3. Co▬Mo▬Zr electrolytic alloys

the ratio of the molybdate and tungstate concentrations in a solution are the tools

Current efficiency decreases in the range of 87–52% when increasing on-time is caused by the hydrogen evolution enhancement (Figure 5a). Prolong the pause as compared with pulse time positively influences the current efficiency as subsequent chemical reactions accompanying the alloying metals discharge are more fully. A larger current interruption than for 20 ms reduces the efficiency of the process (Figure 5b) as it was also observed in [34]. Thus, high refractory metal content in the Co▬Mo▬W alloy while maintaining a reasonable value of current efficiency Ce 70–75% at a current density of 10 A dm<sup>2</sup> is achieved with ton/toff = 5/20 ms.

Figure 6 shows X-ray diffraction patterns for Co▬Mo▬W alloys deposited on a

(Figure 6, black line) and a direct current density of 4 A dm<sup>2</sup> (Figure 6, red line);

The X-ray diffraction patterns indicate an amorphous-crystalline structure of the alloys. The high intensity peaks at 60 and 90° are copper substrate lines. We can see some peaks corresponding to α-Co phase, intermetallic phase Co7W6 as previously for binary Co▬W alloy [32], additional reflections of the intermetallic compound Co7Mo6, and rather wide halo with width about 15° is detected at angles 2θ of 43–58° (Figure 6) that reflects amorphous structure for both coatings deposited at pulse and direct current. The most important fact is the appearance of reflexes of metallic molybdenum and tungsten on XRD patterns for Co▬Mo▬W alloys deposited at pulse current. Such a character of X-ray diffraction patterns for coatings obtained by pulse current confirms our proposed mechanism of alloy formation [14, 28]. Metallic tungsten and molybdenum are formed in the chemical stage of reduction of refractory metal intermediate oxides during the pause of polarization. Such a pattern, along with decreasing tungsten content in the ternary alloy Co▬Mo▬W compared with a binary Co▬W [32], also indicates molybdenum and tungsten competition during deposition in the alloy. The coherent-scattering region

X-ray diffraction patterns for electrolytic alloys Co▬Mo▬W of composition, at. %: (1) Co—75, Mo—16,

; ton/toff = 2/10 ms

copper substrate at a pulse current amplitude of 8 A dm<sup>2</sup>

for ternary alloy composition control.

Applied Surface Science

T = 30°C; coating thickness is 20 μm.

size of the amorphous part is 2–8 nm.

Figure 6.

100

W—9; (2) Co—85, Mo—9, W—5.

The molybdenum content in Co▬Mo▬Zr alloys changes with current density increasing in the range of 2–4 A dm<sup>2</sup> (Figure 7a) similar to the Co▬Mo▬W films. At the same time, we observe a wider range of current densities of 4–8 A dm<sup>2</sup> providing coatings with molybdenum content of 24–25 at. %. Coating enrichment by molybdenum with increasing current density is entirely predictable since the molybdate electrochemical behavior is associated with multi-stage process following by chemical reducing of intermediate molybdenum oxides with hydrogen adatoms Had [27, 29, 35]. The cathode potential at Co▬Mo▬Zr electrodeposition is rather negative (2.0–2.8) V (Figure 7b) and it becomes more negative with current density, that resulting in acceleration of side reaction producing Had which are involved in a chemical step of reducing intermediate molybdenum oxides. Exactly due to above reasons, the molybdenum content in the deposits is increased. However, at current densities above 8 A dm<sup>2</sup> , hydrogen evolution reaction becomes the dominant as evidenced by the decreasing current efficiency (Figure 7b), whereby the molybdenum content in the alloy decreases.

The zirconium content in the ternary coatings reaches 3.6–3.7 at. % with increasing current density up to 4 A dm<sup>2</sup> (Figure 7a). As it follows from the experimental data as well as previous investigations [26], molybdenum and zirconium competition is observed when codeposited to get the ternary alloy at higher current density than of 4 A dm<sup>2</sup> . Such behavior is associated with the different mechanisms of alloying metals reducing from citrate-pyrophosphate electrolyte. Really, molybdate reducing to metallic state proceeds through the six electrons transfer accompanied by the removal of four oxygen atoms. Therefore, deposits may also contain incompletely reduced intermediate molybdenum oxides. Zirconium is likely included in the deposit in the form of oxygen compounds ZrOx due to higher binding energy Zr–O [31]. The EDS analysis data confirm the oxygen availability in the composition of the surface layers [31].

Nonlinear dependences of current efficiency Ce on the deposition current density (Figure 7b) were obtained, and Ce increases by 20% and reaches 63% with a rising current density from 5 to 8 A dm<sup>2</sup> ; however, further increase in i reduces the current efficiency up to 47%. Such behavior may be attributed with acceleration of site hydrogen evolution reaction at more negative potentials.

