A Study on the Effect of Additive Combination on Improving

Article pubs.acs.org/IECR

A Study on the Effect of Additive Combination on Improving Anticorrosion Property of Zinc Electrodeposit from Acid Chloride Bath Nayana K. Onkarappa, Prashanth S. Adarakatti, and Pandurangappa Malingappa* Department of Studies in Chemistry, Bangalore University, Central College Campus, Bengaluru 560 001, India S Supporting Information *

ABSTRACT: The presence of polyethylene glycol (PEG) and syringaldehyde (SGA) mixture in a chloride bath produces nanocrystalline, bright zinc electrodeposition on steel substrate. The effect of the combination of additives PEG and SGA and simultaneous influence of deposition current density in the presence of the PEG+SGA mixture on the deposit surface morphology, texture, and grain size were systematically studied. The effects of these additives on cathodic current efficiency and throwing power of the bath and corrosion resistance of the deposit were analyzed. The presence of the PEG and SGA mixture remarkably changes both the morphology and orientation of growing zinc platelets. This study revealed the synergistic influence of additives on enhancing the corrosion resistance property of the deposit.

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INTRODUCTION Pure zinc electrodeposition on steel is most extensively used for sacrificial anodic protection of iron or steel products against corrosion.1−3 The protection ability of zinc coating in highly aggressive environments is reduced due to the sacrificial corrosion of zinc. In recent years, efforts have been made to improve the consistency of zinc coating as an anticorrosion material under various environments. Thus, nanocrystalline, bright, and leveled zinc coatings were developed which exhibit relatively better corrosion resistance and other functional properties in comparison to those of the coarse grained deposit.4−6 Small amounts of additives are generally added to zinc electroplating bath to obtain nanocrystalline bright zinc coating.7 The additives are organic compounds present in the plating bath which adsorbs on the substrate surface during electrodeposition and modify the electrocrystallization process, thus giving a coating with different microstructural characteristic properties such as grain size, surface morphology, and preferred orientation.8−10 The modification in microstructure and morphology of the deposit had a significant effect on corrosion resistance and other properties of the zinc coating. The bath solutions employed in zinc electrodeposition are usually acid chloride, sulfate, mixed sulfate-chloride, and alkaline types.11−14 Among all of these, acid chloride zinc plating bath provides relatively better plating properties such as high-conductivity, low cost, and nontoxic nature. The acid chloride electrolytes are used especially when high plating rate with maximum current efficiency is required.15 The use of a good additives system in chloride bath generates leveled, bright © XXXX American Chemical Society

deposit over the wide current density range with improved anticorrosion property.16 The additives used in chloride bath are generally a combination of brighteners and levelers or carriers. Brighteners are small organic molecules with aldehyde, ketone, and sulfur containing functional groups, whereas the levelers are cationic, anionic, and nonionic surfactant molecules. Polyethylene glycol (PEG) is largely employed for electrodeposition in chloride baths.17−19 Due to large molecular weight, these molecules control the zinc electrodeposition at the first stages of nucleation process and finally determine the properties of the deposit such as the morphology and corrosion rate.20 Moron et al. described the formation of corrosion resistant zinc coating in the presence of brightener (arenes with phenyl group and aliphatic chains) and PEG200 associates in comparison to the coatings formed in the absence of additives or in the presence of PEG200 alone.18 Hsieh et al. reported the synergistic effect of additives (polyoxyethylene nonyl phenyl ether, o-chloro benzyl aldehyde, and polyoxyethylene lauryl amine) on improving the current efficiency of zinc electrodeposit in a wide range of current density.5 Further, the same group observed that the strong synergistic effect between quaternized pyridine carboxylic acid and polyamine additives in alkaline zinc cyanide-free bath improves the morphology of the deposit at high current density.21 Close et al. studied the effect Received: Revised: Accepted: Published: A

January 14, 2017 March 10, 2017 April 25, 2017 April 25, 2017 DOI: 10.1021/acs.iecr.7b00154 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

