Nanostructured Electrolytic Composites Based on Cobalt Alloys …
745
oxides differ in chemical nature, porosity and electrical conductivity. Therefore, the
equivalent circuit includes at least two R–C chains, one of which corresponds to the
oxide phase and the other describes the corrosion behavior of coating. Such a circuit
may include the following elements: R1 is associated with the electrolyte resistance;
CPE i (i = 1, 2) is the constant phase element (the phase interface capacity); and R i
(i = 2, 3) is the resistance of oxide layer (i = 2) or corrosion one (i = 3). The CPE
elements define the phase fractality that is peculiar for the metals (such as Co, Mo,
W) of a variable valence that forms hydrated oxide films on the surface in alkali
corrosive medium. The degree of deviation from the perfect capacitor (the degree
of surface fractality) is determined by the parameter n (0 < n < 1), and for perfect
capacitor n = 1 [27].
Figure 6 shows the different shape of Nyquist plots for specimens coated with
Co–Mo–W alloy: specimen 1 demonstrates more capacitor behavior in comparison
with specimen 2. The equivalent circuit for specimen 1 includes only three elements:
the electrolyte resistance R1, CPE1 which is associated with oxide layer capacitance,
and R2 which simulates the resistance of the non-porous oxide layer (Fig. 6b). The
values of equivalent circuit parameters are: R1 = 63 ± 1 cm
2 ; CP1 = 0.1 ±
0.02 cm
2 ; R2 = 9100 ± 1100 cm
2 .
The Nyquist plot for specimen 2 (Fig. 6a) is characterized by two flattened semicircles that reflects inhomogeneity of the surface namely some sections with pores
and cracks (see Table 1 and as a result uneven distribution of corrosion process in
the system. Therefore, the equivalent circuit for this specimen consists of two R–C
chains (Fig. 6b), where the first one reflects the compact oxide layer properties as
for specimen 1. The second chain is associated with corrosion behavior of porous
oxide film, where CPE2 is the double-layer capacitance, and R3 simulates the corrosion resistance which is inversely proportional to the corrosion rate. The values of
equivalent circuit parameters and the error (%) of parameters fitting to experimental
data are given in Table 2. They characterize sample 2 as resistant to corrosion in
an alkaline environment, but such that it has less ohmic resistance. Both facts are
favorable for the implementation of electrocatalytic processes.
The Nyquist plots as well as the equivalent circuit for Co–Mo–Zr-coated electrodes (specimens 3–5) are similar to each other and to that of specimen 2, and to
Co–W–Zr alloy (Fig. 2a). Since the difference of parameters does not exceed the
Table 2 Values of equivalent circuit parameters ( cm 2 ) and error (%) of parameters fitting for
coated specimens
Coating
(specimen #)
R1 Error R2
Error CPE1 Error CPE2 Error R3
Error n
Co–Mo–W
(#2)
59 0.2
3700 24
0.74
1.4
0.53
2.2
1800 9
0.47
Co–Mo–Zr
(#3)
61 1.2
7100 1.9
0.76
0.8
0.98
2.4
2500 4
0.48
Co–W–Zr
(#6)
42 0.9
2800 2.1
0.76
0.7
0.9
1.7
1300 8
0.51
745
oxides differ in chemical nature, porosity and electrical conductivity. Therefore, the
equivalent circuit includes at least two R–C chains, one of which corresponds to the
oxide phase and the other describes the corrosion behavior of coating. Such a circuit
may include the following elements: R1 is associated with the electrolyte resistance;
CPE i (i = 1, 2) is the constant phase element (the phase interface capacity); and R i
(i = 2, 3) is the resistance of oxide layer (i = 2) or corrosion one (i = 3). The CPE
elements define the phase fractality that is peculiar for the metals (such as Co, Mo,
W) of a variable valence that forms hydrated oxide films on the surface in alkali
corrosive medium. The degree of deviation from the perfect capacitor (the degree
of surface fractality) is determined by the parameter n (0 < n < 1), and for perfect
capacitor n = 1 [27].
Figure 6 shows the different shape of Nyquist plots for specimens coated with
Co–Mo–W alloy: specimen 1 demonstrates more capacitor behavior in comparison
with specimen 2. The equivalent circuit for specimen 1 includes only three elements:
the electrolyte resistance R1, CPE1 which is associated with oxide layer capacitance,
and R2 which simulates the resistance of the non-porous oxide layer (Fig. 6b). The
values of equivalent circuit parameters are: R1 = 63 ± 1 cm
2 ; CP1 = 0.1 ±
0.02 cm
2 ; R2 = 9100 ± 1100 cm
2 .
The Nyquist plot for specimen 2 (Fig. 6a) is characterized by two flattened semicircles that reflects inhomogeneity of the surface namely some sections with pores
and cracks (see Table 1 and as a result uneven distribution of corrosion process in
the system. Therefore, the equivalent circuit for this specimen consists of two R–C
chains (Fig. 6b), where the first one reflects the compact oxide layer properties as
for specimen 1. The second chain is associated with corrosion behavior of porous
oxide film, where CPE2 is the double-layer capacitance, and R3 simulates the corrosion resistance which is inversely proportional to the corrosion rate. The values of
equivalent circuit parameters and the error (%) of parameters fitting to experimental
data are given in Table 2. They characterize sample 2 as resistant to corrosion in
an alkaline environment, but such that it has less ohmic resistance. Both facts are
favorable for the implementation of electrocatalytic processes.
The Nyquist plots as well as the equivalent circuit for Co–Mo–Zr-coated electrodes (specimens 3–5) are similar to each other and to that of specimen 2, and to
Co–W–Zr alloy (Fig. 2a). Since the difference of parameters does not exceed the
Table 2 Values of equivalent circuit parameters ( cm 2 ) and error (%) of parameters fitting for
coated specimens
Coating
(specimen #)
R1 Error R2
Error CPE1 Error CPE2 Error R3
Error n
Co–Mo–W
(#2)
59 0.2
3700 24
0.74
1.4
0.53
2.2
1800 9
0.47
Co–Mo–Zr
(#3)
61 1.2
7100 1.9
0.76
0.8
0.98
2.4
2500 4
0.48
Co–W–Zr
(#6)
42 0.9
2800 2.1
0.76
0.7
0.9
1.7
1300 8
0.51