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

### **3.1. Coating composition**

(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,

**Figure 1.** The onset of hydrogen evolution in solutions containing metal salts (specified), Mt (specified) 0.1 M borate and

long dash); (e) Zn2+, Ni2+ (green dash dot); (f) Zn2+, Ni2+ Mt (orange solid) [54].

OH (a) Ni2+ (pink short dash); (b) Ni2+ Mt (blue dot); (c) Zn2+ (purple square dot); (d) Zn2+ Mt (black

pH = 9.4 with NH4

194 Nanocomposites - Recent Evolutions

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 solution. ZnNi coatings with Al2 O3 incorporation were found to contain anywhere from trace Al2 O3 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 maximized corrosion protection [57, 59, 70, 83].

#### **3.2. X-ray diffraction (XRD)**

The phase of electrodeposited ZnNi alloy coatings is dependent upon the nickel content in the alloy and can be controlled by a number of factors including electrolytic bath conditions [12]. ƔNi5 Zn21 is known to be the most corrosion resistant ZnNi alloy phase and appears to be preferentially deposited under alkaline conditions in ZnNi systems without nanoparticle incorporation. The γ ZnNi has a preferred orientation with the (330) reflection as main peak in the XRD pattern [42, 59, 65–67, 71, 72]. This preferred orientation continues with the incorporation of nanoparticles although an overall decrease in peak intensity and broadening of peak suggest smaller crystallite size formation [12, 35, 48]. The peak width of the diffraction peak at half maximum height (FWHM) is dependent on crystallite size and lattice strains due to lattice imperfections such as dislocations or atom vacancies with the values dependent most heavily on crystallite size [84, 85]. If we assume there is little strain in the system, we can assume the broadening at FWHM is due to a decrease in crystallite size of the metallic particles [35]. The average crystallite size of ZnNi coatings with TiO2 , SiC, and Al2 O3 nanoparticles are presented in **Table 2**. The trends show an overall decrease in particle size with the increase in nanoparticle incorporation as compared to pure ZnNi coatings.

#### **3.3. Microhardness**

Hardness (HV) values are a measurement of the microhardness or resistance to penetration of a sample and can be used to compare quality of the coatings. All composite coatings studied demonstrate improved microhardness values as compared to the base alloy as presented in **Table 3**. As expected, addition of nanoparticles to the coatings improve the overall hardness values, as demonstrated with an increase of 305 HV to 524 HV for ZnNi coatings with CeO2 treated SiO2 particles, an increase of 35 HV with the addition of TiO2 particles in Praveen's work, a 300% increase in hardness with an incorporation of 11.2 wt % Al2 O3 particles in Zheng's work and noticeable increases in both Ataie's and Ghaziof's work with incorporation of Al2 O3 particles as well [27, 29, 32, 37, 47]. The improved microhardness is believed to be due to dispersive strengthening as the ceramic like particles (TiO2 ) form a barrier to deformation commonly observed in metal matrix systems. As the incorporation of nanoparticles increases, the microhardness also increases [36]. The higher hardness of the coating is due to the fine-grained structure. The dispersed particles in the matrix are able to obstruct easy movement of dislocations [37].

**3.4. Corrosion studies**

An advantage to developing metal matrix composite coatings is for increased corrosion resistance as compared to pure metal coatings. Properties that may contribute to this added protection include a finer coating structure with refined grains, incorporation of electrochemically inert particles dispersed throughout the metallic coating, and filling of crevices, gaps, and micron sized holes on the coatings surface. These could otherwise lead to localized defects which are vulnerable to corrosion. Improvement of self-passivation of the coating is offered through improved barrier protection due to the incorporated particles in the naturally formed defects of the coatings. Common methods to examine the corrosion resistance of a material include open circuit potential (OCP) studies, linear polarization resistance (LPR), potentiody-

The open circuit potential (OCP) is the potential of the working electrode relative to the reference electrode when no external potential or current is being applied to the system. OCP is dependent on the composition of the working electrode, treatment of the electrode prior

namic polarization, and electrochemical impedance spectroscopy (EIS).

