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Electrochemical studies of Zn–Ni alloy coatings from non-cyanide alkaline bath containing tartrate as complexing agent
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Surface & Coatings Technology 202 (2008) 2897 – 2904 www.elsevier.com/locate/surfcoat
Electrochemical studies of Zn–Ni alloy coatings from non-cyanide alkaline bath containing tartrate as complexing agent M.G. Hosseini ⁎, H. Ashassi-Sorkhabi, H.A.Y. Ghiasvand Electrochemistry Research Laboratory, Physical Chemistry Department, Chemistry Faculty, University of Tabriz, Tabriz, Iran Received 29 May 2007; accepted in revised form 22 October 2007 Available online 26 November 2007
Abstract The Zn–Ni alloys have been electro-deposited from a non-cyanide alkaline bath containing tartrate as a complexing agent for Ni2+ ions. A water soluble polymer is used as a brightener. It was prepared by the reaction of epiclorohydrin with hexamethylenetetramine and mercaptobenzimidazol. Its effect on co-deposition process was examined. It was found that adding brightener in plating bath has a great effect on the cyclic voltammogram and galvanostatic measurements during the electrodeposition. Under the examined conditions, the electrodeposition of the alloys was of anomalous type. X-ray diffraction measurements revealed that the alloys consisted δ-phase (Ni3Zn22). The composition and morphology of the deposits were also studied by using scanning electron microscopy (SEM) and energy dispersive analysis X-ray (EDAX), respectively. The effect of optimum plating bath conditions on the corrosion resistance is studied by Tafel polarization. © 2007 Elsevier B.V. All rights reserved. Keywords: Tartrate; Alkaline Bath; Electro-deposition; Zn–Ni; Anomalous
1. Introduction Zn–Ni alloy coatings have attracted much attention because they possess higher corrosion resistance and better mechanical characteristics in comparison with zinc and other zinc alloy coatings [1–10]. The zinc–nickel coating provides improved corrosion protection for steel in relatively aggressive environments. The processes of zinc–nickel alloy electrodeposition can be divided into two types [11–14], acid and the alkaline type. The most of commercial alkaline baths used for electroplating of zinc and zinc alloys contained cyanide compounds. However, alkaline cyanide plating solutions are carcinogenic, corrosive, and toxic. In this paper, we have developed tartate plating bath in alkaline media for Zn–Ni co-deposition. The nickel content of zinc–nickel coatings plated from acid bath is more sensitive to variation of cathode current density. For this reason, acid Zn–Ni coatings can only be used for steel parts of simple shape and their industrial applications have been limited. In contrast, alkaline Zn–Ni baths can give more uniform nickel content in ⁎ Corresponding author. Tel.: +98 411 3393138; fax: +98 411 3340191. E-mail address: [email protected] (M.G. Hosseini). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.10.022
the coatings. In addition, the alkaline zinc–nickel deposition process has several other advantages over the acid process such as less corrosion of equipment and lower product cost [15]. According to Brenner [16], Zn–Ni deposition is sometimes an anomalous co-deposition which has been proved also by Ohtsuka and Komora [17] using an in situ ellipsometry method. It was believed that the initial adsorbed layer of zinc inhibits the nucleation and growth of nickel nucleus [15]. The use of additives in electrodeposition bath is extremely important due to their influence on the growth and structure of the resulting deposits. The presence of additives has been shown to influence physical and mechanical properties of electrodeposits such as grain size, brightness, internal stress, pitting and even chemical composition [1,18]. Nowadays, a very large variety of organic additives is used in electroplating baths, their purpose being (a) to improve the appearance and properties of the deposits and/or (b) to improve the operating performance of the plating bath. The aim of this work is preparing and investigating of the mechanism of Zn–Ni alloy deposition in alkaline electrolytes containing tartrate salt as complexing agent to replace cyanide in cyanide alkaline baths. Also, the effects of brightener and
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auxiliary brightener additives on Zn–Ni Alloys are investigated. In order to study, mechanism of deposition electrochemical methods such as cyclic voltammetry (CV), galvanostatic and potentiostatic are used. X-ray diffraction (XRD) and anodic linear sweep voltammetry (ALSV) are used for determination of phase structure, and SEM for characterization of morphology of the coatings. Finally, the corrosion behaviors of coatings obtained from the optimum bath are studied. 2. Experimental 2.1. Electroplating process A new plating bath, made of analytical reagent-grade chemicals (Merck) and deionized distilled water, was used for each experiment. A large number of experimental runs were realized to develop a suitable bath composition for electrodeposition of Zn–Ni alloys with desirable characteristics and to optimize the operating parameters. The optimized bath composition used in this study was included in Table 1. The samples used in this study were mild steel ST12 (in %W) C (0.10), Mn (0.45) S (0.035) and P (0.06) mounted in a Teflon holder, with an exposed electrode area of 1 cm2. Before each experiment, the working electrode was polished with emery paper (1200 grit) and degreased first in a solution containing NaOH(30 g/l), Na2CO3 (30 g/l), Na3PO4 (20 g/l) and Na2SiO3 (15 g/l) at 340 K for 0.5–1 min; then pickled and activated in a solution of 30% hydrochloric acid and 20% sodium chloride. The etching of substrate was carried out in order to improve the deposition and adhesion of deposits. The deposition experiments were performed at the bath temperature 33 0C and cathode current density of 5–20 mA cm−2. 2.2. Brightener synthesis As a brightener additive for the Zn–Ni electroplating baths, a water-soluble polymer is prepared by the reaction of epiclorohydrin with hexamethylene-tetramine and mercaptobenizimadazol. Full details on synthesis of brightener were described elsewhere [19]. 4-methyl benzaldehyde was used as an auxiliary brightener in electrodeposition bath.
Fig. 1. Cyclic voltammogram for Pt electrode in 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate, at 10mVs− 1. Ec: crossover potential where two current intercept together in the reversal sweep.
used as a counter electrode and the reference electrode was saturated calomel (SCE). A platinum electrode with an area of 0.1 cm2 was used as the working electrode. In order to study the electrochemical behavior of deposition the cathodic linear voltammetry and cyclic voltammetry (CV) techniques are used. Potentials are varying between − 0.8 and − 1.8 V with respect to SCE and scan rates from 10 to 200 mV s− 1 in composition solution as shown in Table 1. CV studies showed that the obtained coatings from bath 1 Table 1 were not well in the view of quality such as appearance, leveling and brightness, hence, Zn–Ni coatings were prepared using from bath 2 Table 1. For phase determination of the alloys, the alloys were dissolved anodically at room temperature (23 ± 1 °C) using the slow sweep voltammetry technique (sweep rate 3 mV s− 1) in a 0.01 M Na2SO4 + 0.01 M EDTA solution. For evaluation of corrosion resistance, Tafel polarization measurements were carried out on Zn–Ni alloys in 0.5 N NaCl using a three electrode cell. The data were recorded over a potential e range of Ecorr ± 50 mV, where Ecorr is the equilibrium corrosion potential (after ∼10 min stabilization). The scan rate was 0.166 mV s− 1, and sweep direction was from cathodic to anodic region. 2.4. X-ray diffractometry (XRD) measurement
2.3. Electrochemical measurements A conventional three-electrode cell was used for the voltammetric measurements. A platinum sheet with high area was Table 1 The composition of used baths for electrochemical studies Material
NaOH ZnO KnaC4H4O6 NiSO4 6H2O Brightener 4-methyl benzaldehyde
Amount Bath 1
Bath 2
380 mM 12 mM 110 mM 5 mM – –
380 mM 12 mM 110 mM 5 mM 3.7 ml L− 1 0.4 ml L− 1
A model D5000 Siemens X-ray diffractometry (XRD) was used to identify the phases of Zn–Ni alloys deposited (The area of sample was 1 cm2 ST12). The instrument is equipped with a copper anode generating Ni-filtered Cu Kα radiation (l = 1.5418 A°, 40 kV, 30 mA). An on-line data acquisition and handling system facilitated an automatic JCPDS library search and match (Diffract software, Siemens) for phase identification purposes. 2.5. Surface morphology and microstructure analyses Surface morphology of the deposits was tested with scanning electron microscope (XL30 SEM Philips) at 20 kV. The composition of the deposits on top surface was measured by energy dispersive spectroscopy (EDS) equipped in the SEM
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Fig. 2. Cathodic Linear Sweep Voltammetry for Pt electrode in 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate at various sweep rates: a) 10 mV s− 1 b) 25 mV s− 1, c) 50 mV s− 1, d) 75 mV s− 1, e) 100 mV s− 1, f) 125 mV s− 1, g) 150 mV s− 1, h) 200 mV s− 1.