farther and nearer cathodes and M is the ratio of deposit weight obtained on the nearer and farther cathodes.24 The cathodic current efficiency (CCE) of the bath solution was determined based on the weight of deposit produced on the cathode using a rectangular cell similar to the one used in throwing power measurement. The steel cathode was placed at an equal distance between two zinc anodes. The electrodeposition was carried out for 10 min, and the weight of the deposit was recorded. The CCE was calculated using the w− w′ equation CCE = w × 100, where w is the theoretical weight based on Faraday’s law and w′ is the weight of the deposit on the cathode.12 Potentiodynamic cathodic polarization studies were carried out using a conventional three-electrode system. Glassy carbon electrode (3 mm dia.) acts as working electrode. The saturated calomel electrode (SCE) and platinum wire (1 mm diameter) were used as reference and counter electrodes, respectively. The glassy carbon electrode surface was cleaned by polishing with 0.05 μm alumina powder, agitated ultrasonically for 15 min in 10% HCl, and then washed with distilled water. All electrochemical measurements were carried out using an electrochemical analyzer (CH Instruments, model: 660C, United States). The surface morphology of the deposit was analyzed using field emission scanning electron microscopy (FESEM, model: LE01530-VP). The reflectivity of coatings was measured using gloss meter (NOVO gloss meter) referenced against a vacuum coated silver mirror. The mirror reflectance was adjusted to 100% at an angle of 60°. The average value of reflectance at different regions on the coating surface was measured and reported. The average grain size and orientation of the deposit were examined by X-ray diffraction (XRD) studies (PANalytical X’pert PRO powder XRD with graphite monochromatized Cu Kα radiation source). The average grain size of the deposit was calculated using the Scherrer equation, which relates the crystallite size of the coating material with broadening of the peak in a diffraction pattern. D = kλ/βcos θ, where k = 0.9, β is full width at half-maximum, θ is reflectance angle, and λ = 1.541 Å wavelength of radiation used. The preferred orientations of the deposits were determined by calculating the texture coefficient (Tc) using equation

of additives such as 4-hydroxybenzaldehyde and ammonium thiocyanate on Zn−Mn electrodeposition in chloride plating baths and obtained Zn−Mn coating containing ε2−Zn−Mn hexagonal close packed single phase and strong fiber texture along (110) plane with good corrosion resistance.22 Ibrahim and Omar concluded that the presence of ninhydrin and iodide ions has a synergistic effect on enhancing the throwing power (TP) and throwing index of zinc sulfate plating baths.23 The study of existing literature reveals that use of additives combination in chloride bath is more beneficial to enhance the anticorrosion property of the deposit and the throwing power and current efficiency of the plating bath. In recent years, very little emphasis has been given to study the influence of the current density on the electrodeposition in the presence of additive mixture. In the present report, we studied zinc electroplating in chloride bath using additives such as polyethylene glycol (PEG) and syringaldehyde (SGA), which are capable of generating bright deposit over a wide range of current density. The individual and combined effect of additives PEG and SGA on cathodic current efficiency; throwing power of the bath; and the morphology, orientation, and corrosion resistance of the bright zinc deposit was systematically examined. Simultaneously, the influence of applied current density on efficiency, throwing power of the bath, and anticorrosion property of the deposit was also analyzed in the presence of PEG and SGA mixture.

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EXPERIMENTAL SECTION The nanocrystalline, bright zinc coating was electrodeposited on steel by galvanostatic current control technique using acid chloride bath. Initially, chloride bath solution was prepared by dissolving analar grade chemicals (SD Fine, Mumbai, India) 32 g L−1 ZnCl2, 180 g L−1 KCl, and 8 g L−1 H3BO3 in distilled water. The analar grade additives such as PEG with an average molecular weight of 4000 and SGA (4-hydroxy-3,5-dimethoxybenzaldehyde) were used as leveler and brightener additives, respectively. The pH of the bath solution was adjusted to 5 using 10% H2SO4 and NaHCO3 solutions. The experiments were carried out at room temperature (25 ± 2 °C) using pure zinc (99.9%) metal sheet with a surface area of 5 × 6 cm as an anode. The anodic surface was activated each time by acid dip method in 10% hydrochloric acid for about 3−5 s followed by washing with running water. The mild steel cathode (Hull cell dimension as well as 4 × 6 cm area for other studies) surface was mechanically polished with 200, 1000, 20 000 grades of water proof emery paper, degreased in trichloroethylene, and then immersed in 10% HCl to remove the dust and rust. The plates were then washed in running water and immediately transferred to plating bath solution for electrodeposition. After electrodeposition, the adsorbed superfluous organic impurities on the coating surface were removed by bright dip in 1% HNO3 solution for few seconds followed by water wash and air drying. The TP of the plating bath was measured using a rectangular Perspex cell (15 × 5 × 5 cm) with the Haring−Blum method. The two steel cathodes were placed on either side of zinc anodes at a distance ratio of 1:5. The weight of the deposit on two steel cathodes was recorded in the absence and presence of additive mixture at 4 Adm−2 and also at different current densities using optimized bath composition. The %TP was L−M calculated from Field’s formula TP(%) = L + M − 2 × 100, where L is the ratio of the distance from the anode of the

TC(hkl) =

I(hkl) ∑ I(hkl)

×

∑ I0(hkl) I0(hkl)

× 100, where I(hkl) is the peak

intensity of zinc electrodeposit and ∑I is the sum of the intensities of independent peaks. The index 0 refers to the intensities for the standard zinc sample (JCPDS 00-004-0831). The orientation with maximum texture coefficient value is the preferred orientation of the deposit.25 The corrosion behavior of deposits was analyzed by weight loss method and electrochemical methods such as potentiodynamic anodic polarization, Tafel plot, and impedance studies. In weight loss method, 4 × 4 cm zinc coated samples were immersed in 3.5% NaCl solution for 240 h. After specified hours of immersion, the weight loss in the coated specimen was determined every 24 h. The corrosion behavior of the coating was also analyzed by electrochemical methods (anodic polarization, Tafel, and electrochemical impedance studies) in 3.5% NaCl solution. The coated steel samples with 1 × 1 cm exposed surface area were employed as working electrode in a three electrode cell. Before each electrochemical measurement, a coated specimen B

DOI: 10.1021/acs.iecr.7b00154 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research was immersed in the corrosive media for about 10 min to ascertain the open circuit potential (OCP).

