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j.matchemphys.2018.08.025

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Materials Chemistry and Physics 219 (2018) 175–181
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Barrier ability and durability of NieCoeP coatings in accordance to the
content of H3PO3 and NaH2PO2 as phosphorus sources
T
Katya Ignatova∗, Stephan Kozhukharov, Momchil Alakushev
University of Chemical Technology and Metallurgy; 8, Kl. Ohridsky blvd., 1756, Sofia, Bulgaria
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
Galvanostatically deposited NieCoeP
• on
copper substrates coatings are investigated.
The impact of H PO and NaH PO as
• P-providers
on the barrier ability is
3
3
2
2
assessed.
and LVA are applied during ex• EIS
tended coatings exposure to 3.5%
NaCl.
of amorphous phase and higher
• Both
Co:Ni ratios improve the coatings
properties.
coincidence of both P-providers
• The
improves the resulting coating performance.
A R T I C LE I N FO
A B S T R A C T
Keywords:
Metal alloys
NickeleCobaltePhosphorus
Electrodeposition
Corrosion resistance
Impedance spectroscopy
The effect of P-providing components (phosphorous acid, H3PO3 and sodium hypophosphite, NaH2PO2) in
modified Watts electrolyte (pH = 2; 80 °C) on the barrier ability and durability of the deposited NieCoeP and
NieCo (as a reference) coatings during extended exposure in 3.5% NaCl until 672 h was studied. The data
acquired for the charge transfer and polarization resistances (Rct and Rp) by electrochemical impedance spectroscopy, EIS and linear voltammetry, LVA, undoubtedly reveal that the best corrosion protective properties
belong to the NieCoeP coating deposited at simultaneous occurrence of H3PO3 and NaH2PO2. It was followed
subsequently by those obtained from electrolyte with only NaH2PO2, then, deposited from electrolyte with only
H3PO3 and finally by the reference NieCo coating. The occurrence of an amorphous phase in the structure of
NieCoeP coatings, combined with the increase of cobalt content and reducing that of nickel, predetermines the
best protective properties of the investigated alloys. The established anomalous Rct and Rp increment during the
exposure of NieCoeP coatings could be related to corrosion products accumulation, most probably of metal
phosphates, and it forms a barrier layer between the coating and the corrosive medium.
1. Introduction
NieCoeP coatings are a new type of alloys [1,2], which combine
the high corrosion resistance and hardness, typical for the NieP compositions [3–5] with the superplastic extensibility, wear resistance, high
∗
saturation magnetization and gоod thermal stability owed by the Со-Р
[6–8] and by the cobalt enriched NieCo coatings [9]. At the beginning,
the triple alloy was investigated as an alternative to solid chromium
coatings obtained from solutions involving carcinogenic Cr6+ ions
[1,2,10,11]. Currently, it is used as a catalytic material for the
Corresponding author.
E-mail address: katya59ignatova@gmail.com (K. Ignatova).
https://doi.org/10.1016/j.matchemphys.2018.08.025
Received 17 February 2018; Received in revised form 21 June 2018; Accepted 10 August 2018
Available online 11 August 2018
0254-0584/ © 2018 Elsevier B.V. All rights reserved.
Materials Chemistry and Physics 219 (2018) 175–181
K. Ignatova et al.
(NaHCO3) at 60°С. Afterwards, the samples were washed thoroughly
with water and subsequently etched in solution with concentrated nitric
and sulfuric acids in the ratio 2:1 v/v with adding of 10 g L−1 sodium
chloride, NaCl (all p.a. Merk), followed by a final washing with distilled
water and drying. The coating thickness was measured using a handportable, caliper (0÷1250 μm) with combined magnetic-induction and
eddy current sensor of type BB20 of the company BioEviBul.
The surface morphology of the samples was analyzed by scanning
electron microscopy (SEM) using an Oxford Instruments, JSM-6390Jeol. Chemical composition of the electrodeposits was determined with
energy dispersive X-ray (EDX) spectroscopy by analyzer, attached to the
SEM device. The structure of the NieCoeP coatings was assessed by XRay diffraction technique (XRD) using Philips PW 1050 vertical automatic diffractometer with secondary graphite monochromator, operating with CuKα radiation and scintillation counter. The diffraction
curves were recorded in angular interval from 10 to 100° 2θ with step
0.04° and exposure 1 s.
