Synthesis, Structure, and Properties of Nanocrystalline Zinc by Pulsed-Current Electrodeposition
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Date
2004-01-08
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Abstract
Square-wave cathodic current electrodeposition was used to produce for the first time nanocrystalline zinc electrodeposits from both zinc chloride and zinc sulfate-based electrolytes. The influence of pulse electrodeposition parameters, namely, pulse on-time, pulse off-time, and peak current density on the grain size, surface morphology, and preferred orientation of zinc deposits was determined. Furthermore, the effect of polyacrylamide (PAA) and thiourea additions was also investigated. The microstructure and surface morphology of the zinc electrodeposits were studied by scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). X-ray diffraction was used to determine the preferred orientation of these deposits.
In zinc chloride-based bath, nanocrystalline zinc deposits with average grain sizes ranging from 50 to 95 nm were produced. The optimized concentrations of PAA and thiourea in the bath that yield the finest grain sizes were 0.7 and 0.05 g/L, respectively. Increasing current on-time in the range of 0.1 to 7 ms resulted in grain refinement which was attributed to increased overpotential. Increasing the current off-time in the range of 9 to 50 ms was found to yield grain growth which was explained by the decrease of the overpotential and by the fact that longer off-times allow zinc adatoms to migrate over the crystal surface and enhance the grain-growth process. Grain refinement was also observed by increasing peak current density, as expected, and 50-nm zinc grains were obtained at a peak current density of 1000 mA/cm2.
In zinc sulfate-based electrolyte, the grain size of zinc deposits decreased gradually with increasing current on-time at constant current off-time and peak current density. An increase in the current off-time at constant current on-time and peak current density resulted in grain growth. A progressive decrease of the grain size was observed with increasing peak current density at constant current on-time and off-time. Nanocrystalline zinc with an average grain size of 38 nm was obtained at current on-time of 7 ms, current off-time of 9 ms, and at peak current density of 1200 mA/cm2. The crystallographic orientations developed were correlated to the change in the cathodic overpotential, the angle between the preferred oriented plane and basal (0002) plane, and the pulse electrodeposition parameters.
The hardness of nanocrystalline zinc deposits increases from 5 to 8 times higher than that of pure polycrystalline zinc (0.29 GPa). This high hardness was correlated with the presence of additives, strong prismatic texture (11 0) , and the internal lattice strain.
Calorimetric investigations of the as-electrodeposited samples using DSC show two exothermic peaks. The first peak, with an onset temperature of 377 K and peak temperature of 429 K, was attributed to the release of internal lattice strain. Abnormal grain growth was observed by the AFM and the second peak from the DSC scan, which begins at 576 K with a peak temperature of 608 K. The abnormal grain growth may be linked with the segregated sulfur at grain boundaries and interfaces.
Potentiodynamic and alternating current impedance testing of nanocrystalline zinc deposits show that the corrosion current density of nanocrystalline zinc was about 60% lower than that of electrogalvanized (EG) steel, 90 mA/cm2 and 229 mA/cm2, respectively. The surface morphology of corroded nanocrystalline zinc was characterized by discrete etch pits; however, uniform corrosion was obtained after potentiodynamic polarization of EG steel. The passive film formed on the nanocrystalline zinc surface seems to be a dominating factor for the corrosion behavior observed.
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Thermal stability, Hardness, Corrosion, Pulse electrodeposition, Zinc, Nanocrystalline
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Degree
PhD
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Materials Science and Engineering
