Study of the corrosion behavior of zinc and Zn–Co alloy electrodeposits obtained from alkaline bath using direct current

Materials Chemistry and Physics 117 (2009) 414–421

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Study of the corrosion behavior of zinc and Zn–Co alloy electrodeposits obtained from alkaline bath using direct current M. Heydari Gharahcheshmeh, M. Heydarzadeh Sohi ∗ School of Metallurgy and Materials Engineering, University of Tehran, P.O. Box 11155-4563, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 11 March 2009 Received in revised form 3 June 2009 Accepted 13 June 2009 Keywords: Alloys Coatings Corrosion

a b s t r a c t The corrosion behavior of Zn and Zn–Co alloy electrodeposits that were obtained from weakly alkaline glycine solutions has been studied. SEM, EDS and XRD were used to study surface morphology, chemical composition and phase structure of the coatings. Corrosion behavior of Zn and Zn–Co alloy coatings was studied by using potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The results showed that increasing current density during deposition, increases cobalt content of the coating. It was also shown that increasing current density, up to 15 mA cm−2 , decreases the grain size and further increase in current density increases the grain size of the deposit. It was also noticed that corrosion resistance of deposits was highly influenced by the composition and morphology of the coatings. Zn–Co deposit containing 0.89 wt.% Co showed the highest corrosion resistance due to its single phase structure and its finer morphology. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The interest in zinc alloy electrodeposits, including Zn–Co, Zn–Fe and, Zn–Ni has been growing as a consequence of their higher corrosion resistance in comparison with ordinary zinc coatings [1–3] and as a substitute for toxic and high-cost cadmium coatings [3–5]. Electrodeposition of Zn–Co alloy coatings has been studied for more than two decades. However, the optimum content of cobalt in the coatings and their protection mechanism are still subjects of many researches [6–12]. Neto et al. [11] studied the corrosion behavior of pure zinc and Zn–Co alloy coatings with 1 to 18 at.% of Co content deposited from an acidic bath. They found that zinc alloy coating with 18 at.% Co had the maximum corrosion resistance. Ramanauskas et al. [12] showed that phase structure of Zn–Co alloy coatings plays an important role in corrosion behavior of these coatings. During zinc co-deposition with metals of the iron group (Ni, Co and Fe), the less noble metal, zinc, is electrodeposited preferentially; this phenomenon has been described as anomalous co-deposition [13–17]. Several theories have been suggested by various researchers about anomalous co-deposition. The most widespread theory in this regard is the so-called hydroxide suppression mechanism (HSM) [13,14].

∗ Corresponding author. Tel.: +98 21 82084077; fax: +98 21 88006076. E-mail addresses: [email protected] (M.H. Gharahcheshmeh), [email protected] (M.H. Sohi). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.06.009

Both acidic and alkaline plating baths have been used for the plating of zinc and zinc alloy deposits. The acidic baths exhibit high current efficiency, high plating rate but they seem to have poor throwing power. The alkaline baths, on the other hand, show uniform distribution of thickness but have low current efficiencies [12,18,19]. In order to improve the properties of the coating, organic additives are often added into the plating bath used for electrodeposition of zinc and zinc alloy coatings [11,20–22]. Barbosa and Carlos [23] used sorbitol as a complex agent in alkaline bath applied for deposition of Zn–Fe. They observed that sorbitol had a beneficial effect in minimizing the potential difference of iron and zinc and codeposition of Fe and Zn occurred successfully. Mouanga et al. [22] used coumarin in an acidic bath applied for deposition of Zn–Co. They found that coumarin affected the reduction of zinc but it had no effect on the reduction of cobalt. They also noticed that the presence of coumarin in the electrolyte resulted in structural refinement of alloy deposits. Ortiz-Aparicio et al. [17] studied the influence of glycine complex agent in the electrodeposition of Zn–Co alloy, using an alkaline bath. They observed that deposition of cobalt shifts to more negative potentials, and co-deposition of Co and Zn occurred successfully. The presence of additives has been shown to have an influence on the physical and mechanical properties of electrodeposits such as grain size, structure, brightness, internal stress, pitting and even the chemical composition and corrosion behavior. The aim of the present work was to develop an alkaline bath containing glycine as complexing agent to replace cyanide in cyanide alkaline baths for electrochemical deposition of the zinc–cobalt alloy coatings.

M.H. Gharahcheshmeh, M.H. Sohi / Materials Chemistry and Physics 117 (2009) 414–421

Fig. 1. Effect of current density on the Co content of Zn–Co alloy electrodeposits.