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RESULTS AND DISCUSSION The concentration of bath constituents and operating conditions were optimized by Hull cell studies (Supporting Information). During Hull cell studies, the concentration of one constituent was varied continuously in constant increments while keeping the other constituents at their selected concentration level. The concentration of constituent or operating condition at which good bright deposit developed over wide range of current density was fixed as optimum. The procedure was repeated for all bath constituents and operating conditions. The optimization process of all bath constituents and operating conditions is illustrated in Figure S1 (Supporting Information). The optimized bath compositions and operating conditions are given in Table S1 (Supporting Information). Under optimized bath conditions, PEG functions as a leveling agent, and SGA serves as brightening agent. The mixture of PEG and SGA additives in the bath produces smooth, uniform, bright deposit over a wide range of current densities compared to that produced in the presence of either SGA or PEG as a single component. The synergistic influence of these additives on bath characteristics such as current efficiency, throwing power, and the deposit property such as corrosion resistance were studied in detail. The electrodeposition current density is one of the important factors which influence the properties of the deposit and performance of the bath solution. Hence, the effect of plating current density on current efficiency, throwing power, and corrosion resistance of the deposit was analyzed. The following bath solutions were used for further studies:

Figure 1. Throwing power and cathodic current efficiency of the bath at 4 Adm−2.

exhibits a good effect on improving the efficiency of the zinc deposit. Shanmugasigamani and Malathy utilized a leveler and brightener mixture such as poly(vinyl alcohol) and piperonal, respectively, which exhibited good cathodic current efficiency of 76−84% in comparison with that of other mixtures.26 However, the proposed method in this report showed superior cathodic current efficiency. The throwing power and cathodic current efficiency of the optimized bath containing additive mixture SPEG+SGA at different current densities are shown in Table 1. Initially, the throwing Table 1. Throwing Power and Cathodic Current Efficiency Values of SPEG+SGA Bath at Different Current Densities

S0 = 40 g L−1 ZnCl 2 + 200 g L−1 KCl + 16 g L−1 H3BO3 (without additive)

SPEG = S0 + 6 g L−1 PEG

current density (Adm−2)

TP (%)

CCE (%)

1 2 3 4 5 6 7 8

30 32 45 48 52 59 48 45

79 89 91 96 98 99 98 97

SSGA = S0 + 0.64 g L−1 SGA

power was found to be 30% at 1 Adm−2. However, the throwing power increases with increasing the applied current density and was found to be maximum at 6 Adm−2; thereafter, it decreases (>6 Adm−2). The maximum throwing power of 59% at 6 Adm−2 was achieved, and the developed bath has the capacity to coat irregular materials more uniformly. At a different current density, the efficiency of SPEG+SGA bath was found in the range 79−99%. These studies revealed that the developed optimized bath has good efficiency during deposition. Deposit Surface Morphology and Orientation. The typical SEM images of the deposit obtained from bath solutions S0, SPEG, SSGA, and SPEG+SGA at 4 Adm−2 are shown in Figure 2. The zinc coating from additive-free bath S0 is dull in appearance (8% reflectance), and the SEM image (Figure 2a) is composed of thin hexagonal platelets of dimension larger than 2.5 μm with thickness less than 0.1 μm, typical of zinc coatings (Figure 2a, inset).7 The plates grow in multiple layers and were randomly distributed on the electrode surface with different degrees of orientation. In contrast to the morphology of the deposit obtained in the presence of PEG alone, SPEG is compact and made up of flakes grouped in hemispherical clusters (Figure 2b) with different sizes of 50−200 μm2,20

SPEG + SGA = S0 + 6 g L−1 PEG + 0.64 g L−1 SGA (optimized bath)

TP and CCE of the Bath. The throwing power and cathodic current efficiency was measured for solutions S0, SPEG, SSGA, and SPEG+SGA at 4 Adm−2, and the values are plotted in bar graph (Figure 1). The additive-free S0 bath showed low TP of 32%, whereas the addition of PEG (SPEG) and SGA (SSGA) individually to S0 results in a rise of the throwing power to 38 and 45% respectively. The mixture of PEG and SGA in the bath (SPEG+SGA) increases the throwing power to 48%. This implies that the presence of additive mixture has a synergistic effect on enhancing the throwing power of the bath. A similar result was reported on improving the throwing power of zinc electroplating in sulfate bath due to synergistic effect of additives such as ninhydrin and iodide ions.23 The cathodic current efficiency of the S0 bath (without additive) was found to be 90%. However, by the addition of PEG (SPEG) and SGA (SSGA) to the S0 bath, the efficiency was found to be 89 and 92%, respectively (Figure 1). The presence of PEG and SGA mixture in the bath SPEG+SGA gave 96% efficiency, which reveals that the combination of additives C