instrument. Each sample was measured at three different locations to assure for uniformity. Each measurement was repeated three times. The error was found to be less than 1 atom%. The average values for each sample are reported here. 3. Result and discussion 3.1. Electrochemical studies on bath 1 3.1.1. Cyclic voltammetry The cyclic voltammetry (CV) measurements gives information regarding the kinetic behavior of the various electroactive species behavior and linear sweep voltammetry (LSV) can be used for determining the characteristics of these species. CV and LSV involve sweeping the potential of the working electrode and measuring the current response. Fig. 1 shows cyclic voltammogram on the Pt electrode in bath 1 in potential range between − 0.75 to − 1.75 V vs. SCE. The peak (C), centered at − 1.52 V, can be with strong probability attributed to simul-
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Fig. 4. Voltammetric curves for Pt substrate in 0.012 M ZnO, 0.38 M NaOH and 0.11 M tartrate (dashed line); 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate (dashed–dotted line); 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate (solid line), at 100 mV s− 1.
taneously reduction of the Ni2+ and to Zn2+ complexes. Beyond peak C, the increase in the current density at potentials more negative than − 1.57 V can be related to hydrogen evolution reaction (HER). The anodic stripping peak (A), centered at − 1.37 V, is attributed to the dissolution of Zn–Ni alloy coating. Beyond peak A the current approaches to zero, indicating that the majority of the deposited Zn–Ni alloy coating has been removed from the substrate surface. Upon the sweep reversal, in crossover potential (Ec), two current crossovers appear indicating the formation of stable growth centers at the substrate surface [20]. The influence of the potential scan rate (υ) on the Zn–Ni codeposition was shown in Fig. 2. The data reveal that upon increasing the sweep rate, the peak potentials (Ep) shifts to more negative values and the peak current (Ip) increases. The relation between the cathodic peak current and the square root of scan rate (υ1/2) is presented in Fig. 3. The variation is linear but the line do not passes through the origin. The linearity is expected for a reduction processes that occurs under mass transfer
Fig. 3. Variation of Ip with υ1/2 for peak C.
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control. However, the non-zero intercept indicates that an additional process other than diffusion occurs [21]. For a further study to confirm in this regard the galvanostatic deposition technique has been used (see Section 3.2). Fig. 2 shows that the peak potential (Ep) shifts negatively with increasing υ. Fig. 4 shows voltammograms recorded on the Pt substrate in plating baths containing the Zn salt (dashed line), the Ni salt (dashed–dotted line) or both Ni and Zn salts (solid line). Analyzing the Ni deposition curve, the current density increases from ∼ − 1.03 V, owing to reduction of nickel complexes; beyond ∼ − 1.2 V, bulk deposition of Ni and HER occur simultaneously. The anodic stripping peak (A3), centered at − 0.95 V, is attributed to the oxidation of metallic Ni to Ni2+. The complexation of nickel by tartrate is beneficial, since the reduction potential (− 1.00 V) is brought close to that of the zincate ion (∼ − 1.38 V), so that co-deposition of nickel and zinc occurs. On the other hand, tartrate salt is used as complexing agent to replace cyanide in cyanide alkaline baths. Moreover,
the Zn–Ni co-deposition (solid line) lies between Zn (dashed line) and Ni (dashed–dotted line). These results imply that in the region of peak C2 (Fig. 4, solid line) Zn–Ni films of different compositions could be obtained. This suggestion corroborates the results of experiments on Fe–Zn alloy electrodeposited from sorbitol [22], Cu–Sn alloy electrodeposited from pyrophosphate [23], tartrate [24], sorbitate baths [25] and Cu–Zn alloy electrodeposited from sorbitate baths [26]. Also, as Zn–Ni deposition is polarized more than Ni deposition, then it can be predicted that Zn–Ni films obtained at − 1.6 V will probably have a zinc content higher than those obtained at − 1.4 V. With these alkaline electrolytes the oxidation of the Zn–Ni alloys occurs at potentials between those of the pure nickel and slightly more positive than pure zinc, contrary to that observed with ammonium baths where the oxidation of the alloys occurred at much more negative potentials than that of pure nickel [27]. It is interesting to mention that the height of the anodic peak (the peak current) of the deposited Zn dissolution was higher
Fig. 5. (A) E–t curves for pt electrode at different current densities. (B) ALSVof dissolution of Zn–Ni alloys electrodeposited at different current densities. Sweep rate 3 mV s− 1.