Pulsed electrolysis allows the use of higher current densities, and not only energy parameter but also polarization time ton and current interruption time toff as well as its ratio are effectively used to control deposit composition and current efficiency. The shortest pulse duration should ensure the achievement of the alloy

#### Figure 7.

Pulse current density influence on the composition (a) and current efficiency (b) for Co▬Mo▬Zr coatings; ton/toff 2/10 ms; T 20–25°C; рН 8; plated time 30 min.

deposition potential, as well as ton is limited by the requirements for the visual quality of coatings and efficiency of electrolysis while minimizing side reactions.

Electrodeposition of Co▬Mo▬Zr alloys is the relevant example to demonstrate the benefits and flexibility of pulsed electrolysis control. Molybdenum and zirconium content in the alloys deposited at a current density of 4 A dm<sup>2</sup> rises when increasing pulse time of 0.5–2 ms while maintaining pause time 10 ms (Figure 8a). Obviously, the real current value increases at the expense of a full signal handling, thereby achieving potential of alloying metal reduction in alloy. However, an increase in the pulse duration of more than 2 ms reduces the zirconium content; therefore, it does not seem appropriate.

As follows from the experimental data (Figure 8b) prolong current interruption time of 5–10 ms everything else being equal promotes zirconium content in the alloy of 2.1 up to 3.7 at. %; although increasing the pause reduces the incorporation of this metal in the alloy. Thus, the top zirconium content in the electrolytic deposits is reached at the ratio ton/toff = 2/10 ms (duty factor q = 10 and f = 85 Hz). It should be stated, the molybdenum percentage in Co▬Mo▬Zr deposits rises from 16.0 to 24.0 at. % with the pause duration (Figure 8b) due to more complete chemical reducing of intermediate molybdenum oxides by Had. It also confirms difference in zirconium and molybdenum reducing mechanism as well as their competition when deposited into the alloy.

As for Co▬Mo▬W alloy, current efficiency of Co▬Mo▬Zr deposition decreases with increasing pulse time due to acceleration of side reaction. Prolonging the pause contributes to the current efficiency as the following chemical reactions accompanying the alloying metal discharge are more full; and a larger current interruption reduces the efficiency of the process. Thus, current efficiency reaches maximum 98% when toff = 50 ms and ton = 2 ms.

Surface morphology of Co▬Mo▬Zr coatings changes with increasing the current density amplified internal stress that leads to fracture grid (Figure 9). The coating surface becomes less smooth and more globular, and the crystallite sizes increase at the higher polarization and respectively at larger molybdenum content (Figure 9a and c). EDS analysis data (Figure 10) show sufficiently uniform distribution of the alloying metals on uneven relief of the deposits which is typical for pulsed electrolysis and emphasizes its advantage over stationary. Increasing in polarization time at a fixed pause contributes growth of irregular spheroids on the surface; and microcracks become larger as observed in [36]. Furthermore, the coatings deposited at the current density of 6–8 A dm<sup>2</sup> and on-time of 10 ms are more porous as compared with other (Figure 9b and c), apparently due to accelerated hydrogen evolution.

#### Figure 8.

Dependence of Co▬Mo▬Zr coating composition on the time of pulse ton (a) (toff 10 ms) and pause toff (b) (ton 2 ms); i = 4 A dm<sup>2</sup> ; T = 20–25°C; рН 8; plated time 30 min.

Figure 11 shows X-ray diffraction patterns for electrolytic alloys Co▬Mo▬Zr deposited at a pulse current amplitude of 4 A dm<sup>2</sup> (Figure 11 black line) and 8 A dm<sup>2</sup> (Figure 11 red line). A series of diffraction lines for α-Co on X-ray diffraction patterns for Co▬Mo▬Zr deposits on steel substrates was obtained (Figure 11). The high intensity peaks at 52, 60, and 90° are copper substrate lines.

,

Distribution of alloying elements at picks and valleys of coating Co▬Mo▬Zr deposited at 4 A dm<sup>2</sup>

Morphology (2000) and composition (at. %) of Co▬Mo▬Zr coatings deposited in pulse mode at current

: 4 (a); 6 (b) and 8 (c); T = 20–25°C; рН 8; plated time 30 min.

Composition Electrolytic Coatings with Given Functional Properties

DOI: http://dx.doi.org/10.5772/intechopen.84519

Figure 9.

Figure 10.

