Tin–zinc alloy electrodeposition from aqueous citrate baths Honorata Kazimierczak a, ⁎, Piotr Ozga a , Aldona Jałowiec b , Remigiusz Kowalik b a Institute of Metallurgy and Material Science, Polish Academy of Sciences, 30-059 Krakow, Reymonta 25, Poland b AGH University of Science Technology, Faculty of Non-Ferrous Metals, 30-059 Krakow, al. Mickiewicza 30, Poland abstract article info Article history: Received 20 September 2013 Accepted in revised form 19 December 2013 Available online 29 December 2013 Keywords: Sn–Zn alloys Coatings Electrodeposition Citrate baths The process of tin–zinc alloy electrodeposition from aqueous citrate electrolytes was studied. The influence of ap- plied potential, current density, hydrodynamic conditions, electrolyte composition and charge transfer on the electrodeposition of Sn–Zn alloy was determined. Depending on these parameters, coatings with different com- positions and appearances can be obtained. The surface composition of deposits was ascertained by chemical analysis (WDXRF). The morphology of coatings was studied by SEM. The electrodeposition of bright, shiny Sn– Zn coatings on steel is possible from the citrate baths studied. The presence of zinc ions in the electrolyte results in a significant inhibition of tin reduction rate, which enables the electrodeposition of tin–zinc alloy layers con- taining from about 0.5 to 75 wt.% of Zn, with high current efficiency, within the range from 80% to almost 100%. Two phases are formed in deposits: a hexagonal zinc phase and a tetragonal β-tin phase. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Tin–zinc alloy layers are widely used in the metal finishing industry because they have a number of attractive properties. They have good corrosion resistance, good friction properties, high wear resistance and excellent solderability [1–3]. They are especially interesting as a re- placement for toxic tin–lead solders, because both of these alloys have similar melting temperatures [4,5]. A further advantage is the substan- tially lower cost of such alloys in comparison to the alternative alloys based on tin and silver. In this case, alloys containing about 8–9 wt.% of zinc are applied. However, due to their very good corrosion resis- tance, Sn–Zn alloy layers are often used as protective coatings. In this case, the optimum zinc content in the alloy is about 20–30 wt.% [6–8]. There is currently a great interest in this type of coating, in relation to the need to eliminate of anticorrosive cadmium coatings, which were widely used but proved to be carcinogenic and highly toxic, and hence their use is restricted by the European Union's Restriction of Hazardous Substances directive. By comparison, the carcinogenicity of tin and its compounds was not observed in animal studies. Moreover the Sn–Zn alloy coatings on steel combine the anti-corrosion barrier properties of tin and zinc (barrier protection) with sacrificial properties of zinc (ca- thodic protection). Tin–zinc alloys can be obtained by electrodeposition, which is a rel- atively straightforward and low cost technique. However, a significant difference in the standard redox potentials of tin and zinc (620 mV [9]) means that it is essential to add an appropriate complexing agent which enables the electrodeposition of Zn–Sn alloy. The first patents describing the electrochemical method of tin–zinc alloy deposition date from the beginning of the 20th century [10–14]. These alloys were already proposed as a replacement for cadmium coat- ings from an economic point of view, because the price of cadmium was 2–3 times greater than the price of tin in that period. Then the alloys were obtained from cyanide baths [15] so the main problem was the toxicity of the electrolytes used. Moreover, electrodeposition of Sn–Zn alloy with a high content of zinc (N 50%) from these baths was not possible. In recent years the number of papers describing the electrodeposi- tion of Sn–Zn alloy from various electrolytes has increased, because it is currently considered important to find an environmentally friendly way of preparing alternative, non-toxic coatings [16–25]. Vitkova et al. [18] describe a gluconate–citrate bath for the prepara- tion of Sn–Zn alloy with low tin content. Guaus et al. [23,24] examine sulphate–gluconate and sulphate–tartate baths. Gluconate baths have also been used for the preparation of Sn–Zn alloy solder by Hu et al. [25], while studies on alkaline baths were conducted by Dubent et al. [6]. In this work, citrate baths are proposed for the electrodeposition of Sn–Zn alloys because citrates are non-toxic and form strong complexes with Zn(II) and Sn(II). The previous investigation by Ozga and Kazimierczak [26,27] proved the possibility of Sn–Zn electrodeposition from citrate baths. The purpose of this work was to study the kinetics of co-reduction of tin and zinc from non-toxic citrate electrolytes, and determine the Surface & Coatings Technology 240 (2014) 311–319 ⁎ Corresponding author. Tel.: +48 12 2952812; fax: +48 12 2952804. E-mail address: h.kazimierczak@imim.pl (H. Kazimierczak). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.12.046 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat