ELECTRODEPOSITION , CHARACTERIZATION AND CORROSION INVESTIGATIONS OF GALVANIC TIN-ZINC LAYERS FROM PYROPHOSPHATE BATHS

Tin-zinc alloy deposits are recognised as a potential alternative to toxic cadmium as corrosion resistant coatings. Tin-zinc alloy layers offer outstanding corrosion protection for steel by combining the barrier protection of tin with the galvanic protection of zinc. Tin-zinc coatings have been used on the chassis of electrical and electronic apparatus and on critical automotive parts such as fuel and brake line components. In this study, tin-zinc alloy deposits were successfully prepared from alkaline, pyrophosphate-based electrolytes. The plating process gives a compact and fine grained deposit. The desired proportions of tin and zinc in the deposited alloy are determined by the bath composition and the operating conditions during plating. Three electrode systems were used for the electrochemical investigation. The mechanism of Sn-Zn electrodeposition was studied by linear and cyclic voltammetry. The corrosion parameters, including open-circuit potential-time curves, corrosion potential and corrosion current density of electrodeposited tin-zinc alloys of different compositions have been examined in a brine medium containing 3 wt. % NaCl. The corrosion resistance depends on the plating composition. The Sn-28Zn deposit showed the best anticorrosive properties.


Introduction
Because of outstanding corrosion resistance, tin-zinc layers are mostly used in aerospace, automobile and electrical applications replacing toxic cadmium (Ashiru and Shirokoff, 1996, pp.159-169). The previously used cadmium is not permitted for usage, except for military applications. Additionally, tin-zinc layers have shown excellent solderability and high electrical conductivity.
Because of some disadvantages, the previously mostly used sodium stannate-zinc cyanide plating bath was not widely accepted for industrial applications. In this work, tin-zinc layers were produced in analkaline, noncyanide, pyrophosphate-based bath. This work contains a description of the synthesis of tin-zinc layers, a detailed analysis of the influence of the deposition parameters and the mechanism of synthesis, a morphological characterization of the formed layers and their corrosion stability. The studied electrolyte contains tin sulphate, zinc sulphate, potassium pyrophosphate as a complexing agent and an additive of gelatine. The formation of tin-zinc layers was performed in a Hull cell. At the same time, X-ray fluorescence was used for a chemical analysis of the electrochemically formed layers. The analyses of the corrosion stability via the open-circuit potential-time curves, the corrosion potential and the corrosion current density of the electrodeposited tin-zinc layers with different chemical compositions have been performed in a brine medium containing 3 wt. % NaCl.
The most important aim of this study was to find a relationship between the process-determining parameters and the characteristics of the electrochemically formed Sn-Zn-layers.
The following partial activities were performed: • Process design and the analysis of the influence of different parameters on the synthesis of Sn-Zn layers • Determination of deposition kinetics as well as the stability (corrosion resistance) of the formed Sn-Zn layers • Investigation of the metal distribution as well as the morphology of layers depending on the chemical composition of the baths used Synthesis of tin-zinc layers in the Hull cell The Hull cells are small transportable electrolytic cells. The simulation of different electrochemical processes was mostly performed in the Hull cell. The investigations of electroplating were performed in a range of current density (Andrle and Jelinek, 2007). As the anode plate and the cathode test panel are placed along the non-parallel sides, it is possible to investigate different current densities at the same time in only one performed experiment (Figure 1).  The container of the Hull cell consists of a non-conductive material (e.g., Plexiglas). A brass or steel test panel is usually used as a cathode (cathode dimensions: 0.3 x 70 x 100 mm). The anode material is adjusted to the used electrolyte. As an anode,either a soluble anode based on the material to be coated can be used or an unsoluble anode made from the material such as stainless steel or lead (anode dimensions: 3 x 70 x 65 mm).

Influence of different parameters on Sn-Zn electroplating
• Influence of the metal ion concentration The concentration of alloying elements in an electrolyte has an important influence on the chemical composition of formed coatings (Despic, & Jovic, 1996, pp.239-248). The metal ion concentration in a bath can be varied in three different ways (Vitkova, et al. 1996, pp.226-231): 1. Change of the metal ions ratio, in which the total metal content stays constant 2. Change of the total metal content, in which the metal ion ratio stays constant 3. Change of the concentration of one alloying element, in which the concentration of the other element stays constant The chemical compositions of the baths studied are given in Table 1. • Influence of temperature The variation of temperature affects the polarization of the electrode and, consequently,the deposition potential. The influence of temperature on the chemical composition of deposited layers and cathodic efficiency was studied at 40°C and 60°C.

