| Steel is one of the most widely used metal materials in industrial production.However,it is easily subjected to corrosion in various corrosive environments.So corrosion not only brings about wasting of resources and economic loss but also can cause environmental pollution and seriously affect human's normal production and life.Therefore,it is of great significance to investigate the corrosion mechanism of metals and develop effective corrosion prevention techniques.Metal surface pretreatment process was very necessary before use(for example paint coating),by means of which chemical conversion coatings can be formed directly on the metal surface,thereby achieving the goal of increasing the anticorrosion properties of themetal substrates and promoting the overall performance of the painting system.At present,the most widely used metal pretreatment techniques are chromate passivation process and phosphating process.However,both techniques can cause serious ecological and environmental pollution.With the increasing of environmental awareness,people have gradually realized that the two technologies can give rise togreat environmental and ecological hazards.Therefore,the two kinds of pretreatment technologies were gradually restricted and even banned.Based on this,it is of great social significance and application value to develop a new environmental-friendly pretreatment technology.On the basis of consulting many related literatures,we have found that some phosphate molecules with multifunctional groups can form ordered molecular layers on metal surface due to strong physical or chemical adsorption of-POH groups on the metal surface,thereby effectively preventing the metal substrate from corrosion attack in different corrosive environments.At present,most studies surrounding this important task still concentrate on the application of phosphate compounds as corrosion inhibitors.Our previous studies have demonstrated phosphate esters may be spontaneously self-assembled on the iron surface in water solution,forming phosphate thin films with good corrosion resistance.Herein we explored the applications of organic phosphonic acid in the fabrication of environmentally-friendly chemical conversion layers as well as in situ phosphorization.Some new results have been obtained.The main research contents and the obtained results are summarized as follows:(1)A simple method to construct new chemical conversion coating on the cold-rolled steel(CRS)substrate was established by making use of good affinity of multifunctional phosphate ester for the oxidized iron surface and strong complexation ability to metal ions Zn2+.In this study,a phosphonic acid with multifunctional groups,amino trimethyphosphonic acid(ATMP),was selected as film-forming material and used to construct conversion layers in the presence of a certain concentration of zinc salt.At first,by adjusting the pH of the solution and the concentration ratio of ATMP and zinc ions,a dense chemical conversion film with a three-dimensional space structure and micron-scale thickness was prepared on the CRS substrate.Next,scanning electron microscope(SEM),X-ray photoelectron spectroscopy(XPS)and electrochemical methods were adopted to systematically investigate the surface morphology,chemical element composition,bonding mode and corrosion resistance of the chemical conversion film.SEM observation confirmed that compact films were fabricated by ATMP-directed assembly technology.But as-formed film contained many cracks caused by the residual stresses because the film was relatively thick.XPS results further demonstrated that the films with layered structures were obtained via the bridging role of zinc ions.Electrochemical tests consisting of polarization curve and electrochemical impendence spectra(EIS)indicated that anticorrosion performance of films were closely associated with the film-forming conditions.It can be achieved best anticorrosion efficiency of 81.2%in the case of assembly time of 21 min,pH 4.0 and addition amount(0.8 mL)of ATMP.In order to examine general applicability of the above film-forming method,we selected another phosphonic acid,diethylene triamine penta(methylene phosphonic acid)(DTPMP),as the film-forming substance and Zn2+ ions as crossking ions to fabricate new conversion film.The results showed that DTPMP molecules displayed the similar behavior like what ATMP molecules did and was also applicable to fabricating phosphonate-metal complex conversion film(defined as DTPMP-Zn film).In particular,the DTPMP-Zn film on CRS surface still presented excellent anticorrosion performance.(2)Seeing that phosphating process is an essential treatment procedure for metal products before paint coatings,here we also explored the technical feasibility of in situ phosphatization with the purpose of changing phosphating mechanisms and completely eliminating environmental pollution caused by conventional phosphating process.Phenyl phosphonic acid was used as in situ phosphating agent,and environment-friendly doped polyaniline as passivation agent,and both agents were added to the PVB coatings.The anticorrosion performance of as-obtained modified PVB coatings was evaluated preliminarily.The results clearly indicated that phenyl phosphonic acid can form a layer of insulated phosphonate film between PVB coating and substrate.In addition,phosphonic acid functional groups were able to react with-OH groups from PVB molecules to form P-O-C bonds,thereby promoting adhesive force of the coating to metal substrate.Meanwhile,the existence of poly aniline may provide a weak passivation effect to metal substrate.The combined effect of in situ phosphatization and weak passivation greatly enhanced the anticorrosion performance of PVB coatings. |
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