The corrosion resistance of Zn–Co alloys with low cobalt content is also studied to optimize DC electroplating parameters and cobalt content of the alloy deposits in the presence of glycine. 2. Experimental AISI 1018 steel with a plated area of 2 cm × 1 cm and high purity zinc were used as cathode and anode, respectively. Before electroplating process, the samples were polished with (emery papers of the following grades: 220, 400, 600, 800, 1000 and 1500) and thoroughly degreased in alkaline solution (containing 10 g L−1 NaOH and 40 g L−1 Na2 CO3 ) for 5 min at 60 ◦ C. After a thorough rinse with distilled water, the samples were etched in 10% HCl for 5 min to neutralize the remnants of alkaline solution and to activate the surface. Finally, the substrates were rinsed again with distilled water and dried by air. After surface preparations, the samples were immediately put in the electrolyte, in order to prevent formation of oxide layer on their surfaces. The chemicals used this study were of Merck analytical grade. The electrolyte used for direct current (DC) electrodeposition contained 0.4 M ZnSO4 , 0.02 M CoSO4 (except for the pure zinc deposition) and 2 M glycine. The pH of electrolyte was adjusted to 11 by adding sodium hydroxide and deposition was carried out at room temperature. The thickness of coatings was fixed to approximately 12 ␮m. The surface morphology of the coatings was investigated by using MV-2300 scanning electron microscope (SEM). The chemical composition of the coatings was determined by energy dispersive spectroscopy (EDS). The phase composition of coatings was also determined via X-ray diffraction (XRD) analysis by using a diffractometer (model Philips X’Pret Pro) with Co–K␣ radiation ( = 1.78897 Å) at 30 kV and 20 mA. The 2 ranged from 20◦ to 120◦ and the scan rate was 0.02◦ s−1 . Corrosion behavior of Zn and Zn–Co alloy coatings was studied by using potentiodynamic polarization test and electrochemical impedance spectroscopy (EIS) in 3.5 wt.% NaCl solution. The tests were carried out in a three-electrode cell using an EG&G potentiostat/galvanostat, model 273A. Platinum plate and saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. potentiodynamic polarization test was carried out by sweeping the potential at a scan rate of 2 mV s−1 from −1800 to 200 mV (vs. SCE). The EIS tests were undertaken using a Solartron Model SI 1255 HF Frequency Response Analyzer (FRA) coupled to a Princeton Applied Research (PAR) Model 273A Potentiostat/Galvanostat. The EIS measurements were obtained at the open circuit potential (OCP) in a frequency range from 0.01 Hz to 100 kHz with an applied AC signal of 5 mV (rms) using EIS software model 398. The equivalent circuit simulation program (ZView2) was used for data analysis, synthesis of the equivalent circuit and fitting of the experimental data.

3. Results and discussions 3.1. Chemical composition and morphology of the deposits Fig. 1 shows the effect of current density on the cobalt content of the electrodeposits. It is observed that by increasing current density, the Co content of the coating increases and approaches the composition reference line (CRL) which is defined according to Eq. (1): CRL =

c(Co2+ ) [c(Co2+ + Zn2+ )]

(1)

