**2. Electro deposition of nanomaterials**

Nano crystalline materials were first reported by Gleiter [3] and due to their attractive properties. The fact that electrochemical deposition "ED", also being an atomic deposition process, can be used to synthesize nanomaterials has generated a great deal of interest in recent years. The obvious advantages of this century-old process of ED are rapidity, low cost, high purity, production of free-standing parts with complex shapes, higher deposition rates, the production of coatings on widely differing substrates. In addition, ability to produce structural features with sizes ranging from nm to μm, and ability to produce compositions unattainable by other techniques [4, 5]. This method also provides for cost-effective production of free‐ standing forms such as ultrathin foil, wire, sheet, and plate, as well as complex shapes.

Electro deposition parameters are bath composition, pH, temperature, over potential, bath additives, etc... Important microstructural features of the substrate include grain size, crystal‐ lographic texture, dislocation density, internal stress. Crystallization (Figure. 1) occurs either by the buildup of existing crystals or the formation of new ones. These two processes are in competition with each other and are influenced by different factors. The two key mechanisms which have been identified as the major rate-determining steps for nanocrystal formation are charge transfer at the electrode surface and surface diffusion of adions on the crystal surface [6]. With increasing inhibition, the deposit structure changes from basis oriented and repro‐ duction type (BR) to twin transition types (TT), to field oriented type (FT), and finally to unoriented dispersion type (UD). A large number of grain refiners have been described in the literature; their effectiveness depends upon surface adsorption characteristics, compatibility with the electrolyte, temperature stability, etc. For example, saccharin, coumarin, thiorea, and HCOOH have all been successfully applied to achieve grain refinement down to the nano‐ crystalline range for nickel electrodeposits. The second important factor in nanocrystal formation during electro crystallization is overpotential; Grain growth is favored at low over potential and high surface diffusion rates. On the other hand, high over potential and low diffusion rates promote the formation of new nuclei [7].

#### **2.1. Electrodeposited nanocrystalline Ni-Fe alloys**

sation, plasma spraying, and electrochemical deposition [1]. Chemical vapour deposition includes chemical reaction of input materials in the gas phase and deposition of the product on the surface. Physical vapour deposition (PVD) includes transforming the material into the gaseous phase and then deposition on the surface [2]. The impact of an atom or ion on a surface produces sputtering from the surface. Unlike many other vapour phase techniques there is no melting of the material. Sputtering is done at low pressure on cold substrate. In laser ablation, pulsed light from an excimer laser is focused onto a solid target in vacuum to boil off a plum of energetic atom. A substrate will receive a thin film of the target material. The sol-gel process

The superiority of electrochemical deposition techniques in synthesizing various nanomate‐ rials that exhibit improved compared with materials produced by conventional techniques, will be discussed. Nanocoatings can be obtained either directly on substrates or by using

Nano crystalline materials were first reported by Gleiter [3] and due to their attractive properties. The fact that electrochemical deposition "ED", also being an atomic deposition process, can be used to synthesize nanomaterials has generated a great deal of interest in recent years. The obvious advantages of this century-old process of ED are rapidity, low cost, high purity, production of free-standing parts with complex shapes, higher deposition rates, the production of coatings on widely differing substrates. In addition, ability to produce structural features with sizes ranging from nm to μm, and ability to produce compositions unattainable by other techniques [4, 5]. This method also provides for cost-effective production of free‐ standing forms such as ultrathin foil, wire, sheet, and plate, as well as complex shapes.

Electro deposition parameters are bath composition, pH, temperature, over potential, bath additives, etc... Important microstructural features of the substrate include grain size, crystal‐ lographic texture, dislocation density, internal stress. Crystallization (Figure. 1) occurs either by the buildup of existing crystals or the formation of new ones. These two processes are in competition with each other and are influenced by different factors. The two key mechanisms which have been identified as the major rate-determining steps for nanocrystal formation are charge transfer at the electrode surface and surface diffusion of adions on the crystal surface [6]. With increasing inhibition, the deposit structure changes from basis oriented and repro‐ duction type (BR) to twin transition types (TT), to field oriented type (FT), and finally to unoriented dispersion type (UD). A large number of grain refiners have been described in the literature; their effectiveness depends upon surface adsorption characteristics, compatibility with the electrolyte, temperature stability, etc. For example, saccharin, coumarin, thiorea, and HCOOH have all been successfully applied to achieve grain refinement down to the nano‐ crystalline range for nickel electrodeposits. The second important factor in nanocrystal formation during electro crystallization is overpotential; Grain growth is favored at low over potential and high surface diffusion rates. On the other hand, high over potential and low

is well adapted for ceramics and composites at room temperature [1].

**2. Electro deposition of nanomaterials**

diffusion rates promote the formation of new nuclei [7].

porous templates.

