| The collecting materials is an important component of cell. In this thesis, the hole-punched steel strip has been studied and its mechanical and electrochemical performance are improved by reducing the thickness, roughening the surface, Ni electrodeposition and heat treatment.For the first time, the reduction of thickness and surface-roughing of hole-punched steel strip are accomplished by electrochemical machining method. By reducing the thicknesses of hole-punched steel strip, and thus the volume occupied by such non-reactive elements are reduced, the winding performance of batteries electrode plate can be improved. The fraction of the battery devoted to holding electrochemically active material therein is increased, with corresponding increase in the capacity of the battery. By roughing of surface, the hole-punched steel strip has a high surface roughness, which result in the increase of the specific surface area, provides the desired improvement in adhesive bonding of the nickel plated layer to the base steel strip, enhances the amount of physical and electrical contact and the better adhesion between the active material and Nickel plated punched steel strip so as to reduce the contact resistance and the electrochemical polarization to increase the battery electrical performance. Under the optimal conditions of electrochemical machining, the thickness of hole-punched steel strip can be reduced to28.44?m(theoretical value:25.63?m), the surface roughness increases from0.23?m to0.49?m. The original deformation layer is partially or completely removed and the metallographic structure of the hole-punched steel strip is not changed. The residual stress decreases from102.5Mpa to24.87Mpa.According to the redox reaction principle and E-pH diagram, ascorbic acid is added to the Fe-riched chloride electrolyte to slow down the oxidation of Fe2+in the baths. In the presence of ascorbic acid, there is a tendency to decrease pH of solution and to reduce ferric to ferrous. The mechanism of the reduction of Fe3+ion by ascorbic acid is analyzed in details and the kinetic equation is derived. The second order reaction rate constant is5.5688(mol/L)-1.min-1. Ascorbic acid is a non-toxic and environmental friendly ingredient, it can be used repeatedly.Using higher buffering capacity and coordination properties of sodium citrate, combined with Ni-citrate system E-pH diagram, a new electrolytes for nickel electrodeposition, which contains sodium citrate instead of boric acid as the butter has been developed. Owing to high toxicity of boron compound, it causes serious pollution to the environment if the direct discharge of wastewaters without treatment. The citrate bath exhibits excellent buttering capacity (0.025mol/L) for bath pH3-5, which are comparable with those of the watts bath (0.011mol/L). The results show that a citrate bath offers an excellent, practical, and more environmentally friendly substitute for boric acid in a Watts bath. The electrolytes provide good electrochemical performance, such as high current efficiency, good butter properties, high values of the throwing power, the electrical conductivity and cathodic polarization.For the first time, the electrodeposition mechanism of Nickel from citrate bath is investigated by means of electrochemical impendence spectroscopy (EIS) method. The influence of the deposition potential, electrolyte composition and technological conditions on the charge transfer resistance and capacitance are systematically investigated during the Nickel electrodeposition. By analyzing the EIS spectroscopy, it can be concluded that the mechanism of Ni electrodeposition as follows:in the acidic citrate bath, it involves two consecutive one-electron charge transfers, the first involving the participation of an anion(assumed to be OH") with formation of an adsorbed complex Ni(OH)ads followed by subsequent reduction to Ni. The rate-determining step is the first electron transfer reaction. The low frequency inductive loop is ascribed to the relaxation of the electrode coverage by an adsorbed intermediate NiOHads, the kinetic equation of electrode reaction can be derivated from the theoretical views and verified by experiment. The results indicate as follows:the Tafel slope of the cathodic process is0.142V decade-1, the apparent transfer coefficient a is0.49, the exchange current density is 7.56×10-6A/cm2, the order of the reaction is1with respect to Ni2+. When the electrolyte pH varies between2-5, no dependence of cathodic rate constants on pH can be found. The coverage of intermediate Ni(OH)ads can not be ignored, the average value of the apparent activation energy of electrochemical reaction is50.3kJ/mol.For the first time, the mechanism of electrocrystalline of nickel on vitreous carbon from citrate bath is investigated by cyclic voltammetry and chronoamperometry methods. Electrochemical tests show that nickel electrodeposition may begin at applied potential-0.9V or so. Under lower overpotential (applied potential:-0.9?