**4.2. ZnNi-TiO2**

ZnNi-TiO2 coatings comprise ~20% of the papers on ZnNi-nanocomposite coatings. As demonstrated with the Al2 O3 composite coatings, improved corrosion and mechanical properties of the ZnNi coatings occur with the incorporation of TiO2 particles into the metal matrix. In a study by Praveen et al. they varied the TiO2 concentration in the bath from 0.5 to 5.0 g/L. Lower current densities were observed at 3 g/L and above this concentration the corrosion current increased so it was chosen as the optimal concentration [37].

The deposition with TiO2 gave coatings with preferential γ-phase alloy, though small amounts of a pure zinc phase are seen in ZnNi coatings without TiO2 incorporation. Textural modifications due to the presence of TiO2 nanoparticles are suggested due to slight changes in peak intensity in the XRD patterns as compared to ZnNi coatings without TiO<sup>2</sup> incorporation. The metallic grain size also decreases with the incorporation of TiO2 , due to changes to nucleation and growth due to disruption of the metallic growth by incorporation of semiconducting particles during coating formation [35]. TiO2 incorporation can also cause a considerable decrease in grain size for the metallic phase, with rough and irregular deposits as demonstrated by SEM and AFM (**Figure 3**) [34, 35]. The ZnNi coating without TiO2 exhibited multiple defects, cracks, gaps, crevices and microholes. The TiO2 nanoparticles fill these gaps, leading to an overall decrease in the corrosion rate. The crystal size of the composite coating also appears smaller as compared to the ZnNi coating [37]. The compact size is preferred as it also better protects from corrosion onset. The effects of soniciaton on morphology were also examined. Ultrasonic vibration during deposition was found to result in increased nanoaparticle incorporation and a more homogeneous coating, suggesting the vibration promotes uniform distribution of the particles and decreased agglomeration of the particles. Improvement of nanoparticle incorporation due to ultrasonicaiton was also noted [36].

TiO2 particles restrained the growth of the ZnNi alloy grains leading to a significantly higher microhardness in the presence of TiO2 [37]. As expected, with increasing nanoparticle incorporation, the hardness increases, which is believed to be due to the dispersion of the ceramic like TiO2 particles throughout the metal matrix [36]. As observed with Al2 O3 addition, sonication of the electrolytic bath lead to increased microparticle incorporation, with hardness

**Figure 2.** Impedance spectra of electrodeposited ZnNi and ZnNi-Al2

, recorded at 0, 24, 48 and 120 h immersion in 0.2 g/L Na2

ZnNi-15 g/L Al2

O3

from [17]. Copyright 2013, John Wiley and Sons."

O3

SO4

coatings, a) ZnNi, b) ZnNi-5 g/L Al2

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219 201

solution. "Reprinted with permission

O3 and c)

of Al2 O3

were studied in 0.2 g/L Na2

200 Nanocomposites - Recent Evolutions

observed with incorporation of Al2

O3

study by Praveen et al. they varied the TiO2

of the ZnNi coatings occur with the incorporation of TiO2

increased so it was chosen as the optimal concentration [37].

of a pure zinc phase are seen in ZnNi coatings without TiO2

metallic grain size also decreases with the incorporation of TiO2

SEM and AFM (**Figure 3**) [34, 35]. The ZnNi coating without TiO2

nanoparticle incorporation due to ultrasonicaiton was also noted [36].

intensity in the XRD patterns as compared to ZnNi coatings without TiO<sup>2</sup>

**4.2. ZnNi-TiO2**

onstrated with the Al2

The deposition with TiO2

tions due to the presence of TiO2

ticles during coating formation [35]. TiO2

microhardness in the presence of TiO2

cracks, gaps, crevices and microholes. The TiO2

ZnNi-TiO2

TiO2

like TiO2

SO4

and varying immersion (0, 24, 48 and 120 h) is presented in **Figure 2**. The coatings

Research). The frequency domain was 10 kHz to 100 mHz and temperature was maintained at 23 ± 2°C. The plots were fit with ZSimpWin 3.21 software. The impedance modulus of the nanocomposite is higher than pure ZnNi films. The charge transfer resistance for the composite coating is higher than ZnNi films, yet the double layer capacitance is smaller. Initially the measurement decreases at a systematic rate, suggesting a rapid degradation of the coating due to corrosion but after 50 h the rate of degradation decreases, likely due to the forma-

still follow an anomalous deposition route. Improved hardness and corrosion properties are

[12, 27, 29, 32, 46].

current densities were observed at 3 g/L and above this concentration the corrosion current

and growth due to disruption of the metallic growth by incorporation of semiconducting par-

in grain size for the metallic phase, with rough and irregular deposits as demonstrated by

overall decrease in the corrosion rate. The crystal size of the composite coating also appears smaller as compared to the ZnNi coating [37]. The compact size is preferred as it also better protects from corrosion onset. The effects of soniciaton on morphology were also examined. Ultrasonic vibration during deposition was found to result in increased nanoaparticle incorporation and a more homogeneous coating, suggesting the vibration promotes uniform distribution of the particles and decreased agglomeration of the particles. Improvement of

particles restrained the growth of the ZnNi alloy grains leading to a significantly higher

poration, the hardness increases, which is believed to be due to the dispersion of the ceramic

cation of the electrolytic bath lead to increased microparticle incorporation, with hardness

particles throughout the metal matrix [36]. As observed with Al2

coatings comprise ~20% of the papers on ZnNi-nanocomposite coatings. As dem-

composite coatings, improved corrosion and mechanical properties

gave coatings with preferential γ-phase alloy, though small amounts

nanoparticles are suggested due to slight changes in peak

incorporation can also cause a considerable decrease

[37]. As expected, with increasing nanoparticle incor-

nanoparticles fill these gaps, leading to an

tion of corrosion products on the surface of the coating [12]. Incorporation of Al2

results in γ-phase zinc-nickel alloys with nanoparticle incorporation. ZnNi-Al2

O3

(pH 5) using a potentiostat PARStat 2273 (Princeton Applied

O3

O3

particles into the metal matrix. In a

incorporation. Textural modifica-

, due to changes to nucleation

exhibited multiple defects,

O3

addition, soni-

incorporation. The

concentration in the bath from 0.5 to 5.0 g/L. Lower

particles

/SiC coatings

**Figure 2.** Impedance spectra of electrodeposited ZnNi and ZnNi-Al2 O3 coatings, a) ZnNi, b) ZnNi-5 g/L Al2 O3 and c) ZnNi-15 g/L Al2 O3 , recorded at 0, 24, 48 and 120 h immersion in 0.2 g/L Na2 SO4 solution. "Reprinted with permission from [17]. Copyright 2013, John Wiley and Sons."

corrosion density (0.4–0.6 mA/cm2

solution, the icorr of the ZnNi-TiO2

decrease in corrosion current density was observed as TiO2

on the corrosion potential, corrosion current and resistivity of the systems.

were spread over the surface of the nutrient agar, and the ZnNi-TiO2

with a decreasing trend following increased sonication of the particles prior to deposition [36]. Coatings throughout literature demonstrate a wide array of corrosion potentials, varying from E = −0.5 to −1.2 V, which follow values found for ZnNi coatings [12, 28, 30–32, 34–37, 39, 43, 44]. The value of the corrosion potential, which can show corrosion tendencies, is indicative of the components of the coatings. The optimal corrosion potential will lie between that of a pure zinc coating and a pure nickel coating, as it will have character of each metal and with that, corrosion behavior of each metal. The corrosion current, which is proportional to the corrosion rate, does decrease with the incorporation of nanoparticles as demonstrated in **Table 4**. The addition of nanoparticles, even in small amounts shows an overall improvement

antibacterial inhibition, specifically the antibacterial resistance toward Gram positive (*Staphylococcus aureus* PTCC1431) and Gram negative (*E. coli* PTCC1394) bacteria through an inhibition zone method (**Figure 4**). The bacterial strains were transferred into flasks containing nutrient broth and bacteria which had been cultured at 37°C under aerated conditions. An agar diffusion test was used to study antibacterial activity. Inoculums of *E. coli* and *S. aureus*

coatings with increase of TiO2

increased by a factor of 3. The ZnNi-TiO2

after 24 h immersion [35]. ZnNi-TiO2

Momeni et al. studied ZnNi-TiO2

**Figure 4.** Inhibition capability of ZnNi-TiO2

Na2 SO4 ) and low polarization resistance. At 24 h immersion in the

coating has decreased by a factor of 5 and the Rp had

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219

coating presented the highest corrosion protection

was incorporated into the coating,

203

sample was placed

coatings were examined in 3.5% NaCl solution and a

coatings on copper substrates as a possible coating for

in the electrolytic bath [14].

