**2. Electrodeposition of zinc-nickel nanocomposite coatings**

#### **2.1. Dispersion of particles**

zinc alloys have become an attractive option. An alloy modifies the composition of a material resulting in different corrosion properties then the original element which can significantly improve the stability of the protective coating [2–5, 52, 57, 58], therefore, by picking the correct combination of alloys, one can greatly increase the corrosion resistance of the material [49, 52]. Alloy formation can result in various phases, dependent upon the experimental conditions at the time of formation. For zinc-nickel, there are 5 known alloy phases: α- and β- (30% Ni, nickel

**Plating bath Incorporated particle and** 

**deposition parameters**

α-SiC powder, ~7.0 μm

, ~80 nm

pulse, Ja = 1.0 A/dm2

. ms, 25°C

Carbon nanotubes

Reversing mode, *i*

α-SiC ~9.5 μm, α-Al2

.

modified SiO<sup>2</sup>

Montmorillonite (Mt)

E1 = −1.45 V, T1 = 10 sec. E2 = −0.9 V, T2 = 2 sec, Room

Deposition current and temp—not

<sup>a</sup> = 1.5 A/dm2

Not specified 25°C

Not specified

specified

1, 5 g/L

Temperature

Cathodic pulse, ip = 5.0 A/dm2 with ton = 4 ms, toff = 16 ms. Anodic

Average current density ~0.67 A/

with ton = 4.

<sup>c</sup> = 6 A/dm2 ,

, 400–500 nm

O3 ~3.4 μm

20–120 g/L 25°C

CeO2

5 g/L

dm2

*i*

0.05 g/L

**Dispersion method**

Stirring 24 h prior to deposition, substrate rotated during deposition.

Stirred 24 h prior to deposition, continued stirring during deposition, 200 rpm.

Not specified

Not specified

Sonicated 1 h prior to deposition, N2

bubbled through solution during deposition.

gas

Stirred 12 h prior to deposition. Substrate rotated during deposition.

and experimental parameters used to form the alloy [50, 55, 59–61]. The γ-phase and δ-phase are predominantly formed through electrochemical methods, with γ-phase showing the strongest protection against corrosion [57, 60, 62–65]. Zinc nickel γ-phase alloys with approximately 8–18% have been found to be optimal for maximum corrosion protection [48, 57, 62, 65].

Zn22) and η- (1% Ni) (zinc rich), all dependent upon the Zn/Ni ratio

rich), γ- (Ni5

**Reference and application**

190 Nanocomposites - Recent Evolutions

Creus et al. [49, 50]

Tseluikin et al. [18,

Xiang et al. [53] Corrosion

Conrad et al. [54] Corrosion

**Table 1.** Survey of literature.

Corrosion

51]

Müller et al. [48] 0.16 M ZnO, 1.7 × 10−<sup>2</sup> NiSO4 . 6H2

63 g/L ZnCl2

220 g/L NH4

NaCH3

NiSO4 . 6H2

0.2 M ZnSO4

0.1 M Na2 B4 O7 . 10H2 O

pH = 9.5

H4 )2 (SO4 )2 . 6H2 O

. H2

Tulio et al. [52] 0.25 M ZnSO4

BO3

10 g/L ZnO, 50 g/L NiCl2

. 7H2

O, 0.4 M H3

Not specified CeO2

0.1 M sodium citrate, pH = 4.9.

COO

NiSO4 . 6H2

20 g/L H3

O, 3.75 M NaOH, 3.4 × 10−<sup>2</sup> M diethylenetriamine, pH = alkaline

, 100 g/L

O, 215 g/L KCl,

, pH = 5.3.

