| Iron and steel is the largest output,the most widely used functional materials,but the shortcomings of its corrosion problem cause industrial production and human life great losses.Galvanizing with chromate treatment and phosphating with organic coating are iron and steel's two kinds of commonly used anti-corrosion technology,but the production process uses highly toxic cyanides and carcinogenic chromates,causing eutrophication phosphates and poisonous and harmful organic solvents,which seriously damages and negatively affects the human ecological environment and human health.Therefore,in order to replace these technologies,two kinds of the conversion coatings with different morphological structure and high corrosion resistance were studied in this paper.In the first part of the paper,based on the previous and successful preparation of granular fluroferrite conversion coating,by the neutral salt spray test for corrosion resistance evaluation standard,the modification on the corrosion resistant of the fluroferrite conversion coating was studied and the results showed that the modification effect of hydrogen peroxide is best.SEM,EDS,XPS,XRD and electrochemical analysis and testing technology of hydrogen peroxide modified process found that fluroferrite conversion coating is mainly composed of columnar K3FeF6 crystals,the film-forming process formed by nucleation(0 ~ 0.5 h),crystal growth(0.5 ~ 2 h)and crystal dissolution(>2 h)consists of three stages.Hydrogen peroxide accelerates the rate of reaction of the control step of the oxidation of Fe2+ into Fe3+ in the process of the coating formation.When the reaction is 2h,the coating reaches the maximum thickness of 70.9 ?m,and the corrosion resistance is the best,and the salt spray time reaches 72 h,which is close to the standard of industrial application of galvanized steel with 96 h salt spray test time.Compared with the bare iron,the corrosion current density of the hydrogen peroxide-modified fluoferrite coated sample reduced by 98.4%,the low frequency impedance value increased by 25.1 times,the charge transfer resistance increased by 37.2 times.For the particulate conversion coating,the coating thickness is the key factor of the corrosion resistance of the coating.In the second part of the thesis,the fluozirconate conversion coating with amorphous structure and its corrosion resistant modification were studied.SEM,EDS,XPS,neutral salt spray,electrochemical,FTIR analysis and test results showed that:The fluozirconate conversion coating is of amorphous structure,is composed of ZrO2?ZrF4?K3ZrF7?Fe F3?Fe3O4 and FeOOH,the coating's element content is 76.85% Fe,14.88% Zr,3.95% O,3.53% F and 0.79% K,the neutral salt spray time was 6h.As compared to the bare steel sample,the corrosion current density of the fluozirconate coated sample reduced by 65.3%,the low frequency impedance value and the charge transfer resistance both increased by an order of magnitude.Sodium bicarbonate's modification effect on the corrosion resistant of the fluozirconate conversion coating was studied.The results showed that the conversion coating is still amorphous structure,consisting of ZrO2?Zr F4?K3ZrF7?FeF3?Fe3O4 and FeOOH.But the coating's elements contain 50.15% Fe?24.62% Zr?12.39% O?11.64% F and 1.2% K.Zr,O and F element,especially Zr,O or ZrO2 increase greatly,indicating that sodium bicarbonate acts as accelerant by increasing the amount of the effective corrosion resistant components,ZrO2.The salt spray time of the fluozirconate coated sample modified with sodium bicarbonate increases to 24 h,the corrosion current density reduced by 35%,the polarization resistance was doubled,the charge transfer resistance increased by 92.5%.On the basis of the fluozirconate conversion coating modified with sodium bicarbonate,phytic acid was used to improve the fluozirconate conversion coating's corrosion resistance.The results found that the conversion coating was still in its amorphous structure.FTIR analysis shows that the characteristic peaks of PO43-,HPO42-,OH-exist in the coating.EDS analysis results show that the coating contains 16.12% Fe?25.69% Zr?27.50% O?10.83% F?6.71% K and 13.15% P.XPS analysis results show that the coating's surface mainly consists of FeF3?ZrF4,metal ions(Fe(II),Fe(?),Zr4+,and K+)and phytic acid complexes.A significant reduction of Fe content,a significant increase of O and K contents,and a large number of P element detected show that phytic acid and metal ion participate in the reaction of coating formation,fluozirconate conversion coating and phytic acid complex deposit on the surface of the steel substrate together to form a composite coating.The thickness of the composite coating increases with reaction time.When the reaction time is 24 h,the composite film's thickness is 9.6?m,and the salt spray time reached 120 h,greater than the industrial application standards of galvanized steel with 96 h salt spray time.When the reaction time continues to increase,due to the increase of the film thickness and stress,then film cracked,its corrosion resistance decreased.For the amorphous conversion coating,the coating composition is the key factor of the corrosion resistance of the coating. |
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