Study On The Nucleation Mechanism Of Tinplate And The Protection Technique

Electrolytic tinplate is widely used in various forms of items such as chemical, paint, spray, food and beverage packaging and manufacturing of various kinds of vessels. For a long time, electroplating mechanism and corrosion resistant properties of tin plate has been the special concern of the production enterprises. Nucleation and growth mechanism has the very big impact on the electrodeposition of coating structure, phase composition, further influence on the performance of the coating corrosion resistance and its wide application. The study on electrodeposition nucleation/growth mechanism by electrochemical methods and technology (chronoamperometry, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), electrochemical noise (EN)) is beneficial to explore and understand metal electrodeposition of electrochemical theory, development of new type electroplating additives, improve the corrosion resistant ability of electrolytic tinplate, etc., which also has extremely important theoretical and practical significance. Meanwhile, reflowing and passivation are two important ways to improve the corrosion resistance of tinplate. The study on the effect of different technological parameters on the corrosion resistance helps to optimize these parameters to get the best corrosion resistance of tinplate.This paper was mainly divided into two parts, one of the works on the nucleation and growth mechanism of tinplate electrodeposition. New mechanism models are established using some electrochemical methods; the other one mainly on the two protection technology:tinplate reflowing and passivation, where the corrosion resistant was used as the target to optimize the related process parameters.This paper first studied the nucleation and growth mechanism of tinplate electrodeposition. The effect of the electrodeposition parameters (main salt concentration, solution pH, temperature, current density, etc.) on the corrosion resistance of tinplate was evaluated mainly by electrochemical impedance spectroscopy to optimize these parameters.Tin electrodeposition behavior follows three-dimensional nucleation and growth mechanism, and mainly controlled by the diffusion, which was confirmed by cyclic voltammetric method. Chronoamperometry was used to differentiate the instantaneous nucleation or progressive nucleation type under different overpotentials, nucleation and growth rate constant, etc. The results show that with the increment of overpotentials, nucleation and growth mechanism changes from progressive nucleation into instantaneous nucleation. New current-time model was established based on both the tin nucleation/growth and the simultaneous hydrogen evolution reaction, which was used to analyze the real nucleation and growth rate constant. Chronoamperometry results show that under the lower deposition overpotentials, the total current is composed of tin nucleation/growth current and current of hydrogen evolution reaction on the substrate, as well as on the tin growth centres; while under the higher deposition overpotentials, the total current is composed of tin nucleation/growth current and current of hydrogen evolution reaction on the tin growth centres.Meanwhile, EIS was also used to study the electrodeposition mechanism of tinplate. Results reveal that the charge transfer resistance of the deposition process decreases exponentially with the applied potential bias. Furthermore, electrochemical potential noise was carried out to explore the relationship between the potential noise features and the tinplate structure. EEN generated during the electroplating of dentritic or large conglomerate zinc deposit has large potential oscillation amplitude and positive potential drift while the compact zinc deposit possesses small noise amplitude and little potential drift. The variation trend of the growth energy obtained from the relative energy distribution plot is the same as the tin crystal size from XRD with variation of deposition current density.Secondly, the paper studied the tinplate reflowing process and performance. Effects of reflowing temperature and time on the alloy layer of tinplate and its electrochemical behavior was investigated in3.5%NaCl solution by electrochemical measurements and surface characterization. It is found that with increase of reflowing temperature and time, the amount of alloy layer increases. Then the corrosion potential of detinned tinplate shifts positively and the corrosion rate decreases. After coupled with tin, detinned tinplate is as cathode initially. However, after exposed for some time, the potential shifts of both detinned tinplate and tin reverse polarity of coupling system. The galvanic current density decreases with increase of reflowing temperature and time.Finally, the paper discusses the passivation process of tinplate. The corrosion resistance was used as the target to evaluate the effect of different passivation parameters, mainly by the method of polarization curve and EIS. The results show that the variation trend of the corrosion resistance with the passivation parameters, is initially rise and then drop, which is the result from the joint action of both the growth of passive film and dissolvation of oxidation film.