The barrier ability and durability of the obtained coatings were
determined by two independent methods: electrochemical impedance
spectroscopy (EIS) and linear voltammetry (LVA). Autolab 30 universal
Galvanostat/Potentiostat, equipped with FRA-2 impedance analyzer,
was used. The experiments were implemented in three-electrode cell,
ISO16773-2, which provided 2 cm2 working area, exposed in the model
corrosion medium (MCM) consisting of 3.5% NaCl in distilled water.
The counter electrode was a platinum net and Ag/AgCl/3M KCl
(E = 0.194 V) of Metrohm served as reference.
The impedance spectra were acquired after certain periods: 24, 168,
336, 505 and 672 h of exposure to MCM at room temperature. The
spectra were recorded in frequency interval from 10 kHz tо 10 mHz,
distributed in 50 measurement points, at excitation signal of 10 mV,
according to the Open Circuit Potential (OCP). Anodic polarization
dependencies in logarithmic coordinates were recorded under the same
conditions as impedance spectra, i.e. after different times of their exposure to the MCM. Potential range was between −270 mV tо 350 mV,
in respect to the Ag/AgCl electrode at potential sweep rate of 5 mV s−1.
This potential range was selected in order to include the corrosion
potential (Ecorr) value.
Hydrogen-Evolution Reaction (HER) for water electrolysis and in the
hydrogen fuel cells [12,13] and also in magnetic data recorders
[14–16]. Recently, the transition metal phosphide nanomaterials have
been used in Li-ion batteries, Na-ion batteries, supercapacitors and solar
cells [17–21].
Many authors point out that with increasing of phosphorus content
over 10–12 mass.% in NieP [3,5,22], CoeP [6–8] and NieCoeP
[1–3,12,23] coatings, its structure becomes amorphous and therefore
their protective capability increases. This fact can be explained assuming lower interatomic distances of the obtained amorphous structures, resulting in lower density of grain boundaries, dislocations and
other surface defects [24,25].
There is contradictive information regarding the nature of the anode
dissolution, the ability to passivation and the sensitivity to pitting of
NieP, CoeP and NieCoeP coatings [2,3,13,26–28]. Some authors
[5,25,26,29] indicate that the NieP coatings do not form any distinguishable passivation layers in acidic media, showing active anodic
dissolution. On the basis of both XPS analyses and anodic behavior of
NieP alloys, M. Crobu et al. [26] have concluded that the active anodic
dissolution leads to phosphorus accumulation on the metallic surface. It
promotes preferential Ni-selective dissolution during the polarization.
According to other authors, this process leads to threshold current
density in the course of dissolution of NieP alloys due either to formation of Ni3(PO4)2 film [5,25] or adsorption of hypophosphite ions
[29]. The accumulation of such products on the surface serves as a
barrier against further dissolution. The phenomenon is known as
“chemical passivity” [29]. The surface layer enrichment by phosphorus
is the most probable reason for the superior corrosion strength of the
CoeP alloy, in respect to the Со one [8,30]. Other authors [27] highlight that during the anodic dissolution of amorphous NieP alloys with
P content exceeding 12 at% a passive film is being formed in chloride
and sulphate neutral solutions.
Nowadays, no sufficient literature data exist about the comparative
evaluation among the P-providing additives, regarding the improvement the protective capability of electrodeposited NieCoeP alloys. This
article highlights on the comparison of the barrier ability and durability
of NieCoeP coatings, deposited at presence of either NaH2PO2 or
H3PO3, as well as in their combination. The analysis was based on
various advanced analytical techniques, including SEM, EDX, X-Ray
and also two independent electroanalytical methods (EIS, LVA).
3. Results and discussion
3.1. EIS measurements
2. Experimental
For easier data interpretation, the investigated coatings have the
same signs as the respective electrolytes in Table 1. (i.e. the coatings A,
B, C, D). The impedance spectra acquired for the entire exposure period
from 24 to 672 h for all the investigated coatings are summarized in
Fig. 1. The spectra are represented in Bode (Fig. 1, a-d) and in Nyquist
(Fig. 1, a’- d’) plots.