103

ton/toff = 2/10.

density, A dm<sup>2</sup>

#### Composition Electrolytic Coatings with Given Functional Properties DOI: http://dx.doi.org/10.5772/intechopen.84519

#### Figure 9.

deposition potential, as well as ton is limited by the requirements for the visual quality of coatings and efficiency of electrolysis while minimizing side reactions. Electrodeposition of Co▬Mo▬Zr alloys is the relevant example to demonstrate the benefits and flexibility of pulsed electrolysis control. Molybdenum and zirconium content in the alloys deposited at a current density of 4 A dm<sup>2</sup> rises when increasing pulse time of 0.5–2 ms while maintaining pause time 10 ms (Figure 8a). Obviously, the real current value increases at the expense of a full signal handling, thereby achieving potential of alloying metal reduction in alloy. However, an increase in the pulse duration of more than 2 ms reduces the zirconium content;

As follows from the experimental data (Figure 8b) prolong current interruption time of 5–10 ms everything else being equal promotes zirconium content in the alloy of 2.1 up to 3.7 at. %; although increasing the pause reduces the incorporation of this metal in the alloy. Thus, the top zirconium content in the electrolytic

deposits is reached at the ratio ton/toff = 2/10 ms (duty factor q = 10 and f = 85 Hz). It should be stated, the molybdenum percentage in Co▬Mo▬Zr deposits rises from 16.0 to 24.0 at. % with the pause duration (Figure 8b) due to more complete chemical reducing of intermediate molybdenum oxides by Had. It also confirms difference in zirconium and molybdenum reducing mechanism as well as their

As for Co▬Mo▬W alloy, current efficiency of Co▬Mo▬Zr deposition decreases with increasing pulse time due to acceleration of side reaction. Prolonging the pause contributes to the current efficiency as the following chemical reactions accompanying the alloying metal discharge are more full; and a larger current interruption reduces the efficiency of the process. Thus, current efficiency reaches

Surface morphology of Co▬Mo▬Zr coatings changes with increasing the current density amplified internal stress that leads to fracture grid (Figure 9). The coating surface becomes less smooth and more globular, and the crystallite sizes increase at the higher polarization and respectively at larger molybdenum content (Figure 9a and c). EDS analysis data (Figure 10) show sufficiently uniform distribution of the alloying metals on uneven relief of the deposits which is typical for pulsed electrolysis and emphasizes its advantage over stationary. Increasing in polarization time at a fixed pause contributes growth of irregular spheroids on the surface; and microcracks become larger as observed in [36]. Furthermore, the coatings deposited at the current density of 6–8 A dm<sup>2</sup> and on-time of 10 ms are more porous as compared with other (Figure 9b and c), apparently due to acceler-

Dependence of Co▬Mo▬Zr coating composition on the time of pulse ton (a) (toff 10 ms) and pause toff (b) (ton

; T = 20–25°C; рН 8; plated time 30 min.

therefore, it does not seem appropriate.

Applied Surface Science

competition when deposited into the alloy.

maximum 98% when toff = 50 ms and ton = 2 ms.

ated hydrogen evolution.

Figure 8.

102

2 ms); i = 4 A dm<sup>2</sup>

Morphology (2000) and composition (at. %) of Co▬Mo▬Zr coatings deposited in pulse mode at current density, A dm<sup>2</sup> : 4 (a); 6 (b) and 8 (c); T = 20–25°C; рН 8; plated time 30 min.

#### Figure 10.

Distribution of alloying elements at picks and valleys of coating Co▬Mo▬Zr deposited at 4 A dm<sup>2</sup> , ton/toff = 2/10.

Figure 11 shows X-ray diffraction patterns for electrolytic alloys Co▬Mo▬Zr deposited at a pulse current amplitude of 4 A dm<sup>2</sup> (Figure 11 black line) and 8 A dm<sup>2</sup> (Figure 11 red line). A series of diffraction lines for α-Co on X-ray diffraction patterns for Co▬Mo▬Zr deposits on steel substrates was obtained (Figure 11). The high intensity peaks at 52, 60, and 90° are copper substrate lines.

#### Figure 11.

X-ray diffraction patterns for electrolytic alloys Co▬Mo▬Zr of composition, at. %: (1) Co—72.2, Mo—24.1, Zr—3.7; (2) 72.9, Mo—24.9, Zr—2.2.

We can see peaks corresponding to intermetallic compounds Co3Mo and Co7Mo6. Furthermore, one can find a small halo with full width at half maximum about 10° at angles 2θ 48–58° 59°, which indicates an XRD amorphous structure of above materials [34]. Thus, the X-ray diffraction patterns indicate an amorphouscrystalline structure of the alloys. The most important fact is the appearance of reflexes of metallic molybdenum on XRD patterns for Co▬Mo▬Zr alloys deposited at higher current density 8 A dm<sup>2</sup> (Figure 11 red line). In addition, the higher intensity of intermetallic compound reflexes is due to the enrichment of the alloy with the refractory component. The coherent-scattering region size of the amorphous part is 2–6 nm.