124 Modern Surface Engineering Treatments

Nanostructured Ni-Fe alloys, produced by electro-deposition technique provide material with significant improved strength and good magnetic properties, without compromising the coefficient of thermal expansion (CTE). Such properties made these alloys to be used in many of the applications where conventional materials are currently used. For such applications a special attention has been made to study the physical, mechanical and chemical properties of such alloys because of the potential for performance enhancement for various applications of Ni-Fe alloys arising from the enhanced properties due to the ultra-fine grain size of these alloys [7-9].

**Figure 1.** Two stages of electro crystallization according to Bockris et al. [6]

According to R. Abdel-Karim et al. [10], nanocrystalline Ni-Fe deposits with different compo‐ sition and grain sizes were fabricated by electrodeposition. Deposits with iron contents in the range from 7 to 31% were obtained by changing the Ni2+/Fe2+ mass ratio in the electrolyte. The deposits were found to be nanocrystalline with average grain size in the range 20–30 nm. The surface morphology was found to be dependent on Ni2+/Fe2+ mass ratio as well as electroplating time. Figure 2 represents SEM of electrodeposited Ni base layers at longer electrode position time (100 min) as a function of Ni2+/Fe2+ mass ratio in the electrolytic bath. From Figure 2(a), in case of Ni2+/Fe2+ mass ratio equal to 20.7, SEM image displayed well defined nodular coarse and fine particles with no appearance of grain boundaries. This nanosized particles can be better illustrated by using higher magnification (100000x), as shown in Figure 2(b). From Figure 2(c), sample of Ni2+/Fe2+ mass ratio equal to 13.8 displayed clusters of fine particles embedded in elongated elliptical ones and some grain boundaries can be seen. By raising the iron content and thus decreasing the Ni2+/Fe2+ mass ratio down to 9.8 (Figure 2(d)), the surface morphology showed rough cauliflower structure. The cauliflower morphology particle is made of coagulate particle distributed all over the surface with a flattened grains. The grains size decreased with increasing the iron content, especially in case of short time electroplating. Increasing the electroplating time had no significant effect on grain size. The microhardness of the materials followed the regular Hall-Petch relationship with a maximum value (762 Hv) when applying Ni2+/Fe2+ mass ratio equal to 9.8.

less noble metal, Fe, to the more noble metal, Ni. In other words, the reduction of nickel is inhibited while the deposition of iron is enhanced when compared with their individual deposition rates. According to Afshar et al. [12], the electrode position of nickel-iron alloys is a diffusion-controlled process with typical nucleation mechanism. According to Krause et al. [13], the anomalous behavior was assumed due to precipitation of iron hydroxide on surface

( ) <sup>+</sup> 2+ Ni° / nFe° + H 1/2 Fe + H - Ni°/ n-1/2 Fe ® atom (1)

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( ) ( ) - H - Ni°/ n-1/2 Fe°+ R- Cl Ni°/ n-1/2 Fe°+R + H– Cl atom ® (2)

() ( ) + 2+ Ni°/n Fe° + 2H Fe + H + Ni°/ n-1 Fe° side reaction ® <sup>2</sup> (4)

( ) - O g +2H O+ 4e 4OH 2 2 ® (5)

( ) () ( ) 2+ H - Ni°/ n-1/2 Fe°+H+ 1/2 Fe + Ni°/ n-1 Fe°+H side reaction atom ® <sup>2</sup> (3)

One of the important observations in the Ni-Fe alloys is the dependence of the crystal structure on the iron and nickel content in the deposited layers. Figure 3 shows X-ray diffraction patterns of various nanocrystalline Ni-Fe electrodeposits ranging in nickel content from 0 to 100%. Ni-Fe deposits with low nickel concentrations were found to have a body centered cubic (BCC) structure, while those with high nickel concentrations had a face-centered (FCC) structure [13]. While a mixed FCC/BCC structure was observed for nickel concentrations ranging from 10wt

The first group of properties are strongly dependent on grain size. These include strength, ductility and hardness, wear resistance and coefficient of friction, electrical resistivity, coercivity, solid solubility, hydrogen solubility and diffusivity, resistance to localized corro‐

On the other hand, the second group of properties including bulk density, thermal expansion, Young's modulus resistance to salt spray environment, and saturation magnetization are little

**2.4. Properties of electrodeposited nanocrystalline Ni-Fe alloys**

sion and intergranular stress corrosion cracking, and thermal stability.

electrode that inhibits the nickel reduction.

**2.3. Phase formation**

% to 40wt% nickel.

affected by grain size [5].

**Figure 2.** SEM of electrodeposited Ni-Fe layers at current density 20 mV/cm2 and deposition time 100 min. as a func‐ tion of Ni2+/Fe2+ mass ratio in the electreolyte [10].