Cl, 20 g/L

. 6H2 O,

O, 0.2 M

O, 0.1 M Ni(N

BO3 ,

Zn21), δ- (Ni3

A variety of particles, including Al2 O3 , TiO2 , SiO2 , SiC, ceria, carbon nanotubes and montmorillonite (Mt) have been successfully incorporated into zinc-nickel coatings. For optimal effect, the nanoparticles need to be dispersed throughout the metallic coating. To accomplish this, the particles first need to be suspended in the electrolytic solution and agglomeration of the particles needs to be kept to a minimum to prevent issues in coating formation. Particle agglomeration is an issue seemingly independent of particle concentration as it occurs under low to high concentrations, though smaller particle size does increase tendency to form agglomerations, leading to less incorporation in the final coating. To prevent agglomeration, various methods can be used such as organic additives, agitation of the solution, current density, etc. Treatment of the nanoparticles prior to deposition is varied throughout the field but the most common methods used for particle suspension are magnetic stirring, sonication or a combination of stirring and sonication prior to and during deposition [11–13, 26–48]. In addition to treatment of the nanoparticles, concentration in the bath also affects the quality of the coatings. As expected, as the concentration of nanoparticles in the bath increases, the concentration of nanoparticles in the resulting coating tends to increase. The small sized particles are easily incorporated into irregularities on the metal surface and positively charged particles are attracted to the cathode, so more easily incorporated into the coating [13]. In the case of oxide nanoparticles, the oxides compete with the metallic ions for adsorption onto the active sites, creating more nucleation sites and perturbing metallic grain growth. Other particles are trapped during deposition, filing holes or gaps within the naturally forming coating [22, 40].

Concentration of nanoparticles in the bath varies from 0.05–300 g/L with most work using around 5–15 g/L. Müller et al., who relied on mechanical stirring to disperse the nanoparticles, found optimal concentration of SiC particles to be 60 g/L, beyond which the particles began to agglomerate. Beyond this concentration, stirring was not sufficient to keep the particles suspended in solution and a decreasing trend of SiC in the coatings was observed [42].

Katamipour et al. studied the effects of ultrasonic conditions to promote uniform dispersion of the coating particles, and to determine if improvement occurred in the corrosion and mechanical properties of the coatings. They found that increasing the ultrasonic power density lead to a decrease in particle size, an increase in nanoparticle incorporation in the coating, and initially, an improvement in corrosion and mechanical properties. The agglomeration often observed with nanoparticles also dissipated with the use of sonication [36]. Nano-alumina particles were found to be uniformly imbedded in the ZnNi-Al2 O3 coating after treatment of ultrasonic vibration [32, 33]. Without sonication, ceria nanoparticles were seen organized in long string-shape agglomerates. These agglomerates became trapped inside voids and pores during coating growth [44]. Though sonication or mechanical disruption of the nanoparticles is needed to distribute them throughout the metal matrix, care must be taken as excessive agitation can lead to a lower quality of particles in the deposit [7].

applied potential is more similar to the free corrosion potential of zinc and zinc-nickel alloys. These systems also present with lower current efficiencies [57, 61, 73–77]. Normal deposition leads to alternate ZnNi phases, which are not preferred for maximized corrosion protection, so the goal is to remain under an anomalous deposition route, to further aid in the deposition of γ-phase ZnNi alloy. Within the research presented on the deposition mechanism with nanopar-

Hydrogen evolution at cathodic potentials is a concern in electrochemical deposition as it can lead to the formation of cracks and defects in the overall coating structure, both during deposition and later during use of the material. Hydrogen evolution competes with metal electrodeposition in this system and can play a major role in determining the composition of ZnNi coatings [35, 46]. In ZnNi deposition systems under alkaline conditions, boric acid was found to suppress hydrogen evolution. Hydrogen evolution is a larger concern for nickel deposition than zinc deposition as a larger overpotential is required for nickel deposition since the deposition is under kinetic control while zinc deposition is thermodynamically controlled [57]. Our previous study examined the change in hydrogen evolution onset with varying borate concentrations in alkaline solutions, and found as the borate concentration is increased, hydrogen evolution is pushed to more cathodic values [48]. A maximum borate concentration of 100 mM was used due to conductivity of borate in the system [57]. In addition to borate, nanoparticles can have an overall effect on hydrogen evolution in the system as well. The hydrogen evolution onset was compared for solutions with and without the presence of Mt in **Figure 1**. For nickel, a large cathodic shift was observed when Mt was added to the system. For zinc and zinc-nickel, small cathodic shifts were observed with Mt. The Mt can help further shift the onset of hydrogen evolution within this system, in addition to borate [48, 57]. Alloy formation typically occurs at or near the onset of hydrogen evolution for this system. By shifting the onset in a cathodic direction, less hydrogen will be produced during alloy formation, leading to less entrapped hydrogen in the overall coating. Hydrogen evolution can hinder adsorption

, Mt and carbon nanotubes on the deposition mecha-

Electrodeposited Zinc-Nickel Nanocomposite Coatings http://dx.doi.org/10.5772/intechopen.80219 193

, SiC, Al2

nism has been explored and is discussed under their individual sections.

