| Metallic Corrosion causes huge economic losses,seriously affects life safety,and causes environmental pollution.The chemical conversion coatings and organic coatings on metal surfaces are the most economical and effective means to prevent corrosion of metal materials.At present,among the commonly used metal substrates,the phosphating treatment on the surface of carbon steel has the problems of large pollution,time-consuming and high energy consumption.The titanium-zirconium conversion coatings on the surface of aluminum alloy have the disadvantage of poor corrosion resistance.In addition,the water epoxy coatings on the surface of the metal substrates are subject to the coating forming mechanism,and a lot of defects such as micropores and shrinkage cavities would be formed during the coatings forming process,which will reduce the corrosion resistance and fail to meet the designed anti-corrosion requirements.In view of the above problems,two representative metal substrates,low carbon steel and aluminum alloy,were selected to construct chemical conversion coatings and organic anti-corrosion coatings on their surfaces,and the anti-corrosion properties were investigated.And the promotion mechanisms of chemical conversion coatings were also studied by Density Functional Theory(DFT)calculation.The main research contents are as follows:(1)The nanomaterial a-zirconium phosphate(α-ZrP)was introduced as a phosphating accelerator to construct nano-phosphate composite conversion coatings on the surface of low carbon steel,and the effects of α-ZrP on the morphology,chemical composition and corrosion resistance of the phosphate coatings were investigated.At the same time,the promotion mechanism of α-ZrP in the phosphating process was discussed.By observing the growth process of phosphate crystals on the surface of the metal substrate and the evolution of the surface morphology of the phosphate coatings,it is found that the phosphate coatings obtained by introducing α-ZrP have smaller porosity and the particles size of the phosphate crystals were refined.And α-ZrP helped to generate more phosphate crystals on the metal surface,thereby forming denser phosphate coatings.This is mainly due to the large specific surface area,excellent adsorption capacity and ion exchange capacity of α-ZrP introduced into the phosphating solution,which promotes the phosphating reactions.The gradient experiments results indicated that the optimal concentration of α-ZrP in the phosphating solution is 0.8 g/L.Potentiodynamic polarization curves and electrochemical impedance spectroscopy(EIS)tests results show that the corrosion rate of the phosphate coating under optimized conditions is an order of magnitude lower than that of the blank sample.(2)Melatonin(MEL)was introduced into the phosphating process to prepare phosphate coatings on the low carbon steel,and the effect of MEL on the morphology,chemical composition and corrosion resistance of the phosphate coatings was investigated.The mechanism of MEL promoting phosphating process was analyzed by DFT.Scanning electron microscope(SEM)and three-dimensional(3D)contourgraph results confirms that the introduction of MEL effectively inhibited the upward growth of phosphate crystals and promoted the formation of denser phosphate coatings on the metal substrates.Compared with the blank phosphate coating sample,the roughness of phosphate coatings incorporated with MEL decreases from 21.1 μm to 10.8 μm,and the coating porosity decreases from 77.5% to0.6%,which indicates that MEL significantly improved the formation process of phosphate coatings.Potentiodynamic polarization curves results obtain that the corrosion rate of phosphate coating with MEL decreased from 5.64 mpy to 0.04 mpy,showing excellent corrosion resistance.In addition,the dry friction test shows that friction coefficient of the phosphate coatings with the introduction of MEL is decreased to 0.3 compared to 0.6 of the blank one,which effectively prevents the phosphate coatings from being damaged.Finally,theoretical calculation shows that the energy gap of MEL is 5.0136 e V,and it has good reactivity.The analysis of Fukui function and other parameters confirmed that MEL has good adsorption capacity for metals,and good promotion effect on phosphating process.(3)The green plant basil leaves extract(BLE)was incorporated to construct new titaniumzirconium conversion coatings(Ti Zr CCs)on the surface of 6061 aluminum alloy(AA6061)substrate,and the influence of the introduction of BLE on the formation and performance of Ti Zr CCs was investigated.AA6061 was immersed in 3.5 wt% Na Cl solution containing BLE for 105 h,and the surface still maintained the original metallic luster,indicating that BLE effectively inhibited the occurrence of corrosion reaction of aluminum alloy substrate.The optimum coatings forming conditions for Ti Zr CCs are as follows: the time is 1 min,and p H of the conversion solution is between 3.5 and 4.0.SEM was used to observe the morphologies of Ti Zr CCs,and found that BLE helped to form more complete Ti Zr CCs and greatly reduced the coatings cracks.Compared with the blank Ti Zr CC,the corrosion rate of conversion coating decreases from 0,043 mpy to 0.007 mpy after the introduction of BLE.The reactivity and reaction sites of several main active substances in BLE including methyleugenol,carvacrol,linalool,rosmarinic acid and eugenol were calculated and analyzed.The rosmarinic acid has the largest electronegativity and the best reactivity,which is helpful for the formation and performance improvement of Ti Zr CCs.(4)Based on the aluminum alloy substrate of the Ti Zr CCs,the corrosion resistance of waterborne epoxy(WEP)coatings that can be used together with the chemical conversion coatings was researched.Graphene oxide(GO)was chemically grafted with Dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride(TPOAC).Then,the modified GO(TGO)was introduced into WEP coatings,and the anti-corrosion properties were investigated.The characterization results of GO and TGO by FTIR,Raman,XRD,UV-vis and XPS indicate that TPOAC mainly reacted with the oxygen-containing functional groups on GO and grafted onto the surface of GO sheets.SEM images show that the surface of TGO/WEP coating is smooth and free of pores,which confirms that TGO greatly reduces the defects inside the WEP coatings.The corrosion tests results indicate that TGO significantly enhanced the corrosion resistance of WEP coatings.The enhancement mechanism of TGO on the corrosion resistance of WEP coatings is mainly attributed that TGO was uniformly dispersed in polymer coatings and exerted excellent shielding effect.At the same time,TPOAC also acted as a latent curing agent,which promoted the compatibility between epoxy resin and water-based epoxy curing agent and reduced the defects in the WEP coatings.Thereby,the corrosion resistance of WEP coatings was greatly enhanced. |
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