415

where c(Co2+ ) and c(Zn2+ ) are the concentration of Co and Zn ions in the electrolyte, respectively [14,16]. The lower Co content of alloy coatings in comparison with the CRL is the result of the preferential deposition of the less noble metal (Zn) comparing with the more noble metal (Co) that results in anomalous deposition of Zn–Co alloy coatings. This phenomenon is explained by the concept of zinc hydroxide oscillation. According to this model, the thickness of the hydroxide layer changes alternately. When hydroxide layer is depleted, the reduction of H+ and Co2+ occurs preferentially to that of Zn2+ . As a result, pH of interfacial increases due to the hydrogen-evolution reaction and the hydroxide layer forms again favoring reduction of zinc, and co-deposition becomes anomalous [13,16,17]. By increasing current density, deposition of Zn2+ which is reduced more easily is accelerated. As a result, the solution that is next to the cathode is depleted from Zn2+ . This causes more Co2+ to have opportunity for reduction and its reduction rate rises, and hence the cobalt content of coating increases (Fig. 1). Fig. 2a–e show the surface morphology of pure zinc deposit obtained at 15 mA cm−2 as well as those of Zn–Co alloy deposits obtained at 10, 15, 20 and 30 mA cm−2 , respectively. According to Fig. 2a, the morphology of pure zinc is a combination of hexagonal crystals. This morphology is usually observed in zinc electrodeposits. With the increase in current density and consequently increase in the cobalt content in the alloyed deposits, it is seen that the morphology is changed from hexagonal to angular that is in agreement with studies by Lodhi et al. [15]. It is also observed that in Zn–Co alloy deposits, the grain size reduces with the increase of current density from 10 to 15 mA cm−2 (Fig. 2b and c). However, further increase in the current density from 15 to 30 mA cm−2 has a reverse effect and causes grain coarsening (Fig. 2d and e). It is postulated that with increase in current density, because of the rise in the overpotential, free energy for formation of new nuclei in Zn–Co alloy deposits increases that leads to higher nucleation rate and hence formation of coatings with smaller grain size [12]. However, further increase in current density from 15 mA cm−2 to higher values, will result in creation of discrete large crystallites. The formation of big and discrete polyhedral grains is attributed to the fast growth of grains and agglomeration on the nuclei in comparing with nucleation rate [12,17]. EDS analyses of different areas (A and B) in Fig. 2d and e, show composition variations in the coatings. According to this analysis, Co contents of areas A and B in Fig. 2d are approximately 1.74 and 2.25 wt.%, respectively. The Co contents for areas A and B in Fig. 2e are 3.06 and 3.88 wt.%, respectively. These results also show that areas with more Co content (B), comparing to areas with less Co (A), have a finer structure. Hence, it appears that cobalt acts as grain refiner in Zn–Co alloy deposits.

3.2. Phase composition of the deposits XRD patterns of pure zinc and Zn–Co alloy deposits obtained at different current densities are shown in Fig. 3. According to XRD pattern of pure zinc (pattern a), the highest intensity in pure zinc belongs to (1 0 1) plane. The presence of cobalt in the coating slightly decreases the intensities of the diffracted planes as compared with those of the pure zinc. With the increase in cobalt content in Zn–Co alloy deposits (due to the increase of current density), the intensity of peaks for planes (0 0 2), (1 0 0) and (1 0 1) decreases further. This decrease of intensity is also observed in other peaks related to hexagonal phase of zinc but in lower extent. The persistence of those peaks although with lower intensity suggests the appearance of a zinc rich phase (␩) which is a solid solution of Co in Zn in these coatings. Diffraction pattern of ␩-phase is very

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Fig. 2. Surface morphology of coatings (a) pure zinc coating at 15 mA cm−2 and Zn–Co alloy coatings at: (b) 10 mA cm−2 (c) 15 mA cm−2 (d) 20 mA cm−2 (e) 30 mA cm−2 .

similar to that of pure zinc [22]. This decrease of intensity in peaks is the result of variation in orientation of the zinc crystallographic planes by virtue of increase in Co content of coating that causes the distortion of hexagonal structure of zinc [17,22,24]. By increasing

current density above 20 mA cm−2 during electroplating and hence increasing the cobalt content in the deposits, a new phase called ␥-phase (CoZn13 ) is formed. The XRD patterns indicated as d and e in Fig. 3 show the presence of this new phase indicated with letter i.

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Table 1 Effect of current density on chemical composition, phase composition and grain size of Zn and Zn–Co alloy coatings. Coating

Current density (mA cm−2 )

Wt.% Co in deposit

Phase composition

Grain size (nm)

Zn Zn–Co Zn–Co Zn–Co Zn–Co

15 10 15 20 30

0.00 0.52 0.89 1.95 3.34

Pure zinc ␩ ␩ ␥+␩ ␥+␩

740 290 140 380 560

3.3. Corrosion behaviors of the deposits

Fig. 3. XRD patterns obtained from (a) pure zinc coating at 15 mA cm−2 and Zn–Co alloy coatings at: (b) 10 mA cm−2 (c) 15 mA cm−2 (d) 20 mA cm−2 (e) 30 mA cm−2 .

