J Appl Electrochem (2015) 45:67–78 DOI 10.1007/s10800-014-0777-9

RESEARCH ARTICLE

Electrodeposition of zinc in the presence of quaternary ammonium compounds from alkaline chloride bath Jose´ Luis Ortiz-Aparicio • Yunny Meas Thomas W. Chapman • Gabriel Trejo • Rau´l Ortega • Eric Chainet

•

Received: 10 April 2014 / Accepted: 20 October 2014 / Published online: 6 November 2014 Ó Springer Science+Business Media Dordrecht 2014

Abstract The effects of several quaternary ammonium compounds on electrodeposition of zinc onto AISI 1018 carbon steel were studied in an alkaline zincate electrolyte. Tetraethylammonium, tetrabutylammonium, N-benzyltriethylammonium and N-benzyl-3-carboxyl pyridinium cations were examined. The electrochemical behavior and the inhibition of dendrite formation are related to the structure of the ammonium compounds. The presence of either longchain aliphatic groups or aromatic groups, i.e., with tetrabutylammonium hydroxide (TBAOH) or N-benzyltriethylammonium chloride (NBT) exerts a more effective inhibition of dendrite formation. N-benzyl-3-carboxylpyridinium (3NCP) and tetraethylammonium hydroxide (TEAOH) additives lead to slightly deformed deposit morphology. Crystallographic measurements of the zinc deposits revealed a highly oriented deposit formed in the presence of 3NCP, which favors the dense atomic packing basal plane (002). The presence of TEAOH diminishes slightly the peak of plane (002) and introduces some

J. L. Ortiz-Aparicio  Y. Meas (&)  T. W. Chapman  G. Trejo  R. Ortega Centro de Investigacio´n y Desarrollo Tecnolo´gico en Electroquı´mica, Parque Tecnolo´gico Quere´taro, Sanfandila, Co´digo Postal 76703 Pedro Escobedo, Quere´taro, Mexico e-mail: [email protected] Present Address: J. L. Ortiz-Aparicio Centro Nacional de Metrologı´a, Carretera a los Cue´s, Km 4.5, C.P. 75246 El Marque´s, Quere´taro, Mexico E. Chainet Laboratoire d’Electrochimie et Physico-chimie des Mate´riaux et Interfaces, UMR 5631 CNRS-Grenoble, Insitut National Polytechnique-Universite´ Joseph Fourier (INP-UJF, PHELMA) BP75, 38402 Saint Martin d’He`res, France

pyramidal (101) orientation. Addition of NBT or TBAOH favors the formation of low-atomic packing prismatic planes. Additives that increase the overpotential for Zn(II) reduction tend to promote the formation of high-energy low-atomic packing crystallographic planes. Comparison of the effects of these ammonium compounds indicates that the observed effects are related to the hydrophobic and steric interactions introduced to the interface by the size and structure of the ammonium compounds. Keywords Zinc  Electrodeposition  Additives  Quaternary ammonium compounds  Adsorption

1 Introduction Zn and Zn-iron-group alloy coatings are used because of their ability to delay corrosion of steel substrates [1, 2]. In plating formulations, the presence of organic molecules and inorganic species added in small amounts, called additives, exerts important effects on metal electrodeposition even at low concentrations. The additives modify the crystal growth habit and therefore the mechanical properties, the corrosion resistance, and the appearance of coatings [3]. Additives used in many metal plating formulations are often classified as brighteners or levelers [1, 3] according to their capability to produce smooth, bright, leveled and fine-grained deposits. The effects of additives have been studied extensively because of their important influence on metal electrodeposition, but their mechanisms of action are still not clearly elucidated, still being matter of study [3, 4]. Some additives may inhibit or catalyze the electrodeposition process by forming complexes with metallic ions or increasing the polarization of metal electrodeposition due to their adsorption on active sites [4].

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For zinc plating formulations, additives classically involve amines, carbonyl compounds, polyols, polymeric amines, and quaternary ammonium compounds (QACs) [1, 5]. Various examples of additive effects have been reported; compounds such as polyvinyl alcohol, anisaldehyde, piperonal, and veratraldehyde [6] produce fine-grained deposits and have a strong impact on the electroplating parameters of zinc. The addition of vanillin or p-anisaldehyde exerts important changes on the electrochemical behavior, morphology, composition, and crystal structure of ZnCo electrodeposits [7]. Electrochemical impedance spectroscopy has revealed the appearance of intermediate species adsorbed on the electrode during Zn(II) ions discharge [8–11]. In these studies, additives that prevent dendritic growth also change the impedance plots (i.e., the time constants that are related to the reduction mechanism, which seem to be related to the final morphology) [9, 11]. Quaternary ammonium compounds are used as brighteners in modern alkaline zinc electrolytes free of cyanide [12]. A first attempt to associate the chemical structure of QACs with the properties of deposits was reported by James and McWhinnie [13], who studied the effects of N-benzyl-3carboxylpyridinium (3NCP), N-propyl-3-carboxylpyridinium, and N-benzyl-4-carboxylpyridinium. They concluded that 3NCP was the best brightener among these additives. In previous studies [14–16], we showed that the presence of different QACs affected the electrochemical behavior of Zn [14, 15] and ZnCo [16, 17] deposition as well as their morphological and crystallographic structures and that observed changes are related to the chemical structure of the additive molecules. Anisaldehyde and 3NCP modify the reduction kinetics of Zn [18] and promote the hydrogen evolution reaction (HER) [19]. The effects of aliphatic and heterocyclic polyamines on Zn electrodeposition in alkaline electrolytes were studied recently [20]. Such additives act as brighteners and levelers, producing fine-grained coatings, with the impact being strongly dependent on the electron density and structure of the additives [20]. Other QACs have also been studied. Diggle and Damjanovic reported the inhibition of Zn dendrite growth by quaternary alkyl ammonium bromides [21]. Other authors [22, 23] found that tetraalkylammonium hydroxides also improve the inhibition of Zn dendrite formation. The addition of Nbenzyltriethylammonium ions modifies the morphology of deposits during alkaline zinc electrowinning [23, 24]. Bockris et al. [25] and Hendrikx et al. [26] proposed that the sequence of zinc electroreduction on zinc electrodes in highly alkaline baths takes place through steps (1) to (4):   ZnðOHÞ2 4 ! ZnðOHÞ3 þ OH

ZnðOHÞ 3

123



þe !

