Mise en page 1 - Laboratoire National des Champs Magnétiques ...

MAGNETO-SCIENCE 2009Magnetohydrodynamic effect on electrodeposition of nickel alloys-catalysts forhydrogen evolutionElectrodeposition is strongly affected by the addition ofnivellant agents or brighteners in the electrolytic bath.Many electroactive species can be used to modify the characteristicsof the deposits. However, the presence of organiccompounds in the baths gives alloys whose properties withrespect to the corrosion resistance are degraded considerably[Meguro et al. J. Electroch. Soc. 147 3003 (2000)].The magnetic forces generated by the passage of a currentin the presence of a magnetic field create additional convectionwhich affect the morphology of the deposits.field influences deposition by generation of additional convectionclose to the electrode surface and by reduction ofthickness of diffusion layer near cathode surface.In theory, the size of the grain of the deposits is a functionof the speed of nucleation and growth of the nucleus;the more numerous nucleuses are, the lower is the grainsize. A magnetic field applied parallel to the surface of theelectrode generates convection (magnetohydrodynamic effector MHD) of the electrolyte; it results in a laminar flowon the surface of the electrode which reduces the diffusionlayer and increases the concentration gradient. This resultsin change in the size of the grains. In principle, the grainsize is a function of the nucleation rate and the growth ofthe nuclei: the more the nuclei the smaller is the resultinggrain size.The influences of magnetic field on composition, structureand morphology of Cu-Ni alloys have been investigated.The XRD diffraction pattern of deposited alloys ispresented in figure 158. Alloys show a fcc structure ofCu 0 .81Ni 0 .19. There is no difference in the alloy structurewith increasing applied magnetic field while the grainsize of Cu-Ni deposit decreases. This could be explainedby the generation of additional convection close to the electrodesurface by the MHD effect. At the same time thenickel content of the alloy is increased with increasing appliedmagnetic field. This could be explained by magneticfield action on the hydrogen evolution. Nickel electrodepositionis always accompanied by violent hydrogen evolution.When a magnetic field is applied the size of hydrogenbubbles is smaller and they are more rapidly removed fromsurface of the electrode. In this way the nickel partial currentefficiency is increased and nickel content of the alloy ishigher. The magnetic field also influences the morphologyof Cu-Ni alloys. Hydrogen evolution enhanced by the appliedmagnetic field leads to a smooth surface of the alloys(figure 159). However, this effect is visible only for appliedmagnetic field of 6 T or more.In conclusion, a magnetic field changes the morphologyand grain size of deposited alloys. There is no influenceof applied magnetic field on structure of deposit. MagneticFigure 158: XRD diffraction pattern of Cu-Ni alloys depositedwith superimposed magnetic field parallel to the surface of electrode.Figure 159: Morphology of Cu-Ni alloys deposited with superimposedmagnetic field parallel to the surface of electrode.F. DebrayP. Zabinski, R. Kowalik, A. Jarek (AGH University of Science and Technology, Krakow, Poland)110