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than that in the case of the co-deposited Zn–Ni alloy dissolution. This means that the amount of Zn in the alloy is less than that in a single metal deposit and gives an indication of the formation of Ni in the alloy deposited. This is in good agreement with the results referred in the literature [28]. The Zn–Ni co-deposition started at about − 1.2 V and there is no Ni cathodic peak at − 1.03 V as mentioned before. The reason for this behavior is ascribed that Zn2+ inhibits the electrodeposition of Ni2+ (anomalous deposition). Similar results were reported by Ohtsuka et al. [29] during study of the initial layer formation of the preferential Zn deposition during Zn–Ni electroplating using another bath. The cyclic voltammogram of Zn2+ in the absence of Ni2+ showed the cathodic peak C1 aswellas the corresponding anodic stripping peak A1 (Fig. 4). In what concerns peak C1, centered at − 1.54 V, it corresponds to the zincate bulk reduction according with the following equation [30]: − ZnðOHÞ2− 4 þ 2e→ZnðsÞ þ 4OH
ð1Þ
In this potential region the HER also occurs as it can be detected by direct observation, and described by the below reaction: 2H2 O þ 2e→H2 þ 2OH−
ð2Þ
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current versus time (I–t) transients for 200 sec time. It can be seen that, each of the “I–t” curves consists of a rising portion due to the nucleation process and a posterior decreasing portion due to the diffusion process. The rising section appears to reach its maximum at increasing shorter times with more negative overpotential steps. The maximum in the current transient at high overpotentials and short times corresponds to the maximum surface area. The ALSV record of alloys deposited from the alkaline bath (Fig. 6(B)) are practically the same as those obtained under galvanostatic control, i.e. they are seen to consist of the one well-defined peak. As the potential is increased that peak shifts from − 0.89 V versus SCE in the positive direction. The deposits from the alkaline bath are single phase alloys. This conclusion is valid for both potentiostatically and galvanostatically obtained alloys. 3.2. Electrochemical studies on bath 2 3.2.1. Cyclic voltammetry Fig. 7 shows the voltammograms obtained in similar conditions bath 1 in the absence and presence of 3.7 mL L− 1 brightener and 0.4 mL L− 1 auxiliary agent. The presence of the additives molecules causes significant changes on the shape of the voltammogram and also on the current intensity values. These results indicate that the additives have affected on both
The anodic stripping peak (A1), centered at − 1.37 V, is attributed to the oxidation of metallic zinc to Zn (OH)42−. 3.1.2. Galvanostatic deposition Deposition of Zn–Ni alloys were evaluated at different constant current densities. Fig. 5(A) shows the potential-time dependence for the deposition of Zn–Ni alloy on Pt at constant current densities for 200 s. The nucleation potential (Eτ) and the nucleation time (Tτ) is where the E–t peak falls because the sudden change in slope is an indication of diffusion controlled reduction system. When current density increases, the over potential value increases for making and growing nucleation. Therefore, the nucleases potential shifts to negative values i.e. with increasing constant current density the nucleation time reduces. Also, when potential shifts to negative values, HER as an undesired reaction leads to a reduction in deposition efficiency current. 3.1.3. Anodic linear sweep voltammetry The anodic linear sweep voltammogram (ALSV) were recorded for deposits obtained from the alkaline electrolyte (Fig. 5B) under galvanostatic condition. The deposits were found to exhibit only one well-defined peak. As the current density is increased the peak shifts from − 0.81 V in the positive direction so that increasing deposition current density leads to a thicker layer formation on substrate surface. When the thicker layer anodically dissolved, the anodic current peak increased and shifted towards the positive direction. 3.1.4. Potentiostatic deposition Zn–Ni alloys were electrodeposited potentiostatically at different potential values. Fig. 6 (A) shows the potentiostatic
Fig. 6. (A) I–t curves for pt at different deposition potential. (B) ALSV of dissolution of Zn–Ni alloys electrodeposited at different deposition potential. Sweep rate 3 mV s− 1.
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grames of Zn–Ni alloy coatings obtained from bath 2 (Fig. 9) have a well-defined peak. These coatings are nobler than those obtained from bath 1, because the dissolution of them is started from potential ∼ − 1 V vs. SCE, while the dissolution of the coatings obtained from bath 1 is beginning from potential ∼− 1.2 V versus SCE. Meanwhile, the alloy coatings obtained from bath 2 are thinner than those obtained from bath 1, because the intensity of current peaks of these has reduced in comparison with those obtained from bath 1. 3.3. X-ray diffraction results
Fig. 7. Cyclic voltammograms for Pt electrode in 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate in present (solid line) and absent (dished line) additives. Scan rate 50 mV s− 1.
the Zn–Ni alloy deposition and stripping processes. The two current crossovers in the cathodic region are evident indicating that nucleation and growth of Zn–Ni alloy centers occur for the tested conditions. The crossover potential (Ec) of cyclic voltammogram is different for the plating baths containing additives and free of the additives and shifted in the positive direction. When additives are present in the plating solution, smaller currents for peak C1 and A1 are recorded. This can be explained by an inhibition effect on the Zn–Ni alloy deposition, by blocking the active sites available for the deposition, due to the additives adsorption on the electrode surface. This inhibition depends on the type and size of the organic molecules and, on the specific interaction between the additives and the substrate. On the anodic sweep, a smaller oxidation peak (A1) is observed at about − 1.38 V vs. SCE. This result pointed out that the rate of zinc dissolution is inhibited by the presence of these additives. This is in agreement with the literature values [17]. The additives have a clear effect on the surface brightness which arises from the easy absorption of the groups in the additives on the cathode surface. As a result, an absorbed layer can form continuously on the surface of the cathode and this layer has an inhibitive effect to deposition. Therefore, the deposition occurs only when the zinc and nickel ions arriving at the surface of the cathode can go through this absorbed layer. This delays the movement and discharge of the zinc and nickel ions. As the micro-protrude regions of surfaces have higher current density, the additives tend to absorb preferentially on these areas. The deposition rate of the zinc and nickel on these points can be reduced. As a result, a more even and bright coating can be obtained [31]. 3.2.2. Galvanostatic deposition, potentiostatic deposition and Anodic linear sweep voltammetry studies on bath 2 The E–t, I–t curves and ALSV voltammograms have shown in Figs. 8 and 9 under different conditions of Zn–Ni alloy deposition in the plating bath containing additives, respectively. These curves corroborate the diffuse mechanism and composition of deposits for this system, as well as, the results obtained from the plating bath free of additives. The ALSV voltammo-
The phase structures of the coatings were analyzed by XRD. The results showed that all coatings are of only a single tetragonal-phase structure (Ni3Zn22). Fig. 10 shows the four X-ray diffraction patterns that correspond to the Zn–Ni alloy deposits obtained under different deposition current densities. There are some other peaks corresponding to the substrate, i.e. mild steel sheets (St12). This is probably due to thinner Zn–Ni deposit. Furthermore, as the nickel content in the coatings obtained from the alkaline bath, was approximately identical, the coating has a single δ-phase structure. In contrast, the nickel content in the coatings obtained
Fig. 8. (A) E–t curves for pt at different current densities, (B) ALSV of dissolution of Zn–Ni alloys electrodeposited at different current densities in present additives. Sweep rate 3 mVs− 1.
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Fig. 9. (A) I–t curves for pt at different deposition potential, (B) ALSV of dissolution of Zn–Ni alloys electrodeposited at different deposition potential in present additives. Sweep rate 3 mVs− 1.
from the acid bath is not constant. Therefore, it is difficult to obtain a single-phase coating. It has been reported that the coatings from the acid bath were thought to be of η-phase and of γ-phase structures [32]. Because the potentials of these two phases are different, the corrosion cell tends to form in corrosive environments. Therefore, the coatings from the acid bath may
Fig. 10. X-ray diffractogram of electrodeposited Zn–Ni on steel, Effect of plating current density on phase structure in (time of electroplating is 15 min).
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Fig. 11. SEM image and EDAX spectra Zn–Ni-coated steel in plating bath containing additives.
exhibit generally lower corrosion protection. The peaks at 45″, 65″ and 83″ are the peaks from the stainless steel substrate. All other peaks were identified as reflections of the δ-phase using the JCPDS card 10-209. X-ray diffraction, along with energy dispersive X-ray spectroscopy, was used to verify that all Zn–Ni deposits were obtained were of the single δ-phase only. The spectra present that the intensity of peaks of δ-phase increase with decreasing the plating current density and (512), (652) reflections of δ-phase appeared in XRD spectra. Zn–Ni alloy normal deposition occurs in low current density thereupon Ni content increases in Zn–Ni alloy coating, so peak intensity of δ
Fig. 12. Potentiodynamic polarization curves of Zn and Zn–Ni -coated steels in 0.5 N NaCl solution.