ZnðOHÞ 2

þ OH

ð1Þ 

ð2Þ

 ZnðOHÞ 2 ! ZnðOHÞ þ OH

ð3Þ

ZnðOHÞ þ e ! Zn þ OH

ð4Þ

At high current densities, Reaction (2) is faster than the incorporation of the Zn adatoms into the growing lattice of zinc, forming dendritic, powdery non-adherent deposits. According to Darken [12], bright deposits would be obtained if the rate of Reaction (2) was reduced. Gerischer 2reported that although both Zn(OH)24 and Zn(CN)4 are able to form in zinc cyanide electrolytes, the electrodeposition takes place via the formation of Zn(OH)2 [27]. Recent studies performed by Rezaite and Vishomirskis [28] indicated the possible formation of partially adsorbed interfacial zinc-hydroxo-cyanide intermediates. The cyanide ions act not only as a complexing agent but also as a grain refiner [2, 12]. A similar mechanism was proposed for polyvinyl alcohol (PVA) [12]. This additive is considered to form an adsorbed barrier that controls the rate of Reaction (2), probably by forming a ZnðOHÞ 2 ðPVAÞ complex. According to this assumption, Reaction (2) is retarded even more because decomplexation must take place and, as a consequence, intermediate Zn(I) is stabilized somewhat. Darken suggested that QACs act in a similar way as PVA during zinc electrodeposition [12]. Wiart et al. [29–31] proposed a more complex mechanism of Zn(II) reduction from alkaline baths, which involves adsorbed Zn(I) intermediates with the formation of a thin zinc oxide layer. The addition of surfactants modifies the kinetics parameters of the interfacial reactions and the geometry of the oxide layer as well as the adsorbed Zn(I) surface concentration. Such effects are reflected in the final morphology of Zn deposits and in the shape of impedance spectroscopy plots [30]. To understand the role of the additives on Zn electrodeposition, systematic studies are required to correlate chemical structure with the electrochemical behavior and the final characteristics of deposits. It is known that molecular structure affects the adsorption mechanisms of organic molecules [32]. The aim of the present work is to explore the influence of several QACs on electrodeposition of Zn onto a carbon steel substrate from an alkaline zincate electrolyte. The effects of 3NCP chloride, N-benzyl-triethylammonium chloride (NBT), tetraethylammonium hydroxide (TEAOH), and tetrabutylammonium hydroxide (TBAOH) were thus considered. These molecules have different sizes (TEAOH vs. TBAOH), the addition of the benzyl group (NBT) increases electron density, whereas 3NCP increases both molecular size and electron density. The structures of these cations are sketched in the inset of Fig. 2.

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3 Results and discussion 3.1 Voltammetric study Line a in Fig. 1 characterizes the electrochemical behavior of the steel cathode in 4 M NaOH. A reduction wave (signal A1) appears at EpA = -1.2 V versus SCE. Some works devoted to the electrochemical study of iron electrodes immersed in alkaline solutions [33–35] concluded that this cathodic peak corresponds to the reduction of iron oxides and/or iron hydroxides that form on the surface. Oxidation peaks such as the anodic peak A2 thus correspond to the formation of these species. A large second rise in cathodic current (A3) begins at -1.40 V versus SCE and can be attributed to the HER, which is a concomitant reaction in metal electrodeposition [36] and specifically during zinc alkaline electrodeposition [19, 37]. Subsequent voltammograms showed that the HER depends on the nature of both the substrate and the growing surface as well as the solution composition [36].

5

Ia

-2

40

A2

Ia

0

j/mAcm

60

j/mAcm

Electrodeposits were elaborated from 0.25 M ZnCl2 (Merck) solutions at room temperature. All the solutions were prepared with deionized water (18 MX cm). Zn coatings were deposited on an AISI 1018 steel substrate. The basic Zn electrolyte solution was S0: 0.25 M ZnCl2 ? 4.0 M NaOH (Merck). The organic compounds NBT, TEAOH, TBAOH, and nicotinic acid (NA) additions were of analytical grade. 3NCP chloride was synthesized from the condensation reaction of equimolar amounts of benzyl chloride and NA sodium salt [16, 17]. The electrochemical experiments were performed with an Autolab potentiostat/galvanostat (PGSTAT 30). Cyclic and linear voltammetry were carried out in a typical threeelectrode cell with a carbon steel rod embedded in a Teflon [poly(tetrafluoroethylene)] sleeve as the working electrode offering a circular geometrical area of 0.032 cm2. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a graphite bar. The trials were carried out under an ultrapure nitrogen atmosphere. Before each run, the steel electrode was polished with 0.05 lm alumina (Buehler) until reaching a mirror surface appearance. After polishing the steel disks were etched in 2 % HCl for 5 s and then rinsed with copious deionized water. The morphology of Zn deposits was examined by scanning electronic microscopy (JEOL DSM5400 LV). X-ray diffraction analyses of the deposits were performed in a Brucker X-ray diffractometer (D8 Advance) using CuKa radiation.