**Reference Coating composition Hardness** 

ZnNi coating with incorporated SiO2

ZnNi coating with incorporated CeO2

ZnNi Coating with incorporated Al2

ZnNi Coating with incorporated Al2

nanoparticles

nanoparticles

treated, SiO2

Ataie et al. [35] ZnNi Coating with incorporated Al2

nanoparticles

nanoparticles

nanoparticles

Ghaziof et al. [33] ZnNi coating with incorporated Al2

nanoparticles

nanoparticles

nanoparticles

nanoparticles

ZnNi with incorporated TiO2

ZnNi coating with incorporated Al2

ZnNi coating with incorporated Al2

**Table 3.** Microhardness values of coatings throughout the literature.

Al2 O3

Xiang et al. [53] Bare substrate 134 Direct deposition

ZnNi coating 305

**values (HV)**

535

524

640

340 Direct deposition

235 Direct deposition

310 Pulsed current deposition, 100 Hz

323 Pulsed current deposition, 500 Hz

640 30 W ultrasonic application during deposition

750 45 W ultrasonic application during deposition

magnetic stirring during

O3

O3

O3

deposition ZnNi coating with incorporated 11.2 wt %

particles 170

O3

O3

O3

Zheng et al. [38] ZnNi coating 215 Ultrasound generation and

Praveen et al. [43] Zinc-nickel coating 135 Direct deposition

**Additional parameters**

197

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



**Table 3.** Microhardness values of coatings throughout the literature.

#### **3.4. Corrosion studies**

**3.2. X-ray diffraction (XRD)**

196 Nanocomposites - Recent Evolutions

average crystallite size of ZnNi coatings with TiO2

ticle incorporation as compared to pure ZnNi coatings.

particles, an increase of 35 HV with the addition of TiO2

ening as the ceramic like particles (TiO2

increase in hardness with an incorporation of 11.2 wt % Al2

[12]. ƔNi5

**3.3. Microhardness**

Al2 O3

Al2 O3

Al2 O3

TiO2

TiO2

TiO2

The phase of electrodeposited ZnNi alloy coatings is dependent upon the nickel content in the alloy and can be controlled by a number of factors including electrolytic bath conditions

be preferentially deposited under alkaline conditions in ZnNi systems without nanoparticle incorporation. The γ ZnNi has a preferred orientation with the (330) reflection as main peak in the XRD pattern [42, 59, 65–67, 71, 72]. This preferred orientation continues with the incorporation of nanoparticles although an overall decrease in peak intensity and broadening of peak suggest smaller crystallite size formation [12, 35, 48]. The peak width of the diffraction peak at half maximum height (FWHM) is dependent on crystallite size and lattice strains due to lattice imperfections such as dislocations or atom vacancies with the values dependent most heavily on crystallite size [84, 85]. If we assume there is little strain in the system, we can assume the broadening at FWHM is due to a decrease in crystallite size of the metallic particles [35]. The

in **Table 2**. The trends show an overall decrease in particle size with the increase in nanopar-

Hardness (HV) values are a measurement of the microhardness or resistance to penetration of a sample and can be used to compare quality of the coatings. All composite coatings studied demonstrate improved microhardness values as compared to the base alloy as presented in **Table 3**. As expected, addition of nanoparticles to the coatings improve the overall hardness values, as

well [27, 29, 32, 37, 47]. The improved microhardness is believed to be due to dispersive strength-

metal matrix systems. As the incorporation of nanoparticles increases, the microhardness also increases [36]. The higher hardness of the coating is due to the fine-grained structure. The dis-

demonstrated with an increase of 305 HV to 524 HV for ZnNi coatings with CeO2

noticeable increases in both Ataie's and Ghaziof's work with incorporation of Al2

persed particles in the matrix are able to obstruct easy movement of dislocations [37].

**Nanoparticle ZnNi (nm) ZnNi nanocomposite (nm)**

(5 g/L) [17] 40.93 26.4

(10 g/L) [17] 40.93 33.2

(15 g/L) [17] 40.93 20.68

[43] — 30

[41] — 19.7

 [40] 15.5 11.7 SiC [48] 28.5 21.0–22.0

**Table 2.** Crystallite size of coatings listed in the literature.

Zn21 is known to be the most corrosion resistant ZnNi alloy phase and appears to

, SiC, and Al2

O3

O3

) form a barrier to deformation commonly observed in

nanoparticles are presented

particles in Praveen's work, a 300%

particles in Zheng's work and

O3

treated SiO2

particles as

An advantage to developing metal matrix composite coatings is for increased corrosion resistance as compared to pure metal coatings. Properties that may contribute to this added protection include a finer coating structure with refined grains, incorporation of electrochemically inert particles dispersed throughout the metallic coating, and filling of crevices, gaps, and micron sized holes on the coatings surface. These could otherwise lead to localized defects which are vulnerable to corrosion. Improvement of self-passivation of the coating is offered through improved barrier protection due to the incorporated particles in the naturally formed defects of the coatings. Common methods to examine the corrosion resistance of a material include open circuit potential (OCP) studies, linear polarization resistance (LPR), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS).