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phase increases. Whereas anomalous deposition occurs in high current density as a result Zn content in Zn–Ni alloy increases and therefore the peak intensity of δ phase reduces. 3.4. SEM and EDAX results The morphology and surface analysis have been shown in Fig. 11. The SEM micrograph of Zn–Ni alloy shows that film totally covers the substrate, and the typical morphology is induced by the additive. The additive used with the alkaline bath leads to smoother and more compact deposits. The surface is very smooth, and the deposit appears as an almost uniform layer without well-defined geometric characteristics. This suggests that the deposit is mainly made of very small crystals that can not be distinguished at that scale. In conclusion, the additives improved the morphology of the Zn–Ni alloy deposits. The fine grain structure and low porosity of in Zn–Ni alloy coatings could greatly improve the corrosion resistance of electrodeposits. Energy dispersive X-ray analysis revealed that in Zn–Ni alloy coatings the composition was 1.79 wt.% Ni and 98.21 wt. % Zn. 3.5. Anticorrosion properties of the zinc–nickel coatings The acid bath may exhibit generally lower corrosion protection. The Zn–Ni alloy coatings were deposited on mild steel from bath 2 and the Zn coatings were deposited on stainless steel from bath 1 in the absence of Ni ions. Fig. 12 shows the potentiodynamic polarization curves. Fig. 12 shows that the corrosion potential of the Zn–Ni coating shifts to a more positive potential than that of the Zn coatings. The corrosion potential for Zn–Ni and Zn coatings in a chloride environment are − 0.966 and − 1.055 V respectively. The Zn–Ni coating has a corrosion current (Icorr) of 34 μA cm− 2 which is much lower than that of the electrodeposited Zn coating on steels i.e. 72 μA cm− 2. The above differences in the corrosion potential and the corrosion current density are responsible for better corrosion resistance for the zinc–nickel coatings. 4. Conclusions 1. In this study a new suitable zinc–nickel plating bath has been introduced in order to replace cyanide bath. Tartrate salt is used as complexing agent to instead of cyanide in cyanide alkaline bathes. 2. The mechanism of Zn–Ni alloy deposition in alkaline bath controlled by diffusion process. Under the examined conditions, the electro-deposition of the alloys is of anomalous type. 3. Although, the deposition of Zn–Ni alloy has not been facilitated in the presence of the additives, but the properties of coating were improved. Ec for Zn–Ni deposition from plating bath containing the additives is more positive than that obtained from the same bath without the additives. 4. ALSV and XRD curves of deposits showed that Zn–Ni coatings have consisted from δ-phase alone.
5. Obtained results from EDAX analysis proved the existence of Ni about 2 wt % in Zn–Ni alloy deposits. 6. Using of the additives allowed obtaining smooth, compact and homogeneous deposits. 7. The zinc–nickel coatings showed excellent corrosion resistance as compared with conventional zinc coatings from an alkaline bath. Acknowledgements The authors would like to acknowledge the financial support of the office of Vice chancellor in charge of research of Tabriz University. References [1] C. Muller, M. Sarret, M. Benballa, Electrochim. Acta 46 (2001) 2811. [2] J.B. Bajat, Z. Kacarevic-Popovic, V.B. Miskovic-Stankovic, M.D. Maksimovic, Prog. Org. Coat. 39 (2000) 127. [3] R. Ramanauskas, L. Muleshkova, L. Maldonado, P. Dobrovolskis, Corros. Sci. 40 (1998) 401. [4] G. Roventi, R. Fratesi, Surf. Coat. Technol. 82 (1996) 158. [5] Z. Zhou, T.J. OKeefe, Surf. Coat. Technol. 96 (1997) 191. [6] M.R. Kalantary, G.D. Wilcox, D.R. Gabe, Electrochim. Acta 40 (1995) 1609. [7] R. Ramanauskas, Appl. Surf. Sci. 153 (1999) 53. [8] Z. Wu, L. Fedrizzi, P.L. Bonora, Surf. Coat. Technol. 85 (1996) 170. [9] M.G. Hosseini, H. Ashassi-Sorkhabi, H.A.Y. Ghiasvand, Iranian Corrosion; ICA International Congress, Tehran: May 14–17, 2007. [10] M.G. Hosseini, H. Ashassi-Sorkhabi, H.A.Y. Ghiasvand, J. Rear Earths 25 (2007) 537. [11] G. Sheela, M. Pushpavanam, S. Pushpavanam, Int. J. Hydrogen Energy 27 (2002) 627. [12] E. Beltowska-Lehman, P. Ozga, Z. Swiatek, C. Lupi, Surf. Coat. Technol. 151 (2002) 444. [13] F.J. Fabri Miranda, O.E. Barcia, S.L. Diaz, O.R. Mattos, R. Wiart, Electrochim. Acta 41 (1996) 1041. [14] S.S. Abd El Rehim, E.E. Fouad, S.M. Abd El Wahab, H. Hamdy Hassan, Electrochim. Acta 41 (1996) 1413. [15] G.Y. Li, J.S. Lian, L.Y. Niu, Z.H. Jiang, Surf. Coat. Technol. 191 (2005) 59. [16] A. Brenner, Electrodeposition of Alloys, vol. 2, Academic Press, New York, 1963, p. 194. [17] T. Ohtsuka, A. Komori, Electrochim. Acta 43 (1998) 3269. [18] R.C. Snowden, J. Phys. Chem. 11 (1907) 369. [19] A. Takahashi, A. Fukuda, T. Igarashi, U.S. Patents 3, 974,045 (1976). [20] A. Gomes, M.I. da Silva Pereira, Electrochim. Acta 52 (2006) 863. [21] G. Trejo, R. Ortega, Y. Meas, V.P. Ozil, E. Chainet, B. Nguyen, J. Electrochem. Soc. 45 (1998) 4090. [22] L.L. Barbosa, I.A. Carlos, Surf. Coat. Technol. 201 (2006) 1695. [23] L. Skibina, J. Stevanovic, A.R. Despic, J. Electroanal. Chem. 310 (1991) 391. [24] I.A. Carlos, C.A.C. Souza, E.M.J.A. Pallone, R.H.P. Francisco, V. Cardoso, B.S. Lima-Neto, J. Appl. Electrochem. 30 (2000) 987. [25] G.A. Finazzi, E.M. Oliveira, I.A. Carlos, Surf. Coat. Technol. 187 (2004) 377. [26] I.A. Carlos, R.H. Almeida, J. Electroanal. Chem. 562 (2004) 153. [27] F. Elkhatabi, M. Sarret, C. Muller, J. Electroanal. Chem. 404 (1996) 45. [28] M. Mortaga, Abou-Krisha, Appl. Surf. Sci. 252 (2005) 1035. [29] T. Ohtsuka, E. Kuwamura, A. Komori, T. Uchida, ISIJ Int. 35 (1995) 892. [30] N.R. Short, S. Zhou, J.K. Dennis, Surf. Coat. Technol. 79 (1996) 218. [31] G.Y. Li, J.S. Lian, L.Y. Niu, Z.H. Jiang, Surf. Coat. Technol. 191 (2005) 59. [32] E. Beltowska-Lehman, P. Ozga, Z. Swiatek, C. Lupi, Cryst. Eng. 5 (2002) 335.
