| Abstract [eng] |
Electroreduction of various Ni(II) and Co(II) complexes in chloride, sulfate, citrate, pyrophosphate and pyrophosphate –ammonia solutions, electrodeposition of Ni and Co and their alloys with W and Mo and composition of obtained coatings and surface morphology, structure and corrosion properties have been studied. W and Mo alloys were electrodeposited from citrate – borate and pyrophosphate –ammonia solutions, and the corrosion behavior of obtained alloys was investigated in sulfate solutions. The shapes of voltammetric curves obtained for Co(II) and Ni(II) electroreduction are similar to the typical shapes of curves for processes occurring under mixed kinetics, and clear plateau presences in voltammogram. However, based on the data obtained by the electrochemical impedance spectroscopy was confirmed that the obtained plateau does not attribute to the diffusion limitations. A slow adsorption stage of electrochemicaly active complex of Co and Ni on the electrode was assumed. It was determined that Ni electrodeposition rate from pyrophosphate baths without ammonia is relatively small. Adding of (NH4)2SO4 and further forming of Ni(II) complexes with ammonia in the solution accelerates sufficiently the rate of Ni electrodeposition. The effect well correlates with increasing the calculated molar fraction of various ammonia complexes with Ni(II). Based on the received data we conclude that electrochemicaly active Ni and Co complexes are different, i.e. CoOH+ and Ni(NH3)162+ particles act as a charge-transfer particle in the pyrophosphate-ammonia solutions, respectively. The deposition rate of cobalt–molybdenum alloys are sufficiently lower than that for pure cobalt in the weakly acidic citrate solutions, and increases sufficiently when ternary Co-Mo-P alloys are electrodeposited. At pH ~5 alloys with low amount of Mo in alloy were obtained; the coatings deposited at low current densities have a needle- shaped surface morphology. The surface becomes more flat (spheroid structure) when current density increases. The increase in Na2MoO4 concentration up to 0.012 M results is a flat surface. For the electrodeposition in wafers the low frequency current pulse mode was chosen. The alloy having composition (in at. %) Co-9.3Mo-3.5P was electrodeposited, and the filling of the recesses is reasonable, and the height of obtained posts is the same, and the surface is flat and consists on the fine crystallites. The dependency of of Co-W alloys composition and structure on the electrodeposition conditions was noticed. The grain size of Co-W alloys depend more on W amount in alloy neither on the electrodeposition conditions: when W amount less than 20 at.%, then Co-W alloy is polycrystalline (grain size >30nm), and when W >22 at.%, then Co-W alloys have nanocrystalline (pseudoamorphous) structure and grain size is < 6nm. Interestingly, the transition from polycrystalline to the nanocrystalline structure occurs in the narrow range of W amounts in alloys, namely ~20-22 at. % . The dependence of the exchange current density for hydrogen evolution as a function of the composition of Co-W alloys has a minimum, corresponding to transitive from polycrystalline to nanocrystalline structure. The exchange current density for Ni-Mo is higher than that for a pure component of alloys and the catalytic evolution of hydrogen is occurring. Corrosion currents of Co-W alloys clearly correlate with exchange current density of hydrogen evolution whereas for Ni-Mo alloys this correlation is rather weak. The values of corrosion current are similar for both Co-W and Ni-Mo alloys. |