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Figure 2. SEM images of the deposit obtained from bath solutions (a) S0, (b) SPEG, (c) SSGA, and (d) SPEG+SGA at 4 Adm−2.

whereas the coating obtained in the presence of SGA alone (SSGA) is semibright and uneven in its appearance. The coating showed greater reduction in grain size, but the surface is nonuniform and has cavities of different dimensions (Figure 2c). Furthermore, coatings obtained in the presence of PEG +SGA mixture (SPEG+SGA) are bright and uniform in appearance with 70% reflectance. The deposit exhibits additional refinement of grain size with microsmoothening and is shown in Figure 2d. The examination of SEM images confirms the effect of additive mixture on both the morphology and grain size of the deposit. The coarse grain, dull deposit (S0) is transferred to smooth, fine grained, nanocrystalline, bright deposit (SPEG+SGA). Also, a PEG + SGA mixture in the bath generated a smooth, compact, and bright deposit, whereas absence or presence of individual additives gives unsatisfactory coatings. The XRD patterns of deposits obtained from solutions S0, SPEG, SSGA, and SPEG+SGA at 4 Adm−2 are given in Figure 3a. The diffraction peaks show line broadening in the presence of additives, which revealed that the additive has a remarkable effect on refining the grain size of the deposit. Hence, the average grain sizes calculated using the Scherrer formula were found to be 245.64, 146.18, 119.3, and 49.36 nm for S0, SPEG, SSGA, and SPEG+SGA deposits, respectively. The average grain size reduces from 245.64 nm (S0) to 146.18 nm (SPEG) and 119.3 nm (SSGA) in the presence of individual additives. The smallest average grain size 49.36 nm in the presence of PEG+SGA mixture indicates the combined effect of additives on refining the grain size of the zinc deposit.

Figure 3. (a) XRD pattern and (b) %Tc values as a function of crystallographic planes of zinc deposits at 4 Adm−2.

D


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Figure 4. (a) XRD patterns of the zinc electrodeposit obtained from SPEG+SGA bath at (i) 1, (ii) 2, (iii) 3, (iv) 4, (v) 5, and (vi) 6 Adm−2 and (b) average grain size of SPEG+SGA coating obtained at different current densities.

The decreases in basal (002) peak and increase in prismatic (100)(110) peak intensities in the XRD pattern confirm the significant effect of additives on crystallographic orientation of the deposit. To know the influence of additives on preferred orientation of the deposit, texture coefficient was calculated for each peak of the diffraction pattern. The corresponding Tc values are shown as a bar diagram (Figure 3b). For additive-free deposit (S0), the Tc values for different orientations were not much larger for any of the planes [19.72% (002), 11.80% (100), 15.06% (101), 17.54% (102), 12.11% (103), and 15.99% (110)]. Hence, the S0 deposit is without significant preferred orientation (SEM, Figure 2a). The presence of additives in the bath lowers the intensity of lower energy planes (002)(102) (103)(004) and enhances the growth of higher energy planes (100)(101)(110) due to preferential adsorption of additives at different crystallographic planes during deposition. The PEG alone in the plating bath (SPEG) generates deposit with prismatic (100)(110) preferred orientation [with Tc value 19% (100) and 25% (110)], whereas SGA alone in SSGA bath diminishes the growth of basal (002) and develops prismatic (100)(110) and pyramidal (101) preferred orientation. The mixture of PEG and SGA in the bath generates 35 and 36.8% of crystals along prismatic (100) and (110) planes, respectively. Hence, prismatic (100)(110) was the preferred orientation of the deposit. When both PEG and SGA were present in the bath, synergistic interaction exists between them and leads to the formation of more predominant (100)(110) preferred orientation compared to that in the presence of PEG alone (lesser Tc value). The morphological and microstructural changes such as refinement of grain size, smoothening, and formation of more predominant preferred orientation along (100) and (110) planes of the zinc deposit (SEM, Figure 2d) in the presence of additive mixture PEG and SGA contributed toward bright appearance of the deposit. These are the essential conditions required for bright appearance of the electrodeposit. Figure 4 shows the effect of deposition current density on XRD pattern and average grain size of the SPEG+SGA deposit, and corresponding Tc values calculated for each peaks of diffraction pattern are given in Table 2. The SPEG+SGA deposit has an average grain size in the range 42−58 nm at different applied current densities from 1 to 6 Adm−2 (Figure 4b). This indicates