O3

of nanoparticles on the surface of the coating material and lead to embrittlement [7].

dency to adsorb onto the SiC particles, leading to a reduction in hydrogen evolution [46].

onto the electrode surface, reducing the active surface area. At lower pH, H+

evolution to occur. This effect is found to be dependent on the concentration of SiC in solution, but

Electrodeposition techniques include potentiostatic and galvanostatic deposition, and further into both methods, applied vs. pulsed deposition parameters in the literature for zinc-nickel nanocomposite coatings [44]. The particles co-deposit with the zinc-nickel coating which has advantages over other methods such as better control of coating thickness, deposition speed, working under controlled temperatures, and it is a single-step method. The nanoparticles are incorporated as the metal species are reduced onto the electrode surface, forming the nanoparticle coating. Applied methods include direct current or direct potential, where a constant current or potential is applied to the electrode. Pulsed methods include pulse current

no dependency is observed. It is believed that SiC and Al2

and SiC was also found to cause a surface blockage preventing hydrogen

O3

are adsorbed

has a higher ten-

ticle presence, the effect of SiO<sup>2</sup>

The addition of Al2

for addition of Al2

**2.3. Deposition methods**

O3

O3

Na-smectites, a type of clay mineral, specifically montmorillonite (Mt) were also examined, for incorporation into metal matrixes. Within aqueous solutions, Na-montmorillonite can be completely exfoliated and incorporated into other materials, forming continuous, crack free films, which is beneficial in corrosion resistant coatings [5, 8–9, 52, 66]. Exfoliation causes the short range order of the clay particles to be disrupted, causing individual clay platelets to exist, unassociated from one another. The resulting clay platelets range 1–2 nm in width with 100–1000 nm in length [66]. These platelets are easily incorporated into the coating during deposition, increasing the overall thermal stability and mechanical strength of the coating, which leads to increased corrosion resistance [2, 8]. As the alloy coating is forming, the exfoliated clay in solution is freely dispersed throughout the electrolytic bath. Mt is a cationic clay with a negatively charged surface which attracts metal ions, increasing incorporation of the platelets into the metal composite during deposition. The clay platelets settle onto the substrate surface as the coating is being formed, allowing them to be incorporated into the coating. Exfoliated Mt, which has a plate-like structure, increases the surface area of the material when imbedded in the coating and leads to a more tortuous mean free path of the corrosion cells upon onset [5]. This technique has previously been successful with the incorporation of montmorillonite platelets into pure nickel, nickel-molybdenum and nickel-copper coatings [2, 5, 8, 9, 52, 58, 66]. However, many traditional particles used in composite coatings are spherical in shape. For example SiO2 nanoparticles coated with a layer of cerium oxide have been introduced into ZnNi coatings to improve corrosion resistant properties [47].

#### **2.2. Influence of nanoparticle addition on deposition mechanism**

Though many researchers use electrochemical deposition as a tool to form a coating of interest, there is little published work on the electrochemical system used for the deposition of zincnickel nanocomposite coatings. A better understanding can lead to an improved deposition system, and an overall superior coating. Work continues to be done in acidic and alkaline conditions with a goal of further improving the materials, longer material lifetimes and a better understanding of the mechanisms involved in various alloy formations [49, 50, 55, 57, 59, 60, 65, 67–71] but little work has been done to examine systems with nanoparticle incorporation.

Zinc-nickel alloy formation follows an anomalous deposition mechanism which occurs when the electrochemically less noble metal deposits preferentially to the more noble metal. This is verified through examination of the voltammetry patterns of the zinc-nickel system as the individual zinc and nickel reduction peaks are shifted based on the presence of the other metal species in solution [48, 59, 62, 70, 72–74]. During deposition, a thin layer of nickel is initially deposited onto the substrate. As the deposition continues, zinc is intercalated into the nickel, leading to formation of the alloy [57, 61, 67]. In acidic systems under low current density, a transition from anomalous to normal codeposition has been noted. Normal codeposition is dominant when the applied potential is more similar to the free corrosion potential of zinc and zinc-nickel alloys. These systems also present with lower current efficiencies [57, 61, 73–77]. Normal deposition leads to alternate ZnNi phases, which are not preferred for maximized corrosion protection, so the goal is to remain under an anomalous deposition route, to further aid in the deposition of γ-phase ZnNi alloy. Within the research presented on the deposition mechanism with nanoparticle presence, the effect of SiO<sup>2</sup> , SiC, Al2 O3 , Mt and carbon nanotubes on the deposition mechanism has been explored and is discussed under their individual sections.