80

-2

2 Experimental

A5 A4

-5 -10

-1. 6

-1.2

-0.8

-0.4

E vs SCE/V

20

A2

0

A1

-20 -40 -60 -1.8

(a) (b)

A1

(a) (b)

Ic A3 -1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

E vs SCE/V

Fig. 1 Cyclic voltammetry obtained for (a) 4 M NaOH, (b) Solution S0. v = 20 mV s-1. S0:0.25 M ZnCl2 ? 4 M NaOH. WE:AISI 1018 carbon steel. Inset amplification of cyclic voltammograms

Line b in Fig. 1 shows the cyclic voltammogram for solution S0 in the absence of additives onto the steel electrode. Electrochemical signals (A1) and (A2) are strongly inhibited and two consecutive small peaks (A4 and A5) appear. These observations indicate that the formation of iron oxides/hydroxides and the HER are highly suppressed in the presence of zinc ions. According to some authors [38–41], the underpotential deposition (upd) of Zn inhibits the kinetics of the HER. The exchange current density of the HER is very sensitive to the substrate nature; HER occurs slowly on Zn, whereas larger exchange current densities are obtained on Co, Ni, and Fe [39]. A previous study reports HER inhibition in the presence of zinc ions in an alkaline bath [39] due to the formation of just a submonolayer of Zn on a ferrous substrate; a surface coverage rate around 0.15 is able to inhibit a significant current from the HER [38]. In the absence of Zn(II) ions, iron oxide/ hydroxide reduction and HER seem to take place simultaneously, as shown by line a in Fig. 1. The addition of Zn(II) ions to the electrolyte slows down both processes, and it seems that Zn upd inhibits HER as well as the formation of iron oxides/hydroxides on the electrode surface. The voltammograms thus show how complex are the processes that take place within this range of potential. At more negative potentials, a single reduction peak Ic is observed at Ep(Ic) = -1.590 V versus SCE due to the bulk deposition of zinc. The nature of the substrate also affects the electrodeposition of metals, as it was evidenced in zinc alkaline electrolyte [42]. When the potential scan is inverted, a single oxidation peak Ia appears, corresponding to the electrooxidation of the coating formed during the cathodic scan Spectroscopic [43] and thermodynamic [44]

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analyses indicate that the dominant Zn(II) species in alkaline electrolytes are ZnðOHÞ2 4 hydroxo-complexes. 3.2 Effects of QACs Figure 2 shows the voltammograms of solution S0 when organic additives are present. In the presence of 3NCP (Fig. 2, line b), the reduction peak Ic decreases slightly, and a shoulder is located at potentials prior to the massive Zn reduction peak. When the potential scan is inverted to the positive direction, oxidation peak Ia appears. The reduction behavior is clearly modified when NBT is added to the bath (Fig. 2, line c), with the appearance of two reduction peaks (Ic and IIc). The presence of TEAOH (Fig. 2, line d) in the bath shifts only slightly the reduction peak potential value (EpIc). On the other hand, a dramatic change is observed in the presence of TBAOH (Fig. 2, line e), with the appearance of three peaks, indicating that this molecule presents a greater effect on zinc reduction. Winand [45] indicated that inhibition of metal reduction takes place due to the presence of foreign substances on the surface. The electrode kinetics parameters are modified when the inhibitor is adsorbed into the double layer [45]. Diggle and Damjanovic suggested that two processes may occur: the inhibitor could physically block the electrode, or

it could modify the electrode kinetic parameters by changing the diffuse layer by the specific adsorption of cationic additives and modifying the position of the reaction plane [21]. Bressan and Wiart [9] observed a displacement of the Zn(II) reduction potential in the presence of TBAOH. This cathodic shift, called cathodic polarization, was observed for other organic compounds [3] and was attributed to blocking of the electrode by additive adsorption. The addition of NBT to an acidic zinc electrolyte also shifted the polarization curves for zinc electrodeposition to more negative potentials [46]. The behavior of Zn reduction in the presence of organic additives is associated with the adsorption of the molecules on the electrode surface. The adsorbed additives form a barrier that partially inhibits the discharge of Zn(II) ions and blocks active sites. The resulting overpotential increase promotes desorption of the additive from the surface and thus allows the reduction of Zn(II) to proceed the active sites vacated by the organic molecules. Previous studies have shown that the presence of QACs inhibits metal electrodeposition partially [21–24]. Specific adsorption of tetraalkylammonium cations [47–49] as well as that of Nalkylpyridinium [50, 51] on the electrode surface was proposed. Then, the gradual shift of the massive metal electrodeposition to more negative values in the presence

100

Ia 80

-

Cl-

(b) 60

+

O Na

+

+

N

N

CH2

CH2

Cl-

N

+

OH-

N

+

OH-

(c) (d)

40

NBT

3NCP

TEAOH

TBAOH

(e)

-2

j/mAcm

O

(a)

20

0

-20

-40

IIc

Ic

IIIc -60 -1.8

-1.6

-1.4

-1.2

-1.0

-0.8

E vs SCE/V Fig. 2 Cyclic voltammetry obtained for solution S0 on AISI 1018 carbon steel electrode: (a) Solution S0 (b) S0 ? 3NCP; (c) S0 ? NBT; (d) S0 ? TEAOH; (e) S0 ? TBAOH. v = 20 mV s-1. Concentration of the additives: 5 mM

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71 60

Ia

Ia'