**Figure 3.** Morphology of ZnNi-TiO2 coatings (a) SEM and (b) AFM. "Reprinted with permission from [41]. Copyright 2012, Springer Nature."

increasing from 253 HV for ZnNi coatings, and 464 for ZnNi-TiO2 coatings without sonication, to 754 HV for ZnNi TiO2 with sonication during deposition, a 60% increase in hardness with sonication and a 200% increase in hardness as compared to coating with the addition of Al2 O3 particles incorporated under sonication. TiO2 particles hinder the dislocation of movement, leading to an increased hardness of the material though a reverse trend which was observed at higher concentrations of TiO2 in the deposition bath and believed to be due to agglomeration of the nanoparticles in solution [37].

The OCP was monitored over time for ZnNi and ZnNi-TiO2 coatings in 3.5% NaCl and nearneutral 0.05 M Na2 SO4 solutions, respectively [35]. The initial OCP values show that OCP of ZnNi and ZnNi-TiO2 coatings appear at more noble values due to the presence of nickel (a more noble metal as compared to zinc) in the coating. Initially, the OCP values were − 1.49, −1.51, −1.43 and − 1.23 V (vs. Hg/Hg2 SO4 ) for Zn, Zn-TiO2 , ZnNi and ZnNi-TiO2 , respectfully. After 24 h submersion, these values changed to −1.47, −1.49, −1.18 and −1.10 V for Zn, Zn-TiO2 , ZnNi and ZnNi-TiO2 , respectfully. The ZnNi coating undergoes the most drastic change in OCP in the 24 h time frame. The ZnNi-TiO2 appears to reach a steady state at a faster rate than ZnNi, possibly due to the smaller grain size of the particles due to nanoparticle incorporation [35]. There is a small positive shift in all coatings, due to dissolution of zinc on the surface of the coating, as zinc undergoes a sacrificial protection method.

The polarization of ZnNi and ZnNi-TiO2 coatings were found to have a larger corrosion current after 24 h of submersion in 0.05 M Na2 SO4 solution than the as deposited coatings but the ZnNi-TiO2 coating still maintained a smaller corrosion current value than the ZnNi coating even after immersion (**Table 4**). The microstructure of as deposited and submerged coatings was examined to determine any structural design which could affect the corrosion current of each coating. The incorporation of TiO2 nanoparticles decreased the grain size of the metallic phase and the coatings appear more rough and irregular in surface morphology [35]. The initial increase in corrosion current observed by ZnNi-TiO2 coatings prior to submersion are attributed to the smaller grain size and more porous structure observed in the coatings. The higher porosity of the coatings could be the cause of the increased corrosion resistance [34, 35]. Polarization curves and kinetic data show ZnNi-TiO2 and ZnNi deposits initially have a high corrosion density (0.4–0.6 mA/cm2 ) and low polarization resistance. At 24 h immersion in the Na2 SO4 solution, the icorr of the ZnNi-TiO2 coating has decreased by a factor of 5 and the Rp had increased by a factor of 3. The ZnNi-TiO2 coating presented the highest corrosion protection after 24 h immersion [35]. ZnNi-TiO2 coatings were examined in 3.5% NaCl solution and a decrease in corrosion current density was observed as TiO2 was incorporated into the coating, with a decreasing trend following increased sonication of the particles prior to deposition [36]. Coatings throughout literature demonstrate a wide array of corrosion potentials, varying from E = −0.5 to −1.2 V, which follow values found for ZnNi coatings [12, 28, 30–32, 34–37, 39, 43, 44]. The value of the corrosion potential, which can show corrosion tendencies, is indicative of the components of the coatings. The optimal corrosion potential will lie between that of a pure zinc coating and a pure nickel coating, as it will have character of each metal and with that, corrosion behavior of each metal. The corrosion current, which is proportional to the corrosion rate, does decrease with the incorporation of nanoparticles as demonstrated in **Table 4**. The addition of nanoparticles, even in small amounts shows an overall improvement on the corrosion potential, corrosion current and resistivity of the systems.

Momeni et al. studied ZnNi-TiO2 coatings on copper substrates as a possible coating for antibacterial inhibition, specifically the antibacterial resistance toward Gram positive (*Staphylococcus aureus* PTCC1431) and Gram negative (*E. coli* PTCC1394) bacteria through an inhibition zone method (**Figure 4**). The bacterial strains were transferred into flasks containing nutrient broth and bacteria which had been cultured at 37°C under aerated conditions. An agar diffusion test was used to study antibacterial activity. Inoculums of *E. coli* and *S. aureus* were spread over the surface of the nutrient agar, and the ZnNi-TiO2 sample was placed

increasing from 253 HV for ZnNi coatings, and 464 for ZnNi-TiO2

particles incorporated under sonication. TiO2

The OCP was monitored over time for ZnNi and ZnNi-TiO2

agglomeration of the nanoparticles in solution [37].

change in OCP in the 24 h time frame. The ZnNi-TiO2

initial increase in corrosion current observed by ZnNi-TiO2

Polarization curves and kinetic data show ZnNi-TiO2

observed at higher concentrations of TiO2

SO4

−1.51, −1.43 and − 1.23 V (vs. Hg/Hg2

, ZnNi and ZnNi-TiO2

The polarization of ZnNi and ZnNi-TiO2

each coating. The incorporation of TiO2

rent after 24 h of submersion in 0.05 M Na2

with sonication and a 200% increase in hardness as compared to coating with the addition of

ment, leading to an increased hardness of the material though a reverse trend which was

more noble metal as compared to zinc) in the coating. Initially, the OCP values were − 1.49,

fully. After 24 h submersion, these values changed to −1.47, −1.49, −1.18 and −1.10 V for Zn,

rate than ZnNi, possibly due to the smaller grain size of the particles due to nanoparticle incorporation [35]. There is a small positive shift in all coatings, due to dissolution of zinc on

SO4

even after immersion (**Table 4**). The microstructure of as deposited and submerged coatings was examined to determine any structural design which could affect the corrosion current of

phase and the coatings appear more rough and irregular in surface morphology [35]. The

attributed to the smaller grain size and more porous structure observed in the coatings. The higher porosity of the coatings could be the cause of the increased corrosion resistance [34, 35].

) for Zn, Zn-TiO2

coating still maintained a smaller corrosion current value than the ZnNi coating

SO4

the surface of the coating, as zinc undergoes a sacrificial protection method.

tion, to 754 HV for ZnNi TiO2

**Figure 3.** Morphology of ZnNi-TiO2

202 Nanocomposites - Recent Evolutions

2012, Springer Nature."

neutral 0.05 M Na2

Zn-TiO2

ZnNi-TiO2

ZnNi and ZnNi-TiO2

Al2 O3 coatings without sonica-

particles hinder the dislocation of move-

, ZnNi and ZnNi-TiO2

appears to reach a steady state at a faster

coatings in 3.5% NaCl and near-

, respect-

in the deposition bath and believed to be due to

with sonication during deposition, a 60% increase in hardness

coatings (a) SEM and (b) AFM. "Reprinted with permission from [41]. Copyright

solutions, respectively [35]. The initial OCP values show that OCP of

, respectfully. The ZnNi coating undergoes the most drastic

coatings were found to have a larger corrosion cur-

nanoparticles decreased the grain size of the metallic

solution than the as deposited coatings but the

coatings prior to submersion are

and ZnNi deposits initially have a high

coatings appear at more noble values due to the presence of nickel (a

**Figure 4.** Inhibition capability of ZnNi-TiO2 coatings with increase of TiO2 in the electrolytic bath [14].

onto this sample and incubated for 24 h at 37°C. The best coating was found to be ZnNi-TiO2 prepared with 3 g/L TiO2 in solution, which had an inhibition zone of 23 mm for *E. coli* and 28 mm for *S. aureus* [11].

