Electroreduction of Ni(II) and Co(II) from Pyrophosphate Solutions

Abstract

Pyrophosphate solutions are of interest for electrodeposition of resistive to corrosion Mo-containing alloys with iron
group metals. Some difficulties to describe the mechanism and kinetics of codeposition are related with rare data about
the mechanism and kinetics of pure iron group metal complexes electroreduction. The aim of this work was to study the
correlations between deposition rates of Ni and Co from pyrophosphate baths and ionic content of baths. Adding of
(NH4)2SO4 and further forming of ammonia in the solution accelerates sufficiently the rate of Ni electrodeposition. The
effect well correlates with increasing the molar fraction of various ammonia complexes with Ni(II). The molar fraction
of Ni(II) complexes with ammonia is 1000 times higher than ones of Co(II). Based on the different pH and (NH4)2SO4
influence on the deposition rate it is possible to assume that CoOH+ and Ni(NH3)2+
1÷6 can act as charge-transfer particles
in the pyrophosphate-ammonia solutions.