The comparison of the spectra in Bode plots reveals that at the initial 24 h of exposure, the NieCoeP coatings, deposited from electrolytes А, B and С (Fig. 1, a-с), possess slightly lower impedance modulus
values at 0.01 Hz, i.e. |Z| = 105 Ω cm2 (Fig. 1, a-c), compared to these
of the referent NieCo coatings, which impedance modulus is
|Z| = 105.5 Ω cm2 (Fig. 1, d). This fact imposes the inference that the
barrier ability, registered for the initial 24 h for the NieCo reference
coatings looks higher than this of the NieCoeP alloyed coatings (А-С).
Nevertheless, the comparison of the spectra, recorded for the entire
672 h exposure period, in both types of Bode (Fig. 1, а-d) and Nyquist
(Fig. 1, a’- d’) plots shows that the impedance modulus and the corresponding capacitance for coatings А, B and С, undergo negligible
change (excluding coating A, which |Z| decreases from 105.3 to
104.6 Ω cm2), whereas for the NieCo composition (i.e. coating D) the |Z|
suffers remarkable decay, reaching complete barrier properties lose
(Fig. 1, d’), corresponding to complete layer failure.
The comparison of the acquired impedance spectra with spectra of
suitable model equivalent circuits has enabled to define the numeric
NieCoeP coatings were galvanonostatically deposited from three
electrolyte compositions (assigned as A, B and C in Table 1) with presence of either H3PO3 or NaH2PO2, (A and B resp.) as well as in their
combination (electrolyte C). NieCo alloy coating was deposited in
electrolyte D from Table 1. The electrolytes were prepared using analytical grade reagents (trademark p.a. Merck). The pH value 2 was
adjusted by adding of solutions NaOH and H2SO4, p.a. Merck.
The coatings were deposited for 40 min on copper electrodes
(Merck, 99.97%) with surface area 4 cm2 (2 × 2 cm) at
j = 62.5 mA cm−2 for 40 min at 80 °C. The Pt foil anode (of 36 cm2) was
concentrically placed around the working (copper) electrode. Before
the experiments, the copper electrode was degreased in a dilute detergent solution with the addition of 20 g L−1 sodium bicarbonate
Table 1
Basic electrolyte composition in [mol L−1], рН = 2.
Electrolyte→ Components↓
А
B
C
D
NiSO4 6H2O
NiCl2
CoSO4 7H2O
NaH2PO2
H3PO3
0.66
0.20
0.35
–
0.60
0.66
0.20
0.35
0.60
–
0.66
0.20
0.35
0.60
0.60
0.66
0.20
0.35
–
–
176
Materials Chemistry and Physics 219 (2018) 175–181
K. Ignatova et al.
Fig. 1. EIS data, represented in both types of Bode (a–d) and Nyquist (a’- d’), for NieCoeP coatings, deposited from electrolytes A (a,a’), B (b,b’), and С (c,c’), as well
as for NieCo coating from electrolyte D (d,d’) at different times of exposure.
investigated compositions. Тhis fact reveals the purely capacitive
character of the electric double layer on the surface for all NieCoeP
and NieCo coatings, being supplemental, obvious indication for their
uniformity.
The „n“-exponential factor tends to 100% for all the investigated
specimens (Table 2). The quantitative analysis of the impedance spectra
has allowed the assessment of the barrier ability of the obtained coatings, on the basis of the Rct values (Таble 2) at the initial 24 h of exposure. Their durability was evaluated using the Rct values detected
after the completion of the exposure experiments (i.e. after 672 h of
exposure).
On the basis of the data represented in Table 2, it could be inferred
that the highest barrier ability at the initial 24 h of exposure belongs to
the NieCoeP composition A, (Rct = 430 kΩ cm2), followed
values of all оhmic and capacitive resistance elements of the electrolyte/coating/substrate system (Table 2). In the present case, the appropriate equivalent circuit (Fig. 2) was composed by the 3.5% NaCl
(MCM) resistance Rel, subsequently connected to a parallel connection
between the charge transfer resistance Rct and non-ideal capacitance
CPEedl of the electric double layer occurring between the electrolyte
and the electrode surface (represented as constant phase element, CPE).