In other nanoparticle coatings, we find similar trends such as finely grained, uniform, clearly

grains were common with even distribution of the nanoparticle in the coatings [42]. SiO2 coatings were examined for hardness changes, and showed an increase in hardness with an increase in current density during deposition of the coatings. Coatings were deposited from

to 210 HV. The improved hardness of the coating was attributed to the incorporation of SiO<sup>2</sup>

Nanoparticle incorporation was found to be improved through pulsed deposition methods [43, 44]. Exbrayat et al. examined ZnNi coatings with ceria incorporation and confirmed the

in other deposition systems. The intensity of the (600) reflection increases with the addition of ceria particles, which could be attributed to the preferential incorporation of ceria nanoparticles at the grain boundaries which affects the overall growth of the crystals [44].

nanoparticles were first added to the electrolytic bath without prior sonication (**Figure 5a**), and the nanoparticles agglomerated into long string-shape structures. Due to the agglomeration tendancies of the nanoparticles, sonication of the nanoparticles prior to depositon was examined. The coatings obtained from the sample post sonication (**Figure 5b**) take on a pyramidal growth pattern and appear more coarse. EDX was used to determine placement of

the electrode surface. The agglomerated nanoparticles appear uniformly trapped inside the metal matrix. Ultrasonic agitation was done at 20°C with an amplitude value of 35 (power of

sonicated prior to deposition, the agglomerated particles dispersed and were able to better fill the voids and pores naturally formed in the matrix, leading to better overall corrosion protection [43, 44]. Improvement of nanoparticle incorporation through the use of ultrasonication,

solutions, respectively and monitored over time [44]. Exbrayat et al. studied two differing

moving from an OCP value more cathodic than E = −0.95 V to E = −0.55 V after ~20 h of submersion. The OCP then begins to decrease steadily before stabilizing at ~E = −0.65 V after

was not sonicated prior to deposition, stayed relatively stable throughout the 4 day submersion test, decreasing in OCP from ~E = −0.82 V to ~E = −0.75 V. Zinc coatings often settle

OCP from ~E = −0.85 V initially to ~−0.57 V after 30 h of submersion, while ZnNi-CeO2

SO4

, output frequency of 20 kHz) for 20 min prior to deposition. As the samples were

and Al2

coatings was measured in 3.5% NaCl and near-neutral 0.05 M Na2

(sonicated) which was found to contain 85% Zn, 12.8% Ni and 2.2% CeO2

O3

which was determined to contain 84% Zn, 14% Ni and 2%

(sonicated). For ZnNi, a significant ennoblement was observed

(sonicated) follows a similar pattern to ZnNi, with a shift in

solution, the OCP values changed drastically for ZnNi,

coatings comprise ~8% of the literature, while ZnNi-SiO2

in increments of 1, and hardness values increased correspondingly from 155

and SiC [26, 38, 40, 41]. Finer

particles [39].

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219

particles appear to be primarily adsorbed onto

was also noted for CeO2

[32, 33,

SO4

which

.

Zn21 with preference to the (330) plane as previously observed

nanoparticles is shown in **Figure 5** for the SEM micrographs. Ceria

/CeO2

comprises ~4%.

205

pronounced crystal structures with the incorporation of SiO2

particles which add mechanical strength due to embedded SiO2

**/CeO<sup>2</sup>**

 **and SiO<sup>2</sup>**

the nanoparticles in the coating and the CeO2

previously noted for other systems including TiO2

samples, ZnNi-CeO2

presence of single phase γ Ni5

The incorporation of CeO2

2.0–5.0 A/dm2

**4.4. ZnNi-CeO2**

ZnNi-CeO2

41 W/cm2

36, 43, 44].

ZnNi-CeO2

ZnNi-CeO2

CeO2

The OCP of ZnNi-CeO2

and ZnNi-Ce2

When first submerged in the Na<sup>2</sup>

96 h of immersion. ZnNi-CeO2

and ZnNi-CeO2

#### **4.3. ZnNi-SiO<sup>2</sup> and ZnNi-SiC**

SiO2 and SiC comprise ~20% of the literature on ZnNi-nanocomposite coatings. SiO2 was examined by Tuaweri and Wilcox. They studied the change in current density as a function of % Ni in the deposit with and without bath agitation, and with varying SiO2 particle size. SiO2 is believed to deposit with the ZnNi coating under a codeposition mechanism. As expected, without SiO2 presence in the bath, nickel appears to follow an anomalous deposition mechanism as the current density of the system is increased from 3 to 6 A/dm2 . When 26 g/L of 20 nm SiO2 particles was added to the system, a transition from anomalous to normal deposition is noted at 4 A/dm2 [38]. The SiO2 colloids have been previously noted to increase deposition rate of Fe group metals [40]. SiO2 colloids shift this deposition from an anomalous mechanism to a normal mechanism. A possible explanation is due to adsorption of the Fe group metals onto the SiO2 nanoparticles in the electrolytic bath. During the deposition process, the pH of the electrolyte at the working electrode surface increases, or becomes more alkaline due to removal of hydrogen by the generation of hydrogen gas, also known as hydrogen evolution. The SiO2 particles tend to agglomerate once a neutral pH is reached, so the agglomerated colloid can suppress Zn(OH)2 formation causing a slowing in the diffusion of zinc ions from the solution, through the inner layer and to the cathode for reduction. As the SiO2 particle size was increased from 20 nm to 2 μm, a slightly higher nickel wt. % was observed in the coatings. Addition of the SiO2 nanoparticles resulted in increased Ni wt. % at all current densities, as compared to coatings without SiO2 . This suggests that the SiO2 in the bath encourages the deposition of nickel in the coating. Throughout the studies SiO2 appears to have an overall effect on the deposition mechanism of ZnNi coatings through emergence of a normal deposition route, while SiC continues to follow an anomalous deposition pattern. Further studies need to be completed in this area to determine if increased particle presence will encourage a transition from anomalous to normal deposition for other systems or if this is unique to the behavior of SiO2 nanoparticles in the ZnNi electrolytic system.

Tuaweri et al. found the corrosion potentials of ZnNi and ZnNi-SiO2 coatings were more anodic as compared to zinc. Under open circuit potential conditions, ZnNi and ZnNi-SiO2 coatings behave in a similar manner, but once the applied potential is increased, the ZnNi-SiO2 coatings shift toward more anodic potentials as compared to ZnNi coatings. This suggests the presence of SiO2 promoted shifting of the dissolution potential to more anodic values as compared to ZnNi due to the inert nature of SiO2 particles and possible changes in the deposition mechanism in the presence of SiO2 . SiO2 appears to have an overall effect on properties such as deposit texture, morphology, microstructure due to the ability of the SiO2 particles to provide barrier protection to the coating through packing of microholes, gaps and crevices in the coating [38]. The incorporation of SiC and SiO2 nanoparticles shows no changes on phase composition, with γ-phase being the predominant phase in the XRD patterns. Some Zn101, Zn102 and δ-phase XRD peaks were observed, but this was expected as these coatings were deposited under acidic conditions. Low intensity peaks corresponding to SiO2 confirms incorporation of the nanoparticles into the coatings without leading to any structural phase changes [39, 42]. In other nanoparticle coatings, we find similar trends such as finely grained, uniform, clearly pronounced crystal structures with the incorporation of SiO2 and SiC [26, 38, 40, 41]. Finer grains were common with even distribution of the nanoparticle in the coatings [42]. SiO2 coatings were examined for hardness changes, and showed an increase in hardness with an increase in current density during deposition of the coatings. Coatings were deposited from 2.0–5.0 A/dm2 in increments of 1, and hardness values increased correspondingly from 155 to 210 HV. The improved hardness of the coating was attributed to the incorporation of SiO<sup>2</sup> particles which add mechanical strength due to embedded SiO2 particles [39].