50

60

30

20

20

j/mAcm-2

j/mAcm-2

40 40

0 -20

-60 -1.8

-20

IIc

-30 -1.8

Eλ3 Eλ2 Eλ1 -1.6

0 -10

Ic

-40

10

-1.4

-1.2

-1.0

E vs SCE/V

Fig. 3 Cyclic voltammetry obtained for solution S0 on AISI 1018 steel electrode in the presence of 5 mM NBT at different inverting potentials scan, Ek:Ek1 = -1.60 V versus SCE; Ek2 = -1.65 V; Ek3 = -1.70 V, v = 20 mV s-1

of the QAC’s, seen in Fig. 2, would be related to the adsorption of these additives (TEAOH, NBT, and TBAOH) on the electrode surface. Figures 3 and 4 show the cyclic voltammograms for Zn reduction in the presence of NBT and TBAOH, respectively, recorded with different potential-switching values (Ek). In Fig. 3, two reduction peaks (Ic and IIc) are observed, and both are associated with zinc electrodeposition because a single oxidation peak (Ia) appears when different switching potentials are applied. Peak Ic can be attributed to the reduction of Zn(II) ions on free active surface. The appearance of peak IIc in the voltammograms of Fig. 3 may be associated with the reduction of Zn(II) ions on active sites blocked previously by the additive. The adsorption properties of tetraalkylammonium cations on electrodes depend on the number of carbon atoms in the alkyl chain [48]: tetramethylammonium (TMA?) cations are not significantly adsorbed. Tetraethylammonium (TEA?) and tetrapropylammonium (TPA?) ions are adsorbed slightly, whereas strong adsorption is observed for tetrabutylammonium (TBA?) cations. Effectiveness in the inhibition of Zn dendrites is similarly related to the structure of the ammonium compounds, TBA? ions being the most effective because of its longer alkyl chains [21– 23]. The presence of aromatic groups with conjugated pbonds tends to promote adsorption on metallic surfaces [32]. The presence of the benzyl group in NBT increases the inhibition of Zn(II) discharge compared with TEAOH. The lesser inhibition observed with 3NCP indicates that the charged pyridinium group (having conjugated p-orbitals)

IIc'

Ic'

Eλ3 Eλ2 Eλ1 -1.6

-1.4

-1.2

-1.0

-0.8

E vs SCE/V

Fig. 4 Cyclic voltammetry obtained for solution S0 on AISI 1018 steel electrode in the presence of 5 mM TBAOH at different inverting potentials scan, Ek:Ek1 = -1.555 V, Ek2 = -1.658 V, Ek3 = -1.750 V versus SCE, v = 20 mV s-1

and the carboxylate groups adsorb to a different extent because of electrostatic repulsion of the negative charge on the carboxylate group at negative potentials. Previous studies [49, 50] showed that the electroreduction of N-alkylpyridinium derivatives is also possible as well as the formation of 3NCP dimers [13]. A neutral product resulted from the electroreduction of a N-butyl-pyridinium derivative, which formed a porous film on a glassy carbon electrode [52]. It was suggested that quaternary pyridinium salts are adsorbed not only electrostatically but also chemisorbed via the unsaturated ring in an oriented manner [53]. The voltammogram recorded in the presence of TBAOH (Fig. 4) reveals a more complex behavior: two small reduction peaks (Ic0 and IIc0 ) are observed prior to massive zinc deposition (peak IIIc in Fig. 2). An oxidation peak Ia0 appears when the potential scan is inverted before the appearance of peak IIIc, i.e., massive reduction of zinc. Therefore, both small cathodic peaks correspond to consecutive zinc reduction processes that occur at different potentials. The appearance of these peaks reflects the fact that the interfacial structure is a dynamic system that changes with applied potential and time. A current crossover is observed, which could indicate Zn nucleation [54]. Avaca et al. [50] noted that the ionic adsorption will be modified when the electrode charge becomes more negative; anions near the surface are repelled, the layer of external quaternary ammonium cations are also by attraction to the anions leaving an adsorbed monolayer of only cations. Such dynamic potential-dependent structure, as well as the presence of an additive such as TBAOH in the diffusion layer, could permit the transport of Zn(II) for a secondary deposition process (e.g., peak IIc in Fig. 2, line

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Table 1 Effect of the additives on the total cathodic charge (Qc), total anodic charge (Qa), and the ratio (Qa/Qc) transferred using solution S0 in the absence and in the presence of additives Solution

Qa/mC cm-2

Qc/mC cm-2

Qa/Qc

Q0c /C cm-2

Epc/V

gd/V

S0

297

327

0.91

-0.166

-1.591

0.091

S0 ? 5 mM 3NCP

282

322

0.87

-0.144

-1.603

0.103

S0 ? 5 mM NBT

319

394

0.81

-0.109

-1.652

0.153

S0 ? 5 mM TEAOH

297

344

0.86

-0.154

-1.631

0.132

S0 ? 5 mM TBAOH

307

552

0.56

-0.048

-1.782

0.283

The cathodic charge (Q0c ) obtained by integration of data presented in Fig. 2 from the rest potential to E = -1.67 V. The massive Zn(II) reduction peak potential (Epc), the deposition overpotential (gd) of Zn(II) solution S0. Where gd = |Epc–E0 |