The open circuit potential (OCP) is the potential of the working electrode relative to the reference electrode when no external potential or current is being applied to the system. OCP is dependent on the composition of the working electrode, treatment of the electrode prior to the study, and the electrolytic bath in which the electrode resides. Linear polarization resistance is the measurement of current in relation to the electrode potential. This can be used to predict the corrosion rate of the coatings within a specific environment. The film is polarized by applying an external potential forcing the system away from equilibrium and monitoring the resulting potential and current. The deviation from an equilibrium potential is called polarization. The polarization resistance (Rp) is experimentally observed between the electrochemical current density and applied potential for the corroding electrode within a few millivolts of the polarization from the open circuit potential (Eocp). Potentiodynamic polarization pushes the potential even further from the equilibrium potential for the anodic and cathodic sweeps. From this data the anodic slope (β<sup>a</sup> ) and cathodic slope (β<sup>c</sup> ) are obtained from the curves. The Ecorr is determined from the intercepts of the curves. The icorr value is obtained by substituting the β<sup>a</sup> , βc and Rp values into a simplified rearranged Stern and Geary equation [86, 87].

**4. Zinc-nickel nanocomposites**

 **and Al2**

Most work to date has examined ZnNi-Al2

**O3 /SiC**

ties, or areas without a deposit present. When the SiC and Al2

O3

/SiC combined in the nanocomposite. Though the deposition mechanism of

coatings was not explicitly discussed, Tulio et al. examined the effects of SiC and

encouraged deposition of both Ni and Zn individually. For nickel, a marked

in slightly acidic pH with rotating disc. They first examined the effect of SiC and Al<sup>2</sup>

on nickel and zinc, without the other metal ion present in solution and found the addition of

increase in current densities was observed. In the Zn system when the solution was scanned cathodically without the presence of nanoparticles, the deposit exhibited many discontinui-

solution, there was a noticeable increase of coating coverage so much that the discontinuities almost disappeared entirely, suggesting encouragement of Zn deposition. SiC and Al2

do not affect the initial nucleation and growth in the ZnNi system when the metal species are combined, though at higher concentrations of nanoparticles, surface blockage has been

to systems free of nanoparticle presence and a positive shift in potential was noted at the onset of secondary nucleation. This is due to an increase in the mass-transport of the particles to the electrode surface during the rotation. During the scans the quantity of particles reaching the electrode increased, leading to an increase in current density. The ZnNi deposition did remain anomalous under all conditions examined [46]. Blejan and Muresan examined the

with deposition giving a preferred (600) orientation [12]. Improvement of nanoparticle incor-

examined the effect of sonication during the deposition. Without sonication, the hardness was 340 HV, with 30 W sonication it was 640 HV and with 45 W sonication it was 750, a 220% increase over the coating with no sonication [29]. The hardness of ZnNi and ZnNi-Al2

coatings under direct current and pulse current deposition conditions was examined. ZnNi under applied current was 235 HV while pulsed ZnNi was 310–323 HV, a 40% increase and

the number of layers improved the hardness values of the coatings but addition of Al2

SO4

incorporated particles [12]. The corrosion current decreases from 1.83 × 10−<sup>5</sup>

content is doubled from 4.5 to 8.9 wt% [32]. EIS of ZnNi-Al2

corrosion potentials of the composite coatings at more positive potential with initial Al2

incorporation as compared to ZnNi alloys which is attributed to the chemical inertia of the

was 338 HV, a slight increase over pulsed ZnNi coatings [27]. Shourgeshty et al.

O3

solution. ZnNi-Al2

observed. Larger current densities are observed for systems with SiC and Al2

O3

poration was noted through the use of ultrasonication [29, 32, 33].

examined multilayer coatings of ZnNi and ZnNi-Al2

coatings were studied in Na2

Zhang and An found an increase of hardness with the addition of Al2

γ-phase ZnNi alloys, showing small growth of the (330) plane with addition of Al2

coatings (~32% of papers) with 4% examining

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

particles were added to the

O3

O3

deposits. As expected, an increase in

coatings (**Table 4**) present

to 0.92 × 10−<sup>5</sup>

with varying incorporation

O3

films (using a Cr x-ray tube), which only exhibited

O3

O3

O3

199

O3

as compared

O3

[32]. Ataie et al.

O3 also

particles

O3

O3

as the

**O3**

O2

XRD patterns of deposited ZnNi-Al<sup>2</sup>

**4.1. ZnNi-Al2**

the effects of Al<sup>2</sup>

O3

O3

ZnNi-Al2

SiC and Al2

ZnNi-Al2

ZnNi-Al2

Al2 O3 O3

O3

had a positive effect [30, 31].

Al2 O3

The anticorrosive ability of ZnNi-nanocomposite coatings can be further investigated with EIS. Nyquist plots show a semicircle shape in the investigated frequency range with an increased axial radius, which is indicative of better corrosion resistance. Equivalent circuit models are used to simulate the metal-solution interface to better understand the system. A few studies have done corrosion work for these ZnNi nanocomposite coatings and shown improvement with addition of the nanoparticles. **Table 4** lists some results which are discussed in sections below.


**Table 4.** Corrosion potential (Ecorr), corrosion current (icorr) and polarization resistance (Rp) of ZnNi and ZnNi-Mt coatings.