Hydrogen evolution at cathodic potentials is a concern in electrochemical deposition as it can lead to the formation of cracks and defects in the overall coating structure, both during deposition and later during use of the material. Hydrogen evolution competes with metal electrodeposition in this system and can play a major role in determining the composition of ZnNi coatings [35, 46]. In ZnNi deposition systems under alkaline conditions, boric acid was found to suppress hydrogen evolution. Hydrogen evolution is a larger concern for nickel deposition than zinc deposition as a larger overpotential is required for nickel deposition since the deposition is under kinetic control while zinc deposition is thermodynamically controlled [57]. Our previous study examined the change in hydrogen evolution onset with varying borate concentrations in alkaline solutions, and found as the borate concentration is increased, hydrogen evolution is pushed to more cathodic values [48]. A maximum borate concentration of 100 mM was used due to conductivity of borate in the system [57]. In addition to borate, nanoparticles can have an overall effect on hydrogen evolution in the system as well. The hydrogen evolution onset was compared for solutions with and without the presence of Mt in **Figure 1**. For nickel, a large cathodic shift was observed when Mt was added to the system. For zinc and zinc-nickel, small cathodic shifts were observed with Mt. The Mt can help further shift the onset of hydrogen evolution within this system, in addition to borate [48, 57]. Alloy formation typically occurs at or near the onset of hydrogen evolution for this system. By shifting the onset in a cathodic direction, less hydrogen will be produced during alloy formation, leading to less entrapped hydrogen in the overall coating. Hydrogen evolution can hinder adsorption of nanoparticles on the surface of the coating material and lead to embrittlement [7].

The addition of Al2 O3 and SiC was also found to cause a surface blockage preventing hydrogen evolution to occur. This effect is found to be dependent on the concentration of SiC in solution, but for addition of Al2 O3 no dependency is observed. It is believed that SiC and Al2 O3 are adsorbed onto the electrode surface, reducing the active surface area. At lower pH, H+ has a higher tendency to adsorb onto the SiC particles, leading to a reduction in hydrogen evolution [46].

#### **2.3. Deposition methods**

particles were found to be uniformly imbedded in the ZnNi-Al2

agitation can lead to a lower quality of particles in the deposit [7].

introduced into ZnNi coatings to improve corrosion resistant properties [47].

Though many researchers use electrochemical deposition as a tool to form a coating of interest, there is little published work on the electrochemical system used for the deposition of zincnickel nanocomposite coatings. A better understanding can lead to an improved deposition system, and an overall superior coating. Work continues to be done in acidic and alkaline conditions with a goal of further improving the materials, longer material lifetimes and a better understanding of the mechanisms involved in various alloy formations [49, 50, 55, 57, 59, 60, 65, 67–71] but little work has been done to examine systems with nanoparticle incorporation.

Zinc-nickel alloy formation follows an anomalous deposition mechanism which occurs when the electrochemically less noble metal deposits preferentially to the more noble metal. This is verified through examination of the voltammetry patterns of the zinc-nickel system as the individual zinc and nickel reduction peaks are shifted based on the presence of the other metal species in solution [48, 59, 62, 70, 72–74]. During deposition, a thin layer of nickel is initially deposited onto the substrate. As the deposition continues, zinc is intercalated into the nickel, leading to formation of the alloy [57, 61, 67]. In acidic systems under low current density, a transition from anomalous to normal codeposition has been noted. Normal codeposition is dominant when the

**2.2. Influence of nanoparticle addition on deposition mechanism**

cal in shape. For example SiO2

192 Nanocomposites - Recent Evolutions

ultrasonic vibration [32, 33]. Without sonication, ceria nanoparticles were seen organized in long string-shape agglomerates. These agglomerates became trapped inside voids and pores during coating growth [44]. Though sonication or mechanical disruption of the nanoparticles is needed to distribute them throughout the metal matrix, care must be taken as excessive