#### **2.2. Mechanism of electro deposition of Ni-Fe alloys**

Electro deposition of Ni-Fe alloys exhibit the phenomenon of "anomalous codeposition". This term introduced by Brenner [11] is being used to describe the preferential deposition of the less noble metal, Fe, to the more noble metal, Ni. In other words, the reduction of nickel is inhibited while the deposition of iron is enhanced when compared with their individual deposition rates. According to Afshar et al. [12], the electrode position of nickel-iron alloys is a diffusion-controlled process with typical nucleation mechanism. According to Krause et al. [13], the anomalous behavior was assumed due to precipitation of iron hydroxide on surface electrode that inhibits the nickel reduction.

$$\text{Ni}^{\circ}/\text{nFe}^{\circ} + \text{H}^{+} \rightarrow \text{1/2}\ \text{Fe}^{2+} + \text{H}\_{\text{atom}} \cdot \text{Ni}^{\circ}/\text{ (n-1/2) Fe} \tag{1}$$

$$\text{H}\_{\text{atom}}\text{ - Ni\text{\textquotedblleft}(n-1/2)}\text{ Fe\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft R-Cl}\rightarrow\text{Ni\textquotedblright}\text{\textquotedblright}\text{\textquotedblleft R-H}+\text{H-Cl}\tag{2}$$

$$\text{H}\_{\text{atom}} \cdot \text{Ni}^{\circ} / \text{(n-1/2)} \cdot \text{Fe}^{\circ} \text{+H} \cdot \rightarrow \text{1/2} \cdot \text{Fe}^{2+} + \text{Ni}^{\circ} / \text{(n-1)} \text{Fe}^{\circ} \text{+H}\_2 \text{(side reaction)} \tag{3}$$

$$\text{Ni}^{\circ}\text{/n }\text{Fe}^{\circ} + 2\text{H}^{+} \rightarrow \text{Fe}^{2+} + \text{H}\_{2} + \text{Ni}^{\circ}\text{/(n-1) }\text{Fe}^{\circ}\text{(side reaction)}\tag{4}$$

$$\text{O}\_2\text{(g)} + 2\text{H}\_2\text{O} + 4\text{e} \to 4\text{OH}^-\tag{5}$$

#### **2.3. Phase formation**

in elongated elliptical ones and some grain boundaries can be seen. By raising the iron content and thus decreasing the Ni2+/Fe2+ mass ratio down to 9.8 (Figure 2(d)), the surface morphology showed rough cauliflower structure. The cauliflower morphology particle is made of coagulate particle distributed all over the surface with a flattened grains. The grains size decreased with increasing the iron content, especially in case of short time electroplating. Increasing the electroplating time had no significant effect on grain size. The microhardness of the materials followed the regular Hall-Petch relationship with a maximum value (762 Hv) when applying

**Figure 2.** SEM of electrodeposited Ni-Fe layers at current density 20 mV/cm2 and deposition time 100 min. as a func‐

Electro deposition of Ni-Fe alloys exhibit the phenomenon of "anomalous codeposition". This term introduced by Brenner [11] is being used to describe the preferential deposition of the

Ni2+/Fe2+ mass ratio equal to 9.8.

126 Modern Surface Engineering Treatments

tion of Ni2+/Fe2+ mass ratio in the electreolyte [10].

**2.2. Mechanism of electro deposition of Ni-Fe alloys**

One of the important observations in the Ni-Fe alloys is the dependence of the crystal structure on the iron and nickel content in the deposited layers. Figure 3 shows X-ray diffraction patterns of various nanocrystalline Ni-Fe electrodeposits ranging in nickel content from 0 to 100%. Ni-Fe deposits with low nickel concentrations were found to have a body centered cubic (BCC) structure, while those with high nickel concentrations had a face-centered (FCC) structure [13]. While a mixed FCC/BCC structure was observed for nickel concentrations ranging from 10wt % to 40wt% nickel.

#### **2.4. Properties of electrodeposited nanocrystalline Ni-Fe alloys**

The first group of properties are strongly dependent on grain size. These include strength, ductility and hardness, wear resistance and coefficient of friction, electrical resistivity, coercivity, solid solubility, hydrogen solubility and diffusivity, resistance to localized corro‐ sion and intergranular stress corrosion cracking, and thermal stability.

