Preparation And Properties Of Superhydrophobic Cerium Salt/Silane Composite Conversion Coating

Since conventional chromate passivation is harmful to the human body and the environment,the chromium-free passivation technology on the surface of the zinc layer has always been a research hotspot.In many chromium-free passivation studies,cerium salt passivation technology has become one of the technologies that may replace chromic acid passivation because of its green non-toxicity,simple process,and good corrosion resistance.However,the anti-corrosion performance of a single cerium salt conversion coating cannot meet the application requirements.The composite coating formation of silane and cerium salt can effectively reduce the cracks of the cerium salt coating and improve the barrier property,ultimately enhance the corrosion resistance of the coating.In this paper,vinyl trimethoxy silane with hydrophobic groups was chosen,and by using two different methods of preparing passivation solution the cerium salt/silane composite conversion coating with different hydrophobic properties was prepared on the surface of the zinc layer by a simple one-step immersion method.The growth process and corrosion resistance of the coating were studied,and the influence of two different preparation methods on the coating formation mechanism of the coating was analyzed.The main findings were as follows:Cerium salt/silane passivation solution modified via CeO₂nanoparticles was synthesized by mixing CeO₂ nanoparticles modified vinyltrimethoxysilane hydrolysate and cerium nitrate solution,thus a hydrophobic cerium salt/silane composite conversion coating was prepared on the surface of the zinc layer.The water contact angle of the coating reached 147.8°after passivation for 60 min,which was close to superhydrophobic,which was higher than the 120.7°of unmodified cerium salt/silane composite coating and the 27.6°of the single cerium salt conversion coating.Meanwhile,the electrochemical low-frequency impedance value(|Z|_j?0)of the coating was 7.71×10⁵?cm²,which was one and two order of magnitude higher than unmodified cerium salt/silane composite coating and single cerium salt conversion coating,respectively.Hence the coating displayed the best corrosion resistance.The polarization curve corrosion current density was 1.50×10^-2?A cm^-2,which was one and two order of magnitude lower than the unmodified cerium salt/silane composite coating and single cerium salt conversion coating,separately.These indicates that the anti-corrosion performance of the modified cerium salt/silane composite hydrophobic coating was much better than that of the unmodified cerium salt/silane composite coating and the single cerium salt conversion coating.This is mainly due to the modified cerium salt/silane composite hydrophobic coating was thicker and denser,and its defects such as cracks and/or holes are less than those of the other two coatings.Furthermore,the improvement of the hydrophobicity of the coating was beneficial to block the penetration of corrosive media.And also improve the corrosion resistance of the coating.On the aforementioned basis,CeO₂ nanoparticles and Ce(NO₃)₃ were added to the hydrolysis solution of vinyltrimethoxysilane at the same time to participate in the hydrolysis reaction,thus forming CeO₂ nanoparticles and Ce³⁺ions co-modified cerium salt/silane passivation solution.The water contact angle of the coating with a passivation time of 60 min and above exceeds 150°,indicating that the coatings was superhydrophobic.This was mainly attributed to synergistic modification of CeO₂ nanoparticles and Ce³⁺could increase the surface roughness of the coating,which provided a suitable micro-nano rough structure for the coating to achieve superhydrophobicity.The contact angle of the coating reached 161.9° after passivation for 90 min,which owned the best hydrophobicity.In the meantime,under the influence of low surface energy modification of silane with hydrophobic vinyl groups,the coating showed superhydrophobic behavior.The superhydrophobic coating showed best corrosion resistance after passivated for 90 min,and its low-frequency impedance value reaches4.80×10⁶?cm²,which was two and three orders of magnitude higher than Ce³⁺ion modified cerium salt/silane composite conversion coating and single cerium salt coating.Its corrosion current density decreased to 3.38×10^-3?A cm^-2,which was one and three order of magnitude lower than Ce³⁺ion-modified cerium salt/silane composite conversion coating and single cerium salt coating.Which could be attributed to the cerium salt/silane composite conversion coating modified by CeO₂ nanoparticles and Ce³⁺ions were dense and owned fewer cracks.At the same time,the coating was superhydrophobic,which could produce an air layer at the interface in the corrosive medium to block the infiltration of corrosive medium,which significantly improved the anti-corrosion of capacity the coating.By comparing two preparation methods,we found that the CeO₂ nanoparticles in the CeO₂nanoparticles and Ce³⁺ions co-modified cerium salt/silane passivation solution owned a better dispersion than the CeO₂ nanoparticles modified cerium salt/silane passivation solution.CeO₂nanoparticles modified cerium salt/silane composite hydrophobic coating formation process mainly was cerium salt coating,which based on cathode deposition mechanism,and silane participates in the coating formation,while the formation process of cerium salt/silane composite superhydrophobic film modified by CeO₂ nanoparticles and Ce³⁺ions is that cerium salt and silane participate in the film formation.Both coatings were composed of CeO₂/Ce(OH)₄,Ce₂O₃/Ce(OH)₃,Si-O-Ce and Si-O-Si silane networks.