#### **4.4. ZnNi-CeO2 and SiO<sup>2</sup> /CeO<sup>2</sup>**

onto this sample and incubated for 24 h at 37°C. The best coating was found to be ZnNi-TiO2

and SiC comprise ~20% of the literature on ZnNi-nanocomposite coatings. SiO2

% Ni in the deposit with and without bath agitation, and with varying SiO2

nism as the current density of the system is increased from 3 to 6 A/dm2

examined by Tuaweri and Wilcox. They studied the change in current density as a function of

is believed to deposit with the ZnNi coating under a codeposition mechanism. As expected,

to a normal mechanism. A possible explanation is due to adsorption of the Fe group metals

the electrolyte at the working electrode surface increases, or becomes more alkaline due to removal of hydrogen by the generation of hydrogen gas, also known as hydrogen evolution.

was increased from 20 nm to 2 μm, a slightly higher nickel wt. % was observed in the coatings.

effect on the deposition mechanism of ZnNi coatings through emergence of a normal deposition route, while SiC continues to follow an anomalous deposition pattern. Further studies need to be completed in this area to determine if increased particle presence will encourage a transition from anomalous to normal deposition for other systems or if this is unique to the

anodic as compared to zinc. Under open circuit potential conditions, ZnNi and ZnNi-SiO2 coatings behave in a similar manner, but once the applied potential is increased, the ZnNi-SiO2 coatings shift toward more anodic potentials as compared to ZnNi coatings. This suggests the

barrier protection to the coating through packing of microholes, gaps and crevices in the coat-

sition, with γ-phase being the predominant phase in the XRD patterns. Some Zn101, Zn102 and δ-phase XRD peaks were observed, but this was expected as these coatings were deposited

of the nanoparticles into the coatings without leading to any structural phase changes [39, 42].

promoted shifting of the dissolution potential to more anodic values as com-

particles and possible changes in the deposition

appears to have an overall effect on properties such as

nanoparticles shows no changes on phase compo-

the solution, through the inner layer and to the cathode for reduction. As the SiO2

nanoparticles in the ZnNi electrolytic system.

Tuaweri et al. found the corrosion potentials of ZnNi and ZnNi-SiO2

. SiO2

deposit texture, morphology, microstructure due to the ability of the SiO2

under acidic conditions. Low intensity peaks corresponding to SiO2

deposition of nickel in the coating. Throughout the studies SiO2

particles was added to the system, a transition from anomalous to normal deposition is

presence in the bath, nickel appears to follow an anomalous deposition mecha-

nanoparticles in the electrolytic bath. During the deposition process, the pH of

particles tend to agglomerate once a neutral pH is reached, so the agglomerated

. This suggests that the SiO2

in solution, which had an inhibition zone of 23 mm for *E. coli* and

colloids have been previously noted to increase deposition

colloids shift this deposition from an anomalous mechanism

formation causing a slowing in the diffusion of zinc ions from

nanoparticles resulted in increased Ni wt. % at all current densities, as

was

particle size. SiO2

particle size

in the bath encourages the

appears to have an overall

coatings were more

particles to provide

confirms incorporation

. When 26 g/L of 20 nm

prepared with 3 g/L TiO2

 **and ZnNi-SiC**

[38]. The SiO2

28 mm for *S. aureus* [11].

204 Nanocomposites - Recent Evolutions

**4.3. ZnNi-SiO<sup>2</sup>**

without SiO2

onto the SiO2

The SiO2

noted at 4 A/dm2

rate of Fe group metals [40]. SiO2

colloid can suppress Zn(OH)2

compared to coatings without SiO2

pared to ZnNi due to the inert nature of SiO2

ing [38]. The incorporation of SiC and SiO2

mechanism in the presence of SiO2

Addition of the SiO2

behavior of SiO2

presence of SiO2

SiO2

SiO2

ZnNi-CeO2 coatings comprise ~8% of the literature, while ZnNi-SiO2 /CeO2 comprises ~4%. Nanoparticle incorporation was found to be improved through pulsed deposition methods [43, 44]. Exbrayat et al. examined ZnNi coatings with ceria incorporation and confirmed the presence of single phase γ Ni5 Zn21 with preference to the (330) plane as previously observed in other deposition systems. The intensity of the (600) reflection increases with the addition of ceria particles, which could be attributed to the preferential incorporation of ceria nanoparticles at the grain boundaries which affects the overall growth of the crystals [44]. The incorporation of CeO2 nanoparticles is shown in **Figure 5** for the SEM micrographs. Ceria nanoparticles were first added to the electrolytic bath without prior sonication (**Figure 5a**), and the nanoparticles agglomerated into long string-shape structures. Due to the agglomeration tendancies of the nanoparticles, sonication of the nanoparticles prior to depositon was examined. The coatings obtained from the sample post sonication (**Figure 5b**) take on a pyramidal growth pattern and appear more coarse. EDX was used to determine placement of the nanoparticles in the coating and the CeO2 particles appear to be primarily adsorbed onto the electrode surface. The agglomerated nanoparticles appear uniformly trapped inside the metal matrix. Ultrasonic agitation was done at 20°C with an amplitude value of 35 (power of 41 W/cm2 , output frequency of 20 kHz) for 20 min prior to deposition. As the samples were sonicated prior to deposition, the agglomerated particles dispersed and were able to better fill the voids and pores naturally formed in the matrix, leading to better overall corrosion protection [43, 44]. Improvement of nanoparticle incorporation through the use of ultrasonication, previously noted for other systems including TiO2 and Al2 O3 was also noted for CeO2 [32, 33, 36, 43, 44].

The OCP of ZnNi-CeO2 coatings was measured in 3.5% NaCl and near-neutral 0.05 M Na2 SO4 solutions, respectively and monitored over time [44]. Exbrayat et al. studied two differing ZnNi-CeO2 samples, ZnNi-CeO2 which was determined to contain 84% Zn, 14% Ni and 2% CeO2 and ZnNi-Ce2 (sonicated) which was found to contain 85% Zn, 12.8% Ni and 2.2% CeO2 . When first submerged in the Na<sup>2</sup> SO4 solution, the OCP values changed drastically for ZnNi, ZnNi-CeO2 and ZnNi-CeO2 (sonicated). For ZnNi, a significant ennoblement was observed moving from an OCP value more cathodic than E = −0.95 V to E = −0.55 V after ~20 h of submersion. The OCP then begins to decrease steadily before stabilizing at ~E = −0.65 V after 96 h of immersion. ZnNi-CeO2 (sonicated) follows a similar pattern to ZnNi, with a shift in OCP from ~E = −0.85 V initially to ~−0.57 V after 30 h of submersion, while ZnNi-CeO2 which was not sonicated prior to deposition, stayed relatively stable throughout the 4 day submersion test, decreasing in OCP from ~E = −0.82 V to ~E = −0.75 V. Zinc coatings often settle

**4.5. ZnNi-carbon nanotubes**

ZnNi-carbon nanotubes comprise ~8% of the literature to date. The dispersion, linear sweep voltammetry, surface morphology and friction properties of ZnNi coatings with nanotube incorporation was discussed. When carbon nanotubes were introduced into a ZnNi electrolytic solution, a positive shift (~0.1 V) in the polarization curves were observed and the deposition current of the system increased. The transport of the carbon nanotubes to the cathode surface and their incorporation into the coating is believed to be due to adsorption of Zn2+ and Ni2+ ions onto the nanotubes which are then reduced onto the coating, thereby entrapping the nanotubes in the coating. Initially the nanoparticles are weakly adsorbed onto the cathode, but once the particles lose their ionic and solvation shells, they become securely attached to the surface of the deposit. The adsorbed metal ions on the surface of the dispersed phase discharge at this point permanently attaching the nanotube to the coating [13, 45]. The actual deposition mechanism is not discussed in this work, so it is unclear if the nanotubes have an overall effect on the deposition mechanism or if anomalous deposition is still followed for this system.