e). It was proposed that benzyl-dimethyl-phenylammonium ion adsorbs via the mediation of chloride ions [55], forming an intermediate bridge at potentials positive with respect to the potential of zero charge. A similar phenomenon was proposed for the adsorption of tetraalkylammonium halides on mercury electrodes [48]. Signal Ic0 in Fig. 4 could be explained as the electrodeposition of zinc on free active sites. The second peak of zinc deposition IIc0 observed would be attributed to the partial desorption of the cationic additives due to the repulsion of anions close to the electrode at negative potential values. In the potential range for zinc reduction, the HER plays an important role. At more negative potentials, bulk zinc deposition takes place with the concomitant HER (peak IIIc in Fig. 2), and complete desorption of TBAOH occurs. Gas bubbles were observed to form during Zn plating in the presence of the additives. To estimate the current efficiency for zinc deposition, the anodic and cathodic charges passed (Qa and Qc) were evaluated by integration of the currents and compared. Table 1 summarizes the effects the different additives on the observed results. According to these results, all of the additives decreased the current efficiency for zinc reduction. The lowest efficiency was obtained with TBAOH, which shifted the bulk electrodeposition of Zn to the most negative values and presumably promotes the HER [36]. The massive zinc peak current also diminishes when incorporating additives. Additives also increase the overpotential of Zn(II) reduction (gd). The cathodic charge (Q0c ), estimated by integrating the cathodic current from the rest potential E0 to -1.670 V versus SCE (Q0c ), diminished in the presence of additives, presumably due to the active surface blocked by the QACs. The peak potential values for massive zinc reduction (EpIc) and the corresponding overpotential values for Zn deposition (gd) clearly show that the additive 3NCP presents the least inhibitory influence on the Zn reduction, followed by TEAOH, NBT, and TBAOH. The aliphatic chains present in the structure of TEAOH and TBAOH and the aromatic groups in NBT and 3NCP exert different inhibitory effects on Zn(II) reduction. It is

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important to note the significant shift in the overpotential for Zn reduction (gd) observed when the length of the aliphatic chain changes from ethyl (TEAOH) to butyl (TBAOH) and when the aromatic group is present (NBT). The hydrophobic character increases with the size of the alkyl chain present in the QAC, and therefore, it tends to be strongly adsorbed on the electrode surface. Hydrophobic interactions of the tetraalkylammonium additives on the interface and between the organic molecules seem to act in an important way on the inhibition of the metal discharge. 3.3 Morphology of the deposits The electrodeposition conditions play an important role in establishing deposit morphology [1, 2, 56–60]. Addition of foreign substances such as metal ions, anions, or organic additives to the plating bath modifies the crystal growth and therefore the final morphology and mechanical properties of the coatings [1, 2]. This phenomenon is often associated with surface adsorption by some species [3] that block the propagation of lattice steps formed during the crystal growth [61], thus producing leveled and finegrained deposits. Kardos and Foulke [62] proposed three mechanisms: diffusion controlled leveling, grain refining, and randomization of crystal growth. Hoar [63] suggested that selective or random nucleation mechanisms take place during electrodeposition. The former is favored on sites such as kinks, steps, and dislocations that are energetically available. On the other hand, random deposition occurs over the entire surface, and it is often promoted by additives that are adsorbed on the active sites and favor random nucleation. According to Winand [2, 45, 64], the shape of crystals results from a competition between parallel and perpendicular growth with respect to the substrate, i.e., between two- (2D) and three-dimensional (3D) crystal growth, the former requiring lower overpotentials (and/or current densities) than the latter. Adcock et al. [65] pointed out the importance of nucleation overpotential on the formation of fine-grained deposits. Leveling additives increase the

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Fig. 5 SEM images for zinc electrodeposits obtained onto AISI 1018 steel electrode from the following solutions: a Solution S0; b S0 ? 5 mM 3NCP; c S0 ? 5 mM NBT; d S0 ? 5 mM TEAOH;

e S0 ? 5 mM TBAOH; f S0 ? 5 mM TBAOH; g S0. Applied potential: Eappl = -1.65 V) for Figures (a–e), Eappl = -1.78 V for figures (f–g). t = 300 s

charge transfer overpotential and therefore affect the Tafel parameters [65]. Brighteners or grain refiners reduce the nucleation overpotential and promote secondary nucleation events, often referred to as frequent 3D nucleation on top of existing grains [65]. Fine-grained deposits are obtained when secondary nucleation creates active sites on the growing surface and therefore the number density of crystals formed [63]; i.e., crystal nucleation is favored over crystal growth. Bockris et al. [56] observed a transition from mossy-type Zn coatings to layer-like, boulder, and dendritic structure as the overpotential increases. Froment and Maurin [57] identified the evolution of Zn coatings as the potential increases from spongy to compact to dendritic. Wang et al. [66] presented a systematic morphology characterization for Zn deposits obtained under different conditions. They proposed five possible morphological categories: heavy spongy (under diffusion and convection control), dendritic (diffusion control), boulder (diffusion and activation control), layer like (activation control), and mossy (mixed charge transfer and nucleation control). These structures are obtained from very high to very low current density in the order presented. In the present study, thin Zn films were elaborated from solution S0 without or with QACs in order to observe the effects of the QACs on the deposit morphology. Figure 5 presents SEM images showing the additive effects on the Zn deposit morphology under potentiostatic conditions (Eappl = -1.65 V vs. SCE). Hexagonal Zn crystals [56] with a stepped boulder-type structure [66] were obtained with solution S0 as shown in Fig. 5a. Such morphology is characteristic of deposition under mixed control (activation–diffusion