Table 2. Texture Coefficient (Tc) of SPEG+SGA Coating at Different Current Densities TC

1 Adm−2

2 Adm−2

3 Adm−2

4 Adm−2

5 Adm−2

6 Adm−2

(002) (100) (101) (102) (103) (110) (004)

3.63 40.42 13.28 4.33 0 38.33 0

7.51 35.06 16.10 9.87 0 31.45 0

8.04 30.92 17.26 12.14 0 31.64 0

6.79 35.04 13.72 7.64 0 36.8 0

6.34 31.55 13.8 8.03 0 40.03 0

6.76 32.06 15.11 8.06 0 39.00 0

the reduction in the average grain size of the deposit obtained from optimized bath (SPEG+SGA) at all current densities compares to that of the S0 coating. The observation made from Table 3 show that deposits obtained from optimized bath (SPEG+SGA) at all current densities have maximum Tc values for prismatic (100)(101) planes. Thus, prismatic (100)(101) was the preferred orientation of the SPEG+SGA deposit from 1 to 6 Adm−2. Cathodic Polarization Measurements. To examine the effect of PEG and SGA on zinc reduction, a cathodic polarization study was carried out for solutions S0, SPEG, and SPEG+SGA by sweeping the potential in the negative direction from −0.4 to −2.0 V at the scan rate 25 mVs−1. Figure 5 shows the cathodic polarization curves for solutions S0, SPEG, and SPEG+SGA. The additive-free solution S0 has shown zinc reduction peak at −1.38 V, while in the presence of SGA alone, zinc reduction peak potential was shifted to −1.64 V, and two well-resolved cathodic peaks were obtained in the presence of PEG alone: for zinc reduction, one at −1.39 V, which corresponds to zinc reduction in absence of additives (S0), and another peak potential at −1.68 V, which is more negative than that obtained for S0. The additives SGA and PEG adsorb on the electrode surface, creating a barrier for discharge of zinc ions at the vicinity of the electrode and thus shifting the reduction potential to more negative value (more negative polarization).8 Whereas, in the presence of PEG and SGA mixture (SPEG+SGA), the zinc reduction peak was shifted to −1.75 V more negative (polarization) than the potential obtained in absence of additive or presence of either SGA or PEG alone. The more negative shift in zinc deposition is observed due to the synergistic adsorption of additives on the electrode surface. With an E


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Industrial & Engineering Chemistry Research Table 3. Corrosion Parameters from Tafel Plots sample S0 SPEG SSGA 1 2 3 4 5 6

Adm−2 Adm−2 Adm−2 Adm−2 Adm−2 Adm−2

Ecorr (V/SCE)

Icorr 10−3 (A cm−2)

βc (mV dec−1)

βa (mV dec−1)

RP (Ω cm2)

CR 10−3(g/h)

−1.088 −1.086 −1.074

4.178 3.315 2.047

1.916 3.115 1.601

5.662 5.563 6.361

13.7 15.1 26.7

5.091 4.039 2.494

−1.074 −1.072 −1.069 −1.062 −1.054 −1.050

1.776 2.022 1.849 1.648 1.565 1.459

SPEG+SGA 1.900 1.983 1.651 0.865 1.361 1.057

6.155 6.230 5.984 6.028 5.976 6.269

30.4 26.2 30.8 38.3 37.9 40.7

2.164 2.464 2.253 2.008 1.907 1.778

increase in polarization, more activation energy is required for reduction of zinc ions, which leads to increase in the nucleation rate and retardation of growth rate. This results in refinement of grain size of the deposit. Thus, higher polarization was responsible for refinement of grain size of the deposit.27−29 The PEG molecule along with SGA adsorbing synergistically on electrode surface not only increases polarization of zinc deposition but also inhibits absorption of hydrogen bubbles into the deposit, which plays an important role in forming a smooth, pore-free, adherent, bright zinc deposit with improved CCE and TP.20−23 Corrosion Study. The developed coatings are mainly employed for corrosion protection application of base metal. In this study, we examined the effect of PEG+SGA mixture and deposition current density on protection ability of coating by adopting both chemical (weight loss) as well as electrochemical methods in 3.5% NaCl corrosion medium. The zinc hydroxyl chloride pseudopassive layer was formed on coating surface in

Figure 5. Cathodic polarization studies for zinc deposition.