In the case of carbon nanotubes, they are believed to act as nuclei for crystallization, further promoting even distribution of the nanotubes throughout the cathode surface. Microcracks are often observed in ZnNi coatings, but once carbon nanotubes have been added to the elec-

Another property examined for ZnNi-carbon nanotube nanocomposites was the sliding friction coefficient of the coatings as compared to ZnNi coatings. The ZnNi-carbon nanotube coatings were found to have a sliding friction coefficient 1.3–1.5 times smaller than ZnNi coatings without nanotube disbursement both in direct current and reverse current deposition modes. ZnNi coatings showed a decrease in friction coefficient values from 0.30 to 0.24

nanotube coatings decreased from 0.23 to 0.17 for the same current density values. Under a reverse current mode, ZnNi coatings had friction coefficients starting at 0.31 and decreasing to 0.23 as the ratio between cathodic and anodic periods was increased from 10:1 to 16:1 while for ZnNi-carbon nanotube coatings under the same conditions, the friction coefficients

The effect of montmorillonite (Mt) addition to the ZnNi bath was examined through anodic linear sweep voltammetry (ALSV) as presented in **Figure 6**. Montmorillonite is a smectite mineral and has a 2:1 layered structure, with two layers of silicon tetrahedral sandwiching one layer of aluminum octahedral. The layers can be stacked together, but when the van der Waals forces holding the individual clay layers together are overwhelmed, the individual layers become exfoliated (also known as delaminated). For this work, mechanical agitation and/or sonication was used to exfoliate the layered silicate and produce individual nanoplatelets. Individual montmorillonite nanoplatelets exist as coordinated layers, measuring 1–2 nm thick. Mt is a hydrous aluminum silicate with approximate formula (Na,Ca)

cations, causing the montmorillonite structure to have an excess of electrons. The negative

O. The Al3+ and Si4+ locations can be replaced by lower valent

) while the corresponding ZnNi-carbon

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219 207

trolytic mixture, the surface appears uniform and dense.

for current densities changing from 1.0 to 2.5 (A/dm2

decreased from 0.24 to 0.15 [13, 45].

**4.6. ZnNi-Mt**

(Al, Mg)6

(Si4 O10)3


.nH2

**Figure 5.** SEM micrographs of electrodeposited ZnNi-CeO2 nanoparticle coatings without prior sonication (a) and with prior sonication (b), and X-ray maps of the main elements in the coating. "Reprinted with permission from [50]. Copyright [2017], John Wiley and Sons."

over the initial submersion due to the formation of corrosion products, which then begin to protect the coating [43, 44]. EIS of ZnNi-CeO2 coatings were examined using a PGP 301 Autolab potentiostat after 24, 48, 96 h immersion in 35 g/L saline solution at 25°C with a frequency range of 64 kHz to 1 mHz, AC voltage amplitude of ±10 mV. Analysis was completed with Zview software. The Nyquist diagrams exhibit two capacitive loops at middle and low frequencies, with similar time constants. The loop diameter of the ZnNi coating remains relatively constant, suggesting stability in the corrosion rate. In the nanocomposite coatings, the loop diameter increased with immersion time. The incorporation of ceria enhances the corrosion resistance by ennoblement of the surface through reduction of galvanic corrosion of the steel [44].

#### **4.5. ZnNi-carbon nanotubes**

ZnNi-carbon nanotubes comprise ~8% of the literature to date. The dispersion, linear sweep voltammetry, surface morphology and friction properties of ZnNi coatings with nanotube incorporation was discussed. When carbon nanotubes were introduced into a ZnNi electrolytic solution, a positive shift (~0.1 V) in the polarization curves were observed and the deposition current of the system increased. The transport of the carbon nanotubes to the cathode surface and their incorporation into the coating is believed to be due to adsorption of Zn2+ and Ni2+ ions onto the nanotubes which are then reduced onto the coating, thereby entrapping the nanotubes in the coating. Initially the nanoparticles are weakly adsorbed onto the cathode, but once the particles lose their ionic and solvation shells, they become securely attached to the surface of the deposit. The adsorbed metal ions on the surface of the dispersed phase discharge at this point permanently attaching the nanotube to the coating [13, 45]. The actual deposition mechanism is not discussed in this work, so it is unclear if the nanotubes have an overall effect on the deposition mechanism or if anomalous deposition is still followed for this system.

In the case of carbon nanotubes, they are believed to act as nuclei for crystallization, further promoting even distribution of the nanotubes throughout the cathode surface. Microcracks are often observed in ZnNi coatings, but once carbon nanotubes have been added to the electrolytic mixture, the surface appears uniform and dense.

Another property examined for ZnNi-carbon nanotube nanocomposites was the sliding friction coefficient of the coatings as compared to ZnNi coatings. The ZnNi-carbon nanotube coatings were found to have a sliding friction coefficient 1.3–1.5 times smaller than ZnNi coatings without nanotube disbursement both in direct current and reverse current deposition modes. ZnNi coatings showed a decrease in friction coefficient values from 0.30 to 0.24 for current densities changing from 1.0 to 2.5 (A/dm2 ) while the corresponding ZnNi-carbon nanotube coatings decreased from 0.23 to 0.17 for the same current density values. Under a reverse current mode, ZnNi coatings had friction coefficients starting at 0.31 and decreasing to 0.23 as the ratio between cathodic and anodic periods was increased from 10:1 to 16:1 while for ZnNi-carbon nanotube coatings under the same conditions, the friction coefficients decreased from 0.24 to 0.15 [13, 45].

#### **4.6. ZnNi-Mt**

over the initial submersion due to the formation of corrosion products, which then begin

with prior sonication (b), and X-ray maps of the main elements in the coating. "Reprinted with permission from [50].

Autolab potentiostat after 24, 48, 96 h immersion in 35 g/L saline solution at 25°C with a frequency range of 64 kHz to 1 mHz, AC voltage amplitude of ±10 mV. Analysis was completed with Zview software. The Nyquist diagrams exhibit two capacitive loops at middle and low frequencies, with similar time constants. The loop diameter of the ZnNi coating remains relatively constant, suggesting stability in the corrosion rate. In the nanocomposite coatings, the loop diameter increased with immersion time. The incorporation of ceria enhances the corrosion resistance by ennoblement of the surface through reduction of galvanic corrosion

coatings were examined using a PGP 301

nanoparticle coatings without prior sonication (a) and

to protect the coating [43, 44]. EIS of ZnNi-CeO2

**Figure 5.** SEM micrographs of electrodeposited ZnNi-CeO2

Copyright [2017], John Wiley and Sons."

206 Nanocomposites - Recent Evolutions

of the steel [44].

The effect of montmorillonite (Mt) addition to the ZnNi bath was examined through anodic linear sweep voltammetry (ALSV) as presented in **Figure 6**. Montmorillonite is a smectite mineral and has a 2:1 layered structure, with two layers of silicon tetrahedral sandwiching one layer of aluminum octahedral. The layers can be stacked together, but when the van der Waals forces holding the individual clay layers together are overwhelmed, the individual layers become exfoliated (also known as delaminated). For this work, mechanical agitation and/or sonication was used to exfoliate the layered silicate and produce individual nanoplatelets. Individual montmorillonite nanoplatelets exist as coordinated layers, measuring 1–2 nm thick. Mt is a hydrous aluminum silicate with approximate formula (Na,Ca) (Al, Mg)6 (Si4 O10)3 -(OH)6 .nH2 O. The Al3+ and Si4+ locations can be replaced by lower valent cations, causing the montmorillonite structure to have an excess of electrons. The negative

ZnNi is known to undergo anomalous deposition, the dependence of the metal dissolution peaks relative to one another was expected [48, 50, 65, 68]. The added Mt appeared electrochemically inactive itself and has no overall effect on the anomalous deposition previously