On the other hand, the second group of properties including bulk density, thermal expansion, Young's modulus resistance to salt spray environment, and saturation magnetization are little affected by grain size [5].

elsewhere for nanocrystalline materials. Others have only reported a reduction in the Hall-

The grain size dependence of the proof stress was found to obey the Hall-Petch relationship; however, at constant grain size, lower values were always obtained with the equiaxed geometry, are shown in Table 2. In addition to the remarkable increases in hardness, yield strength, and ultimate tensile strength with decreasing grain size, it is interesting to note that the work hardening coefficient decreases with decreasing grain size to virtually zero at a grain size of 10 nm. The ductility of the material decreases with decreasing grain size from 50% elongation to failure in tension for conventional material to 15% at 100nm grain size and about 1% at 10 nm grain size. Generally somewhat greater ductility was observed in bending. A slight recovery in ductility was observed for grain sizes less than 10 nm. Compared to conventional polycrystalline Ni, nanocrystalline Ni electrodeposits exhibited drastically reduced wear rates and lower coefficients of friction as determined in dry air pin-on-disc tests. Contrary to earlier measurements on nanocrystalline materials prepared by consolidation of precursor powder particles, nanocrystalline nickel electrodeposits do not show a significant reduction in Young's modulus [4, 14]. This result provides further support for earlier findings of Krstic et al. [15], and Zugic et al. [16], which demonstrated that the previously reported reductions in modulus

**Property Conventional Nano Ni, 100 nm Nano Ni, 10 nm**

900 -- 2000 -- 1 -- 204 650 0.0 -- 79 0.5

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103 -- 403 -- 50 -- 207 140 0.4 241 1330 0.9

Due to Hall-Petch strengthening, nanocrystalline alloys offer significantly increased strength and hardness over conventional alloys. The table 3 summarizes tensile test data for Prem alloy (Fe-80% Ni- 4.8% Mo) and a nanocrystalline Ni-Fe alloy close to the Prem alloy in composition with average grain size between 10-15 nm. It is obvious that the yield strength, ultimate tensile strength and Vickers hardness values of the nanocrystalline Ni~20% Fe alloy significantly exceed those for the conventional Prem alloy. While the ductility represented in the elongation percentage of the conventional Prem alloy is much greater than that of the nanocrystalline

with nanoprocessing were likely the result of high residual porosity.

Petch slope in the nanometer range [5].

Yield strength, MPa (25 ⁰C) Yield strength, MPa (350 ⁰C)

Ultimate tensile strength, MPa (25 ⁰C) Ultimate tensile strength, MPa (350 C) Tensile elongation, % (25 ⁰C) Elongation in bending, % (25 ⁰C) Modulus of elasticity, GPa (25 ⁰C) Vickers Hardness, Kg/mm2 Work hadnening coefficient

Fatigue strength, MPa (108 cycles/air/ 25 ⁰C) Wear rate (dry air pin on disc, μm3/μm Coefficient of friction (dry air pin on disc)

**Table 2.** Mechanical properties of conventional and nanocrystalline Nickel

**Figure 3.** X-Ray diffraction patterns of electrodeposited Ni-Fe alloys with various Fe concentrations [13]

#### *2.4.1. Mechanical properties*

The plastic deformation behavior of electrodeposited nanocrystalline materials is strongly dependent on grain size. Initial increases, followed by significant decreases in hardness are observed with decreasing grain size (d) in the nanocrystal range, i.e., d ≤20 nm. The observed decreases in hardness are contrary to Hall-Petch behavior and consistent with results reported elsewhere for nanocrystalline materials. Others have only reported a reduction in the Hall-Petch slope in the nanometer range [5].

The grain size dependence of the proof stress was found to obey the Hall-Petch relationship; however, at constant grain size, lower values were always obtained with the equiaxed geometry, are shown in Table 2. In addition to the remarkable increases in hardness, yield strength, and ultimate tensile strength with decreasing grain size, it is interesting to note that the work hardening coefficient decreases with decreasing grain size to virtually zero at a grain size of 10 nm. The ductility of the material decreases with decreasing grain size from 50% elongation to failure in tension for conventional material to 15% at 100nm grain size and about 1% at 10 nm grain size. Generally somewhat greater ductility was observed in bending. A slight recovery in ductility was observed for grain sizes less than 10 nm. Compared to conventional polycrystalline Ni, nanocrystalline Ni electrodeposits exhibited drastically reduced wear rates and lower coefficients of friction as determined in dry air pin-on-disc tests. Contrary to earlier measurements on nanocrystalline materials prepared by consolidation of precursor powder particles, nanocrystalline nickel electrodeposits do not show a significant reduction in Young's modulus [4, 14]. This result provides further support for earlier findings of Krstic et al. [15], and Zugic et al. [16], which demonstrated that the previously reported reductions in modulus with nanoprocessing were likely the result of high residual porosity.


