Effect of benzylideneacetone on the electrodeposition mechanism of Zn–Co alloy G. TREJO 1 , R. ORTEGA 1 , Y. MEAS 1 , E. CHAINET 2 and P. OZIL 2 1 Centro de Investigacio ´n y Desarrollo Tecnolo ´gico en Electroquı´mica (CIDETEQ), Parque Tecnolo ´gico Sanfandila, Pedro Escobedo Quere ´taro AP 064, CP 76700, Me´xico 2 Laboratoire d’Electrochimie et de Physico-Chimie des Mate´riaux et des interfaces (UMR INPG-CNRS 5631 associe´e a ` l’UJF), ENSEEG, BP 45, 38402 Saint Martin d’He`res, France Received 25 May 2002; accepted in revised form 19 December 2002 Key words: additives, benzylideneacetone, electrodeposition, zinc–cobalt alloys Abstract The influence of benzylideneacetone (BA) on the mechanism of Zn–Co alloy electrodeposition onto AISI 1018 steel was studied in chloride acidic solutions. Results indicate that BA modifies the exchange current densities of zinc and cobalt such that the alloy is electrodeposited via a normal codeposition mechanism. Analysis of the deposits by Auger spectroscopy and X-ray diffraction shows that BA increases the cobalt concentration in the electrodeposited alloys and gives deposits with a constant concentration profile of both Zn and Co. BA also inhibits the formation of zinc hydroxide in the initial deposition stages, which supports the proposed mechanism of normal codeposition. Finally, it is shown that BA modifies the morphology of the deposits by inducing a reduction in the cluster size, leading to compact, smooth and shiny coatings. 1. Introduction Zn–Co alloys are widely used in industry as protective coatings on account of their excellent corrosion resis- tance, which is superior to that of traditional pure zinc coatings. The electrodeposition of Zn–Co alloys is generally classified as an anomalous codeposition pro- cess [1] due to the preferential deposition of the less noble metal (Zn). Several hypotheses have been pro- posed to explain this anomalous codeposition. The most accepted hypothesis explains the anomalous behaviour in terms of the precipitation and adsorption of a zinc hydroxide film at the electrode surface [2] that induces zinc reduction. This zinc hydroxide film is formed due to the increase in interfacial pH during the simultaneous electrochemical reaction of hydrogen evolution. An alternative hypothesis, proposed by Nicol et al. [3], suggests that the underpotential deposition (UPD) of Zn inhibits the deposition of Co. Recently, Yan et al. [4] developed the model of hydroxide oscillation, in which the anomalous codeposition of Zn–Co alloys results from the formation of alternating zinc hydroxide layers whose thickness changes periodically. Other authors [5– 8] have attributed the anomalous codeposition to the fact that the exchange current densities of the iron group metals (Fe, Co, Ni) are generally lower than that of zinc, which favours the preferential deposition of zinc. Higashi et al. [2] showed that the transition from anomalous to normal codeposition occurs as a result of changes in the applied current density during the electrodeposition of zinc alloys in acidic medium. Similar results were obtained by Fratesi et al. [9], who studied the influence of current density, tempera- ture and cobalt concentration in the bath on the composition and morphology of electrodeposited Zn– Co alloys. Yunus et al. [10] found that the codeposition mechanism depended on the Zn(II)/Co(II) concentration ratio in a sulfate medium, and Valle´s et al. [11– 15] observed similar behaviour in chloride based solu- tions. In the present study, the effect of benzylideneacetone (BA) on the electrodeposition of Zn–Co alloy onto AISI 1018 steel is investigated. The focus of this work is the codeposition mechanism, with attention also being given to deposit morphology and composition. Voltammetry was used to study the codeposition mechanism, and techniques such as scanning electron microscopy (SEM), X-ray diffraction (XRD) and Auger electron spectro- scopy were used to characterize the deposits obtained under different conditions. 2. Experimental details Zn–Co deposits were obtained from a base solution of 0.1 M ZnCl 2 + 0.1 M CoCl 2  6H 2 O in a 2.8 M KCl + 0.32 M H 3 BO 3 electrolyte, at pH 5, with a range of BA concentrations (0.0, 0.01, 0.1, 0.2, 0.4 g L )1 ). All chemicals were of analytic grade and solutions were prepared using deionized water (18 MW cm )1 ). Journal of Applied Electrochemistry 33: 373–379, 2003. 373 Ó 2003 Kluwer Academic Publishers. Printed in the Netherlands.