Though the effect of pH was not discussed in most works, it was studied with the addition of Mt nanoparticles by monitoring the pH of the baths with and without Mt addition over a period of days to determine overall stability of the system. More acidic plating conditions lead to nonuniform coatings, specifically areas of low to no corrosion protection on the underlying substrate [49]. In **Figure 7**, line A represents the system with zinc, nickel and ammonium hydroxide (starting pH = 9.40), line B represents the system with zinc, nickel, ammonium hydroxide and borate (starting pH = 9.40) and line C represents the line with zinc, nickel, ammonium hydroxide, borate and Mt (starting pH = 9.40). The horizontal dotdash line represents pH 9.21, where the zinc equilibrium species exists (Zn2+ and HZnO2

[57]. Since this work is based at a pH range near this equilibrium, careful control of the pH is needed throughout all studies. The systems were closed to air for 7 days, then opened to atmosphere and monitored for an additional 32 h. Upon exposure to atmosphere, there was a definite decrease in pH as compared to closed systems for the previously stable baths. Line C (containing Mt in the system) decreased in pH at a slower rate than line B (not containing Mt) suggesting the Mt has an additional effect on the stabilization of metal species in solution. The system without borate or Mt addition passed through pH 9.21 (zinc equilibrium) even

as a closed system (Line A). After 7 days the pH of the system with Zn, Ni and NH4

**Figure 7.** pH studies of electrochemical bath solutions in atmosphere and in a closed system over time. (A) Zn2+, Ni2+, and

OH (solid); (C) Zn2+, Ni2+, Mt, borate and NH4

decreased from pH = 9.40 to 9.17, the system with Zn, Ni, borate and NH4

from pH = 9.40 to 9.37 and the system with Zn, Ni, borate, NH4

NH4

OH (dot); (B) Zn2+, Ni2+, 0.1 M borate and NH4

−)

209

OH had

OH had decreased

OH and Mt had decreased

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219

OH (dash) [54].

observed for ZnNi coating deposition [48].

**Figure 6.** Anodic linear sweep voltammetry (ALSV) data of 1:1 molar ratio equivalent of ZnSO4 . H2 O: Ni(NH4 ) 2 (SO4 )2 . 6H2 O all solutions prepared in 0.1 g/100 mL Mt in 0.1 M borate solution, pH = 9.40 with NH4 OH, sweep rate of 50 mV/S. (A) Zn2+; (B) Ni2+; (C) Zn2+ and Ni2+; (D) no Zn2+ or Ni2+ present; (1a, 1b) anodic stripping potentials of Zn2+; (2a, 2b) anodic stripping potentials of Zn2+ in presence of Ni2+; (3) anodic stripping potential Ni2+ in presence of Zn2+; (4) Ni2+ anodic stripping potential [54].

charge is compensated through loosely held cations from the associated water. Sodium montmorillonite, the clay mineral in which the loosely held cation is the Na+ ion, was the clay source used throughout the work. ALSV was used to obtain initial dissolution data of Zn2+ and Ni2+ ions in solution, as well as any electrochemical effect of the Mt nanoparticles on the metal dissolution peaks and the electrochemical behavior of Mt. The potential was scanned from OCP to E = −1.5 V (vs. SCE) at a sweep rate of 50 mV/s, held briefly and scanned back to OCP. During the anodic scan, the metals of interest were stripped back into the electrolytic solution. As previously observed for zinc-nickel systems under anomalous deposition control [57, 62, 69, 79], the anodic stripping peaks of the metals in solution are shifted based on other metal species in solution. According to the linear sweep voltammetry (LSV) data, zinc in the electrolytic solution had two anodic dissolution peaks present at potentials of E = −1.12 V and E = −1.08 V. During the cathodic scan, a small Zn(OH)2 layer deposits on the steel surface, slowing down dissolution kinetics. The dissolution of this species caused the second peak in the LSV [48, 57, 88]. Nickel had an anodic dissolution peak present at a potential of E = −0.48 V. When combined in solution, the zinc anodic dissolution peaks were shifted to potentials of E = −0.91 V, E = −0.83 V and the nickel anodic dissolution peak was shifted to a potential of E = −0.55 V. As previously stated, the zinc-nickel dissolution peaks of zinc and nickel are shifted in potential with respect to the individual metals in solution and this is indicative of an anomalous deposition system [51, 70, 76]. With the presence of Ni2+ in the system, Zn2+ is able to deposit at a more positive potential, and the nickel potential is shifted cathodically as previously observed in ZnNi systems [57, 62, 79, 82]. As ZnNi is known to undergo anomalous deposition, the dependence of the metal dissolution peaks relative to one another was expected [48, 50, 65, 68]. The added Mt appeared electrochemically inactive itself and has no overall effect on the anomalous deposition previously observed for ZnNi coating deposition [48].

Though the effect of pH was not discussed in most works, it was studied with the addition of Mt nanoparticles by monitoring the pH of the baths with and without Mt addition over a period of days to determine overall stability of the system. More acidic plating conditions lead to nonuniform coatings, specifically areas of low to no corrosion protection on the underlying substrate [49]. In **Figure 7**, line A represents the system with zinc, nickel and ammonium hydroxide (starting pH = 9.40), line B represents the system with zinc, nickel, ammonium hydroxide and borate (starting pH = 9.40) and line C represents the line with zinc, nickel, ammonium hydroxide, borate and Mt (starting pH = 9.40). The horizontal dotdash line represents pH 9.21, where the zinc equilibrium species exists (Zn2+ and HZnO2 −) [57]. Since this work is based at a pH range near this equilibrium, careful control of the pH is needed throughout all studies. The systems were closed to air for 7 days, then opened to atmosphere and monitored for an additional 32 h. Upon exposure to atmosphere, there was a definite decrease in pH as compared to closed systems for the previously stable baths. Line C (containing Mt in the system) decreased in pH at a slower rate than line B (not containing Mt) suggesting the Mt has an additional effect on the stabilization of metal species in solution. The system without borate or Mt addition passed through pH 9.21 (zinc equilibrium) even as a closed system (Line A). After 7 days the pH of the system with Zn, Ni and NH4 OH had decreased from pH = 9.40 to 9.17, the system with Zn, Ni, borate and NH4 OH had decreased from pH = 9.40 to 9.37 and the system with Zn, Ni, borate, NH4 OH and Mt had decreased

charge is compensated through loosely held cations from the associated water. Sodium

Zn2+; (B) Ni2+; (C) Zn2+ and Ni2+; (D) no Zn2+ or Ni2+ present; (1a, 1b) anodic stripping potentials of Zn2+; (2a, 2b) anodic stripping potentials of Zn2+ in presence of Ni2+; (3) anodic stripping potential Ni2+ in presence of Zn2+; (4) Ni2+ anodic

clay source used throughout the work. ALSV was used to obtain initial dissolution data of Zn2+ and Ni2+ ions in solution, as well as any electrochemical effect of the Mt nanoparticles on the metal dissolution peaks and the electrochemical behavior of Mt. The potential was scanned from OCP to E = −1.5 V (vs. SCE) at a sweep rate of 50 mV/s, held briefly and scanned back to OCP. During the anodic scan, the metals of interest were stripped back into the electrolytic solution. As previously observed for zinc-nickel systems under anomalous deposition control [57, 62, 69, 79], the anodic stripping peaks of the metals in solution are shifted based on other metal species in solution. According to the linear sweep voltammetry (LSV) data, zinc in the electrolytic solution had two anodic dissolution peaks present at potentials of E = −1.12 V and E = −1.08 V. During the cathodic scan, a small Zn(OH)2

deposits on the steel surface, slowing down dissolution kinetics. The dissolution of this species caused the second peak in the LSV [48, 57, 88]. Nickel had an anodic dissolution peak present at a potential of E = −0.48 V. When combined in solution, the zinc anodic dissolution peaks were shifted to potentials of E = −0.91 V, E = −0.83 V and the nickel anodic dissolution peak was shifted to a potential of E = −0.55 V. As previously stated, the zinc-nickel dissolution peaks of zinc and nickel are shifted in potential with respect to the individual metals in solution and this is indicative of an anomalous deposition system [51, 70, 76]. With the presence of Ni2+ in the system, Zn2+ is able to deposit at a more positive potential, and the nickel potential is shifted cathodically as previously observed in ZnNi systems [57, 62, 79, 82]. As

ion, was the

. H2

O: Ni(NH4 )2 (SO4 )2 . 6H2 O

OH, sweep rate of 50 mV/S. (A)

layer

montmorillonite, the clay mineral in which the loosely held cation is the Na+

**Figure 6.** Anodic linear sweep voltammetry (ALSV) data of 1:1 molar ratio equivalent of ZnSO4

all solutions prepared in 0.1 g/100 mL Mt in 0.1 M borate solution, pH = 9.40 with NH4

stripping potential [54].

