Reserch Of Modiifed Cerium Conversion Coating With Citric Acid On Galvanized Steel

Rare earth metal conversion coating is considered as one of hot topics of non-chromatetreatments adhesive agents on hot dip galvanized steel, which has many advantages, such aslow toxicity, no pollution, having excellent corrosion resistance. But the corrosion resistanceof the typical rare earth conversion coatings is lower than that of chromate conversion coating.The rare earth composite coatings which are obtained by two or more steps treatment are notsuitable to be used in practical industry, though the coatings have better corrosion resistancethan chromate conversion coating. Now an effective research idea is to improve the traditionalpassivation technology of rare earth. So, in this paper, citric acid was added into the ceriumsalt solution to modified the composition of the coating, and make the modified ceriumconversion coating has a similar corrosion nesistance as that of chromate conversion coating.This dissertation is focused on the following points: studying main effect factors forpreparation of modified cerium conversion coatings and the optimal process parameters rangefor the coatings; testing and characterizing the properties and structure of the coatings.Investigating the formation, corrosion resistance mechanism and self-healing of the coaings.The optimal process parameters range for the modified coating are as following:Ce（NO₃）₃·6H₂O25g/L, H₂O₂4-6g/L, H3Cit10²0g/L, T65⁷5℃, t10min。The chemical structure of cerium coating and composite coating were characterized andanalyzed by a series of testing methods including SEM（scanning electron microscope）,EDS（energy-dispersive X-ray spectroscopy）, AFM（atomic force microscope）, RA-IR（reflection absorption infrared spectra）, XPS（X-ray photoelectron spectroscopy） and XRD（X-Ray Powder Diffraction）. The results show that:（1） The growth rate of modified ceriumconversion coating on zinc grain boundaries is same as that in the gain of zine, and grows fastwithin10minutes treatment. Cracks occurred during the initial stage of the growth, and anuniform coating are formed. Increasing the treatment time （more than10min）, the growth rateof the coating is slow down, and the width of the cracks still increased. The bonding forcebetween the coating and the matrix decreases gradually. The coating becomes loose, and iseasy to fall off.（2） The modified cerium conversion coating with citric acid is mainly consistof CeCit、Ce₂O₃/Ce（OH）₃、CeO₂/Ce（OH）₄and little Zn（OH）₂/ZnO. At early stage of thecoating growth, the layer of CeCit adsorbed on the surface of zinc, with a little slightlysoluble compounds Zn₃（Cit）₂; at later stages of the coating growth, Ce（OH）₃/Ce₂O₃、Ce（OH）₄/CeO₂and little Zn（OH）₂/ZnO deposit on the coating. The modified cerium salt conversion coating is formed as amorphous mixture.The anti-corrosion property of modified cerium conversion coating as tested by a seriesof corrosion testing methods which included NSS（neutral salt spray test）, salt waterimmersion test, linear polarization and electrochemical impedance spectroscopy （EIS）technique. The results show that:（1） the corrosion resistance property of modified ceriumconversion coating was excellent in NSS and salt water immersion test; the polarizationresistance and of coatings was23kΩ·cm². Cerium conversion coating obtained by the optimaltreatment process was uniform、compact and suitable thickness. It is very obvious that coatinghinders electrolyte solution ion spread to the Membrane layer/metal matrix interface.Thecorrosion resistance of coating reduce, when the treat time is more than10min.The scratching corrosion test shows that the improved cerium conversion coating hasgood self-heal ability.