Na-smectites, a type of clay mineral, specifically montmorillonite (Mt) were also examined, for incorporation into metal matrixes. Within aqueous solutions, Na-montmorillonite can be completely exfoliated and incorporated into other materials, forming continuous, crack free films, which is beneficial in corrosion resistant coatings [5, 8–9, 52, 66]. Exfoliation causes the short range order of the clay particles to be disrupted, causing individual clay platelets to exist, unassociated from one another. The resulting clay platelets range 1–2 nm in width with 100–1000 nm in length [66]. These platelets are easily incorporated into the coating during deposition, increasing the overall thermal stability and mechanical strength of the coating, which leads to increased corrosion resistance [2, 8]. As the alloy coating is forming, the exfoliated clay in solution is freely dispersed throughout the electrolytic bath. Mt is a cationic clay with a negatively charged surface which attracts metal ions, increasing incorporation of the platelets into the metal composite during deposition. The clay platelets settle onto the substrate surface as the coating is being formed, allowing them to be incorporated into the coating. Exfoliated Mt, which has a plate-like structure, increases the surface area of the material when imbedded in the coating and leads to a more tortuous mean free path of the corrosion cells upon onset [5]. This technique has previously been successful with the incorporation of montmorillonite platelets into pure nickel, nickel-molybdenum and nickel-copper coatings [2, 5, 8, 9, 52, 58, 66]. However, many traditional particles used in composite coatings are spheri-

O3

nanoparticles coated with a layer of cerium oxide have been

coating after treatment of

Electrodeposition techniques include potentiostatic and galvanostatic deposition, and further into both methods, applied vs. pulsed deposition parameters in the literature for zinc-nickel nanocomposite coatings [44]. The particles co-deposit with the zinc-nickel coating which has advantages over other methods such as better control of coating thickness, deposition speed, working under controlled temperatures, and it is a single-step method. The nanoparticles are incorporated as the metal species are reduced onto the electrode surface, forming the nanoparticle coating. Applied methods include direct current or direct potential, where a constant current or potential is applied to the electrode. Pulsed methods include pulse current

adsorbed metallic adatoms are able to reorganize and minimize surface energy. The grain growth continues during this time due to desorption of impurities leading to changes in grain

Coating composition and quality is dependent on the pH of the system at the time of formation. Although extensive work has been done on zinc-nickel coatings in both acidic and alkaline conditions, less work has been done on zinc-nickel coatings with nanoparticle incorporation. A review of the literature shows most studies being performed under acidic conditions [32–37,

predominantly deposited under acidic conditions (pH = 4, 4.9 and 5.0) with one group examin-

was done under acidic conditions with pH = 2.5, 4 and 4.6 from a variety of groups [11, 34–37].

ZnNi-SiC was done under an unspecified alkaline pH [26, 38, 39–42]. The deposition of ZnNiceria particles was undertaken with a pH = 5.3 [43, 44]. ZnNi-carbon nanotubes, though not specified are believed to have been deposited under alkaline conditions due to specified bath components [13, 45] and the deposition of ZnNi-Mt coatings was done at pH = 9.4 [48]. Though the bulk of the work has been done under acidic conditions, focus of the research may benefit from pushing into the realm of alkaline deposition as throughout literature, optimal coating formation is realized under alkaline conditions. Although zinc-nickel coatings deposited under acidic conditions tend to have a higher current efficiency, alkaline processes tend to lead to better substrate coverage [12, 57, 62, 79]. A drawback of alkaline conditions is stabilizing agents are needed to keep the metal species from precipitating as metal hydroxides from the solution.

Zinc and nickel content and nanoparticle incorporation were examined with various techniques including atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), energy dispersive spectroscopy (EDS) energy dispersive x-ray (EDX) and EDX mapping. Uptake of the nanoparticles is of interest as varying concentrations of nanoparticles are found, dependent upon the character of the particle being added to the

 up to 8.9 wt % throughout the literature [12, 27–33]. Zinc-nickel coatings with TiO2 incorporation were found to contain on average 80–85% Zn, 12–17% Ni and 1.25–2.5% Ti [35–37]. ZnNi-SiC coatings contained 11% SiC [42]. ZnNi coatings with ceria incorporation contained 10–11% Ni, with 2–3% ceria content [43, 44]. ZnNi-Mt coatings contained 86–90% Zn, 10–14% Ni with trace amounts of Mg and Al from Mt nanoparticles confirmed in ICP-MS analysis [48]. Throughout the studies, the coatings maintain the Ni% needed (8–18%) for

particles was performed at pH = 2, 3 and 4 while the deposition of

incorporation were found to contain anywhere from trace

ing deposition at pH = 13 [12, 27–33]. The literature for the deposition of ZnNi-TiO2

O3

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

coatings were

coatings

195

morphology and size while chemical composition remains relatively constant [20].