The grain sizes of pure zinc and Zn–Co alloy deposits were also calculated by measuring the peak’s width in XRD patterns and using Sherrier Eq. (2):

3.3.1. Potentiodynamic polarization tests Fig. 5a and b illustrates the polarization curves obtained from potentiodynamic polarization tests in 3.5 wt.% NaCl solution for Zn and Zn–Co alloy coatings. Typical active-passive-transpassive behavior can be clearly observed in Zn and Zn–Co alloy deposits. Two stages of passivation are observed in Zn–Co alloy coatings. This can be an indication of formation of protective film in two stages. While, there is only one stage of passivation observable in pure zinc coating. When Zn–Co alloy is corroded, zinc begins to dissolve preferentially according to Eqs. (3)–(6): Zn(s) → Zn2+ (aq) + 2e−

(3)

O2 (g) + 2H2 O(l) + 4e− → 4OH− (aq)

(4)

2+

Zn 0.9 ˇ= Dcos 

(2)

−

(aq) + 2OH (aq) → Zn(OH)2 (s)

Zn(OH)2 (s) → ZnO(s) + H2 O

(5) (6)

where ˇ is the full width of half maximum height,  is the wave length of X-ray, D is the grain size and  is the reflection angle of peak [25]. Fig. 4 shows the effect of current density on grain size values of Zn–Co alloy coatings. It is noticed that with increase in current density from 10 to 15 mA cm−2 , the grain size decreases from 290 to 140 nm. However, as the current density further increases from 15 to 30 mA cm−2 , the grain size increases from 140 to 560 nm that confirms the SEM results. Table 1 summarizes effect of current density on chemical composition, phase composition and grain size of the coatings on which potentiodynamic polarization and electrochemical impedance spectroscopy tests were carried out.

Fig. 4. Effect of current density on grain size values of Zn–Co alloy coatings.

Fig. 5. Polarization curves obtained for various coatings in 3.5% NaCl (a) Zn and Zn–Co alloys with single phase structure, (b) Zn–Co alloys with dual phase nature.

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Table 2 Corrosion current density (icorr ), corrosion potential (Ecorr ), passive current density (ipass ), passivation potential (Ep ), breakdown potential (Eb ), and passive range (Eb –Ep ) measured in 3.5 wt.% NaCl solutions. Coating

icorr (␮A cm−2 )

Ecorr (mV)

ipass (␮A cm−2 )

Ep (mV)

Eb (mV)

Eb –Ep (mV)

Pure zinc Zn–0.52 wt.% Co Zn–0.89 wt.% Co Zn–1.95 wt.% Co Zn–3.34 wt.% Co

15.95 12.59 0.84 15.85 17.84

−1120 −1124 −1148 −1135 −1130

676 2168 1298 3800 3880

−712 −784 −799 −731 −740

−586 −474 −336 −603 −619

126 310 463 128 121

Therefore, the protective film is a complex composition including Zn(OH)2 and ZnO that covers the surface of corroded coatings and creates an area that current density is independent of potential [11,26]. The presence of cobalt in the coating enhances dissolution of zinc in NaCl solution. Consequently, the increase in zinc dissolution causes its reaction with chloride ions present in NaCl solution and formation of zinc hydroxy chloride (ZHC) according to Eq. (7): 5Zn2+ + 8OH− + 2Cl− + H2 O → Zn5 (OH)8 Cl2 ·H2 O 10−14.2 ,

(7)

ZHC has a very low product of solubility, and ensures higher protective ability for this alloy compared to the pure zinc [9–11]. Normally, passive films are semiconductor. These insoluble passive films cover the corroded surface and as a result, electrochemical corrosive reactions become more difficult [27]. The results obtained from polarization tests for Zn and Zn–Co alloy coatings are summarized in Table 2. The corrosion potential (Ecorr ) and corrosion current density (icorr ) calculated from the intersection of the cathodic and anodic Tafel slopes in the range of ± 50 mV about the open-circuit potential (Eocp ). Passivate potential (Ep ), breakdown potential (Eb ) and passive current density (Ip ) are shown in Fig. 5a and b. The potential at which the current density becomes virtually independent of potential and remains virtually stationary is called the passivate potential (Ep ). The min-

imum current density required to maintain the metal in a passive state is called passive current density (Ip ). The potential at which the breakdown or rupture of protective film takes place and the current density rises sharply is called breakdown potential (Eb ) and in the passive range (Eb –Ep ), the current density approximately remains constant and independent of the potential. Generally, corrosion current density and corrosion potential are used to determine the active dissolution ability of materials, while passivation potential are used to determine the passivation ability of materials and passive current density and passive range are used to evaluate the chemical stability and corrosion resistance of passive films. According to the results summarized in Table 2, it is noticed that Zn–0.89 wt.% Co alloy coating has the lowest corrosion current density among all coatings and creates the protective passive film with a wider passive range. Moreover, it is observed that passive current density of Zn–0.89 wt.% Co alloy coating is lower than those of other alloy coatings, indicating that this coating passivates much easier than other alloy coatings. Therefore, Zn–0.89 wt.% Co has the highest corrosion resistance among all coatings. It is also noticed that the protective passive film that is formed on the surface of Zn–0.52 wt.% Co possesses widespread passive range that shows corrosion resistance of this coating is higher than those of Zn–Co alloy coatings with 1.95 and 3.34 wt.% Co.