**Table 2.** Mechanical properties of conventional and nanocrystalline Nickel

*2.4.1. Mechanical properties*

128 Modern Surface Engineering Treatments

The plastic deformation behavior of electrodeposited nanocrystalline materials is strongly dependent on grain size. Initial increases, followed by significant decreases in hardness are observed with decreasing grain size (d) in the nanocrystal range, i.e., d ≤20 nm. The observed decreases in hardness are contrary to Hall-Petch behavior and consistent with results reported

**Figure 3.** X-Ray diffraction patterns of electrodeposited Ni-Fe alloys with various Fe concentrations [13]

Due to Hall-Petch strengthening, nanocrystalline alloys offer significantly increased strength and hardness over conventional alloys. The table 3 summarizes tensile test data for Prem alloy (Fe-80% Ni- 4.8% Mo) and a nanocrystalline Ni-Fe alloy close to the Prem alloy in composition with average grain size between 10-15 nm. It is obvious that the yield strength, ultimate tensile strength and Vickers hardness values of the nanocrystalline Ni~20% Fe alloy significantly exceed those for the conventional Prem alloy. While the ductility represented in the elongation percentage of the conventional Prem alloy is much greater than that of the nanocrystalline Ni~20% Fe alloy. Also figure 4 shows the Vickers hardness of nanocrystalline Ni-Fe alloys as a function of Iron content in the deposits along with the hardness values of for various conventional Ni-Fe alloys. The average hardness of the nanocrystalline Ni-Fe alloys is approximately 4 to 7 times higher than that of the conventional alloys as seen in table 3 and Figure 4. Besides, Figure 4 shows that there is a moderate decrease in the hardness with increasing the Iron content in the FCC range and a significant increase with Fe- content in the BCC range and a minimum hardness at the FCC-BCC transition [14].

nanocrystalline materials produced by crystallization of amorphous precursor materials; both beneficial and detrimental effects of the nanostructure formation on the corrosion performance were observed. The conflicting results are, to a large extent, due to the poorly characterized microstructures of the crystallized amorphous materials. On the other hand, for nanostruc‐ tured materials produced by electro deposition, considerable advances in the understanding

In previous studies, potentiodynamic and potentiostatic polarizations in de-aerated 2N

Ni at grain sizes of 32, 50, and 500 nanometers and compared with polycrystalline pure Ni (grain size of 100 μm). Figure 5 shows the potentiodynamic anodic polarization curves of these specimens. The nanocrystalline specimens exhibit the same active-passive-transpassive behavior typical of conventional Ni. However; differences are evident in the passive current density and the open circuit potential. The nanocrystalline specimens show a higher current density in the passive region resulting in higher corrosion rates. These higher current densities were attributed to the higher grain boundary and triple junction content in the nanocrystalline specimens, which provide sites for electrochemical activity. However, this difference in current density diminishes at higher potentials (1100 mV SCE) at which the overall dissolution rate

coupons, 0.2 mm thick) nanocrystalline pure

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of microstructure on the corrosion properties have been made in recent years [18].

overcomes the structure-controlled dissolution rate observed at lower potentials [19].

**Figure 5.** Potentiodynamic polarization curves for nanocrystalline and polycrystalline Ni in 2N H2SO4 at ambient tem‐

Figure 6 shows scanning electron micrographs of nickel with a) 32 nm and b) 100 μm grain size, held potentiostatically at 1200 mV (SCE) in 2NH2SO4 for 2000 seconds. Both specimens exhibit extensive corrosion but the nanocrystalline Ni is more uniformly corroded while the specimen with 100 μm grain size shows extensive localized attack along the grain boundaries and triple junctions. X-ray photoelectron spectroscopy of the specimens polarized in the

H2SO4 (pH = 0) were conducted on bulk (2 cm2

perature [19].


**Table 3.** Mechanical properties of nanocrystalline Ni-20%Fe and conventional Prem alloy

**Figure 4.** Vickers hardness as a function of iron content for various conventional and nanocrystalline Ni-Fe alloys [14].

#### *2.4.2. Corrosion properties*

In general, the corrosion resistance of nanocrystalline materials in aqueous solutions is of great importance in assessing a wide range of applications. To date, research in this area is still scarce and relatively few studies have addressed this issue. For the case of the corrosion behavior of nanocrystalline materials produced by crystallization of amorphous precursor materials; both beneficial and detrimental effects of the nanostructure formation on the corrosion performance were observed. The conflicting results are, to a large extent, due to the poorly characterized microstructures of the crystallized amorphous materials. On the other hand, for nanostruc‐ tured materials produced by electro deposition, considerable advances in the understanding of microstructure on the corrosion properties have been made in recent years [18].

Ni~20% Fe alloy. Also figure 4 shows the Vickers hardness of nanocrystalline Ni-Fe alloys as a function of Iron content in the deposits along with the hardness values of for various conventional Ni-Fe alloys. The average hardness of the nanocrystalline Ni-Fe alloys is approximately 4 to 7 times higher than that of the conventional alloys as seen in table 3 and Figure 4. Besides, Figure 4 shows that there is a moderate decrease in the hardness with increasing the Iron content in the FCC range and a significant increase with Fe- content in the

**Ultimate tensile**

2250 550

**Figure 4.** Vickers hardness as a function of iron content for various conventional and nanocrystalline Ni-Fe alloys [14].

