| The Hot-dip galvanizing coating (HDG) is widely used in industrial production, while itis easy to produce white rust that affects the corrosion resistance. Chromates are Chromatesare the most effective way used in anticorrosive treatments for HDG However, hexavalentchromium is carcinogenic and contaminated to the environment, so the chromium freeconversion coatings become a research trend. In this paper, a kind of passivation treatmentwhich consists of film-forming agent (fluorotitantic acid (60wt%)), forming accelerator(manganese dihydrogen phosphate) was prepared. Then polyacrylic acid was added into thebasic passivation solution to obtain the improved fluorotitantic acid conversion coating. Theprocess technologies, microstructure, growth and of the basic fluorotitantic acid conversioncoating and improved fluorotitantic acid conversion coating on HDG steel were investigatedby SEM, EDS, XPS, XRD, IR, and the electrochemical polarization, electrochemicalimpedance spectroscopy (EIS) and neutral salt spray (NSS) aimed to examine the corrosionresistance.The optimal process parameters of the basic fluorotitantic acid conversion coating are asfollows: H2TiF6(60wt%)5ml/L, Mn(H2PO4)2·2H2O15g/L, pH2.5，treating time1min. Theoptimal concentration of polyacrylic acid additives to the basic passivation solution is10ml/Lto form the improve fluorotitantic acid conversion coating.The Surface micrographs show thatthe fluorotitantic acid conversion coatings grow rapidly at room temperature, and thethickness increase with the increase of treatment time. Compared to the fluorotitantic acidconversion coatings, the improved fluorotitantic acid conversion coating the appearance of theimproved conversion coating is lighter, thicker and has fewer holes. While the processingtime exceeds1min, the film appears holes which affect the microstructure morphology andthe corrosion resistance.The results of NSS analysis show that the salt spray corrosion resistance of the basicconversion coating is greatly improved over the zinc layer. After three cycles of neutral saltspray corrosion, the sample only appears12%white rust. While the improved fluorotitanateconversion coating only appears12%the corroded area, which further improved the corrosionresistance. The corrosion current density of the basic and improved conversion coatingsdecreases, while the polarization resistance and the electrochemical impedance increasesignificantly compared to HDG. Compared to untreated HDG sample, it is observed from thebasic conversion coating that the polarization resistance and electrochemical impedance increases the order of two majorities. The corrosion current density Jcorr reduces the order ofa majority. The improved fluorotitantic acid conversion coatings greatly improved theinhibition of the anode process of the corrosion reaction process compared to the basicconversion coatings, and polarization parameters further optimized; the impedance resistanceincrease from20kΩ·cm2to60kΩ·cm2.The results of EDS, XPS and XRD analysis show that the basic fluorotitantic acidconversion coating contains Zn, O, P, F, Ti and Mn elements, and the surface of theconversion coating is composed of ZnO, Zn3(PO4)2, ZnF2, TiO2and the oxides of Mn.Combined the IR analysis, the improved fluorotitantic acid conversion coating is composed ofthe compound of Zn3(PO4)2and the the oxides of Zn, Ti and Mn. Polyacrylates also presentsin surface of the coating, which means polyacrylic acid engage in the formative process of theconversion coating.Adhesion, wettability, service life of the passivation bath and the coating performance ofthe basic and the improved fluorotitantic acid conversion coatings were studied tocharacterize the physicochemical properties of the coatings. Rotating method and grid methodresults show that the two kinds of coatings improve the adhesion and the basic one is betterthan the improved one. It can be seen from the passivation effect that the service life of thetwo passivation solutions is long, which mainly because that the F-in the passivationsolutions is good for the solution stability, but the solution will precipitate after place a periodof time. The service life of the improved fluorotitanate conversion coating passivationsolution is longer than the base one, which related to this added organic polymer acid.Wetting angle analysis shows that the wetting angle of HDG, the basic fluorotitantic acidconversion coating and the improved fluorotitantic acid conversion coating is less than90°,meaning hydrophilic. Excellent performance for industrial subsequent coating is related to thehydrophilic coatings observed from the varnish cross cut test. The fluorotitantic acidconversion coatings are feasible for industrial applications. |
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