Preparation And Properties Of Ti-P/Silane Composite Film On Tinplates

Tinplates have found wide applications in the packaging of food and beverages.Passivation is one necessary procedure in the production of tinplates.Although the traditional hexavalent chromium process is characterized with excellent performance and low cost,the development of chromium-free passivation process is an inevitable development trend.This thesis is based on a corporate cooperation project to develop the chromium-free passivation process on tinplates.Firstly,scanning electron microscopy(SEM)and X-ray energy dispersive spectroscopy(EDS)were employed to characterize the pores on the surface of tinplates(2.8 g/m2).With corrosion resistance as the evaluation standard,KH-170,an amino-double-claw silane coupling agent and KH-566,an epoxy-based silane coupling agent which can formate and have good corrosion protective performance on tinplates are selected from urea-based,amino-based,aminodouble-claw and epoxy-based silane coupling agents.The opitium process for silane film preparation was determined by single factor test and orthogonal test: drying temperature 180 ?,drying time 15 min,silane treatment solution p H = 5.0 ? 6.0,the compositions of the two silane treatment solution were 2.0 wt.% KH-170 and 1.0 wt.% KH-170 + 1.0 wt.% KH-566 respectively.Through the Tafel polarization curve test and EIS impedance spectrum test,it was showed that the self-corrosion current density of tinplate treated by KH-170 decreased by 59.8 %,and the self-corrosion current density of tinplate treated by KH-170 + KH-566 decreased by 81.3 %,indicating that the corrosion protection effect of KH-170 + KH-566 composite silane film was better than that of KH-170 silane film.After measurement by optical contact goniometer,it was found that the contact angle of the tinplate treated with two kinds of silane increased by 64.0 % and 59.3 % respectively compared with the untreated tinplate,indicating that the silane film is to improve the corrosion resistance of tinplates by changing the hydrophobicity of tinplates.Through SEM and EDS tests,it was found that although the prepared silane films could be generated uniformly on the surface of tinplates,it was verified that silane films did not have the sealing effect on tinplates by glow discharge spectrometer(GDS)and iron dissolution value test.A chromium-free passivation solution with sealing effect was then designed and developed with titanium salt and phosphide as the main salt,and the optimal process was determined by orthogonal test: 1.5 g/L Ti OSO4,10 ml/L H3PO4,15 g/L H2O2,drying temperature 40 ? 50 ?,drying time 1.5 min.It was found by SEM and EDS that Ti-P conversion film can form irregular particulate matter on the surface of tinplate and it had sealing effect.Further characterized by X-ray photoelectron spectroscopy(XPS)indicated that the film was mainly composed of Ti O2,Ti3(PO4)4 and Fe PO4.In order to verify the sealing effect of Ti-P film on tinplates,it was found by GDS test that when the measured depth reached 0.75 ?m,the concentration curve of P element showed a second peak,and the porosity test showed that the Ti-P conversion film reduced the iron disssolution value of the tinplate by 42.5 %,indicating that the Ti-P conversion film had sealing effect on tinplates.After measured by Tafel polarization curve test,three-dimensional imaging optical microscope and bond strength tester,it was showed that the selfcorrosion current density of the tinplate decreased by 45.6 %,the surface roughness decreased by 26.2% and the bonding strength increased by 63.2% after coated with the Ti-P conversion film.Combined with the sealing effect of Ti-P film and the uniform and hydrophobic effect of silane film,a Ti-P/silane composite film was prepared by a two-step method in which first preparing Ti-P conversion film and second preparing silane film on the tinplate.Through the evaluation of salt spray test and Tafel polarization curve test,it was demonstrated that the corrosion resistance of the composite film was better than that of the individual Ti-P film or silane film,which was close to the corrosion resistance of the tinplate after chromate passivation.Results of SEM and EDS indicated that the surface morphology of the Ti-P/silane composite film was similar to that of the Ti-P conversion film,irregular spherical particles can be observed on the tinplate,and the composite film can cover uniformly on the tinplate.The GDS test of the tinplate treated with Ti-P/silane composite film showed that the concentration curve of P element had a second peak in the depth range of 0.6 ? 1.0 ?m.Combined with the iron dissolution value test results,it was verified that the composite film also had the function of sealing pores.After testing the EIS impedance spectroscopy,it was found that the Ti-P/KH-170 composite film improved the corrosion resistance of the composite film by increasing the charge transfer resistance,and Ti-P/KH-170 + KH-566 composite film improved the corrosion resistance of the composite film by increasing the film resistance.The contact angle of the tinplate treated by two Ti-P/silane composite films were further increased after tests on the optical contact angle goniometer,indicating that the Ti-P/silane composite film had a good hydrophobic effect as the silane film.