In general, the corrosion resistance of nanocrystalline materials in aqueous solutions is of great importance in assessing a wide range of applications. To date, research in this area is still scarce and relatively few studies have addressed this issue. For the case of the corrosion behavior of

**strength % Elongation**

3-4% 30%

**Vickers Hardness (VHN)**

> 550-600 100

BCC range and a minimum hardness at the FCC-BCC transition [14].

**Table 3.** Mechanical properties of nanocrystalline Ni-20%Fe and conventional Prem alloy

**0.2% Offset MPa**

1785 207

**Material Yield strength**

Nano-Ni-20% Fe Conv Premalloy

130 Modern Surface Engineering Treatments

*2.4.2. Corrosion properties*

In previous studies, potentiodynamic and potentiostatic polarizations in de-aerated 2N H2SO4 (pH = 0) were conducted on bulk (2 cm2 coupons, 0.2 mm thick) nanocrystalline pure Ni at grain sizes of 32, 50, and 500 nanometers and compared with polycrystalline pure Ni (grain size of 100 μm). Figure 5 shows the potentiodynamic anodic polarization curves of these specimens. The nanocrystalline specimens exhibit the same active-passive-transpassive behavior typical of conventional Ni. However; differences are evident in the passive current density and the open circuit potential. The nanocrystalline specimens show a higher current density in the passive region resulting in higher corrosion rates. These higher current densities were attributed to the higher grain boundary and triple junction content in the nanocrystalline specimens, which provide sites for electrochemical activity. However, this difference in current density diminishes at higher potentials (1100 mV SCE) at which the overall dissolution rate overcomes the structure-controlled dissolution rate observed at lower potentials [19].

**Figure 5.** Potentiodynamic polarization curves for nanocrystalline and polycrystalline Ni in 2N H2SO4 at ambient tem‐ perature [19].

Figure 6 shows scanning electron micrographs of nickel with a) 32 nm and b) 100 μm grain size, held potentiostatically at 1200 mV (SCE) in 2NH2SO4 for 2000 seconds. Both specimens exhibit extensive corrosion but the nanocrystalline Ni is more uniformly corroded while the specimen with 100 μm grain size shows extensive localized attack along the grain boundaries and triple junctions. X-ray photoelectron spectroscopy of the specimens polarized in the

compared to conventional polycrystalline Ni; however, the nanostructured materials were

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Comparing bulk nickel to nanocrystalline nickel, it is found that the bulk nickel was more resistant to anodic dissolution once the free corrosion potential had established. This interest‐ ing result because it indicates that once free corrosion conditions have been established, the surface of nanocrystalline nickel is more susceptible to corrosion than bulk nickel. It is known that the primary passivation potential of binary Ni-Fe alloys generally increase with increasing nickel concentration, comparatively little study has been conducted on the corrosion behavior of these alloys in nanocrystalline form. A study on the pitting behavior of nanocrystalline Ni-18% Fe found that it was more susceptible to pitting corrosion after significant grain growth had occurred during annealing [20]. Another study of the corrosion resistance of electrode‐ posited nanocrystalline Ni-W and Ni-Fe-W alloys reported poor corrosion resistance for the ternary alloy because of preferential dissolution of Fe. While alloy concentration effects on the corrosion rate of electrodeposited nanocrystalline Ni-Fe alloys remain to be clearly established, as the Iron content in the alloy is increased, the corrosion rate is increased simultaneously [21].

As the average grain size in the nanocrystalline materials is reduced to the extent that the domain wall thickness is comparable to the grain size, the coercively is found to dramatically decrease while for the permeability of such alloys will increase. Another consequence of the ultra-fine grain size of nanocrystalline materials is an increase in the electrical sensitivity over the polycrystalline materials due to the high volume of grain boundaries. The electrical resistivity of electrodeposited nanocrystalline Ni-Fe alloys has been found to increase consid‐

The grain size reduction to about 10nm in fully dense electrodeposited material has no major effect on the thermal expansion. Comparing between the coefficients of thermal expansion of nanocrystalline Fe-43wt% Ni to that of Ni-Fe conventional alloys it was found that both have

Nickel-iron alloys are of great commercial interest as a result of their low thermal expansion

The nanocrystalline Ni-Fe alloys are used in the integrated circuit packaging and shadow masks for cathode ray tubes where they require a low coefficient of thermal expansion and also additional strength would be beneficial. For the integrated circuit packaging materials, they require materials with thermal coefficient matched to those of silicon to prevent the

erably as the grain size decreases to less than 100 nm [5, 22, 23].

much more immune to localized attack which often can lead to catastrophic failures.