control) [66], and the sizes of the crystal particles vary from 1 lm to around 5 lm. According to Wang et al. [66], boulder morphology is the result of a competition of the growth into the diffusion field with lateral layer growth, i.e., between nonepitaxial and epitaxial growth, respectively [56]. In the presence of 3NCP (Fig. 5b), crystals are larger having sizes around 3–4 lm, the lateral growth seems to be enhanced a bit, and the hexagonal structure is lost. The crystal structure can also be described as a boulder-type morphology [66], with mixed control dominating the deposition process in these conditions. Although 3NCP is considered a brightener [13, 15–19], white or semibright boulder deposits are obtained in the presence of this compound alone, with additional ions like cobalt [16, 17] as well as other organic additives [15, 19] required to improve the bright appearance. A noteworthy change is observed when NBT is dissolved in the bath (Fig. 5c) with the appearance of a needle-type leaf structure that grows perpendicular to the substrate. Inhibition of dendritic growth was observed previously in alkaline electrolytes [23]. Under mass transport control the direction of deposit growth is perpendicular to electrode surface [65], i.e., following the diffusion direction. The additive would adsorb on the electrode, impeding lateral motion of crystals and thus promoting vertical growth of crystals. Zn deposits obtained from baths with TEAOH modify slightly the hexagonal morphology observed previously without additives (Fig. 5d) and also present boulder morphology. Additions of TBAOH induce important morphology changes leading to fine-grained (but not bright) Zn deposits (Fig. 5e) that seem to present filamentous mossy-type morphology. Such morphology appears when the deposit initiation becomes highly selective

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on specific nucleation sites and corresponds to a deposition process under mixed charge transfer and nucleation control [66]. This is in accord with the loop observed in Fig. 4. Other authors observed similar effects: for example, compact platelets were observed without alkyl ammonium additives while spongy-like structures were obtained in their presence [24]. Hydrophobic interactions associated with long-chain tetraalkylammonium ions (e.g., TBAOH) seem to be more important than the interactions of compounds with shorter aliphatic chains (e.g., TEAOH) and lead to formation of an adsorbed barrier on the electrode surface. On the other hand, tetraethylammonium ions would be stable within the solution due to the hydrophilic character of TEAOH. Zn deposition from solutions without and with TBAOH was performed at more negative potentials (Eappl = –1.78 V vs. SCE), i.e., around peak IIIc of Fig. 2-line e and corresponding to massive Zn(II) reduction. Figure 5f shows that a somewhat spongy morphology is then obtained in the presence of the additive. Without additive, dendritic zinc morphologies were observed (Fig. 5g), which would be attributed to mass transfer control according to Wang et al. [66]. During the dendritic Zn growth, hydrogen bubbles were observed on the electrode surface, which, also play a role on Zn deposition according to a previous study [58]. This result clearly points out the influence of TBAOH as an inhibitor of dendritic growth, diminishing the grain size. 3.4 Crystallographic structure Properties of metal deposits such as brightness, corrosion resistance, and others are related to the structure of electrodeposits [67]. The preferential adsorption of organic compounds on some crystallographic planes also depends on the nature of the electrolyte, of the organic compound present and of the metallic substrate [32]. Several theories have been proposed to explain the factors that influence the preferential texture during electrodeposition. Pangarov [68] suggested that 2D crystal growth occurs initially, and the orientation is defined by the applied overpotential. Planes with low energy of formation would develop at low overpotentials; then formation of the most closely packed plane Zn(002) is expected. Youssef et al. [69] argued that 2D nucleation theory proposed by Pangarov does not account for the effects of adsorbed foreign substances. Reddy [70] indicated that the development of preferential textures is due to the differences in the propagation rate between different planes; planes with little or no adsorption effects will grow preferentially. Sato indicated that favorable orientations are determined by the growth rates of various crystallographic planes [71], which depend on the degree of interaction of organic compounds with the growing surface. Amblard et al. [72, 73] explained the development of textures in terms of the selective adsorption of different

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species that may block different growth sites, allowing at least one orientation to grow preferentially. Li and Szpunar [74, 75] argued that the development of different textures is possible due to differences surface energies; some textures are favored because they minimize the surface energy of the system, i.e., the surface energy anisotropy. As the deposit grows, grains having high surface energy tend to reduce their surface area and those with low surface energy increase their surface area [74]. Amblard et al. [76] explained the texture formation as the result of a competition between epitaxial and non-epitaxial growth, which depends on the nature of the substrate. The early stages of electrodeposition involve epitaxial or independent nucleation, followed by crystal growth that goes from layer-bylayer growth, pursuing the substrate texture, to polycrystalline non-epitaxial growth induced by substrate-independent nucleation [76]. The effect of additives on the crystallographic structure of zinc deposits has been reported [77–86] in terms of the orientation of the preferential crystal growth as determined by X-ray diffraction. It is known that the different planes of hexagonal Zn have different energies of formation which depend on the atomic packing [87]. To analyze the texture of Zn deposits, Muresan et al. [88] proposed the texture coefficient parameter defined in Eq. 5. Be´rube´ and L’Espe´rance [89] proposed the computation of a relative texture coefficient to describe preferential orientation of zinc deposits (Eq. 6), which is based on the model of Muresan et al. [88].  IðhklÞ I0ðhklÞ P TcðhklÞ ¼ P ð5Þ IðhklÞ I0ðhklÞ TcðhklÞ RTcðhklÞ ¼ P TcðhklÞ

ð6Þ

In these equations, I(hkl) is the relative diffraction peak intensity of the zinc electrodeposits, RI is the sum of the relative intensities of the independent peaks, and n is the number of reflections. The index 0 denotes the relative intensities for a standard powder sample; in this case, the pattern reported for pure zinc powder (JPDF 04-0831) was used as the standard. The ratio Ihkl/I0(hkl) represents the normalized relative intensities of the reflection of a plane (hkl) of the deposit with respect to the same crystallographic orientation of zinc powder. The expression RIhkl/RI0(hkl) is the overall normalized relative intensities. In these models, the texture coefficient (or relative texture coefficient) represents the degree of deviation of the relative intensities with respect to those reported for zinc powder. Figure 6 shows the crystallographic patterns of the Zn coatings produced here (Fig. 6A) and the variation of the texture coefficient (Tc) calculated from those crystallographic diffraction patterns (Eq. 6) with and without the

