| At present,sodium chromate solution is mostly used in the process of tin plate passivation,which draws into highly toxic hexavalent Chromium,and that is not good for environment protection.With the improvement of people's awareness of environmental protection,chromate passivation process will eventually be substituted with non-Chromium passivation process.The research focused on the chromate free passivation process of tin plate.The main research contents and conclusions are as follows:The paper studied the influence law of passivation concentration,drying temperature and time on passivation effect by orthogonal experiment and single factor experiment in laboratory.The article compared and analyzed resistance and corrosion resistance of the tin plate comprehensively and observed the microstructure of each sample plate to determine optimum parameters of chromate free passivation.The article drew the following conclusions:The tin plate have the best performance when the passivation solution was at the concentration of 5%and at the temperature 120?to dry 1 second in orthogonal experiment.In single factor experiment,when only the passivation solution concentration changed,with the increase of the concentration of passivation solution,the properties of tin plate increased to the peak value first,then began to decrease,and then the performance began to rise again.The tin plate have the best performance when the passivation solution was at the concentration of 10%Meanwhile,when only the passivation solution temperature changed,with the increase of passivation temperature,the change law of tin plate performance firstly increased and then decreased.The optimum passivation temperature was 35?.The article studied the structure and formation mechanism of chromium free passivation film:The surface morphology of the tin plate was observed by scanning electron microscope.The surface of the tin plate was found to be a dense film structure,which diffused from the site to the surrounding area.It was found that the passive film contained many elements such as Ti,Zr,F,Mn,N,P and so on by electron probe.X-ray photoelectron spectroscopy analyzer was used to measure element type and valence of surface passivation film.Ti elements should be Ti4+ ions,Zr elements should be Zr4+ions,F elements should be F-ions.Glow discharge spectrometer was used to analyse surface elemental distribution in the 1.0?m depth range.It was found that Chromium free passivation film contained a high content of P and Mn elements,and the surface was rich in Si elements.The passive film have a clear layered structure.The formation process of chromium free passivation film was divided into three stages:anodic acid dissolution process,hydrogen evolution reaction process and film deposition process.The membrane forming process was firstly carried out at the cathode active site,and then each site diffused to surrounding areas after the film-forming,until the film was connected to each other,and the passivation film deposited on the tin plate surface.The Chromium free passivation tests were carried out twice in an electrolytic tin production line in a steel plant.The Chromium free passivation process was realized by spraying.The article compared and analyzed resistance and corrosion resistance of the tin plate comprehensively and obtained the better process parameters of chromate free passivation under the condition of field production:When the concentration of the passivation solution was 13%and the drying temperature was 120? and the extrusion roller was put into operation,the passivation film reached the target thickness.The acid resistance of tin plate was poot,but the sulfur resistance and painting performance met the requirements. |
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