*2.4.3. Electric and magnetic properties*

*2.4.4. Coefficient of thermal expansion*

**2.5. Applications of Ni-Fe alloys**

*2.5.1. Low thermal expansion applications of Ni-Fe alloys*

and soft magnetic properties.

similar values [18].

**Figure 6.** SEM micrographs of Ni with (a) 100 μm and (b) 32 nm grain size held potentiostatically at 1200 mV (SCE) in 2N H2SO4 for 2000 seconds [19].

passive region proved that the passive film formed on the nanostructured specimen is more defective than that formed on the polycrystalline specimen, while the thickness of the passive layer was the same on both specimens. This higher defective film on the nanocrystalline specimen allows for a more uniform breakdown of the passive film, which in turn leads to a more uniform corrosion. In contrast, in coarse-grained Ni the breakdown of the passive film occurs first at the grain boundaries and triple junctions rather than the crystal surface, leading to preferential attack at these defects [19].

The corrosion behavior of nanocrystalline Ni was also studied in 30 wt% KOH solution and pH neutral solution containing 3 wt% sodium chloride. The results were similar to the corrosion behavior observed in sulfuric acid. The general corrosion was somewhat enhanced compared to conventional polycrystalline Ni; however, the nanostructured materials were much more immune to localized attack which often can lead to catastrophic failures.

Comparing bulk nickel to nanocrystalline nickel, it is found that the bulk nickel was more resistant to anodic dissolution once the free corrosion potential had established. This interest‐ ing result because it indicates that once free corrosion conditions have been established, the surface of nanocrystalline nickel is more susceptible to corrosion than bulk nickel. It is known that the primary passivation potential of binary Ni-Fe alloys generally increase with increasing nickel concentration, comparatively little study has been conducted on the corrosion behavior of these alloys in nanocrystalline form. A study on the pitting behavior of nanocrystalline Ni-18% Fe found that it was more susceptible to pitting corrosion after significant grain growth had occurred during annealing [20]. Another study of the corrosion resistance of electrode‐ posited nanocrystalline Ni-W and Ni-Fe-W alloys reported poor corrosion resistance for the ternary alloy because of preferential dissolution of Fe. While alloy concentration effects on the corrosion rate of electrodeposited nanocrystalline Ni-Fe alloys remain to be clearly established, as the Iron content in the alloy is increased, the corrosion rate is increased simultaneously [21].

#### *2.4.3. Electric and magnetic properties*

As the average grain size in the nanocrystalline materials is reduced to the extent that the domain wall thickness is comparable to the grain size, the coercively is found to dramatically decrease while for the permeability of such alloys will increase. Another consequence of the ultra-fine grain size of nanocrystalline materials is an increase in the electrical sensitivity over the polycrystalline materials due to the high volume of grain boundaries. The electrical resistivity of electrodeposited nanocrystalline Ni-Fe alloys has been found to increase consid‐ erably as the grain size decreases to less than 100 nm [5, 22, 23].

#### *2.4.4. Coefficient of thermal expansion*

The grain size reduction to about 10nm in fully dense electrodeposited material has no major effect on the thermal expansion. Comparing between the coefficients of thermal expansion of nanocrystalline Fe-43wt% Ni to that of Ni-Fe conventional alloys it was found that both have similar values [18].

#### **2.5. Applications of Ni-Fe alloys**

passive region proved that the passive film formed on the nanostructured specimen is more defective than that formed on the polycrystalline specimen, while the thickness of the passive layer was the same on both specimens. This higher defective film on the nanocrystalline specimen allows for a more uniform breakdown of the passive film, which in turn leads to a more uniform corrosion. In contrast, in coarse-grained Ni the breakdown of the passive film occurs first at the grain boundaries and triple junctions rather than the crystal surface, leading

**Figure 6.** SEM micrographs of Ni with (a) 100 μm and (b) 32 nm grain size held potentiostatically at 1200 mV (SCE) in

The corrosion behavior of nanocrystalline Ni was also studied in 30 wt% KOH solution and pH neutral solution containing 3 wt% sodium chloride. The results were similar to the corrosion behavior observed in sulfuric acid. The general corrosion was somewhat enhanced

to preferential attack at these defects [19].

2N H2SO4 for 2000 seconds [19].

132 Modern Surface Engineering Treatments

Nickel-iron alloys are of great commercial interest as a result of their low thermal expansion and soft magnetic properties.