208 Nanocomposites - Recent Evolutions

**Figure 7.** pH studies of electrochemical bath solutions in atmosphere and in a closed system over time. (A) Zn2+, Ni2+, and NH4 OH (dot); (B) Zn2+, Ni2+, 0.1 M borate and NH4 OH (solid); (C) Zn2+, Ni2+, Mt, borate and NH4 OH (dash) [54].

from pH = 9.40 to 9.38. Once opened to atmosphere, it took the borate system 4.5 h to reach a pH of 9.21 and a pH of 8.73 after 32 h, a total decrease of 0.64 pH units. The borate/Mt system reached pH 9.21 after 10 h of exposure to atmosphere and a pH of 9.05 after 32 h, a decrease of 0.33 pH units. The nonstabilized system (no borate or Mt) reached pH 7.94 after 32 h of exposure to atmosphere, a total decrease of 1.46 pH units. The decrease in pH in the closed system is due to formation of metal hydroxide species forming and precipitating out of solution. The large decrease in pH upon exposure to the atmosphere is due to absorption of carbon dioxide from the air [48, 57]. The system with borate demonstrates a clear stabilization of the system when in a closed system, and absorbs CO2 at a slower rate as compared to the system without borate. The addition of exfoliated Mt nanoparticles further stabilized the system, as shown in the relatively slow pH decrease in this system when closed to air and when opened to atmosphere. The nanoparticles stabilize the pH of the bath improving the deposition of the nanocomposite coating [48].

In the case of ZnNi-Mt nanocomposites, **Figure 8**, XRD pattern had a strong (330) reflection present at 2θ = 42.9°, indicative of ZnNi γ-phase alloy formation with a (330) preferred orientation as previously observed in coatings without nanoparticle incorporation [57, 62, 79]. The coating of pattern B was formed under the same conditions as pattern A, but Mt nanoparticles were dispersed into the electrolyte solution and incorporated into the resulting coating. Since Mt nanoparticles do not give diffraction peaks upon exfoliation, no additional peaks were observed due to its presence [2, 8, 52, 58, 66, 89]. The coating with Mt

incorporation was thin as compared to the alloy coating without Mt but the γ-phase alloy was still formed even in the presence of Mt, confirming the Mt did not affect the deposition of the alloy phase of interest. The incorporation of Mt into the coating is shown in **Figure 9**. Films with Mt incorporation have strong adherence, small grain size and overall good coverage of the stainless steel substrate. Small spherical particles covered the surface and no voids appeared present in the coating. The structure was not affected by the incorporation of Mt under these conditions. A strong overall coverage of the substrate material was observed, and particles of exfoliated clay were observed, confirming clay presence within the coating.

**Figure 9.** SEM micrographs of (A) ZnNi γ-phase alloy; (B) ZnNi Mt γ-phase nanocomposite alloy [54].

The potentiodynamic polarization curves of the electrodeposited zinc-nickel and zinc-nickel-Mt nanocomposite alloys in 3.5% NaCl solution are illustrated in **Figure 10** and the corrosion

In previous studies ZnNi coatings with optimal corrosion resistance was found to have a corrosion potential (Ecorr) more anodic as compared to pure zinc but more cathodic as compared to pure nickel. The optimal coatings had a corrosion potential around E = −0.74 V in that study [57]. In this study, the ZnNi-Mt nanocomposite coating had a corrosion potential of E = −0.73 V, which is in agreement with previous findings (**Table 4**). This value is slightly more cathodic (10 mV) than the coatings without Mt incorporation. The high zinc content (~90%) of the coating (confirmed with AAS and ICP-MS) but more cathodic corrosion potential are in the optimal range for improved protection. Corrosion current density is the primary parameter used for evaluating the kinetics of the corrosion reaction. The lower corrosion current density, the better corrosion protection. The corrosion current density for

A/cm2

of the nanocomposite alloy was lower as compared to the alloy without Mt denoting an

improved corrosion resistance. Rp of the coating with Mt was 34,900 Ω cm2

A/cm2

for the coating without Mt further confirming the results of improved protec-

and the corrosion current density for the

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219 211

. The corrosion current density

as compared to

current and potential are given in **Table 4**.

the zinc-nickel γ-phase alloy was 1.06 × 10−<sup>5</sup>

tion with incorporation of Mt [48].

30,485 Ω cm2

ZnNi-Mt γ-phase nanocomposite alloy was 3.72 × 10−<sup>6</sup>

**Figure 8.** X-ray diffraction patterns of (A) ZnNi and (B) ZnNi Mt scanned from 20 to 70 2Ɵ at a step size of 0.05° and a dwell time of 1 second [54].

**Figure 9.** SEM micrographs of (A) ZnNi γ-phase alloy; (B) ZnNi Mt γ-phase nanocomposite alloy [54].

from pH = 9.40 to 9.38. Once opened to atmosphere, it took the borate system 4.5 h to reach a pH of 9.21 and a pH of 8.73 after 32 h, a total decrease of 0.64 pH units. The borate/Mt system reached pH 9.21 after 10 h of exposure to atmosphere and a pH of 9.05 after 32 h, a decrease of 0.33 pH units. The nonstabilized system (no borate or Mt) reached pH 7.94 after 32 h of exposure to atmosphere, a total decrease of 1.46 pH units. The decrease in pH in the closed system is due to formation of metal hydroxide species forming and precipitating out of solution. The large decrease in pH upon exposure to the atmosphere is due to absorption of carbon dioxide from the air [48, 57]. The system with borate demonstrates a clear stabilization of the

without borate. The addition of exfoliated Mt nanoparticles further stabilized the system, as shown in the relatively slow pH decrease in this system when closed to air and when opened to atmosphere. The nanoparticles stabilize the pH of the bath improving the deposition of the

In the case of ZnNi-Mt nanocomposites, **Figure 8**, XRD pattern had a strong (330) reflection present at 2θ = 42.9°, indicative of ZnNi γ-phase alloy formation with a (330) preferred orientation as previously observed in coatings without nanoparticle incorporation [57, 62, 79]. The coating of pattern B was formed under the same conditions as pattern A, but Mt nanoparticles were dispersed into the electrolyte solution and incorporated into the resulting coating. Since Mt nanoparticles do not give diffraction peaks upon exfoliation, no additional peaks were observed due to its presence [2, 8, 52, 58, 66, 89]. The coating with Mt

**Figure 8.** X-ray diffraction patterns of (A) ZnNi and (B) ZnNi Mt scanned from 20 to 70 2Ɵ at a step size of 0.05° and a

at a slower rate as compared to the system

system when in a closed system, and absorbs CO2

nanocomposite coating [48].

210 Nanocomposites - Recent Evolutions

dwell time of 1 second [54].

incorporation was thin as compared to the alloy coating without Mt but the γ-phase alloy was still formed even in the presence of Mt, confirming the Mt did not affect the deposition of the alloy phase of interest. The incorporation of Mt into the coating is shown in **Figure 9**. Films with Mt incorporation have strong adherence, small grain size and overall good coverage of the stainless steel substrate. Small spherical particles covered the surface and no voids appeared present in the coating. The structure was not affected by the incorporation of Mt under these conditions. A strong overall coverage of the substrate material was observed, and particles of exfoliated clay were observed, confirming clay presence within the coating.

