Investigation Of Thickness Distribution Of The Passivation Film On Tinplate Surface And Study On The Film Composition And Performances

As one of the raw materials in the field of metal packaging,tinplate is mainly used for packaging of foods,beverages,and tea leaves.Chromate passivation treatment is an essential process in the production of tinplate,resulting in the formation of a protective film on tinplate surface,which can also improve the adhesion between the paint and the tinplate in painting process.With the increasing demand of the high-end and customized tinplate,and the decreasing thickness of the tin plating and the increasing speed of production,higher requirements have been placed on the control of passivation process and film quality.However,in the study of the passivation film,the composition,structure and formation process of the film are still controversial.The morphology,thickness and distribution state of the film are also neglected and even avoided.In addition,the mechanism of production processes affecting the performances of the passivation film is not yet clear.This study is based on enterprise cooperation projects,targeting at the above problems and difficulties to carry out research.Firstly,the optimum passivation process was determined by orthogonal test as follows: current density was 1.0 A/dm2,and p H and temperature of the passivation solution were 4.2 and 42 °C respectively,and concentration of Na2Cr2O7 was 25 g/L.The passivation treatment was carried out on tinplate surface using the above mentioned process.It was difficult to observe the true morphology of the ultra-thin passivation film on the rough substrate surface using SEM or 3D optical surface profilometer.In view of this,the surface morphology and roughness of the bright tin plating before and after passivation were tested by the 3D profilometer,and the contact potential difference between the tinplate and the probe was tested by the Scanning Kelvin Probe Microscope(SKPM)and Kelvin Probe Force Microscope(KPFM).The results showed that the dipped dichromate passivation film was thin and uniform,and the electrolytic dichromate passivation film was thick while the thickness distribution was not uniform.With the prolonging of the passivation time,the distribution of the dipped dichromate passivation film tended to be uniform,while the uniformity of the thickness distribution of the electrolytic dichromate passivation film decreased.X-ray photoelectron spectroscopy(XPS)analysis results suggested that the main components of the electrolytic passivation film were Cr(OH)3 and Cr2O3,together with a small amount of metal Cr and Cr(VI)compounds,and the main component of dipped dichromate passivation film was Cr2O3.Combining the potential-p H diagram of Cr-H2 O system and the properties of Cr(III)compounds in aqueous solution,the film formation mechanism was proposed that the Cr(III)ions underwent aquatiopn,hydrolysis,olation,oxolation and polymerization steps during passivation process finally to form a 3D network Cr(III)oxides film with bridge structure.The Cr atoms in the film were linked by bridging-OH groups or O-groups,and the Cr atoms were combined with the tinplate through the-Sn-O-bond.XPS indepth analysis revealed that the content of bridging-OH groups in the electrolytic passivation film gradually decreased from the surface to the inside,and the bridging O-groups gradually increased,and the bottom of the film contained almost no bridging-OH groups.The metal Cr dispersedly distributed on the microscopic bulges on the tinplate surface and was directly reduced by Cr(VI)ions.The component change of the passivation film was tracked as a function of the relative content ratio of Cr(OH)3/Cr2O3.XPS analysis results of different performance samples were statistically analyzed,and it was found that as the content ratio increased,corrosion resistance of the film was improved and the paint's adhesion declined.The surface work function(contact potential difference)of the samples with different performance was tested by SKPM,and the average value and the fluctuation range of the work function were setted as quantitative indexes to measure the thickness and distribution uniformity of the passivation film.The results showed that the thickness of the film was the premise to ensure corrosion resistance.If the film was thin,the film must be uniformly distributed.A thin and uniform passivation film had excellent paint's adhesion,thick or non-uniformly distributed passivation film had poor paint's adhesion.The passivation film was prepared under different passivation current density,electric quantity and solution p H,and the composition and chromium content of the film were tested by XPS,and the surface work function of the tinplate was tested by the SKPM,and the film performances were tested by salt spray test and paint's adhesion test.The results showed that under a constant electric quantity,as the current density increased,the Cr(OH)3/Cr2O3 content ratio decreased,and the film became thinner,and the distribution uniformity of the film thickness declined,resulting in a decline in the corrosion resistance of the film.Whether high or low current density leaded to the decline of the paint's adhesion.Under a constant current density,as the electric quantity increased,the Cr(OH)3/Cr2O3 content ratio increased,and the film became thicker,and the film uniformity became poorer,resulting in a decline in the paint's adhesion and corrosion resistance.Under a constant electric quantity and current density,whether high or low solution p H leaded to the decline of the film performances.The influence of electrolytic and dipped passivation combination methods,surface tin oxides and the state of passivation solution on the performances of the passivation film showed that under the method of dipped passivation followed by electrolytic passivation,the Cr(OH)3/Cr2O3 content ratio decreased significantly,and the film thickness and distribution uniformity improved,resulting in enhancement of the corrosion resistance and paint's adhesion.Whether the increase of the tin oxides content or improvement of its oxidation degree,or the opacification of the passivation solution leaded to significant decline of the film performances.