/DTMS composite coating had excellent corrosion resistance.The electrochemical impedance spectrum and hydrogen evolution immersion experiments showed that the Mg（OH）₂/DTMS composite coating could provide long-term corrosion protection for Mg alloys.（3）To enhance corrosion resistance,a polypropylene（PP）layer was fabricated on the DTMS coating on AZ31 Mg alloy.Briefly,the DTMS coating was first fabricated on AZ31by electrodeposition.Subsequently,the PP coating was constructed on the surface of the DTMS coating to prepare the Mg（OH）₂/DTMS composite coating.The PP layer was composed of micro-scale spherical particles and had high superhydrophobicity and chemical stability.The DTMS layer significantly improved the adhesion between PP and AZ31.The scratch tests revealed that the critical load for the DTMS/PP composite coating was 1482 m N.The corrosion resistance experiments showed that the DTMS/PP composite coating remarkably enhanced the anti-corrosion performance of Mg alloys.（4）Si O₂nanoparticles were modified by DTMS and added into the PP layer.The Mg（OH）₂/PP-60m Si O₂ composite coating was prepared by the hydrothermal method and dip-coating method.DTMS modification could convert hydrophilic silica to hydrophobic one which improved the dispersibility of silica in xylene.Therefore,the integration of DTMS-modified silica could enhance the thermal stability and wear resistance of the PP layer.Meanwhile,the interlayer Mg（OH）₂ could significantly enhance the adhesion force between PP and AZ31 substrate.The composite coatings could serve as a barrier to prevent the penetration of corrosive chloride ions into the matrix,thereby remarkably improving the corrosion resistance of magnesium alloys.The composite coating could remain complete and compact after 250 h of immersion,indicating that it was highly stable and could effectively protect Mg alloys from corrosion for a long time.