Surface & Coatings Technology 202 (2008) 2897 – 2904 www.elsevier.com/locate/surfcoat
Electrochemical studies of Zn–Ni alloy coatings from non-cyanide alkaline bath containing tartrate as complexing agent M.G. Hosseini ⁎, H. Ashassi-Sorkhabi, H.A.Y. Ghiasvand Electrochemistry Research Laboratory, Physical Chemistry Department, Chemistry Faculty, University of Tabriz, Tabriz, Iran Received 29 May 2007; accepted in revised form 22 October 2007 Available online 26 November 2007
Abstract The Zn–Ni alloys have been electro-deposited from a non-cyanide alkaline bath containing tartrate as a complexing agent for Ni2+ ions. A water soluble polymer is used as a brightener. It was prepared by the reaction of epiclorohydrin with hexamethylenetetramine and mercaptobenzimidazol. Its effect on co-deposition process was examined. It was found that adding brightener in plating bath has a great effect on the cyclic voltammogram and galvanostatic measurements during the electrodeposition. Under the examined conditions, the electrodeposition of the alloys was of anomalous type. X-ray diffraction measurements revealed that the alloys consisted δ-phase (Ni3Zn22). The composition and morphology of the deposits were also studied by using scanning electron microscopy (SEM) and energy dispersive analysis X-ray (EDAX), respectively. The effect of optimum plating bath conditions on the corrosion resistance is studied by Tafel polarization. © 2007 Elsevier B.V. All rights reserved. Keywords: Tartrate; Alkaline Bath; Electro-deposition; Zn–Ni; Anomalous
1. Introduction Zn–Ni alloy coatings have attracted much attention because they possess higher corrosion resistance and better mechanical characteristics in comparison with zinc and other zinc alloy coatings [1–10]. The zinc–nickel coating provides improved corrosion protection for steel in relatively aggressive environments. The processes of zinc–nickel alloy electrodeposition can be divided into two types [11–14], acid and the alkaline type. The most of commercial alkaline baths used for electroplating of zinc and zinc alloys contained cyanide compounds. However, alkaline cyanide plating solutions are carcinogenic, corrosive, and toxic. In this paper, we have developed tartate plating bath in alkaline media for Zn–Ni co-deposition. The nickel content of zinc–nickel coatings plated from acid bath is more sensitive to variation of cathode current density. For this reason, acid Zn–Ni coatings can only be used for steel parts of simple shape and their industrial applications have been limited. In contrast, alkaline Zn–Ni baths can give more uniform nickel content in ⁎ Corresponding author. Tel.: +98 411 3393138; fax: +98 411 3340191. E-mail address: [email protected] (M.G. Hosseini). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.10.022
the coatings. In addition, the alkaline zinc–nickel deposition process has several other advantages over the acid process such as less corrosion of equipment and lower product cost [15]. According to Brenner [16], Zn–Ni deposition is sometimes an anomalous co-deposition which has been proved also by Ohtsuka and Komora [17] using an in situ ellipsometry method. It was believed that the initial adsorbed layer of zinc inhibits the nucleation and growth of nickel nucleus [15]. The use of additives in electrodeposition bath is extremely important due to their influence on the growth and structure of the resulting deposits. The presence of additives has been shown to influence physical and mechanical properties of electrodeposits such as grain size, brightness, internal stress, pitting and even chemical composition [1,18]. Nowadays, a very large variety of organic additives is used in electroplating baths, their purpose being (a) to improve the appearance and properties of the deposits and/or (b) to improve the operating performance of the plating bath. The aim of this work is preparing and investigating of the mechanism of Zn–Ni alloy deposition in alkaline electrolytes containing tartrate salt as complexing agent to replace cyanide in cyanide alkaline baths. Also, the effects of brightener and
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auxiliary brightener additives on Zn–Ni Alloys are investigated. In order to study, mechanism of deposition electrochemical methods such as cyclic voltammetry (CV), galvanostatic and potentiostatic are used. X-ray diffraction (XRD) and anodic linear sweep voltammetry (ALSV) are used for determination of phase structure, and SEM for characterization of morphology of the coatings. Finally, the corrosion behaviors of coatings obtained from the optimum bath are studied. 2. Experimental 2.1. Electroplating process A new plating bath, made of analytical reagent-grade chemicals (Merck) and deionized distilled water, was used for each experiment. A large number of experimental runs were realized to develop a suitable bath composition for electrodeposition of Zn–Ni alloys with desirable characteristics and to optimize the operating parameters. The optimized bath composition used in this study was included in Table 1. The samples used in this study were mild steel ST12 (in %W) C (0.10), Mn (0.45) S (0.035) and P (0.06) mounted in a Teflon holder, with an exposed electrode area of 1 cm2. Before each experiment, the working electrode was polished with emery paper (1200 grit) and degreased first in a solution containing NaOH(30 g/l), Na2CO3 (30 g/l), Na3PO4 (20 g/l) and Na2SiO3 (15 g/l) at 340 K for 0.5–1 min; then pickled and activated in a solution of 30% hydrochloric acid and 20% sodium chloride. The etching of substrate was carried out in order to improve the deposition and adhesion of deposits. The deposition experiments were performed at the bath temperature 33 0C and cathode current density of 5–20 mA cm−2. 2.2. Brightener synthesis As a brightener additive for the Zn–Ni electroplating baths, a water-soluble polymer is prepared by the reaction of epiclorohydrin with hexamethylene-tetramine and mercaptobenizimadazol. Full details on synthesis of brightener were described elsewhere [19]. 4-methyl benzaldehyde was used as an auxiliary brightener in electrodeposition bath.
Fig. 1. Cyclic voltammogram for Pt electrode in 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate, at 10mVs− 1. Ec: crossover potential where two current intercept together in the reversal sweep.
used as a counter electrode and the reference electrode was saturated calomel (SCE). A platinum electrode with an area of 0.1 cm2 was used as the working electrode. In order to study the electrochemical behavior of deposition the cathodic linear voltammetry and cyclic voltammetry (CV) techniques are used. Potentials are varying between − 0.8 and − 1.8 V with respect to SCE and scan rates from 10 to 200 mV s− 1 in composition solution as shown in Table 1. CV studies showed that the obtained coatings from bath 1 Table 1 were not well in the view of quality such as appearance, leveling and brightness, hence, Zn–Ni coatings were prepared using from bath 2 Table 1. For phase determination of the alloys, the alloys were dissolved anodically at room temperature (23 ± 1 °C) using the slow sweep voltammetry technique (sweep rate 3 mV s− 1) in a 0.01 M Na2SO4 + 0.01 M EDTA solution. For evaluation of corrosion resistance, Tafel polarization measurements were carried out on Zn–Ni alloys in 0.5 N NaCl using a three electrode cell. The data were recorded over a potential e range of Ecorr ± 50 mV, where Ecorr is the equilibrium corrosion potential (after ∼10 min stabilization). The scan rate was 0.166 mV s− 1, and sweep direction was from cathodic to anodic region. 2.4. X-ray diffractometry (XRD) measurement
2.3. Electrochemical measurements A conventional three-electrode cell was used for the voltammetric measurements. A platinum sheet with high area was Table 1 The composition of used baths for electrochemical studies Material
NaOH ZnO KnaC4H4O6 NiSO4 6H2O Brightener 4-methyl benzaldehyde
Amount Bath 1
Bath 2
380 mM 12 mM 110 mM 5 mM – –
380 mM 12 mM 110 mM 5 mM 3.7 ml L− 1 0.4 ml L− 1
A model D5000 Siemens X-ray diffractometry (XRD) was used to identify the phases of Zn–Ni alloys deposited (The area of sample was 1 cm2 ST12). The instrument is equipped with a copper anode generating Ni-filtered Cu Kα radiation (l = 1.5418 A°, 40 kV, 30 mA). An on-line data acquisition and handling system facilitated an automatic JCPDS library search and match (Diffract software, Siemens) for phase identification purposes. 2.5. Surface morphology and microstructure analyses Surface morphology of the deposits was tested with scanning electron microscope (XL30 SEM Philips) at 20 kV. The composition of the deposits on top surface was measured by energy dispersive spectroscopy (EDS) equipped in the SEM
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Fig. 2. Cathodic Linear Sweep Voltammetry for Pt electrode in 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate at various sweep rates: a) 10 mV s− 1 b) 25 mV s− 1, c) 50 mV s− 1, d) 75 mV s− 1, e) 100 mV s− 1, f) 125 mV s− 1, g) 150 mV s− 1, h) 200 mV s− 1.
