Defect Mechanism And Preventive Measurement Of Yellow Staining In Electrolytic Tinplate

Tinplate has become being one of the major packaging materials used in the food industry nowadays, because of its excellent formability, solderability and nontoxicity. Although modern technology of electrolyte tinning line is very mature, there is several defects influence the quality of tinplate. Yellow staining is a special defect of tinplate, and its factors were studied systemtically by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), electrochemical workstation and metalloscope in the thesis. Meanwhile, the formation mechanism and preventive solution of the yellow staining were also presented in this thesis.XPS analysis showed that the passive films of yellow staining tinplates were damaged and the free tin layers were excessive oxidized. The main component of yellow staining was stannic oxide. By the SEM analysis, it showed the roughness of the yellow staining sample was larger than the normal tinplate sample, and the passive films of defect tinplates were destroyed. The electrochemical tests, chronopotentiometry and electrochemical impedance spectroscopy, indicated that there formed a large number of tin oxide on the yellow staining tinplate and the corrosion resistance of the defect tinplates were reduced.The cause for the formation of yellow staining was studied systematically in the thesis. The technology of electroplating procedure was the inner cause which impacted the yellow staining. The influence of electrotinning procedure, fluxing procedure, reflowing procedure, soda-cleaning procedure and cathode passivation procedure were researched. The oxidation resistance of tinplate was improved by increasing the concentration of sodium dichromate, lowering the temperature of passivation and increasing the charge of passivation, which increased the thickness of passive film. In the reflow procedure, the content of tin oxide reduced effectively by controlling the maximum temperature of reflow, accelerating the heating rate before the temperature reached the melting point of tin. Fluxing and soda-cleaning procedure played an important role in controlling the content of tin oxide. The concentration of salt in fluxing procedure should maintain at a low level, such as 1g/L~3g/L, and the charge of cleaning in cleaning procedure should increase to 3C/dm2. On the other hand, the tin sludge which produced in the plating procedure was also an important factor resulting in high content of tin oxide, and the increasement of clean-up frequency got a good result. These measurements can effectively reduce the generation of yellow staining.At the same time, the external cause of yellow staining was analyzed in the thesis. When the thick tin oxide film which reached to a nanometer scale, was formed on the tinplate surface, the whole surface was stained yellow because of its interference color under the hot and humid conditions. The differential aeration corrosion happened when the tinplate surface exposed to different oxygen concentration. This corrosion was electrochemical corrosion which caused the free tin layer excessive oxidation.Finally, this thesis proposed the formation mechanism of yellow staining on defect of tinplate. Oxidation corrosion was caused by the poor corrosion resistance of passive film, which led to over-oxidation of the free tin layer. Electrochemical corrosion occured when the surface of tinplate was covered by corrosive electrolyte. In this situation, the localized corrosion brought about anodic dissolution of free tin layer. According to formation mechanism of yellow staining, the measures to reduce it were proposed from electrotinning procedure, reflowing procedure, passivation procedure.