Study Of Chromate-free Passivation For Pre Nickel Plated Battery Shell

As the rapid development of electronic information technology and increase of environmental awareness amongst the general public, a new generation of safe, green and efficient battery has been developed and applied. The battery shell, which is the one of the most important parts of the battery, should possess good corrosion resistance. In order to improve the corrosion resistance of the battery shell, a nickel coating is usually plated on the shell. There are two types of nickel-plated shells: rolled-nickel plated shell and pre-nickel plated shell. As the high product rate and uniform performance, the pre-nickel plated shell has been applied more and more widely. But the coating of the pre-nickel plated battery shell usually cracks during processing, resulting in the decrease of the corrosion resistance. Therefore, in order to improve the corrosion resistance of the pre-nickel plated battery shell, we need to treat the surface of shell. For a long time, chromate passivation is widely used. But the hexavalent chromium is toxic and carcinogenic, seriously polluting the environment and harming the human health. For this problem, we studied the chromium-free passivation for the surface of the pre-nickel plated shell in this thesis.In this thesis, the effect of phytic acid on the corrosion resistance of pre-nickel plated battery shell was studied. And then the passivation effect of phytic acid combined with other inhibitors was studied. Finally, the corrosion resistance of the passive film was studied by the comparative experiments. The main contents and results of this thesis are as follows:(1) The phytic acid corrosion inhibition for the pre-nickel plated battery shell was studied. We changed phytic acid concentration and pH of the passive solution, and the relationship between phytic acid coverage rate and corrosion resistance of pre-nickel plated battery shell was studied by means of linear sweep voltammetry and electrochemical impedance spectroscopy. The results show that the coverage rate of the phytic acid conversion film achieves the maximum and the corrosion resistance is the best when the phytic acid concentration is 10 g/L and pH=5. The surface morphology and composition of the phytic acid conversion film was characterized under the optimum phytic acid concentration and pH of the passive solution. Furtherly, the mechanism of the corrosion resistance of phytic acid conversion film was analysed in terms of the unique molecular structure of phytic acid. (2) A composite passivation agent containing phytic acid was studied. A new passivation formula has been screened out by the orthogonal experiment: phytic acid 10 g/L, sodium molybdate 6 g/L, corrosion inhibitor 5⁸ g/L, sodium dodecylbenzenesulfonate 3⁵ g/L, complexant 5⁸ g/L. Then we studied the passivation process by electrochemical workstation and the optimum conditions were confirmed: passivation time: 1200¹500 s, passivation solution pH=5, passivation temperature: 40⁶0℃.(3) The corrosion resistance of unpassivated, phytic acid passivated and composite passivated pre-nickel plated shells were compared by 5 wt% CuSO₄ drip experiment, polarization curve and electrochemical impedance spectroscopy. The results indicate that the composite passivated pre-nickel plated shell shows the best corrosion resistance. The surface morphology and composition of the composite conversion film was characterized. Finally, the mechanism of the corrosion resistance of the composite conversion film was analysed in terms of the synergy between the inhibitor molecules.Through the above-mentioned studies, we find the corrosion resistance of the composite conversion film is better than the phytic acid conversion film. This thesis provides a reference for the chromate-free passivation process.