Electrochimica Acta 52 (2007) 4742–4751 Electrodeposition of zinc–cobalt alloy from a complexing alkaline glycinate bath J.L. Ortiz-Aparicio a , Y. Meas a,∗ , G. Trejo a , R. Ortega a , T.W. Chapman a , E. Chainet b , P. Ozil b a Centro de Investigaci´ on y Desarrollo Tecnol´ ogico en Electroqu´ ımica, S.C., Parque Tecnol´ ogico Quer´ etaro, Sanfandila, Pedro Escobedo, Quer´ etaro, C.P. 76703, Mexico b Laboratoire d’Electrochimie et Physico-chimie des Mat´ eriaux et Interfaces (UMR 5631 CNRS-INPG-UJF), Ecole Nationale Sup´ erieure d’Electrochimie et d’Electrometallurgie de Grenoble BP75, 38402 Saint Mart´ ın d’H` eres, France Received 22 August 2006; received in revised form 10 January 2007; accepted 11 January 2007 Available online 26 January 2007 Abstract The influence of cobalt on the electrodeposition of zinc onto AISI 1018 steel was studied in weakly alkaline glycine solutions. Thermodynamic calculations were performed to construct predominance-zone diagrams to identify the stability of the zinc and cobalt glycine complexes, and experimental studies of electrochemical behavior and deposit properties were conducted. When zinc is present, cobalt deposition shifts to more negative potentials, producing ZnCo alloys. Two main reduction steps were observed for electrodeposition from the ZnCo bath: the first at low potentials was due to ZnCo electrodeposition. In the second, at more negative potentials, cobalt content in the deposit increased forming a range of intermediate phases, and the hydrogen-evolution reaction became significant. The presence of Co(II) in the bath modified the morphology of the deposits as well as reducing the faradaic metal-deposition efficiency. ZnCo-deposit morphology was modified by the applied current density as well as the metal composition of the coating. X-ray diffraction studies revealed that cobalt oxide or hydroxide is formed during ZnCo electrodeposition, indicating that an elevation of the interfacial pH plays a role in the alloy deposition process. © 2007 Elsevier Ltd. All rights reserved. Keywords: Zinc–cobalt alloys; Electrodeposition; Glycine; Anomalous deposition 1. Introduction Zinc coatings are used widely to protect iron and steel sub- strates against corrosion [1]. Metals like iron, cobalt and nickel have been incorporated in zinc plating baths to obtain coatings with higher corrosion resistance [1,2]. During zinc codeposition with metals of the iron group, the less noble metal, zinc, is elec- trodeposited preferentially; this phenomenon has been described as anomalous codeposition by Brenner [3]. According to Dahms and Croll [4], in the anomalous code- position of Fe–Ni alloys a layer of iron hydroxide is formed, which adsorbs on the electrode and suppresses the nickel reduc- tion. The metal hydroxide layer is formed as a consequence of elevated interfacial pH caused by the hydrogen-evolution reaction. Decroly and co-workers [5,6] proposed a similar mech- ∗ Corresponding author. Tel.: +52 442 211 6070. E-mail address: yunnymeas@cideteq.mx (Y. Meas). anism for Zn–Co codeposition, involving the formation of a zinc hydroxide layer on the electrode surface, which suppresses cobalt reduction. Higashi et al. [7] carried out a study of the elec- trodeposition of Zn in the presence of Co; the variation of the interfacial pH was determined, and the transition from normal to anomalous codeposition was associated with an increment of the interfacial pH. Stankeviciute et al. [8] applied electrochemical quartz-crystal microbalance measurements to analyze the depo- sition of Zn–Co alloys in terms of the mechanism of hydroxide suppression. On the other hand, other authors have argued that during the anomalous codeposition of Ni–Fe, Ni–Zn and Fe–Zn alloys, only soluble intermediate species are formed [9–14]. The concept of hydroxide oscillation [15] has also been used to explain anomalous deposition. According to this model, the thickness of the hydroxide layer changes periodically. When the zinc hydroxide layer is depleted, the reduction of both H + and cobalt occurs preferentially to that of Zn. Then, as the interfacial pH increases due to the hydrogen-evolution reaction, 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.01.010