-1.0V), the electrocrystalline process of Ni from citrate bath in the initial stage follows the mechanism of Scharitker-Hill three dimensional progressive nucleation and growth manner. Under higher overpotential (applied potential:-1.1?-1.5V), the deposition nucleation of Ni in the initial stage follows the instantaneous nucleation and growth of Scharitker-Hill mechanism with three-dimensional. The nucleation time of Ni-citrate solution system may gradually be shortened with the increase of overpotential. By analyzing of the potentiostatic transients, the diffusion coefficient D of the depositing nickel ions is (1.63±0.48)×10-7cm2·s-1under the progressive nucleation and growth mechanism, and (1.074±0.093)×10-6cm2·s-1under the instantaneous nucleation and growth mechanism respectively. Sodium citrate may causes an inhibition of outward growth rate of Ni deposits, and nickel sulfate may accelerate both nucleation and crystal growth.The performances of nickel plated punched steel strip are tested, such as microhardness, adhesion, porosity, surface roughness and rating of test specimens subjected to corrosion tests, and so on. The results show that products meet the requirements of "nickel plated punched steel strip for rechargeable battery" standard. Under the optimal electrodeposition conditions, the surface roughness of the as-deposited Ni is0.41?m when the thickness of the deposit layer is about6?m, smaller than that of base steel strip processed by electrochemical machining.The influence of electrolyte compositions and technological conditions on electrochemical corrosion behavior of the nickel plated punched steel strips in5%NaCl corrosion media is characterized by immersion corrosion tests, potentiodynamic polarization tests and electrochemical impedance spectroscopy methods. The results reveal that the Ni deposited from citrate bath has higher polarization resistance, smaller corrosion current, higher charge transfer resistance and smaller corrosion rate, compared with that from Watts bath. It exhibits superior corrosion resistance, and the results calculated from potentiodynamic polarization tests are in good agreement with those obtained from impedance measurements. The magnitude of corrosion rate of the Ni deposits decreases with increasing of sodium citrate concentration. The Ni deposits with rough surfaces may accelerate the corrosion reaction rate in5%NaCl solution. All the nickel samples exhibits active-passive potentiodynamic polarization behavior. The results shows that passive film formed on the Ni deposits, and confirmed by X-ray photoelectron spectroscopy (XPS) surface analysis. The XPS analysis indicates that the passive film is composed of stable, continuous Ni hydroxide and NiO. The improved corrosion resistance of Ni in NaCl solution can be explained as follows:Ni prevent the punching steel strip from direct imposing in the corrosion media and passive film formation seems to be another dominant factor, because Ni is a metal that can be passivated easily. The insoluble Ni(OH)2and NiO cover the surface of the corroded samples and create the passivation region so as to reduce the corrosion rate.The concentration distribution profiles in the neighborhood of interface after heat-treatment are measured by EDS. The interdiffusion coefficients at different temperature and concentration of Fe are calculated by Boltzmann-Matano analysis. Within the range of about600?to750?for a soaking time of2hours, the diffusion frequency factor, and activation energy of diffusion are estimated.The anti-corrosion properties of nickel plated punched steel strip in5%NaCl solution are measured by potentiodynamic polarization tests and electrochemical impedance spectroscopy methods after various heat-treatment process. The results show that the corrosion resistance and resistance to penetration of aqueous medium are far better than those of as-deposited Ni. However, in this heat-treatment, the thickness of a Fe-Ni diffusion layer should be controlled so that the Fe/Ni ratio is within the range of30%or less on the surface of the nickel plated punched steel strip. When the Fe/Ni ratio exceeds30%, the corrosion resistance substantially decreases as the exposure rate of iron increases. |
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