4. Corrosion behavior of binary and ternary cobalt electrolytic alloys

control of corrosion process. Such corrosion performance is mainly due to the basic nature of cobalt (II) oxides and hydroxides, which hinders the oxygen transport. The difference in corrosion rate of substrate and coated samples in such conditions is somewhat less—the deep corrosion index k<sup>h</sup> is reduced by almost two orders of

Corrosive medium based on 1 M Na2SO4 pH 3 3% NaCl (pH 7) pH 11 Eoc, V kh, mm/year Eoc, V kh, mm/year Eoc, V kh, mm/year

Functional properties Application area Reference

High hard, wear, heat resistant and corrosion protective coatings

Corrosion protective coatings, electrocatalytic films

[12, 15, 32]

[26, 31]

Steel substrate 0.62 1.85 0.56 0.93 0.43 0.12 Co76▬Mo24 0.42 0.023 0.25 0.029 0.43 0.003 Co84▬W16 0.32 0.024 0.35 0.01 0.44 0.004 Co83▬Mo12▬W5 0.22 0.012 0.37 0.007 0.47 0.008 Co79▬Mo16▬W5 0.25 0.012 0.38 0.005 0.50 0.007 Co72▬Mo24▬Zr4 0.46 0.003 0.5 0.002 0.47 0.003 Co73▬Mo25▬Zr2 0.47 0.004 0.5 0.003 0.48 0.005

Corrosion indicators (Еoc, V; kh, mm per year) of testing materials in different media.

Composition Electrolytic Coatings with Given Functional Properties

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

Summarizing and relying on the results of earlier studies [12, 15, 26, 31, 32] of the ternary cobalt alloy functional properties, it is possible to present their application areas in the table form (Table 3). This table reflects the composition, appropriative properties (microhardness Hv, corrosion resistance, and catalytic activity in hydrogen evolution reaction indicated as hydrogen exchange current density

Ternary Co▬Mo▬W electrolytic alloys deposited from citrate-pyrophosphate

bath at pulse current composition and surface morphology were shown to be dependent on the current density and on/off time. The top refractory metal content in deposits was obtained at the current density of 9–10 A dm<sup>2</sup> and on/off time of 5/20 ms, but increasing current density diminishes efficiency of electrolysis. Tungsten content in the alloy was found to be much lower than molybdenum: W—2–7 at. % vs

H), and technological application of designed coatings.

5. Summarizing the observations

magnitude.

Electrolytic alloy composition, at. %

Table 2.

Co Mo W

Co Mo Zr

Table 3.

Electrolytic alloy composition, at. %

> 45–80 14–34 5–15

70–80 15–25 2–4

lgi 0

lgi 0

[A/cm<sup>2</sup> ]

[A cm2 ]

Application areas for the cobalt ternary coatings.

Hv 450–1100 MN/m2

Hv 450–900 MN/m<sup>2</sup>

<sup>H</sup> (Co71Mo16W13) = 3.35

H (Co75Mo21Zr4) = 3.1

lgi 0

105

It is obvious that hydrogen ions H<sup>+</sup> are the oxidizing agents for cobalt-based electrolytic coatings in acidic medium, as well as in neutral and alkaline environment, only oxygen is a strong oxidizing agent [37]. Open circuit (corrosion) potentials Еoc of coated samples in corrosive solution with pH 3 are more positive than the potential of steel substrate which indicates the anodic control of corrosion process. The deep corrosion index k<sup>h</sup> is reduced by almost three orders of magnitude (Table 2) and such inhibition in alloy corrosion is due to the acidic nature of refractory metals and zirconia oxides forming on the surface in oxygen containing media. The corrosion index of ternary alloys in acidic and neutral chloridecontaining environment is almost halved compared with binary systems (Table 2). Such corrosion performance is associated with a decrease in roughness and smoothing out the relief of ternary coatings as compared with binary. In addition, the combined presence of molybdenum and tungsten or zirconium in the coatings provides a synergistic effect due to molybdenum and tungsten enhancing corrosion resistance to pitting as well as zirconium increasing tend to passivity. Moreover, with increasing total content of alloying metals, corrosion resistance increases.

Open circuit potentials of coated samples in corrosive solution with pH 11 are slow negative as compared with steel substrate (Table 2), that stipulates cathodic

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


Table 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 amor-

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

4. Corrosion behavior of binary and ternary cobalt electrolytic alloys

It is obvious that hydrogen ions H<sup>+</sup> are the oxidizing agents for cobalt-based electrolytic coatings in acidic medium, as well as in neutral and alkaline environment, only oxygen is a strong oxidizing agent [37]. Open circuit (corrosion) potentials Еoc of coated samples in corrosive solution with pH 3 are more positive than the potential of steel substrate which indicates the anodic control of corrosion process. The deep corrosion index k<sup>h</sup> is reduced by almost three orders of magnitude (Table 2) and such inhibition in alloy corrosion is due to the acidic nature of refractory metals and zirconia oxides forming on the surface in oxygen containing media. The corrosion index of ternary alloys in acidic and neutral chloride-

containing environment is almost halved compared with binary systems (Table 2).

Such corrosion performance is associated with a decrease in roughness and smoothing out the relief of ternary coatings as compared with binary. In addition, the combined presence of molybdenum and tungsten or zirconium in the coatings provides a synergistic effect due to molybdenum and tungsten enhancing corrosion resistance to pitting as well as zirconium increasing tend to passivity. Moreover, with increasing total content of alloying metals, corrosion resistance increases. Open circuit potentials of coated samples in corrosive solution with pH 11 are slow negative as compared with steel substrate (Table 2), that stipulates cathodic

phous part is 2–6 nm.

104

Zr—3.7; (2) 72.9, Mo—24.9, Zr—2.2.

Figure 11.

Applied Surface Science

Corrosion indicators (Еoc, V; kh, mm per year) of testing materials in different media.


#### Table 3.

Application areas for the cobalt ternary coatings.

control of corrosion process. Such corrosion performance is mainly due to the basic nature of cobalt (II) oxides and hydroxides, which hinders the oxygen transport. The difference in corrosion rate of substrate and coated samples in such conditions is somewhat less—the deep corrosion index k<sup>h</sup> is reduced by almost two orders of magnitude.

Summarizing and relying on the results of earlier studies [12, 15, 26, 31, 32] of the ternary cobalt alloy functional properties, it is possible to present their application areas in the table form (Table 3). This table reflects the composition, appropriative properties (microhardness Hv, corrosion resistance, and catalytic activity in hydrogen evolution reaction indicated as hydrogen exchange current density lgi 0 H), and technological application of designed coatings.
