Evaluation of nickel deposition by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy was used to characterize the deposition of nickel from unbuffered acid sulfate electrolytes, from which a wide range of deposit morphologies and current efficiencies are possible. The operating parameters were in the range of 40 to 60 g L^–1 Ni²⁺, 40 to 60 °C, and the pH from 2.0 to 3.5. The resulting impedance spectra for nickel deposition at 20 mA cm^–2 consisted of one or two characteristic loops whose frequency and capacitance were dependent upon the electrowinning conditions and indicative of the resultant deposit morphology. A single high frequency capacitive loop, on the order of 1 kHz, correlated to good quality deposits which were flat, smooth and ductile. The presence of a low frequency loop, on the order of a few hertz, indicated a degraded deposit morphology which showed localized dark, glassy areas and were cracked, curled and brittle. The second loop may be associated with a diffusion controlled component in the reaction mechanism. Deposits of intermediate quality had impedance spectra consisting of both type loops. No apparent trend between the impedance spectra and the deposit current efficiency was determined, but a correlation with deposit quality was clearly established.