39–40, 44, 46] with little work in alkaline conditions [12, 42, 48]. ZnNi-Al2

**3. Characterization of the zinc-nickel nanocomposite coatings**

O3

**2.4. pH studies**

The deposition of ZnNi-SiO2

**3.1. Coating composition**

solution. ZnNi coatings with Al2

maximized corrosion protection [57, 59, 70, 83].

Al2 O3

**Figure 1.** The onset of hydrogen evolution in solutions containing metal salts (specified), Mt (specified) 0.1 M borate and pH = 9.4 with NH4 OH (a) Ni2+ (pink short dash); (b) Ni2+ Mt (blue dot); (c) Zn2+ (purple square dot); (d) Zn2+ Mt (black long dash); (e) Zn2+, Ni2+ (green dash dot); (f) Zn2+, Ni2+ Mt (orange solid) [54].

(PC), pulse reversed current (PRC), pulse potential (PP) and pulse reversed potential (PRP). PC and PP involve alternatively applying two or more cathodic direct current or potentials during the deposition, with off times, when no current or potential is being applied. PRC and PRP are similar to PC and PP as a cathodic pulse is applied but during the off times, an anodic pulse is applied to the electrode. Previous studies show an increase in incorporation of particles through a pulse deposition method with better overall coverage of the underlying material compared to a constant applied potential technique [7, 45, 48, 57, 62, 68, 78–80]. The nanoparticles are incorporated in a higher percentage because of the partial dissolution of the metal deposit during the anodic pulse. Pulse plating was found to improve overall quality of deposits and reduce grain size which inherently increases the corrosion protection of the coating [48, 57–58, 79, 81]. Pulse deposition includes the following attributes: (1) better inclusion of nanoparticles in the metal matrix, (2) lower concentration of nanoparticles needed in the electrolytic solution, (3) selective entrapment based on size of nanoparticles, (4) release of trapped hydrogen prior to coating use which leads to longer coating lifetime and (5) a more opened grain structure which allows hydrogen to escape from the deposit without forming holes or pits in the coatings which could otherwise be used as corrosion cell development sites [7, 48, 57, 80]. Pulsed deposits help embed higher concentrations of nanoparticles because it helps eliminate a fraction of the electrodeposited metal during the off time [7]. Pulse durations affect the shape and size of crystallite formation [21, 81, 82]. During off time, adsorbed metallic adatoms are able to reorganize and minimize surface energy. The grain growth continues during this time due to desorption of impurities leading to changes in grain morphology and size while chemical composition remains relatively constant [20].

## **2.4. pH studies**

Coating composition and quality is dependent on the pH of the system at the time of formation. Although extensive work has been done on zinc-nickel coatings in both acidic and alkaline conditions, less work has been done on zinc-nickel coatings with nanoparticle incorporation. A review of the literature shows most studies being performed under acidic conditions [32–37, 39–40, 44, 46] with little work in alkaline conditions [12, 42, 48]. ZnNi-Al2 O3 coatings were predominantly deposited under acidic conditions (pH = 4, 4.9 and 5.0) with one group examining deposition at pH = 13 [12, 27–33]. The literature for the deposition of ZnNi-TiO2 coatings was done under acidic conditions with pH = 2.5, 4 and 4.6 from a variety of groups [11, 34–37]. The deposition of ZnNi-SiO2 particles was performed at pH = 2, 3 and 4 while the deposition of ZnNi-SiC was done under an unspecified alkaline pH [26, 38, 39–42]. The deposition of ZnNiceria particles was undertaken with a pH = 5.3 [43, 44]. ZnNi-carbon nanotubes, though not specified are believed to have been deposited under alkaline conditions due to specified bath components [13, 45] and the deposition of ZnNi-Mt coatings was done at pH = 9.4 [48]. Though the bulk of the work has been done under acidic conditions, focus of the research may benefit from pushing into the realm of alkaline deposition as throughout literature, optimal coating formation is realized under alkaline conditions. Although zinc-nickel coatings deposited under acidic conditions tend to have a higher current efficiency, alkaline processes tend to lead to better substrate coverage [12, 57, 62, 79]. A drawback of alkaline conditions is stabilizing agents are needed to keep the metal species from precipitating as metal hydroxides from the solution.