The potentiodynamic polarization curves of the electrodeposited zinc-nickel and zinc-nickel-Mt nanocomposite alloys in 3.5% NaCl solution are illustrated in **Figure 10** and the corrosion current and potential are given in **Table 4**.

In previous studies ZnNi coatings with optimal corrosion resistance was found to have a corrosion potential (Ecorr) more anodic as compared to pure zinc but more cathodic as compared to pure nickel. The optimal coatings had a corrosion potential around E = −0.74 V in that study [57]. In this study, the ZnNi-Mt nanocomposite coating had a corrosion potential of E = −0.73 V, which is in agreement with previous findings (**Table 4**). This value is slightly more cathodic (10 mV) than the coatings without Mt incorporation. The high zinc content (~90%) of the coating (confirmed with AAS and ICP-MS) but more cathodic corrosion potential are in the optimal range for improved protection. Corrosion current density is the primary parameter used for evaluating the kinetics of the corrosion reaction. The lower corrosion current density, the better corrosion protection. The corrosion current density for the zinc-nickel γ-phase alloy was 1.06 × 10−<sup>5</sup> A/cm2 and the corrosion current density for the ZnNi-Mt γ-phase nanocomposite alloy was 3.72 × 10−<sup>6</sup> A/cm2 . The corrosion current density of the nanocomposite alloy was lower as compared to the alloy without Mt denoting an improved corrosion resistance. Rp of the coating with Mt was 34,900 Ω cm2 as compared to 30,485 Ω cm2 for the coating without Mt further confirming the results of improved protection with incorporation of Mt [48].

particles on the other hand, would combine loosely with the metal matrix, or fall off the coating entirely, leading to gaps and pores in the coating which were easily attacked by corrosion cells. All studies treated the nanoparticles prior to deposition to cause disruption and dispersion of the particles in the plating bath through stirring or sonication, but no studies have been found discussing the overall effect on the particles and any benefits or drawbacks of one method compared to another. It was noted that sonication during deposition leads to better incorporation and higher concentrations of nanoparticles in the final coatings. In addition to sonication, pulse current and pulse potential deposition tends to lead to better incorporation of the nanoparticles in the resulting coating. Up to 11% nanoparticle incorporation was noted

XRD results confirm formation of γ-phase ZnNi-nanocomposite coatings throughout the studies. Interestingly, in acidic conditions without nanoparticle incorporation, acidic electrolytic baths tended to give impure ZnNi coatings, with a mixture of γ and δ-phase ZnNi coatings. The studies with ZnNi nanoparticles show almost exclusive γ-phase ZnNi alloys, which previously was primarily observed under alkaline conditions. The crystallite size decreased with the increase of nanoparticle incorporation in the coatings. Mt nanoparticles have been successfully incorporated into the alloy coatings with no disruption in the crystal structure of the zincnickel γ-phase alloys, deposited with a preferred (330) orientation. The incorporation of TiO2

poration of the nanoparticles, with small, compact like structures and few cracks or holes. The hardness of the coatings increased as nanoparticle concentration in the coating was increased. Corrosion studies all show nanoparticle incorporation into ZnNi coatings leads to lower corrosion currents, suggesting a lower corrosion rate for the coatings. The nanoparticles are believed to fill the crevices, gaps and holes within and on the surface of the coatings, leading to improved corrosion resistance. Overall the corrosion protection offered by the ZnNi-

[1] Musiani M. Electrodeposition of composites: An expanding subject in electrochemical materials science. Electrochimica Acta. 2000;**45**:3397-3402. DOI: 10.1016/S0013-4686(00)

nanocomposite coatings was improved as compared to pure ZnNi coatings.

\* and Teresa D. Golden2

1 Texas Christian University, Fort Worth, Texas, United States

2 University of North Texas, Denton, Texas, United States

\*Address all correspondence to: h.conrad@tcu.edu

, Al2 O3 , SiO2

was also confirmed with XRD. The morphology of the coatings shows incor-

, SiC, CeO2

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219

> , Al2 O3 /SiC,

> > ,

213

throughout the studies, with confirmation of ZnNi-TiO<sup>2</sup>

, Mt and carbon nanotubes.

CeO2

Al2 O3

/SiO2

and CeO2

**Author details**

Heidi Conrad1

**References**

00438-2

**Figure 10.** Potentiodynamic polarization curves of ZnNi and ZnNi-Mt nanocomposite coatings, electrolyte NaCl, 3.5 wt %, at scan rate 0.1667 mV/s [54].

#### **5. Conclusions**

ZnNi nanocomposites can be formed by incorporating nanoparticles into the coating during an electrochemical deposition. The nanoparticles under study do not appear to affect the electrochemical behavior or electrochemical deposition mechanism of zinc-nickel γ-phase alloy formation. Anomalous deposition of the zinc-nickel alloy was observed which is consistent with formation of the γ-phase alloy, but small anodic shifts were observed in the ALSV scans of the metal species in the ZnNi-Mt bath as compared to baths without Mt nanoparticles present. Al2 O3 also noted no overall effect on the electrochemical behavior of the system. The addition of nanoparticles, including Mt, SiC and Al2 O3 also affected the onset of hydrogen evolution, pushing the onset to more cathodic potentials, which can be an advantage in an aqueous plating system as it broadens the working window for the deposition. Zinc-nickel γ-phase deposition requires a high overpotential to overcome the kinetic limitations of nickel deposition, this added benefit of shifting the reduction of the metals anodically with the onset of hydrogen appearing more cathodically, leads to alloy formation with less entrapped hydrogen.

Particle dispersion in the electrolytic bath is an important factor when considering deposition. Optimal corrosion protection is acquired from systems with better dispersion of nanoparticles in the system. When nano-Al2 O3 particles were dispersed uniformly throughout the coating, and incorporated in the matrix, they were able to protect the coating from corrosive medium, increasing the corrosion potential and retarding corrosion onset. Agglomerated nano-Al2 O3

particles on the other hand, would combine loosely with the metal matrix, or fall off the coating entirely, leading to gaps and pores in the coating which were easily attacked by corrosion cells. All studies treated the nanoparticles prior to deposition to cause disruption and dispersion of the particles in the plating bath through stirring or sonication, but no studies have been found discussing the overall effect on the particles and any benefits or drawbacks of one method compared to another. It was noted that sonication during deposition leads to better incorporation and higher concentrations of nanoparticles in the final coatings. In addition to sonication, pulse current and pulse potential deposition tends to lead to better incorporation of the nanoparticles in the resulting coating. Up to 11% nanoparticle incorporation was noted throughout the studies, with confirmation of ZnNi-TiO<sup>2</sup> , Al2 O3 , SiO2 , SiC, CeO2 , Al2 O3 /SiC, CeO2 /SiO2 , Mt and carbon nanotubes.

XRD results confirm formation of γ-phase ZnNi-nanocomposite coatings throughout the studies. Interestingly, in acidic conditions without nanoparticle incorporation, acidic electrolytic baths tended to give impure ZnNi coatings, with a mixture of γ and δ-phase ZnNi coatings. The studies with ZnNi nanoparticles show almost exclusive γ-phase ZnNi alloys, which previously was primarily observed under alkaline conditions. The crystallite size decreased with the increase of nanoparticle incorporation in the coatings. Mt nanoparticles have been successfully incorporated into the alloy coatings with no disruption in the crystal structure of the zincnickel γ-phase alloys, deposited with a preferred (330) orientation. The incorporation of TiO2 , Al2 O3 and CeO2 was also confirmed with XRD. The morphology of the coatings shows incorporation of the nanoparticles, with small, compact like structures and few cracks or holes. The hardness of the coatings increased as nanoparticle concentration in the coating was increased.

Corrosion studies all show nanoparticle incorporation into ZnNi coatings leads to lower corrosion currents, suggesting a lower corrosion rate for the coatings. The nanoparticles are believed to fill the crevices, gaps and holes within and on the surface of the coatings, leading to improved corrosion resistance. Overall the corrosion protection offered by the ZnNinanocomposite coatings was improved as compared to pure ZnNi coatings.
