Co-Ni Anomalous Codeposition Studies: Mechanism and Effects of Glycine
Authors: Graciano Vinicius Primo , Cordoba de Torresi Susana I. 
Published: February 2020

Magnetic alloys, mainly the ones made of Fe, Ni or Co, have great technological importance such as in magnetic recording devices. The electrodeposition of these alloys, which enables a greater control over the process as compared to other techniques, occurs with the anomalous codeposition phenomenon: the film has a greater amount of the less noble metal than the electroactive species in the bath, something not expected thermodynamically. This has been widely studied, though no consensus about a single mechanism has been reached. Additives have been used in these baths but outside of controlling the composition of the film, little is known of their role. Here we present studies attempting to comprehend the anomalous codeposition mechanism of Co-Ni alloy and to understand the influence of glycine (Gly) on it.
Species distribution was analyzed using the software Hydra/Medusa [1],[2] in solutions with 0.05 mol/L MSO4 (M = Co, Ni or Co+Ni), 0.5 mol/L Na2SO4 and different concentrations of Gly. Gly deeply affects the bath composition in the alkaline region by forming complexes with Co2+ and Ni2+ that reduce the metallic hydroxides formation. The presence of Gly in alloy baths prevents the formation of Ni(OH)x while Co(OH)x formation is unaffected. This is due to the differing complexation constants (β) that the amino acid has with both metals (βNi>βCo). With Gly the electrodeposited alloy films have an increased amount of Co, the less noble metal, independent of the bath’s pH and Gly concentration, [Gly].
The EQCM studies were done mainly through M/z analysis (combining Sauerbrey’s equation and Faraday’s law of electrolysis). Gly (mostly in the zwitterion form when 3 < pH < 7, the pH range studied) buffering action leads to the formation of Gly-, the species that complexes with the metallic ions, which results in complexation even in acidic pH, where [Gly-] is low. This effect can be noted by the similar values in the M/z ratios in all baths, independent of pH, which differs from the species distribution determined previously. Gly also affects Ni cyclic voltammetries allowing a more behaved pattern in the cathodic region while approaching the M/z ratios to that of Co reduction. Its effect on the alloy deposition is limited, and the M/z ratios between the species of Co and Ni aren’t different enough to determine which reactions are taking place.
Thin layer FT-IR measurements were made in baths with 0.5 mol/L Na2SO4 and varying concentrations of Gly. Due to the thin layer, metal reduction cannot be analyzed as the H2 from the hydrogen reduction reaction (HER) will accumulate and block the surface. Co and Ni electrodes were electrodeposited and used instead and the effect of potentials on baths without the metallic ions was observed. OH- formation, due to the HER, is limited in the presence of Gly and Gly- is formed in acidic baths, confirming the buffering effect suggested by the EQCM. No evidence of adsorption was observed in any bath on any electrode used, which is consistent with some studies in the literature that observed this phenomenon only in positive potential regime [3].
Zn-Co alloys, which present the anomalous codeposition phenomena, were deposited in the presence of cysteine (Cys) to confirm the effects of the amino acids in this process. In this case the amount of the least noble metal in the film, Zn, was lowered with the additive in bath. This can be related to the relationship between the complexation constants (βZn>βCo), supporting rather than interfering with the anomalous codeposition mechanism, these additives impede the reduction of one metallic ion through complexation. This effect is observed in a wide pH range due to the concomitant hydrogen reduction reaction, which increase Cys- concentration ([Cys-]) or [Gly-].

[1] I. Puigdomenech, Hydra/Medusa Chemical Equilibrium Database and Plotting Software, KTH Royal Institute of Technology, 2004.
[2] A. E. Martell, R. M. Smith, R. J. Motekaitis, NIST Critically slected stability constants of metal complexes database, Version 4.0, Texas A&M University, College Station, 1997.
[3] A. P. Sandoval, J. M Orts, J. M. Feliu, J. Phys. Chem. C, 115 (2011), 16439.

Figure 1