Fig. 6. Surface micrographs of (a) pure zinc and Zn–Co alloy coatings with: (b) 0.52 (c) 0.89 (d) 1.95 (e) 3.34 wt.% Co, after potentiodynamic polarization test followed by 10-h immersion in 3.5 wt.% NaCl solution.

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Fig. 7. Cross-section micrographs of Zn–Co alloy coatings with: (a) 0.52 (b) 0.89 (c) 1.95 wt.% Co, after potentiodynamic polarization test followed by 10-h immersion in 3.5 wt.% NaCl solution.

Zn–3.34 wt.% Co alloy coating, possessed the lowest corrosion resistance compared to other alloy coatings and showed the lowest passive range and the highest corrosion current density among the alloy coatings. The lower corrosion resistance of Zn–Co alloy coatings with 1.95 and 3.34 wt.% Co are due to their dual phase nature (␩ + ␥) as confirmed by their XRD patterns (Fig. 3), that promotes galvanic corrosion. The dual phase nature of the se two alloy coatings even reduced their corrosion resistance to a value lower than that of pure zinc coating. In alloy coatings with single phase structure (␩-phase) i.e. the coatings with 0.52 and 0.89 wt.% Co, the coating with smaller grain size (i.e. the one with 0.89 wt.% Co) exhibited higher corrosion resistance than the coating with coarser grain size. This is due to the higher grain boundary density in the coating with smaller grain size that speeds up the formation of a stable and protective passive film. Similar behaviors have been reported by Youssef and Wang [26,27] who worked on pure zinc and pure cobalt. It was noticed that there was no significant difference in corrosion potential (Ecorr ) of the alloy coatings. The difference in initial open-circuit potential (Eocp ) and corrosion potential (Ecorr ) is due to the difference in the chemical composition, phase structure and

grain size of the coatings [26,28,29]. The grain refinement should increase the surface area and consequently causes an increased dissolution of Zn when the anodic polarization started. This may be the reason for the more negative shift of corrosion potential (Ecorr ) of Zn–0.89 wt.% Co coating. It is evident that, although the corrosion potential of Zn–0.89 wt.% Co is more negative than that of other coatings, the Zn–0.89 wt.% Co sample exhibits a more protective film (improved passivation behavior) than that of other coatings. Apparently, the fine structure enhances both the kinetics of passivation and the stability of the passive film formed [26,28]. This finding indicates that Zn–0.89 wt.% Co exhibits a higher activation energy for dissolution than those of other coatings. The high activation energy results in faster transition from the active dissolution to the passivation during polarization which facilitates the formation of a stable and protective passive film. Therefore Zn–0.89 wt.% Co provided the best sacrificial type of protection. The surface micrographs of the coatings after potentiodynamic polarization test followed by 10-h immersion in 3.5 wt.% NaCl solution shown in Fig. 6a–e confirming the aforementioned polarization results. These figures also confirm pitting corrosion in all coatings

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Fig. 8. Nyquist impedance curves obtained for (a) pure zinc and Zn–Co alloy coatings with: (b) 0.52, (c) 0.89, (d) 1.95, (e) 3.34 wt.% Co, in 3.5 wt.% NaCl solution and (f) equivalent circuit model for these coatings.

except for the Zn–0.89 wt.% Co coating (Fig. 6c). This is related to formation of a more protective passive film on Zn–0.89 wt.% Co coating compared to other coatings. The pitting corrosion could be related to the absorption of halide anions on the passive films formed on the coatings. The anions that are absorbed on the passive film create an electrostatic field across the interface of passive film and electrolyte. When the electrostatic field reaches a critical value, the absorbed anions start to penetrate in the passive film especially at the point defects of the film. This process leads to breakdown of the passive and creates local dissolution on surface of passive film [30,31]. The cross-sectioned SEM images of Zn–Co alloy deposits after potentiodynamic polarization test followed by 10-h immersion in 3.5 wt.% NaCl solution are shown in Fig. 7a–c. It is observed that surface of Zn–0.89 wt.% Co alloy (Fig. 7b) is covered with corrosion products (mainly ZHC) that act as barriers against further corrosion. It is also noticed that in Zn–Co alloy coatings with 0.52 and 1.95 wt.% Co, the corrosion damages in cross-section are in the form of uneven pits and surface of these coatings is covered with discrete corrosion products. Cross-sectioned micrograph of the Zn–1.95 wt.% Co alloy coating, shows cracks from the surface down to the substrate. Presence of such cracks is associated with the dissolution of zinc-rich phases.

