| Self-assembled film is a highly ordered ultrathin film formed by the self-assembly of organic molecules on a solid surface or at solid-liquid interface,which can effectively isolate the metal substrate from the corrosive medium and retard the corrosion of the base metal.Molecular self-assembly processes are mostly carried out in organic solvents,but aqueous solutions containing corrosive substances are the most commonly used solutions.Therefore,it is very important to develop a typical self-assembly method on the iron surface in water phase.In recent years,the molecular self-assembly technology related to the coordination of metal ions has made great progress,showing a good application prospect in the fields of sensing,separation,catalysis and metal protection.The application of the new self-assembly technology induced by coordination of metal ion is beneficial to the construction of multi-layer assembly films(chemical conversion films)with certain thickness on the surface of metal substrates,so as to replace environmentally hazardous phosphating film and chromate passivation film.In this paper,we confirmed the feasibility and superiority of the self-assembly technologies in water phase and constructed a series of chemical conversion films with micron thickness based on phosphonate(organic phosphonic acid)-metal complex on the surface of iron by adopting self-assembly technology induced by coordination of metal ions.Herein,natural phosphate esters(organic phosphonic acid)with multiple P-OH functional groups,were used as major film-forming substance.This film has excellent corrosion resistance,and can avoid the precipitation problem in the preparation process of traditional chemical conversion film.It is expected to achieve the purpose of using organic phosphating film(low phosphorus)to replace the traditional inorganic phosphating film.Then,the application of self-assembly technology induced by coordination was extended to polymer-polyacrylic acid molecules to prepare environmentally-friendly organic-inorganic anti-corrosion chemical conversion coating on the surface of iron substrate,realizing the goal of preparation of phosphate-free chemical conversion film.The main research content and experimental results of this paper are summarized as follows:(1)Study in the morphology and protective properties of two different bis(2-ethylhexyl)phosphate self-assembled thin films prepared on an iron substrate in water and ethanol solventsAlkyl phosphate esters contain both O and P atoms and phosphate group has a good affinity for oxidized iron surface.Therefore,they can be used as amphiphilic molecule,which can diffuse,adsorb and assemble at the solid/liquid interface.In this paper,bis(2-ethylhexyl)phosphate(BEP)self-assembled thin films were prepared on iron substrates in water and ethanol solutions respectively and the formation mechanisms were investigated by means of ATR-FTIR,XPS and AFM.Polarization curves and EIS tests were used to study the corrosion protection performance of the film to the iron substrate.The results showed that in the ethanol solution,the BEP SAMs formed on the iron surface by both electrostatic interaction and chemical covalent binding,produced island-like molecular aggregates;while in the aqueous solution,the BEP SAMs formed on the iron surface only through P-O-Fe(substrate)bond between iron substrate and BEP molecules,generated nodules aggregates.When used as anticorrosive film in 0.5 M H2SO4 solution,the self-assembly film formed in water phase owned better protection performance than that formed in ethanol phase.(2)One-step self-assembly method to fabricate new types of phytic acid-metal conversion films with excellent corrosion resistance on iron substrateThe coordination-driven self-assembly technique was used to prepare a series of phytic acid-metal complex coatings by immersing the iron sample in the mixed solution containing phytic acid and metal ions.Phytic acid has a good affinity to iron surface and can be assembled to iron substrate surface to form phytic acid film.Because the Phytic acid film is thin(0.56 ?m)and porous,the corresponding corrosion resistance is poor and protection efficiency in 3.5wt.%NaCl is lower than 41%.Layer-by-layer growth of phytic acid-metal complexes on iron surface can be realized through the bridging effect of the co-existing metal ions.The prepared phytic acid-metal complexes films are thicker(>15?m)and denser.They have superior corrosion protection properties and protection efficiencies are over 70%.The present method was applicable to the film-forming process of a variety of metal ions,such as 2Zn2+,Ca2+,Co2+ and Ni2+.Phytic acid-metal films mainly contain phytate-metal complexes and the corresponding metal oxide or hydroxide.Among them,phytic acid-Zn film shows the best corrosion resistance and the protection efficiency is greater than 95%(3)Fabrication and insight into the self-strengthening mechanism of 1-hydroxyethylidene-1,1-diphosphonic acid(HEDP)-Zn complex anticorrosion coating on the surface of cold-rolled steel(CRS)In this chapter,based on the properties of HEDP to form insoluble complexes with iron ions or zinc ions,we designed and fabricated a new environmentally-friendly chemical conversion coating(HEDP-Zn coating).The strong complexation ability of HEDP to iron ions allowed the HEDP molecules to adsorb onto iron substrate in the form of self-assembly,and the participation of zinc ions in the film-forming process made the coating grow in three dimensions.When immersed in NaCl solution,the cold-rolled steel modified by HEDP-Zn film goes through three stages:strengthening period of corrosion resistance,balance stage and weakening stage of corrosion resistance.The HEDP-Zn coating prepared with film-forming time of 10 s exhibited the optimal self-strengthening effect in NaCl corrosive solution:the protection efficiency of the HEDP-Zn coating increased from 93.4%to 99.3%after 2 h immersion,and after 6 h immersion,the corrosion resistance was still better than original coating.XPS,ATR-FTIR,FE-SEM and EDS were used to analyze the changes of surface microstructure and chemical composition of HEDP-Zn coating in NaCl solution under different immersion time.Finally,A possible self-strengthening mechanism was proposed.With the increase in immersion time in NaCl solution,the HEDP molecules and zinc ions from the destroyed HEDP-Zn complexes escaped from the coating,accompanied by the intercalation and departure of Na+throughout the whole process.Thus,the HEDP-Zn coating became compact and produced additional binding sites,which resulted in a greatly enhanced anticorrosion ability(4)Anticorrosion organic-inorganic hybrid films constructed on iron substrates using self-assembled polyacrylic acid as a functional bottom layerConsidering the distinguishing features of PAA polymer in terms of structure and performance,the self-assembled PAA layer was first prepared on iron substrates by a spontaneous assembly process of PAA polymer in alcohol-water mixed solution.The PAA film was further functionalized using cerium nitrate solution,zinc nitrate solution or mixed solution to construct based on PAA-metal complexes organic-inorganic hybrid film.On the basis of XPS analyses,PAA films are mainly formed by coordination of iron ions dissolved under weak acidic conditions to deprotonated carboxyl.When using Ce3+ or Zn2+to modify the PAA bottom layer,a part of metal ions is bound onto PAA molecules as PAA-metal complex,and another part of metal ions is transformed into metal oxides.The electrochemical results showed that in 3.5wt.%NaCl solution,the protective efficiency of PAA film on the iron substrate was less than 40%.After further modification with metal ions,the hybrid films present excellent corrosion protection performance and the protective efficiency can reach 98%,Moreover,the anticorrosion performance of both films strongly depended on the chemical compositions and microstructure of hybrid films. |
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