#### *2.5.1. Low thermal expansion applications of Ni-Fe alloys*

The nanocrystalline Ni-Fe alloys are used in the integrated circuit packaging and shadow masks for cathode ray tubes where they require a low coefficient of thermal expansion and also additional strength would be beneficial. For the integrated circuit packaging materials, they require materials with thermal coefficient matched to those of silicon to prevent the formation of cracks, de-lamination and/or de-bonding of the different materials during thermal cycles to which the components are exposed [14].

from water electrolysis [28]. Nanocrystalline Ni-Mo alloys have been processed by different techniques such as RF magnetron sputtering, mechanical alloying and electrodeposition techniques. H. Jin-Zhao et al. [29] reported that nanocrystalline Ni-Mo alloys prepared by RF magnetron sputtering technique to be promising electrodes for hydrogen evolution reactions. While P. Kedzierzawski et al. [30] were able to produce successfully nanocrystalline Ni-Mo alloys by mechanical alloying method, as well as indicating the positive contribution of the large surface area in increasing the catalytic effect as electrodes for hydrogen production and

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Furthermore, several attempts have been made in order to produce nanostructured Ni-Mo alloys by electro deposition technique to be used as cathodes for hydrogen production from water electrolysis. This is because electro deposition is considered to be cheaper than other production techniques -being mentioned previously- from the aspect of initial capital invest‐ ment and running costs [31, 32]. While it also require minor modifications to existing conven‐ tional plating lines to be able to produce nanocrystalline films or stand free objects, in addition to that scaling up is relatively easy and high production rates can also be achieved [33].

Ni-Mo deposits have been well known for their use as cathodes for hydrogen production from water by electrolysis as well as catalysts for hydrogen production by steam reforming of hydrocarbons [7]. Arul Raj and Venkatesan [34] showed an increased electrocatalytic effect of Ni-Mo electrodeposited alloys for the hydrogen evolution reaction than that showed by nickel and other nickel-based binary alloys such as Ni-Co, Ni-W, Ni-Fe, and Ni-Cr. In addition, Ni-Mo alloys are considered as highly corrosion resistant due to the good corrosion protection characteristics of molybdenum in non oxidizing solutions of hydrochloric, phosphoric, and hydrofluoric acid at most concentrations and temperatures and in boiling sulfuric acid up to about 60% concentration [35]. The nickel-molybdenum alloys normally containing 26–35wt% Mo are among the few metallic materials that are resistant to corrosion by hydrochloric acid at all concentrations and temperatures [36]. Electro deposition is one of the most promising techniques for producing nanostructure materials owing to its relative low cost compared to the other methods. Electro deposition produces nanocrystalline materials when the deposition parameters (e.g., plating bath composition, pH, temperature, current density, etc.) are opti‐ mized such that electrocrystallization results in massive nucleation and reduced grain growth [37, 38]. Due to better anticorrosive in several aggressive environments, mechanical and thermal stability characteristics of Ni-Mo alloys, the electro deposition of these alloys plays an

important role. It is an example of the induced codeposition mechanism [39].

According to J. Halim et al [40], Ni-Mo nanocrystalline deposits (7–43 nm) with a nodular morphology (Figure 7) were prepared by electro deposition using direct current from citrateammonia solutions. They exhibited a single Ni-Mo solid solution phase. Increasing the applied current density led to a decrease of the molybdenum content in the deposited alloys, increase in crystallite size, and increase of the surface roughness. The highest microhardness value (285 Hv) corresponded to nanodeposits with 23% Mo. The highest corrosion resistance accompa‐ nied by relatively high hardness was detected for electrodeposits containing 15% Mo. Mo content values between 11 and 15% are recommended for obtaining better electrocatalytic activity for Hydrogen evolution reaction "HER" with the lowest cathodic Tafel's constant.

decrease of the exchange current density.

In the color cathode ray tube televisions and computer monitors, the shadow mask is a perforated metal sheet which the electrons from the electron gun must pass through before reaching the phosphor screen. Its role is to ensure that the electron beam hits only the correct colored dots and does not illuminate more than the one that was intended. Only 20% of the electrons pass through the shadow mask, and the other 80% are being absorbed by the mask which leads to an increase in the temperature of the mask. The resulting thermal expansion can disturb the alignment between the apertures and the phosphor triads, leading to a distorted image. This effect is known as "doming [14].

The main advantages of using electrodeposited nanocrystalline over conventional Ni-Fe alloys -for the use in integrated circuit packaging or shadow masks- for cathode ray tubes include:


#### *2.5.2. Magnetic applications of Ni-Fe alloys*

For soft magnetic applications, such as electromagnetic shielding, transformers materials, read-write heads, and high efficiency motors, magnetic materials- that exhibit small hysteresis losses per cycle- are required. More specifically, materials that have high permeability, low coercivity, high saturation and remnant magnetization, high electrical resistivity (to minimize losses due to eddy current formation [23-25]
