1 Nanostructured Functional Coatings of Iron Family Metals with Refractory. . .
27
Table 1.4 Corrosion indexes of Fe-Co-Mo and Fe-Co-W coatings
Ó° 3
Ó° 5
Ó° 9.5
Composition of
alloy, ω, at.%
E cor , V
lg i cor ,
A/cm 2
E cor , V
lg i cor ,
A/cm 2
¨ cor , V
lg i cor ,
A/cm 2
´ÕÃÎß
−0.34
−2.8
−0.35
−3.0
−0.32
−2.8
Fe51Co36Mo13 −0.25
−3.7
−0.30
−4.9
−0.46
−5.5
Fe31Co31Mo38 −0.16
−4.5
−0.31
−4.8
−0.32
−5.1
Fe54Co36W10
−0.54
−4.7
−0.36
−5.1
−0.53
−5.2
Fe59Co33W8
−0.35
−4.1
−0.39
−4.8
−0.36
−5.5
The Fe-Co-Mo alloy corrosion mechanism in acidic and neutral media differs
by open circuit potential shift to the positive side compared with substrate material
(Fig. 1.22, b; Table 1.4). The corrosion rate decreases due to the formation of the
dense film of acidic molybdenum oxide on the surface. On the contrary, the open
circuit potential shifts toward the negative side in the alkaline environment, which is
evidence of the cathode reaction inhibition by insoluble iron and cobalt hydroxides
formed at the alloy surface. Accordingly, the coatings with a molybdenum content of
38 at.% and iron of 31 at. % are more resistant in acidic environments; and coatings
containing molybdenum of 13 at.% and iron of 51 at. % are more resistant in alkaline
media as one can see from Table 1.4.
Verification of polarization method results was carried out using electrode
impedance spectroscopy (¨IS) technique. It was found that in neutral environment
the Nyquist plot for Fe-Co-Mo(W) alloy (Fig. 1.23) is a connection of two conjugate
semicircles at high frequencies complicated by the diffusion Warburg impedance
Z W at low frequencies. The fact that semicircles are not overlapping is due,
presumably, to near values of the time constants τ = RC of parallel circuits, which
reflect the kinetic nature of the charge transfer inhibition on parts of the surface with
different composition. Indeed, part of the surface is covered with chemically stable
neutral media oxides, which correspond to elements of the substitution scheme
C OX and R OX (Fig. 1.24), while Faraday reactions on the other surface parts are
visualized by the elements C d and R f . A conclusion on the inhibition of transport
stages, the presence of which reflects Warburg impedance, is quite identical to the
results of polarization measurements. The relatively near values of the corrosion
current density, calculated from data of polarization resistance technique and EIS,
are also evidence of correctness of both the parameters of the corrosion process
determination and the validity of the electrochemical systems equivalent circuit.
Parameter R f for tested materials decreases in the range:
R f (Fe − Co − W) > R f (Fe − Co − Mo) >> R f (steel substrate).
Open circuit potential of coatings alloyed with zirconium becomes more negative
in all environments (Figs. 1.25 and 1.26; Table 1.5). At the same time, the corrosion
rate of a ternary alloy in the acidic and neutral chloride-containing environment
27
Table 1.4 Corrosion indexes of Fe-Co-Mo and Fe-Co-W coatings
Ó° 3
Ó° 5
Ó° 9.5
Composition of
alloy, ω, at.%
E cor , V
lg i cor ,
A/cm 2
E cor , V
lg i cor ,
A/cm 2
¨ cor , V
lg i cor ,
A/cm 2
´ÕÃÎß
−0.34
−2.8
−0.35
−3.0
−0.32
−2.8
Fe51Co36Mo13 −0.25
−3.7
−0.30
−4.9
−0.46
−5.5
Fe31Co31Mo38 −0.16
−4.5
−0.31
−4.8
−0.32
−5.1
Fe54Co36W10
−0.54
−4.7
−0.36
−5.1
−0.53
−5.2
Fe59Co33W8
−0.35
−4.1
−0.39
−4.8
−0.36
−5.5
The Fe-Co-Mo alloy corrosion mechanism in acidic and neutral media differs
by open circuit potential shift to the positive side compared with substrate material
(Fig. 1.22, b; Table 1.4). The corrosion rate decreases due to the formation of the
dense film of acidic molybdenum oxide on the surface. On the contrary, the open
circuit potential shifts toward the negative side in the alkaline environment, which is
evidence of the cathode reaction inhibition by insoluble iron and cobalt hydroxides
formed at the alloy surface. Accordingly, the coatings with a molybdenum content of
38 at.% and iron of 31 at. % are more resistant in acidic environments; and coatings
containing molybdenum of 13 at.% and iron of 51 at. % are more resistant in alkaline
media as one can see from Table 1.4.
Verification of polarization method results was carried out using electrode
impedance spectroscopy (¨IS) technique. It was found that in neutral environment
the Nyquist plot for Fe-Co-Mo(W) alloy (Fig. 1.23) is a connection of two conjugate
semicircles at high frequencies complicated by the diffusion Warburg impedance
Z W at low frequencies. The fact that semicircles are not overlapping is due,
presumably, to near values of the time constants τ = RC of parallel circuits, which
reflect the kinetic nature of the charge transfer inhibition on parts of the surface with
different composition. Indeed, part of the surface is covered with chemically stable
neutral media oxides, which correspond to elements of the substitution scheme
C OX and R OX (Fig. 1.24), while Faraday reactions on the other surface parts are
visualized by the elements C d and R f . A conclusion on the inhibition of transport
stages, the presence of which reflects Warburg impedance, is quite identical to the
results of polarization measurements. The relatively near values of the corrosion
current density, calculated from data of polarization resistance technique and EIS,
are also evidence of correctness of both the parameters of the corrosion process
determination and the validity of the electrochemical systems equivalent circuit.
Parameter R f for tested materials decreases in the range:
R f (Fe − Co − W) > R f (Fe − Co − Mo) >> R f (steel substrate).
Open circuit potential of coatings alloyed with zirconium becomes more negative
in all environments (Figs. 1.25 and 1.26; Table 1.5). At the same time, the corrosion
rate of a ternary alloy in the acidic and neutral chloride-containing environment