Experimental work and procedure
For each experiment, a freshly prepared electrolyte was used. The main components in the bath were metal salts: tin sulphate (as 95.5 % SnSO 4 ) and zinc sulphate (as ZnSO 4 * 7H 2 O), as well as potassium pyrophosphate (as 98 % K 4 P 2 O 7 ) as a complexing agent.Firstly, a volume of 200 ml de-ionized water was put into a glass beaker placed on a heating plate with a magnetic stirrer. After a heating up to 60°C, the complexing agent was firstly added in order to be completely dissolved. Then the metal salts were put into the glass beaker and filled to 300 ml with de-ionized water. The prepared electrolyte was finally added to the Hull cell, where the pH value was adjusted. Finally, the filled electrolyte was mixed with gelatin hydrolysate.The experimental work was performed in the equipment shown in Figure 2. The electroplating parameters in the Hull cell are shown in Table 2. The characterization of layers was performed using the X-ray fluorescence analysis XRF, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy EDXS. The X-ray fluorescence analysis was used for the determination of the thickness and the chemical composition of the prepared Zn-Sn layers. The measurements were performed using aFischerscope X-ray-XDL-device (Company Fischer, Germany). Before the measurement, a required calibration was done using the Fischer standards. The morphology of the deposits and the elemental composition was examined with a ZEISS DSM 982 Gemini scanning electron microscope with an EDX-System, Oxford.
Observations, results and discussion of the Sn-Zn electrodeposition in the Hull cell During the experimental work, the gas formation was observed on both electrodes. The foam formation on the bath surface was observed during the electroplating process. The experimental design in the Hull cell with the simplified anode and cathode reactions is shown in Figure 3. All coatings have a bright gray/silver appearance. The influence of the addition of gelatine was investigated, as well. In the absence of gelatine, a gray sponge-like layer was formed in the range of the studied current density (as shown in Figure 4). In Figure 5, a dependency of the chemical composition of the Sn-Zn layers on the electrolyte chemical compositionand the current density is presented. An increase in the current density and the zinc content in the bath led to a decrease of the Sn-content in the deposited Sn-Zn coatings in all cases. For nearly all studied current densities, the electroplating at 40°C in comparison to that at 60°C led to a decreased content of tin in the layer ( Figure 6). This is probably connected with a faster dissociation of the zinc pyrophosphate complex compared to that of the tin pyrophosphate complex, at higher temperature. As shown in Figure 7, the deposited layer thickness and, consequently,the current efficiency increased with an increase in temperature from 40°C to 60°C. This behavior can be explained by an inhibition of diffusion of metal ions for discharging from the electrolyte inside the cathode, as well as by a greater reaction overvoltage at a lower temperature, which is caused through a slow dissociation of metal complexes. The increase in the current density led to a reduction in the cathodic current efficiency, which can be caused by the hydrogen favored deposition at higher current densities, probably provoked by the decrease of metal ions to be discharged at the cathode. The EDXS analysis of the electrochemically deposited Sn-Zn-layers has confirmed the presence of both metals (Figure 8). 0.375 mol/l K4P2O7 and 1 g/l gelatine (60°C; pH = 9.3; 20 min) at 2 A/dm2 Рис. 8 -EDX спектр Sn-Zn слоев, осажденных при электрохимическом процессе из электролита с 0.05 mol/l Sn; 0.1 mol/l Zn; 0.375 mol/l K4P2O7 и1 g/l желатина (60°C; pH=9.3; 20 мин.); pri 2 A/dm2 Slika 8 -EDX spektar Sn-Zn sloja nataloženog elektrohemijskim procesom iz elektrolita sa 0.05 mol/l Sn; 0.1 mol/l Zn; 0.375 mol/l K4P2O7 i 1 g/l želatina (60°C; pH = 9.3; 20 min) pri 2 A/dm2 The SEM images of the Sn-Zn layers, deposited in two different baths at 2 A/dm 2 and 4 A/dm 2 are presented in Figure 9 and Figure 10. The SEM analysis of the Sn-Zn layers shows a fine-grained structure. An increase of the Zn-content in the layer led to the formation of a rough granular structure. 0.2 mol/l Zn; 0,625 mol/l K4P2O7 и 1 g/l желатина (60°C; pH=9.3; 20 мин.); масштаб увеличения: 1.000 x Slika 10 -SEM analiza Sn-Zn slojeva dobijenih iz elektrolita sa 0.05mol/l Sn; 0.2 mol/l Zn; 0,625 mol/l K4P2O7 i 1 g/l želatina (60°C; pH=9.3; 20 min.); uvećanje: 1.000 x A SnZn28layer was analyzed using the EDX analysis and a mapping function. Figure 11 shows a SEM image in gray. The element distribution images are shown in different colors: Zn-(red) and Sn-(green). The constituent parts are identified in the common layer in accordance wththeir ratios. The mixed elemental map shows that the layer is composed of a zinc layer located at the bottom and of many granules rich in tin. During the electrodeposition, zinc is preferentially deposited at the beginning, which is caused probably by an irregular co-deposition mechanism. Electrochemical studies of the electrodeposition of Sn-Zn layers The most important aim of the electrochemical investigations is a detailed analysis of different partial processes in order to arrange and understand them. The electrochemical behavior is studied using the following methods: • Linear-Sweep-Voltammetry (LSV) • Cyclovoltammogrammetry /CV)