Figure 6. Variation of the corrosion rate with immersion time for (a) S0, SPEG, SSGA, and SPEG+SGA coating at 4 Adm−2 in 3.5% NaCl solution and SEM images of (b) S0 and (c) SPEG+SGA coatings at 4 Adm−2 after weight loss measurement. F


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region (SEM image Figure 2a) of the dull coating obtained in absence of additives. Whereas, the coating obtained in the presence of PEG and SGA mixture was smooth and uniform without any sharpe edges (SEM image Figure 2d) with surface zinc atoms containing a large number of nearest neighboring atoms for binding, which hinders dissolution. Thus, dissolution peak at −1.2 V was completely absent in bright zinc coating. This also confirms improvement of the corrosion resistance of the deposit in the presence of additive mixture. To understand the dissolution behavior of the zinc coating, SEM images (Figures 8b and c) of S0 and SPEG+SGA deposits after anodic polarization were recorded. Both of the images show the presence of corrosion product. The deposit S0 undergoes nonuniform localized corrosion with larger dissolution of zinc hexagonal platelets from the deposit surface (Figure 8b). The SEM image of SPEG+SGA coating (Figure 8c) exhibited uniform type of corrosion with dissolution zones and corrosion products distributed uniformly on the surface. The bright SPEG+SGA coating obtained from optimized bath exhibited more positive shift in anodic polarization at all current densities from 1 to 6 Adm−2 (Figure 9). This shift indicates that the deposit obtained from SPEG+SGA bath at different current densities showed more noble character. Tafel Extrapolation Method. The anticorrosive nature of developed zinc deposits can be investigated by Tafel extrapolation method in 3.5% NaCl solution (Figure 10). During Tafel studies, the zinc coated working electrode surface was polarized between −0.2 to +0.2 V from OCP. The corrosion parameters such as corrosion potential (Ecorr), corrosion current (Icorr), corrosion rate (CR), and anodic/ cathodic Tafel slopes (βa and βc) derived from each Tafel plots have been compiled in Table 3. The polarization resistance (Rp) values were determined using the relation Icorr = β/Rp, where β is a constant calculated by the following equation:

3.5% NaCl medium, which prevents further dissolution of the zinc coating.30−32 1. Weight Loss Measurement. Figure 6a shows the corrosion rate of S0, SPEG, SSGA, and SPEG+SGA coated samples immersed in 3.5% NaCl solution. The additive-free S0 coating displayed a higher corrosion rate, whereas SPEG and SSGA coatings obtained in the presence of PEG and SGA individually exhibit moderately lower corrosion rates when compared to that of the S0 coating. The SPEG+SGA coating has shown more corrosion resistance than the deposits obtained either in absence or presence of single additive (PEG or SGA). These results imply that the presence of additive mixture generates deposit with higher corrosion resistance. Hence, additive combination PEG + SGA in the plating bath improves the corrosion resistance property of the electrodeposit. The SEM images of S0 and SPEG+SGA zinc coatings after 10 days of weight loss measurements are shown in Figures 6b and c. The larger corrosion with large number of holes and more corrosion product is noticed on the SEM image obtained for S0 coating obtained in absence of additives (Figure 6b). However, lesser corrosion is observed on the bright deposited specimen (Figure 6c). The corrosion rate of bright SPEG+SGA coating obtained at different current densities is given in Figure 7. The deposit

β=

βa βc 2.3(βa + βc)

The corrosion rate in gh−1 was calculated by the 0.13I

(eq wt)

corr where eq wt is the following equation: CR = d equivalent weight and d is the density of the zinc metal in g/ cm3. The Tafel curves recorded for S0, SPEG, SSGA, and SPEG+SGA zinc coated in 3.5% NaCl solution are presented in Figure 10a. The curves demonstrated observations similar to those of anodic polarization. The shift in corrosion potential toward noble direction with decrease in corrosion current and corrosion rate was noticed in the presence of SPEG, SSGA, and SPEG+SGA compared to that of the S0 coating. The presence of additive enhances corrosion resistance of the zinc coating. The more positive Ecorr and lesser Icorr values with 60% reduction in corrosion rate were noticed for the SPEG+SGA coating. Thus, SPEG+SGA coating exhibits more corrosion resistance compared to that of SPEG and SSGA coatings. This confirms that the additive combination (PEG+SGA) generates deposits with good corrosion resistance. Similarly, Moron et al. showed lower corrosion current density ((14.29 μA cm−2) for zinc coatings obtained in the presence of brightener-PEG200 associates (brighteners: benzylideneacetone, benzylacetone, and butylbenzene).18 Synergism of Saccharum of f icinarum and Ananas comosus extract additives in zinc deposition showed good corrosion resistance in a seawater test when compared with the unplated samples.33 Bright SPEG+SGA coatings were also obtained at different current densities from 1 to 6 Adm−2, and Tafel curves were

Figure 7. Variation of the corrosion rate with immersion time for SPEG+SGA coating at different current densities in 3.5% NaCl solution.