The insignificant deviations established between the spectra acquired
by measurements of the corroding coatings and the model equivalent
circuit evince the uniformity and the nano-size distribution of the investigated NieCoeP coatings. Тhis result coincides with the data obtained by the XRD analysis. Besides, the phase shift between the input
potential sinusoid and the resulting current density output sinusoid
tends to φ = 90°, corresponding to almost ideal capacitance for all the
177
Materials Chemistry and Physics 219 (2018) 175–181
K. Ignatova et al.
Fig. 2. Model equivalent circuit of the EIS method.
composition B (shown in Table 2). Thus, the Rct of coating С decreases
from 271.4 to 246.0 kΩ cm2, for the entire 672 h of exposure, whereas
for composition B, these values increased from 224.0 to 250.0 kΩ cm2.
The high phosphorous content possessed by coatings А-С is a possible
reason for their superior barrier ability and extended durability, compared to these of the referent NieCo coating (D), because the coating
dissolution results in the phosphorus accumulation on the electrode
surface, confirmed by other authors [8,26,30], as well. Comparing the
data obtained for the barrier ability of the coatings А-D shown in
Table 2, with their elemental compositions represented in Table 3, it
could be concluded that the high phosphorus content is not the unique
consecutively by the NieCo coating D (Rct = 295 kΩ cm2), the NieCoeP coating С (Rct = 271.4 kΩ cm2) and finally the NieCoeP coating
В (Rct = 224 kΩ cm2). Although the rather high initial values of the
barrier ability recorded for composition A, it loses its superior properties much faster than the rest NieCoeP layers. The charge transfer Rct
of this coating undergoes 4 folds decrement from its initial value. For
the referent NieCo coating D, this decrement achieves 15 folds, coincided by complete film failure, established after 504 h of exposure.
The compositions B and C reveal very high average Rct values
during the entire exposure period, especially for coating C.
Furthermore, even slight Rct increment was registered particularly for
Table 2
Results from the impedance modeling for the NieCoeP (А-С) and NieCo (D) coatings regarding the MCM (3.5% NaCl): electrolyte resistance, Rel; Constant phase
element, СРЕedl; n - exponential factor and the charge transfer resistance, Rct.
Hours of exposure
SAMPLE
24 h
168 h
336 h
504 h
672 h
SAMPLE
24 h
168 h
336 h
504 h
672 h
SAMPLE
24 h
168 h
336 h
504 h
672 h
SAMPLE
24 h
168 h
336 h
504 h
672 h
СРЕedl (×10−6)
Rel
n
Rct
[Ω.cm2]
E, %
[sn.Ω−1.cm−2]
E, %
[−]
E, %
[kΩ.cm2]