Experimental setup for measurements
The experimental setup for the recording of the current-potential curves contains the following components (as shown in Figure 12): • electrochemical three-electrode cell • waterbath with thermostat • potentiostat (typ: Iviumsoft) • electronic dataacquisition The electrochemical cell consists of a double walled glass beaker of 100 ml volume, connected with a waterbath and covered by a plastic lid. The five bore holes on the lid are required for the positioning of three electrodes, a thermometer and an injection tube for gas. The usedmeasuringcellisshown in Figure 13. In the voltammetric studies, a copper disc was used as a working electrode (surface area 0,785 cm 2 ). The working electrode potentials were referred to as an Ag/AgCl electrode. The counter electrode was a platinum rod.
The cathodic current-potential curves of tin, zinc, and tin-zinc deposition from a pyrophosphate electrolyte are shown in Figure 14.
In the mixed Sn-Zn electrolyte, the deposition began at a more positive potential in comparison with the electrolytes containing only one metal. and pH = 9.3; v = 1 mV/s (0.05 mol/l Me + 0.25 mol/l K4P2O7 with 1 g/l gelatine) Рис. 14 -Поляризационные кривые осажденного олова, цинка и сплава олово-цинк на меди при 60°C, со значением pH = 9.3; v = 1 mV/s (0.05 mol/l Me + 0.25 mol/l K4P2O7 с применением 1 g/l желатина Slika 14 -Polarizacione krive nataloženog kalaja, cinka i legure kalaja i cinka na bakru na 60°C pri vrednosti pH = 9.3; v = 1 mV/s (0.05 mol/l Me + 0.25 mol/l K4P2O7 sa 1 g/l želatina The cyclovoltammetry allows a determination of electrode kinetics in a wide potential range using only one measurement. All measurements were performed at a scan rate of 5 mV/s, scanning towards negative potentials. In Figure 15, the cyclic voltammograms obtained from pyrophosphate electrolytes are given. After comparing the single diagrams for pure zinc, pure tin and the Sn-Zn alloy, it can be concluded that, in the case of a co-deposition, the reduction peaks for both alloying elements were shifted about 100 mV to the positive potential value in comparison to the single deposition. This indicates that, in a mixed electrolyte, a strong interdependency happens between the adsorbed zinc and tin species. Electrochemical behavior of the Sn-Zn layers in 3 wt.%NaCl The corrosion behavior of the Sn-Zn layers with different chemical compositions was studied by means of potentiodynamic polarization tests (Wang, Pickering, &Weil, 2004, pp.34-37). An evaluation of the corrosion parameters with the aid of the Tafel extrapolation method has shown that all investigated Sn-Zn layers can be utilized for the protection of steel substrate in chloride-containing media as a sacrificial anode (Taguchi, Bento &Mascaro, 2008, pp.727-733) Between the three Sn-Zn layers, the layer with 28.5 wt. % zinc has exhibited the lowest corrosion rate (Table 3).

Conclusions
This work studied the electrodeposition of Sn-Zn layers from a weak alkaline sulphate-pyrophoshate electrolyte. The deposition of the layers was performed in the Hull cell. The electrochemical characterization was performed in a measuring cell with a three electrode setup employing Linear-Sweep-Voltammetry (LSV) und Cyclovoltammetry (CV). The corrosion behavior of the deposited Sn-Zn-layers was investigated using the potentiodynamic method in order to determine the current-potential curves.
The most important findings are presented below: • During electroplating, a gas formation was observed on both electrodes. All coatings had a gray/silver bright look with a fine striation.
• The chemical composition of deposits varied depending on different bath compositions and current densities.
• An increase in current density and zinc content in the bath increased the content of zinc in the deposited Sn-Zn layer. For example, an increase in current density from 1 A/dm 2 to 5 A/dm 2 at the same chemical composition of the bath led to an increase of about 20 wt. % of zinc in the layer. At the same current density, an increase of tin content from 30 to 65 wt. % in the bath led again to an increase of tin content of about 35 wt % in the layer.
• An increase of current density led to a decrease of current efficiency at all studied bath compositions.
• The SEM analysis of the Sn-Zn layers showed a dense, finegrained structure. An increase of the zinc content influenced the formation of a rough crystalline arrangement.
• In the absence of gelatin-hydrolysate, the sponged structure was formed, which confirms that the limited current density was overpassed. An addition of colloid admixture led to a formation of smooth layers.
• In the mixed Sn-Zn electrolyte, the deposition began at the more positive potential in comparison to that for each single metal, which means that a strict interdependence exists between absorbed tin and zinc species. An increase of zinc in the bath led to a replacement of the deposition curves again to the negative values of potential.
• The obtained results of electrochemical investigations reveal that the corrosion potential of all studied Sn-Zn alloys was more negative than the one for the steel sample. Therefore, it is possible to use all three investigated tin-zinc alloys as an anode for protection of steel substrate in a chloride medium.