obtained at lower current density showed higher corrosion rate, and that obtained at higher current density showed lower corrosion rate. Hence, corrosion resistance of the deposit is higher for coatings obtained at higher current densities. Anodic Polarization Study. The anodic polarization profile of deposits obtained from S0, SPEG, SSGA, and SPEG+SGA baths in 3.5% NaCl are as shown in Figure 8a. The S0, SPEG, and SSGA coatings exhibited smaller polarization and hence easily undergo dissolution. Further, coatings obtained in the presence of PEG and SGA together (SPEG+SGA) shift the dissolution potential to less negative direction (+ve shift) when compared to other coatings. Hence, the presence of PEG and SGA combination gave coatings with better corrosion resistance behavior compared to the zinc coating obtained in absence or presence of individual additive. The anodic polarization curve comprises initially a small dissolution peak at −1.2 V which has been attributed to the oxidation of loosely bound surface zinc atoms surrounded by a lower number of nearest neighboring atoms at the sharp edge G


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Figure 8. Anodic polarization profile of (a) S0, SPEG, SSGA, and SPEG+SGA coatings at 4 Adm−2 in 3.5% NaCl solution and SEM images of (b) S0 and (c) SPEG+SGA coatings at 4 Adm−2 after anodic polarization.

frequency range 10 mHz to 100 kHz with a 5 mV sine wave as an excitation signal. The data were recorded and analyzed using ZSimp-Win 3.21 software. EIS data were represented as Nyquist (-Z″ versus Z′) and Bode plots (frequency versus phase angle/|Z|) and are shown in Figures 11 and 12, respectively. The electrically equivalent circuit (EEC) was proposed for impedance spectra analysis (Figure 13), and calculated impedance parameters are listed in Table 4. The three capacitive loops observed for coatings in Nyquist plot indicate the occurrence of three relaxation processes during corrosion. The capacitance elements at higher frequency end Cdl and CF in EEC are related to constant phase elements (CPE), respectively, Qdl and Qf. The CPE represent all the frequency dependent electrochemical phenomena, namely double layer capacitance and diffusion process. The coefficients (n) of the CPE n = 1 gave a real capacitive sense to the electrical element and n < 1 shows nonuniform distribution of current due to surface defects and surface roughness.34−37 In the proposed electrically equivalent circuit RS(Cf(Rf(Qdl(Rct(QfRf))))), each element is attributed to the following contributions.38−40 RS is the resistance of electrolyte between reference and working electrode surface. The high frequency couple (Cf − Rf) is attributed to the dielectric character of the thin porous corrosion product layer formed during corrosion on the coating surface (Cf) and Rf due to electrical leakage by ionic conduction through its pores. The medium frequency contribution is due to double layer capacitance (Qdl) at electrolyte/coating (zinc) interface at the bottom of the porous corrosion product layer, coupled with Rct, the charge transfer resistance (corrosion rate). The lower

Figure 9. Anodic polarization profile of SPEG+SGA coatings in 3.5% NaCl solution at different current densities.

recorded (Figure 10b). The bright zinc coating obtained from optimized bath at all current densities showed noble Ecorr with an average 58% decrease in Icorr compared to that of the dull S0 deposit. Even though there is not much difference in corrosion current and corrosion rate at different current densities, lesser Icorr and CR with more positive Ecorr values were noticed at higher current density values. Thus, SPEG+SGA coatings become noble at higher current densities. Electrochemical Impedance Study (EIS). The electrochemical impedance is an in situ and nondestructive probe for assessing the corrosion behavior of the metal coating. The impedance of the coating is analyzed at OCP over the H


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Figure 10. Potentiodynamic polarization curves for (a) S0, SPEG, SSGA, and SPEG+SGA coating at 4 Adm−2 in 3.5% NaCl solution and (b) SPEG+SGA bright zinc coatings at different current densities.

Figure 11. Nyquist plots for (a) S0, SPEG, SSGA, and SPEG+SGA zinc coating at 4 Adm−2 and (b) SPEG+SGA bright zinc coatings at different current densities in 3.5% NaCl solution.

Figure 12. Bode plots (a) frequency versus phase angle and (b) frequency versus |Z| for zinc coatings at different current densities in 3.5% NaCl solution.

frequency couples (Qf − Rf) are related to a redox process occurring at the coating surface, containing the thin corrosion product layer formed at the electrolyte/coating interface. The total polarization resistance (Rp) is the sum of Rf, Rct, and Rf, i.e., Rp = Rf + Rct + Rf. From Figure 11a and Table 4, it is clear that the SPEG+SGA coating exhibits higher charge transfer resistance, total polarization resistance, and a smaller double layer capacitance value compared to those of S0, SPEG, and SSGA coatings. The coating obtained in the presence of PEG+SGA mixture was more resistant for transfer of electron to external corrosion medium.

Figure 13. Electrical equivalent circuit used to simulate the recorded EIS data.