instrument. Each sample was measured at three different locations to assure for uniformity. Each measurement was repeated three times. The error was found to be less than 1 atom%. The average values for each sample are reported here. 3. Result and discussion 3.1. Electrochemical studies on bath 1 3.1.1. Cyclic voltammetry The cyclic voltammetry (CV) measurements gives information regarding the kinetic behavior of the various electroactive species behavior and linear sweep voltammetry (LSV) can be used for determining the characteristics of these species. CV and LSV involve sweeping the potential of the working electrode and measuring the current response. Fig. 1 shows cyclic voltammogram on the Pt electrode in bath 1 in potential range between − 0.75 to − 1.75 V vs. SCE. The peak (C), centered at − 1.52 V, can be with strong probability attributed to simul-
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Fig. 4. Voltammetric curves for Pt substrate in 0.012 M ZnO, 0.38 M NaOH and 0.11 M tartrate (dashed line); 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate (dashed–dotted line); 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate (solid line), at 100 mV s− 1.
taneously reduction of the Ni2+ and to Zn2+ complexes. Beyond peak C, the increase in the current density at potentials more negative than − 1.57 V can be related to hydrogen evolution reaction (HER). The anodic stripping peak (A), centered at − 1.37 V, is attributed to the dissolution of Zn–Ni alloy coating. Beyond peak A the current approaches to zero, indicating that the majority of the deposited Zn–Ni alloy coating has been removed from the substrate surface. Upon the sweep reversal, in crossover potential (Ec), two current crossovers appear indicating the formation of stable growth centers at the substrate surface [20]. The influence of the potential scan rate (υ) on the Zn–Ni codeposition was shown in Fig. 2. The data reveal that upon increasing the sweep rate, the peak potentials (Ep) shifts to more negative values and the peak current (Ip) increases. The relation between the cathodic peak current and the square root of scan rate (υ1/2) is presented in Fig. 3. The variation is linear but the line do not passes through the origin. The linearity is expected for a reduction processes that occurs under mass transfer
Fig. 3. Variation of Ip with υ1/2 for peak C.
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control. However, the non-zero intercept indicates that an additional process other than diffusion occurs [21]. For a further study to confirm in this regard the galvanostatic deposition technique has been used (see Section 3.2). Fig. 2 shows that the peak potential (Ep) shifts negatively with increasing υ. Fig. 4 shows voltammograms recorded on the Pt substrate in plating baths containing the Zn salt (dashed line), the Ni salt (dashed–dotted line) or both Ni and Zn salts (solid line). Analyzing the Ni deposition curve, the current density increases from ∼ − 1.03 V, owing to reduction of nickel complexes; beyond ∼ − 1.2 V, bulk deposition of Ni and HER occur simultaneously. The anodic stripping peak (A3), centered at − 0.95 V, is attributed to the oxidation of metallic Ni to Ni2+. The complexation of nickel by tartrate is beneficial, since the reduction potential (− 1.00 V) is brought close to that of the zincate ion (∼ − 1.38 V), so that co-deposition of nickel and zinc occurs. On the other hand, tartrate salt is used as complexing agent to replace cyanide in cyanide alkaline baths. Moreover,
the Zn–Ni co-deposition (solid line) lies between Zn (dashed line) and Ni (dashed–dotted line). These results imply that in the region of peak C2 (Fig. 4, solid line) Zn–Ni films of different compositions could be obtained. This suggestion corroborates the results of experiments on Fe–Zn alloy electrodeposited from sorbitol [22], Cu–Sn alloy electrodeposited from pyrophosphate [23], tartrate [24], sorbitate baths [25] and Cu–Zn alloy electrodeposited from sorbitate baths [26]. Also, as Zn–Ni deposition is polarized more than Ni deposition, then it can be predicted that Zn–Ni films obtained at − 1.6 V will probably have a zinc content higher than those obtained at − 1.4 V. With these alkaline electrolytes the oxidation of the Zn–Ni alloys occurs at potentials between those of the pure nickel and slightly more positive than pure zinc, contrary to that observed with ammonium baths where the oxidation of the alloys occurred at much more negative potentials than that of pure nickel [27]. It is interesting to mention that the height of the anodic peak (the peak current) of the deposited Zn dissolution was higher
Fig. 5. (A) E–t curves for pt electrode at different current densities. (B) ALSVof dissolution of Zn–Ni alloys electrodeposited at different current densities. Sweep rate 3 mV s− 1.