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75

organic additives (Fig. 6B). The Zn diffraction pattern obtained was the characteristic reported for Zn (JPDF 04-0831). Basal orientation (002) and pyramidal (103) and (101) are the dominant textures (Fig. 6, line a). High-angle pyramidal Zn(101) plane is also observed but in a minor intensity. Addition of additive 3NCP provokes the preferential growth of the basal plane Zn(002), obtaining a highly oriented deposit where low-energy high-atomic packing basal peak (002) becomes the most dominant (Fig. 6A, line b). The presence of additive NBT inhibits the growth of the basal plane (Fig. 6A, line c), leading to formation of Zn deposits oriented to high-angle pyramidal texture (101) and prismatic (110) and (100) increases. The addition of TEOH suppresses a bit basal plane (002), increases the high-angle pyramidal (112) and markedly pyramidal (101), being the dominant orientation (Fig. 6A, line d). The addition of TBAOH increases pyramidal (101) even more and prismatic (100) orientation somewhat, i.e., the high-energy low-atomic packing that has a greater energy of formation (Fig. 6A, line e). From the texture analysis (Fig. 6B), it can be seen that the additives (except 3NCP) inhibit parallel growth of the new phase and favor growth of the pyramidal orientation (101) as well as the prismatic (100) to a minor extent. The orientation seems to be related to the overpotential required to reduce zinc ions. The formation of deposits with basal and low-angle pyramidal orientations (with lower energy of formation and high-atomic packing) is promoted in conditions that favor lower overpotentials. On the other hand, in conditions that require greater overpotentials for zinc reduction, the formation of highangle pyramidal and prismatic orientations (with high energy of formation and low-atomic packing) is favored.

(a)

Although TBAOH promotes the highest overpotential, prismatic planes are not favored. Such behavior could be explained if the HER is considered as an additional factor that affects zinc deposition, as seen previously [36, 58]. These findings are in agreement with other earlier results [6, 15, 77–86]. Gomes et al. [77, 78] observed the transition of Zn texture from basal (002) dominant peak to pyramidal (101) and prismatic (100) when additives were added to the electrolyte. The addition of additives inhibits Zn basal plane (002); thiamine favors the formation of texture (100) and gelatin (100), (110) and (101) orientations [79]. Nakano et al. [80] reported that the presence of polyethylene glycol decreased peak (002) and increased (110), (101), and (100). Silva-Filho and Lins [81] reported the inhibition of basal (002) and pyramidal (105), (104), and (103) planes when the current increases, favoring the increase of (101) and (112) planes on steel substrates. The increase of (100) texture was observed in the presence of PVA and piperonal in an alkaline bath, which was attributed to the adsorption of the additives [6]. Such a trend was also reported for Zn electrodeposition in the presence of oxalate ions; basal plane dominates in deposits obtained without oxalate, whereas high-angle pyramidal and prismatic planes are formed with this additive [82]. Chandrasekar et al. [83] found the transition from basal Zn (002) plane (obtained without additives) to pyramidal (102) and (112) planes (obtained in the presence of PVA) obtained from alkaline conditions. The addition of piperonal and PVA gives Zn deposits with preferential prismatic (100) and pyramidal (112) planes. Sato observed the transition of Zn basal (002) and (11.l) to prismatic (110) when the concentration of additives increased [71]. Yu et al. [84] observed the

(b) 0.7 Zn

0.6

(e)

Zn TEAOH

I/Imax

(d)

50

60

)

(1

10

(1

Fe

70

2)

Fe

0.4

Zn NBT Zn 3NCP

0.3 0.2 0.1

1 (1

)

03

40

(1

30

) 02

Fe

0.5

(b) (1

) 02 (0

00 ) (1 01 )

(c)

Texture coefficient

Zn TBAOH

80

Fe

(a)

0

90

2θ/°

Fig. 6 a X-rays diffraction patterns of the Zn deposits, obtained on AISI 1018 steel electrode under potentiostatic conditions. (a) Solution S0; (b) S0 ? 5 mM 3NCP; (c) S0 ? 5 mM NBT; (d) S0 ? 5 mM TEAOH; (e) S0 ? 5 mM TBAOH. Eappl = -1.65 V, t = 300 s. (I) represents the intensity of crystallographic signals and (Io) is the

Crystal plane orientation (hkl)

maximum peak value for a given pattern. b Variation of the texture coefficient for Zn deposits obtained in the absence and in the presence of organic additives. Method reported by Be´rube´ and L’Espe´rance, ref. [83]