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Industrial & Engineering Chemistry Research Table 4. Impedance Data specimen S0 SPEG SSGA 1 2 3 4 5 6

Adm−2 Adm−2 Adm−2 Adm−2 Adm−2 Adm−2

Rf (Ω cm2)

Cf (10−7 μF cm−2)

Rct (Ω cm2)

6.22 5.73 2.32

16.82 7.365 2.128

92.6 255.5 202.6

3.91 5.31 4.27 4.69 5.87 6.01

1.015 6.008 5.947 1.906 4.407 2.335

209.3 244.9 250.6 314.5 321.5 492.0

Qdl (10−5 μF cm−2)

SPEG+SGA

ndl

15.77 0.7348 18.04 0.7554 7.794 0.6768 at different current densities 2.973 0.8025 4.657 0.7805 4.892 0.7789 3.257 0.8011 4.738 0.743 12.14 0.5845

Rf (Ω cm2)

Qf (10−3 μFcm−2)

nF

Rp (Ω cm2)

2.4 88.6 216.6

42.89 2.841 2.249

0.4535 0.7788 1

101.22 349.83 421.52

75.09 108.8 154.4 234.3 215.2 243.1

16.951 10.620 7.756 1.314 8.028 2.916

1 1 1 0.8464 1 1

288.30 359.01 409.27 553.49 542.57 741.11

size 52 nm) with preferred orientation along (100) and (101) low index planes. The additive mixture improves the efficiency and throwing power of the bath. The Tafel study revealed the 60 percent reduction in corrosion rate in the case of the SPEG +SGA coating compared to that in either the absence or presence of individual additives. The additives PEG and SGA interact synergistically with each other and furnished desired anticorrosion properties in the deposit. The absence or existence of either PEG or SGA alone in plating bath produces unsatisfactory deposit in terms of appearance or corrosion resistance property. At different applied current densities from 1 to 6 Adm−2, the developed bath showed good throwing power (30−59%) and efficiency (79−98%), and the deposit exhibited excellent anticorrosion properties. The corrosion studies by different methods revealed the decrease in corrosion rate with increase in charge transfer resistance and polarization resistance for SPEG+SGA coating obtained at higher current densities.

Thus, SPEG+SGA coating show superior anticorrosion behavior. The coating obtained from optimized SPEG+SGA bath at higher current density (4−6 Adm−2) showed larger Rp and smaller capacitance values compared to those of the coating obtained at lower current density (Figure 11b). This indicates improvement of corrosion resistance of the deposit at higher current density. The three humps observed in frequency versus phase angle Bode plots (Figure 12a) confirm the existence of three relaxation process during corrosion. In frequency versus impedance modulus |Z|, the Bode plot revealed that the |Z| values are larger for SPEG+SGA coating obtained at higher current densities compared to those for the S0 coating (Figure 12b). Thus, the PEG and SGA mixture makes zinc deposit considerably resistant toward external aggressive media compared to the zinc coating without or with additives. The corrosion behavior of metallic zinc coating depends on the texture, morphology, and chemical composition of the deposit. The smooth, compact, and nanocrystalline SPEG+SGA coating showed a zinc dissolution rate lower than that of the coarse grain, dull deposit from the S0 bath. Due to its compact and nanocrystalline nature, SPEG+SGA coating contains larger number of nearest neighboring atoms for binding; hence, the total energy required for breaking of the bond and dissolution of the zinc atom is higher.41 Also, in the presence of additives PEG + SGA, fine grained deposit was obtained, which increases the number of active atoms on the deposit surface, and this accelerates the formation of an oxide protective layer during the initial stages of dissolution. This oxide layer inhibits further oxidation of zinc from the coating surface, as reported by Zhang.42 The atoms in different crystallographic planes possess different binding energies because atoms in different crystallographic planes have different atomic coordination. Thus, relative rate of dissolution of atoms from different crystallographic planes is different. The deposit in which the preferential growth takes place in the direction normal to low index crystallographic planes requires higher energy for dissolution of atoms due to higher binding energy of atoms.43 In the present work, the SPEG+SGA coating possesses preferred orientation along (100) and (101) low index planes compared to the orientation of other coatings. This accounts for the superior anticorrosion property of the developed SPEG+SGA coating compared to that of the S0 coating.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00154. Hull cell studies, optimization (syringaldehyde, polyethylene glycol, zinc chloride, potassium chloride, boric acid, cell current, pH, and temperature), and optimized bath (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: 080-22961352; E-mail: [email protected]. ORCID

Pandurangappa Malingappa: 0000-0001-7492-3893 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors are grateful to the University Grants Commission, New Delhi, India for the award of Post Doctoral Fellowship to N.K.O. [Award F.15-1/2015-16/PDFWM-2015-17-KAR31527(SA-II)]. The authors express gratitude to Prof. T. V. Venkatesha, Department of Chemistry, Kuvempu University, Shimoga, India for his useful discussions during manuscript preparation. We acknowledge the Department of Chemistry, Bangalore University, Bengaluru, India for providing the laboratory facilities.

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CONCLUSIONS The PEG and SGA additive mixture in zinc chloride bath develops a good bright zinc deposit (70% reflectance) over a wide range of current density from 0.2 to 14 Adm−2. The deposit was smooth, compact, and fine grained (average grain J


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