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than that in the case of the co-deposited Zn–Ni alloy dissolution. This means that the amount of Zn in the alloy is less than that in a single metal deposit and gives an indication of the formation of Ni in the alloy deposited. This is in good agreement with the results referred in the literature [28]. The Zn–Ni co-deposition started at about − 1.2 V and there is no Ni cathodic peak at − 1.03 V as mentioned before. The reason for this behavior is ascribed that Zn2+ inhibits the electrodeposition of Ni2+ (anomalous deposition). Similar results were reported by Ohtsuka et al. [29] during study of the initial layer formation of the preferential Zn deposition during Zn–Ni electroplating using another bath. The cyclic voltammogram of Zn2+ in the absence of Ni2+ showed the cathodic peak C1 aswellas the corresponding anodic stripping peak A1 (Fig. 4). In what concerns peak C1, centered at − 1.54 V, it corresponds to the zincate bulk reduction according with the following equation [30]: − ZnðOHÞ2− 4 þ 2e→ZnðsÞ þ 4OH
ð1Þ
In this potential region the HER also occurs as it can be detected by direct observation, and described by the below reaction: 2H2 O þ 2e→H2 þ 2OH−
ð2Þ
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current versus time (I–t) transients for 200 sec time. It can be seen that, each of the “I–t” curves consists of a rising portion due to the nucleation process and a posterior decreasing portion due to the diffusion process. The rising section appears to reach its maximum at increasing shorter times with more negative overpotential steps. The maximum in the current transient at high overpotentials and short times corresponds to the maximum surface area. The ALSV record of alloys deposited from the alkaline bath (Fig. 6(B)) are practically the same as those obtained under galvanostatic control, i.e. they are seen to consist of the one well-defined peak. As the potential is increased that peak shifts from − 0.89 V versus SCE in the positive direction. The deposits from the alkaline bath are single phase alloys. This conclusion is valid for both potentiostatically and galvanostatically obtained alloys. 3.2. Electrochemical studies on bath 2 3.2.1. Cyclic voltammetry Fig. 7 shows the voltammograms obtained in similar conditions bath 1 in the absence and presence of 3.7 mL L− 1 brightener and 0.4 mL L− 1 auxiliary agent. The presence of the additives molecules causes significant changes on the shape of the voltammogram and also on the current intensity values. These results indicate that the additives have affected on both
The anodic stripping peak (A1), centered at − 1.37 V, is attributed to the oxidation of metallic zinc to Zn (OH)42−. 3.1.2. Galvanostatic deposition Deposition of Zn–Ni alloys were evaluated at different constant current densities. Fig. 5(A) shows the potential-time dependence for the deposition of Zn–Ni alloy on Pt at constant current densities for 200 s. The nucleation potential (Eτ) and the nucleation time (Tτ) is where the E–t peak falls because the sudden change in slope is an indication of diffusion controlled reduction system. When current density increases, the over potential value increases for making and growing nucleation. Therefore, the nucleases potential shifts to negative values i.e. with increasing constant current density the nucleation time reduces. Also, when potential shifts to negative values, HER as an undesired reaction leads to a reduction in deposition efficiency current. 3.1.3. Anodic linear sweep voltammetry The anodic linear sweep voltammogram (ALSV) were recorded for deposits obtained from the alkaline electrolyte (Fig. 5B) under galvanostatic condition. The deposits were found to exhibit only one well-defined peak. As the current density is increased the peak shifts from − 0.81 V in the positive direction so that increasing deposition current density leads to a thicker layer formation on substrate surface. When the thicker layer anodically dissolved, the anodic current peak increased and shifted towards the positive direction. 3.1.4. Potentiostatic deposition Zn–Ni alloys were electrodeposited potentiostatically at different potential values. Fig. 6 (A) shows the potentiostatic
Fig. 6. (A) I–t curves for pt at different deposition potential. (B) ALSV of dissolution of Zn–Ni alloys electrodeposited at different deposition potential. Sweep rate 3 mV s− 1.
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grames of Zn–Ni alloy coatings obtained from bath 2 (Fig. 9) have a well-defined peak. These coatings are nobler than those obtained from bath 1, because the dissolution of them is started from potential ∼ − 1 V vs. SCE, while the dissolution of the coatings obtained from bath 1 is beginning from potential ∼− 1.2 V versus SCE. Meanwhile, the alloy coatings obtained from bath 2 are thinner than those obtained from bath 1, because the intensity of current peaks of these has reduced in comparison with those obtained from bath 1. 3.3. X-ray diffraction results
Fig. 7. Cyclic voltammograms for Pt electrode in 0.012 M ZnO, 0.005 M NiSO4, 0.38 M NaOH and 0.11 M tartrate in present (solid line) and absent (dished line) additives. Scan rate 50 mV s− 1.
the Zn–Ni alloy deposition and stripping processes. The two current crossovers in the cathodic region are evident indicating that nucleation and growth of Zn–Ni alloy centers occur for the tested conditions. The crossover potential (Ec) of cyclic voltammogram is different for the plating baths containing additives and free of the additives and shifted in the positive direction. When additives are present in the plating solution, smaller currents for peak C1 and A1 are recorded. This can be explained by an inhibition effect on the Zn–Ni alloy deposition, by blocking the active sites available for the deposition, due to the additives adsorption on the electrode surface. This inhibition depends on the type and size of the organic molecules and, on the specific interaction between the additives and the substrate. On the anodic sweep, a smaller oxidation peak (A1) is observed at about − 1.38 V vs. SCE. This result pointed out that the rate of zinc dissolution is inhibited by the presence of these additives. This is in agreement with the literature values [17]. The additives have a clear effect on the surface brightness which arises from the easy absorption of the groups in the additives on the cathode surface. As a result, an absorbed layer can form continuously on the surface of the cathode and this layer has an inhibitive effect to deposition. Therefore, the deposition occurs only when the zinc and nickel ions arriving at the surface of the cathode can go through this absorbed layer. This delays the movement and discharge of the zinc and nickel ions. As the micro-protrude regions of surfaces have higher current density, the additives tend to absorb preferentially on these areas. The deposition rate of the zinc and nickel on these points can be reduced. As a result, a more even and bright coating can be obtained [31]. 3.2.2. Galvanostatic deposition, potentiostatic deposition and Anodic linear sweep voltammetry studies on bath 2 The E–t, I–t curves and ALSV voltammograms have shown in Figs. 8 and 9 under different conditions of Zn–Ni alloy deposition in the plating bath containing additives, respectively. These curves corroborate the diffuse mechanism and composition of deposits for this system, as well as, the results obtained from the plating bath free of additives. The ALSV voltammo-
The phase structures of the coatings were analyzed by XRD. The results showed that all coatings are of only a single tetragonal-phase structure (Ni3Zn22). Fig. 10 shows the four X-ray diffraction patterns that correspond to the Zn–Ni alloy deposits obtained under different deposition current densities. There are some other peaks corresponding to the substrate, i.e. mild steel sheets (St12). This is probably due to thinner Zn–Ni deposit. Furthermore, as the nickel content in the coatings obtained from the alkaline bath, was approximately identical, the coating has a single δ-phase structure. In contrast, the nickel content in the coatings obtained
Fig. 8. (A) E–t curves for pt at different current densities, (B) ALSV of dissolution of Zn–Ni alloys electrodeposited at different current densities in present additives. Sweep rate 3 mVs− 1.
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Fig. 9. (A) I–t curves for pt at different deposition potential, (B) ALSV of dissolution of Zn–Ni alloys electrodeposited at different deposition potential in present additives. Sweep rate 3 mVs− 1.
from the acid bath is not constant. Therefore, it is difficult to obtain a single-phase coating. It has been reported that the coatings from the acid bath were thought to be of η-phase and of γ-phase structures [32]. Because the potentials of these two phases are different, the corrosion cell tends to form in corrosive environments. Therefore, the coatings from the acid bath may
Fig. 10. X-ray diffractogram of electrodeposited Zn–Ni on steel, Effect of plating current density on phase structure in (time of electroplating is 15 min).
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Fig. 11. SEM image and EDAX spectra Zn–Ni-coated steel in plating bath containing additives.
exhibit generally lower corrosion protection. The peaks at 45″, 65″ and 83″ are the peaks from the stainless steel substrate. All other peaks were identified as reflections of the δ-phase using the JCPDS card 10-209. X-ray diffraction, along with energy dispersive X-ray spectroscopy, was used to verify that all Zn–Ni deposits were obtained were of the single δ-phase only. The spectra present that the intensity of peaks of δ-phase increase with decreasing the plating current density and (512), (652) reflections of δ-phase appeared in XRD spectra. Zn–Ni alloy normal deposition occurs in low current density thereupon Ni content increases in Zn–Ni alloy coating, so peak intensity of δ
Fig. 12. Potentiodynamic polarization curves of Zn and Zn–Ni -coated steels in 0.5 N NaCl solution.