3.3.2. Electrochemical impedance spectroscopy measurements The Nyquist impedance curves obtained for pure zinc and Zn–Co alloy coatings in 3.5 wt.% NaCl solution are shown in Fig. 8a–d. These curves show a single semicircle at high frequencies. The diameter of semicircle corresponds to a charge-transfer resistance (Rct ). In addition, a nearly ideal Warburg tail at an angle of 45◦ is obtained at low frequency range corresponding to diffusion-controlled charge transfer [11,26,27]. The distance between origin and starting point of semicircle represents solution resistance (Rs ) among reference electrode and working electrode. The double layer capacitance (Cdl ) is calculated according to Eq. (8): Z(ω) =

1 (iωCdl )

n

(8)

where Cdl is independent of ω, and 0.5 < n < 1, i = (−1)1/2 , ω = 2␲f (if n = 1, Eq. (8) becomes the impedance of a pure capacitor) [32–34]. In order to study the corrosion behavior of the coatings, and to simulate the metal/solution interface, an equivalent circuit model was obtained by using ZView2 software. The equivalent circuit model consisting of solution resistance (Rs ), double layer capacitance (Cdl ), charge-transfer resistance (Rct ) and Warbrug impedance (Zw ) has been shown in Fig. 8f, in which the Cdl and Rct are parallel to each other.

M.H. Gharahcheshmeh, M.H. Sohi / Materials Chemistry and Physics 117 (2009) 414–421 Table 3 Equivalent circuit parameters that were determined by modeling impedance spectra of Zn and Zn–Co alloy coatings in 3.5 wt.% NaCl solution. Coating

Rs ( cm2 )

Rct ( cm2 )

Cdl (␮F cm−2 )

Pure zinc Zn-0.52 wt.% Co Zn–0.89 wt.% Co Zn–1.95 wt.% Co Zn–3.34 wt.% Co

14 14.5 14 15 14.5

467 1185 11322 96 53

85 34 27 78 89

The results are obtained from the impedance curves are summarized in Table 3. According to these results the solution resistance of all coatings shows no significant difference. As for the doublelayer capacitance of the coatings, the alloy coating with 0.89 wt.% Co shows the lowest value. However, the charge-transfer resistance of Zn–0.89 wt.% Co alloy coating is dramatically higher than those of other coatings, that demonstrates its significantly higher corrosion resistance compared to other coatings. On the whole, it can be concluded that EIS results strongly support potentiodynamic polarization tests. 4. Conclusions (1) Increase in current density resulted in increase of cobalt in Zn–Co alloy electrodeposits. The lower Co content in Zn–Co deposits in comparison with the CRL is the result of the preferential deposition of the less noble metal (Zn) compared with the more noble metal (Co) that is an indication of anomalous deposition of Zn–Co alloy coating. (2) In Zn–Co alloy deposits, the grain size reduces with the increase in current density from 10 to 15 mA cm−2 . However, further increase in the current density from 15 to 30 mA cm−2 has a reverse effect and causes grain coarsening. (3) Zn–Co alloy coatings with 1.95 and 3.34 wt.% Co are consisted of two phases (␩ and ␥), but Zn–Co alloy coatings with lower cobalt content have single phase structure (␩-phase) which is a solid solution of cobalt in zinc. (4) Among alloy coatings with single phase structure (␩-phase), the coating with smaller grain size (i.e. the one with 0.89 wt.% Co) exhibited higher corrosion resistance than the coating with coarser grain size (i.e. the one with lower cobalt value of 0.52 wt.%). This is due to the higher grain boundary density in the coating with smaller grain size that speed up the formation of a stable and protective passive film. (5) The lower corrosion resistance of Zn–Co alloy coatings with 1.95 and 3.34 wt.% Co are due to their dual phase nature (␩ + ␥), that promotes galvanic corrosion. The dual phase nature of these two alloy coatings even reduced their corrosion resistance to a value lower than that of pure zinc coating.

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