**4. Zinc-nickel nanocomposites**

#### **4.1. ZnNi-Al2 O3 and Al2 O3 /SiC**

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

from the curves. The Ecorr is determined from the intercepts of the curves. The icorr value is

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.

**Coating [ref] Ecorr (V)/SCE icorr (A) Rp (Ω cm2**

(sonicated) −0.78 2.80 × 10−<sup>5</sup> —

ZnNi −1.05\* 4.30 × 10−<sup>5</sup> 352.0

ZnNi (24 h immersion) −1.03\* — 94.1 ZnNi [17] −0.62 2.51 × 10−<sup>6</sup> 1167.6

[41] −1.09\* 9.90 × 10−<sup>5</sup> 122.2

(24 h immersion) −1.11\* 1.25 × 10−<sup>5</sup> 97.3

5 g/L −0.52 1.23 × 10−<sup>6</sup> 4024.9

10 g/L −0.63 2.37 × 10−<sup>6</sup> 2038.3

15 g/L −0.70 2.57 × 10−<sup>6</sup> 1190.0

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

Zn [54] −1.17 2.09 × 10−<sup>4</sup> 1333 Ni −0.45 2.75 × 10−<sup>5</sup> 6790 ZnNi γ phase −0.74 1.06 × 10−<sup>5</sup> 30,485 ZnNi-Mt γ phase −0.73 3.72 × 10−<sup>6</sup> 34,900 ZnNi [50] −0.92 6.20 × 10−<sup>5</sup> — ZnNi-CeO2 −0.77 3.30 × 10−<sup>5</sup> —

) and cathodic slope (β<sup>c</sup>

and Rp values into a simplified rearranged Stern and Geary

) are obtained

**)**

and cathodic sweeps. From this data the anodic slope (β<sup>a</sup>

, βc

obtained by substituting the β<sup>a</sup>

198 Nanocomposites - Recent Evolutions

equation [86, 87].

ZnNi-CeO2

ZnNi-TiO2

ZnNi-TiO2

ZnNi-Al2 O3

ZnNi-Al2 O3

ZnNi-Al2 O3

Corrected to SCE.

\*

Most work to date has examined ZnNi-Al2 O3 coatings (~32% of papers) with 4% examining the effects of Al<sup>2</sup> O2 /SiC combined in the nanocomposite. Though the deposition mechanism of ZnNi-Al2 O3 coatings was not explicitly discussed, Tulio et al. examined the effects of SiC and Al2 O3 in slightly acidic pH with rotating disc. They first examined the effect of SiC and Al<sup>2</sup> O3 on nickel and zinc, without the other metal ion present in solution and found the addition of SiC and Al2 O3 encouraged deposition of both Ni and Zn individually. For nickel, a marked 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 discontinuities, or areas without a deposit present. When the SiC and Al2 O3 particles were added to the 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 O3 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 observed. Larger current densities are observed for systems with SiC and Al2 O3 as compared 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 XRD patterns of deposited ZnNi-Al<sup>2</sup> O3 films (using a Cr x-ray tube), which only exhibited γ-phase ZnNi alloys, showing small growth of the (330) plane with addition of Al2 O3 particles with deposition giving a preferred (600) orientation [12]. Improvement of nanoparticle incorporation was noted through the use of ultrasonication [29, 32, 33].

Zhang and An found an increase of hardness with the addition of Al2 O3 [32]. Ataie et al. 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 O3 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 ZnNi-Al2 O3 was 338 HV, a slight increase over pulsed ZnNi coatings [27]. Shourgeshty et al. examined multilayer coatings of ZnNi and ZnNi-Al2 O3 deposits. As expected, an increase in the number of layers improved the hardness values of the coatings but addition of Al2 O3 also had a positive effect [30, 31].

ZnNi-Al2 O3 coatings were studied in Na2 SO4 solution. ZnNi-Al2 O3 coatings (**Table 4**) present corrosion potentials of the composite coatings at more positive potential with initial Al2 O3 incorporation as compared to ZnNi alloys which is attributed to the chemical inertia of the incorporated particles [12]. The corrosion current decreases from 1.83 × 10−<sup>5</sup> to 0.92 × 10−<sup>5</sup> as the Al2 O3 content is doubled from 4.5 to 8.9 wt% [32]. EIS of ZnNi-Al2 O3 with varying incorporation of Al2 O3 and varying immersion (0, 24, 48 and 120 h) is presented in **Figure 2**. The coatings were studied in 0.2 g/L Na2 SO4 (pH 5) using a potentiostat PARStat 2273 (Princeton Applied 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 formation of corrosion products on the surface of the coating [12]. Incorporation of Al2 O3 particles results in γ-phase zinc-nickel alloys with nanoparticle incorporation. ZnNi-Al2 O3 /SiC coatings still follow an anomalous deposition route. Improved hardness and corrosion properties are observed with incorporation of Al2 O3 [12, 27, 29, 32, 46].