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phase increases. Whereas anomalous deposition occurs in high current density as a result Zn content in Zn–Ni alloy increases and therefore the peak intensity of δ phase reduces. 3.4. SEM and EDAX results The morphology and surface analysis have been shown in Fig. 11. The SEM micrograph of Zn–Ni alloy shows that film totally covers the substrate, and the typical morphology is induced by the additive. The additive used with the alkaline bath leads to smoother and more compact deposits. The surface is very smooth, and the deposit appears as an almost uniform layer without well-defined geometric characteristics. This suggests that the deposit is mainly made of very small crystals that can not be distinguished at that scale. In conclusion, the additives improved the morphology of the Zn–Ni alloy deposits. The fine grain structure and low porosity of in Zn–Ni alloy coatings could greatly improve the corrosion resistance of electrodeposits. Energy dispersive X-ray analysis revealed that in Zn–Ni alloy coatings the composition was 1.79 wt.% Ni and 98.21 wt. % Zn. 3.5. Anticorrosion properties of the zinc–nickel coatings The acid bath may exhibit generally lower corrosion protection. The Zn–Ni alloy coatings were deposited on mild steel from bath 2 and the Zn coatings were deposited on stainless steel from bath 1 in the absence of Ni ions. Fig. 12 shows the potentiodynamic polarization curves. Fig. 12 shows that the corrosion potential of the Zn–Ni coating shifts to a more positive potential than that of the Zn coatings. The corrosion potential for Zn–Ni and Zn coatings in a chloride environment are − 0.966 and − 1.055 V respectively. The Zn–Ni coating has a corrosion current (Icorr) of 34 μA cm− 2 which is much lower than that of the electrodeposited Zn coating on steels i.e. 72 μA cm− 2. The above differences in the corrosion potential and the corrosion current density are responsible for better corrosion resistance for the zinc–nickel coatings. 4. Conclusions 1. In this study a new suitable zinc–nickel plating bath has been introduced in order to replace cyanide bath. Tartrate salt is used as complexing agent to instead of cyanide in cyanide alkaline bathes. 2. The mechanism of Zn–Ni alloy deposition in alkaline bath controlled by diffusion process. Under the examined conditions, the electro-deposition of the alloys is of anomalous type. 3. Although, the deposition of Zn–Ni alloy has not been facilitated in the presence of the additives, but the properties of coating were improved. Ec for Zn–Ni deposition from plating bath containing the additives is more positive than that obtained from the same bath without the additives. 4. ALSV and XRD curves of deposits showed that Zn–Ni coatings have consisted from δ-phase alone.
5. Obtained results from EDAX analysis proved the existence of Ni about 2 wt % in Zn–Ni alloy deposits. 6. Using of the additives allowed obtaining smooth, compact and homogeneous deposits. 7. The zinc–nickel coatings showed excellent corrosion resistance as compared with conventional zinc coatings from an alkaline bath. Acknowledgements The authors would like to acknowledge the financial support of the office of Vice chancellor in charge of research of Tabriz University. References [1] C. Muller, M. Sarret, M. Benballa, Electrochim. Acta 46 (2001) 2811. [2] J.B. Bajat, Z. Kacarevic-Popovic, V.B. Miskovic-Stankovic, M.D. Maksimovic, Prog. Org. Coat. 39 (2000) 127. [3] R. Ramanauskas, L. Muleshkova, L. Maldonado, P. Dobrovolskis, Corros. Sci. 40 (1998) 401. [4] G. Roventi, R. Fratesi, Surf. Coat. Technol. 82 (1996) 158. [5] Z. Zhou, T.J. OKeefe, Surf. Coat. Technol. 96 (1997) 191. [6] M.R. Kalantary, G.D. Wilcox, D.R. Gabe, Electrochim. Acta 40 (1995) 1609. [7] R. Ramanauskas, Appl. Surf. Sci. 153 (1999) 53. [8] Z. Wu, L. Fedrizzi, P.L. Bonora, Surf. Coat. Technol. 85 (1996) 170. [9] M.G. Hosseini, H. Ashassi-Sorkhabi, H.A.Y. Ghiasvand, Iranian Corrosion; ICA International Congress, Tehran: May 14–17, 2007. [10] M.G. Hosseini, H. Ashassi-Sorkhabi, H.A.Y. Ghiasvand, J. Rear Earths 25 (2007) 537. [11] G. Sheela, M. Pushpavanam, S. Pushpavanam, Int. J. Hydrogen Energy 27 (2002) 627. [12] E. Beltowska-Lehman, P. Ozga, Z. Swiatek, C. Lupi, Surf. Coat. Technol. 151 (2002) 444. [13] F.J. Fabri Miranda, O.E. Barcia, S.L. Diaz, O.R. Mattos, R. Wiart, Electrochim. Acta 41 (1996) 1041. [14] S.S. Abd El Rehim, E.E. Fouad, S.M. Abd El Wahab, H. Hamdy Hassan, Electrochim. Acta 41 (1996) 1413. [15] G.Y. Li, J.S. Lian, L.Y. Niu, Z.H. Jiang, Surf. Coat. Technol. 191 (2005) 59. [16] A. Brenner, Electrodeposition of Alloys, vol. 2, Academic Press, New York, 1963, p. 194. [17] T. Ohtsuka, A. Komori, Electrochim. Acta 43 (1998) 3269. [18] R.C. Snowden, J. Phys. Chem. 11 (1907) 369. [19] A. Takahashi, A. Fukuda, T. Igarashi, U.S. Patents 3, 974,045 (1976). [20] A. Gomes, M.I. da Silva Pereira, Electrochim. Acta 52 (2006) 863. [21] G. Trejo, R. Ortega, Y. Meas, V.P. Ozil, E. Chainet, B. Nguyen, J. Electrochem. Soc. 45 (1998) 4090. [22] L.L. Barbosa, I.A. Carlos, Surf. Coat. Technol. 201 (2006) 1695. [23] L. Skibina, J. Stevanovic, A.R. Despic, J. Electroanal. Chem. 310 (1991) 391. [24] I.A. Carlos, C.A.C. Souza, E.M.J.A. Pallone, R.H.P. Francisco, V. Cardoso, B.S. Lima-Neto, J. Appl. Electrochem. 30 (2000) 987. [25] G.A. Finazzi, E.M. Oliveira, I.A. Carlos, Surf. Coat. Technol. 187 (2004) 377. [26] I.A. Carlos, R.H. Almeida, J. Electroanal. Chem. 562 (2004) 153. [27] F. Elkhatabi, M. Sarret, C. Muller, J. Electroanal. Chem. 404 (1996) 45. [28] M. Mortaga, Abou-Krisha, Appl. Surf. Sci. 252 (2005) 1035. [29] T. Ohtsuka, E. Kuwamura, A. Komori, T. Uchida, ISIJ Int. 35 (1995) 892. [30] N.R. Short, S. Zhou, J.K. Dennis, Surf. Coat. Technol. 79 (1996) 218. [31] G.Y. Li, J.S. Lian, L.Y. Niu, Z.H. Jiang, Surf. Coat. Technol. 191 (2005) 59. [32] E. Beltowska-Lehman, P. Ozga, Z. Swiatek, C. Lupi, Cryst. Eng. 5 (2002) 335.