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76

increase of the (101) and (110) orientations in the presence of some additives. On the other hand, one of the additives tested promoted the increase of basal (002) texture, similar to the effect of additive 3NCP in this study. The trend of the dominant crystallographic planes observed in the study by Nayana et al. [85] was analyzed in terms of its energy of formation: basal plane (002) dominates in the absence of additives, and pyramidal (101), (102), and (112) are favored with the brightener. The addition of the leveler favors prismatic (110) plane, and finally the combination favors the (100) prismatic plane, which possesses the greater energy of formation. In a previous study [15], the addition of polymers favored the formation of Zn coatings with high-angle pyramidal (101) orientations. In the presence of levelers and a brightener, the texture developed involved the low-packing high-energy prismatic (110) and (100) orientations. Compounds with aromatic groups exert greater effects on the zinc reduction overpotential and promote the formation of fine-grained crystals with prismatic orientations with low-packing high energy [15]. Coverage of the electrode by adsorbed additive increases the reduction overpotential of metal ions, increasing the energy of the system, thus provoking a high degree of supersaturation of the surface by adatoms [3]. Youssef et al. [85] reported a transition from the basal (002) plane (which has the lowest energy of formation) to low-angle pyramidal Zn(104) and Zn(103) planes that appear when the overpotential of Zn deposition increases. At greater overpotentials, high-angle pyramidal (112) and (101) planes were formed. Prismatic (110) and (100) orientations are expected at the highest overpotentials. The angle of planes formed with respect to the basal (002) increases as the overpotential does [86]. These results were explained in terms of atomic packing; low overpotentials favor planes with high-atomic packing, and textures with low-atomic packing require high overpotentials [86]. Then, minimization of the surface energy (more specifically the integral $cdS because of the dependence of the surface tension (c) with crystal planes and direction) is an important factor in the morphology and crystallography of surfaces. Kamei and Ohmori [90] reported the epitaxial growth of Zn onto Fe substrates, which follow specific orientation relationships between the Zn deposit and the steel surface, called Burger’s orientation relationships: Fe(100)// Zn(101), Fe(110)//Zn(002), and Fe(111)//Zn(002) [90]. In this work, the texture of the carbon steel electrode was identified as a body-centered cubic (bcc) a-Fe, (JPDF 06-0696), Fe(110) being the dominant plane. Then, the development of Zn(002) would be favored, being in accordance with previous results [90]. Li and Szpunar indicated that the hydrogen adsorbed during metal electrodeposition changes the anisotropic

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surface energy [74, 75]. The authors indicate that the anisotropic factors determine the competition between different planes and are, as a consequence, responsible for texture development. The degree of this change will depend on the amount of hydrogen adsorbed, and it will be different for different crystallographic planes due to their different adsorptive properties [32, 70]. On this basis, it is expected that additives and adsorbed hydrogen exert the same effect on the anisotropic surface energy, and therefore, on the different crystallographic planes that develop during zinc deposition. Additives modify the electrochemical voltammograms and exert important effects on the morphology and texture of zinc deposits. The presence of large alkyl chain or aromatic groups seems to be an important element leading to an increase of the Zn reduction overpotential. Additive NBT, with both benzyl and aliphatic groups, strongly affects the electrochemical behavior by acting on kinetics as well as on the deposit morphology and texture. The presence of an aromatic functional group seems to exert an additional inhibition effect relative to that observed with TEAOH but to a lesser extent than the effect of TBAOH. Probably steric interactions interfere with the adsorption of NBT on the electrode [32], whereas TBAOH would form stable hydrophobic interactions. On the other hand, 3NCP does not affect the voltammograms appreciably but offers interesting textural properties of deposits: a high orientation to high-atomic packing basal plane (002). The addition of TEAOH favors the development of pyramidal (101) orientation. Additives NBT or TBAOH favor the formation of low-atomic packing textures through an increase in overpotential, which gives fine-grained deposits. The morphological and crystallographic findings agree with the fact that at greater overpotentials, high-energy surface and low-atomic packing textures are formed; these grains tend to reduce their surface area [74, 75]. Such observations indicate that random nucleation takes place [63], whereas in the presence of 3NCP granular 3D boulder crystals are formed. Further studies will be necessary to achieve a better understanding of the complex processes that occur during Zn electrodeposition in the presence of these additives.

4 Conclusions The present study showed that with the use of QACs as additives to form Zn coatings, molecular structure of these cationic additives can influence significantly the morphology and texture of the Zn coatings formed. Zinc electrodeposition is strongly inhibited by the addition of quaternary ammonium cations with large aliphatic chain groups, such as TBA?, whereas cations with a shorter

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aliphatic chain, TEA?, exert a lesser inhibition. The addition of NBT increased a little the inhibition of zinc reduction, but its effect was lower than that of TBAOH due to possible steric repulsive interactions of the benzene ring. 3NCP presents a lower inhibition on a carbon steel substrate. Morphology is affected by adding the QACs: Zn deposits formed in the absence of additives or with 3NCP exhibit a boulder-type structure, whereas needle-type structures are observed with NBT. TEAOH changes slightly the Zn deposit morphology. Mossy deposits are obtained when TBAOH is present in the electrolyte with a decreased grain size due to conditions where charge transfer and nucleation control become dominant. A spongy morphology appears at greater potentials, the additive inhibiting dendritic growth while promoting the HER. Additives NBT and TBAOH inhibit dendritic growth and may be used as levelers, whereas TEAOH has a minor effect. The study of X-ray diffraction patterns showed that the presence of additives affects the dominant zinc basal plane (002). Additive 3NCP, often used as a brightener, promotes basal growth of low-energy high-atomic packing basal plane (002). Additives that increase overpotential of Zn(II) reduction (NBT or TBAOH) tend to promote the formation of high-energy low-atomic packing crystallographic planes. Hydrophobic and steric interactions between QACs appear to act in an important way on deposit properties and on the electrochemical behavior of the plating system. Acknowledgments The authors thank Consejo Nacional de Ciencia y Tecnologı´a (CONACyT), Me´xico, Project 31411, for financial assistance. J. L. O.-A. is also grateful for CONACyT scholarships. The authors also thank M. Vega-Gonza´lez and M. Aguilar-Franco of the Instituto de Fı´sica, Universidad Nacional Auto´noma de Me´xico, for help in obtaining the X-rays diffraction patterns.

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