Preparation Of Superhydrophobic Coating On Zinc And Its Corrosion Resistance

Corrosion of hot-dip galvanized steel is getting more serious,especially in humidity and wet conditions,which result in the farbration of white rust,affecting the appearance quality and performance of the steel.Therefore,passivation treatment is commonly used on hot-dip galvanized steel to improve its resistance to white rust.Being dangerous to environment and human,chromate passivasion is not suitable for the industry.Superhydrophobic material,of which the water contact angle larger than 150?,is widely used in self-cleaning,oil-water separation,anti-fog,anti-fouling,drag reduction and corrosion resistance.However,few researches have been done on constructing a superhydrophobic coating with good corrosion resistant on zinc substrate to replace coatings with toxic chromate passivasion.In this paper,two kinds of micro-nano composite rough structures and different compositions were formed on the galvanized steel using immersion and pulling method,and the superhydrophobic surface of the coatings was prepared successfully after low surface energy material modification.The prepared surface was characterized on the surface composition,structure and wettability,and a further study on the corrosion resistance,chemical stability and mechanical stability and other properties were also carried out.Sol-gel method was used to prepare transparent aluminum oxide sol using aluminum isopropoxide?AIP?as precursor,EAcAc as chelating agent,nitric acid?HNO₃?as catalyst and peptizer at 85?.The alumina coating with rough porous structure was got on zinc at room temperature by immersion and pulling method.Then after the modification treatment with 1.0%1H,1H,2H,2H-perfluorooctyltrimethoxysilane?FAS?-ethanol solution,a superhydrophobic alumina coating was finally obtained.The result of laser particle size analyzer showed that the sol with n?AIP?:n?EAcAc?:n?HNO₃?:n?H₂0?:n?EtOH?=1:1:0.5:200:5 had an average diameter of 99.8 nm,and was stable and transparent;The results of contact angle meter tests showed that the coating had high adhesion to drops and exhibited the nature with the model of rose petals,It was found that the hydrophobicity of the coatings increased at the initial time and finally decreased as the immersion time increased.With the immersion time of 24 h,the coating had the highest contact angle of 152?.Field emission scanning electron microscope?FE-SEM?results showed the alumina roughness film had a micro/nano composite structure with many alumina gel particles on which there is a porous alumina layer,and then obtained the superhydrophobic surface after modified by low surface energy compound;The results of corrosion tests showed that the prepared superhydrophobic surface had provided the diffusion path.The phosphate-cerium coating was prepared by using the cerium salt to carry on the roughness treatment on the traditional phosphate film,and then modified with the sample obtained superhydrophobicity after modified with stearic acid-ethanol solution to get the superhydrophobic surface.The results of contact angle test results showed that the samples exhibited lotus adhesion to droplet and followed the model of petal leaf.The superhydrophobicity of the sample increased first and then decrease with the immersion time,and the sample with cerium treat time of 300 s had the largest hydrophobicity of 162+2?.The results of FE-SEM and energy spectrum analyzer?EDS?results showed that the traditional phosphating film partly dissolved and the cerium salt deposited on the surface of the phosphating film,generating the phosphate-cerium salt composite coating.The coating with rough and porous structure is suitable to obtain the the superhydrophobic surface after low energy modification.The results of corrosion tests showed that the sample immersed in corrosion medium formed a kind of air film layer on the boundary of the solution and the sample,and prevented the corrosive media from getting through the sample.The impedance value of the modified sample was about two orders of magnitude higher than that of pure zinc,showing a good corrosion resistance.However,the corrosion resistance of the prepared superhydrophobic sample decreased with the immersion time,but was still better than that of zinc after 6 h immersion,indicating that the samples can still protect the substrate through the internal phosphating film when the superhydrophobic coating was destroyed.The prepared coating can provide the double protection of hydrophobicity and passivasion on zinc.