E,%
21.00
15.46
13.76
15.96
12.98
0.82
0.60
2.42
0.96
0.59
23.5
23.6
38.9
60.3
91.9
0.60
0.42
1.71
0.78
0.49
0.96
0.97
0.91
0.94
0.95
0.14
0.96
0.41
0.20
0.13
430.00
539.00
180.40
152.40
113.6
1.81
0.96
4.44
2.26
1.54
26.70
23.86
24.00
31.34
38.16
0.52
0.47
0.30
0.36
0.62
43.45
46.43
46.58
47.02
51.13
0.43
0.38
0.30
0.30
0.53
0.95
0.95
0.95
0.95
0.93
0.12
0.10
0.11
0.08
0.15
224.00
278.40
333.20
318.20
250.40
1.31
1.39
1.29
1.24
1.99
17.16
14.28
15.96
21.80
18.74
0.91
0.82
0.73
0.87
1.28
31.34
30.71
31.45
32.84
37.47
0.69
0.57
0.53
0.67
0.97
0.95
0.95
0.95
0.95
0.93
0.16
0.13
0.13
0.17
0.24
271.40
390.00
346.00
323.60
246.80
1.87
1.97
1.69
2.11
2.85
0.56
0.85
1.07
0.83
–
33.60
33.97
40.24
74.65
–
0.13
0.20
0.25
0.21
–
295.00
230.40
86.60
19.36
–
1.65
2.18
1.81
0.94
–
A
B
C
D
16.78
13.32
14.10
19.90
–
0.73
1.15
1.27
0.75
–
178
0.96
0.95
0.93
0.87
–
Materials Chemistry and Physics 219 (2018) 175–181
K. Ignatova et al.
(Таble 2). The first probable reason is related with its higher thickness
compared to those of the rest coatings. Consequently, for coating B, it is
possible with the highest probability to the other coatings, at longer
residence times in MCM to reach phosphorus enriched sublayers with
enhanced barrier ability due to their distinguishable structures and
compositions. The other probable reason is that these sublayers are
capable to form corrosion products, such as Ni3(PO4)2 and Co3(PO4)2,
which act as a barrier against the dissolution, as proposed by other
authors [5,25].
Table 3
Elemental compositions of the investigated coatings А-D (in mass.%).
Probe/electrolyte
А
В
С
D
%P
17.67 (18.99)
14.84 (15.63)
31.20 (38.4)
–
[a]
% Co
% Ni
11.67 (12.16)
22.89 (21.01)
35.19 (33,34)
27.71
70.66 (68.85)
62.27 (63.36)
33.61 (28.26)
72.29
a
The brackets show the coating composition on the most protrusive part of
their surface.
reason for the superior barrier ability of coatings A-C.
The data in Table 3 reveal that there is an increase of the cobalt
amounts, in direction from sample A to sample C, which is another
possible reason for the increase of the barrier ability in this direction. In
coating C, the phosphorus content reaches 31.2%, which is comparable
to both the Co (35.19%) and Ni (33.61%) concentrations. In this sense,
our data are concordant to the results reported in Ref. [3].
The lowest thickness is established for coating A (2.5 μm) in comparison with these of coatings В (7.8 μm) and С (5.6 μm). This is a
possible reason for the quick loss of barrier ability of coating A.
Coatings A and B possess typically amorphous structures, whereas the
XRD pattern for coating С, deposited at j = 62.5 mA cm−2 reveals
mixed amorphous/crystalline structure, comprising nano-dimensional
amorphous phase and crystal hexagonal phase of solid solutions Ni2P
and Ni2P-Со2Р. The dimensions of the individual domains for the
amorphous phase are below 3 nm. The occurrence of hexagonal crystals
(Fig. 3a) with elevated P-content (Table 3, content in brackets) is explained assuming recrystallization processes, after its deposition. Undoubtedly, it could be inferred that the combination among the highest
phosphorus content, registered for coating C, the relatively similar Со
and Ni contents and its specific microstructure is the reason for the
superior barrier properties, compared to all the rest investigated compositions. These distinguishable structural and compositional features
of coating C are consequence of the fact that it is deposited at presence
of both P-providing electrolyte ingredients.
The Rct increment during the exposure of coatings А, B and С (Таble
2), could be related to corrosion products accumulation, which forms a
barrier layer between the coating and the corrosive medium. The values
in Table 3, enclosed in brackets, correspond to the coating composition
data from the most protrusive part of the coating's surface, acquired
shortly before the corrosion tests. The most considerable data dissipations of entire 8% are observed for the P-content in coating C. Although
this result does not give an idea for the composition of the inner layers
of the coating, it suggests that there is a differentiation of the composition and structure of different layers of the NieCoeP coatings.
From the statements mentioned above, it could be concluded that
there are two possible reasons for the observed Rct increment for
coating B until the highest values among the investigated coatings
3.2. LVA measurements
Linear Voltammetry (LVA) was applied as another electrochemical
analytical method, in order to confirm the inferences done above, regarding the durability of the investigated coatings А-D. It allows to
register the Tafel anodic plots (in logarithmic coordinates), shown in
Fig. 4. The LVA curve analysis has enabled to calculate the data for the
corrosion potential Ecorr and the polarization resistance Rp for each one
of the investigated coatings after different periods of exposure to МСМ
(Таble 4).
The weakest current density increase within the anodic polarization
was registered for the coatings B and C. This weak current increment
was preserved for the entire exposure period (Fig. 4b and c). For
comparison, coating A, deposited from electrolyte with only H3PO3
containing solution suffers gradual active dissolution until the 168th
hour of exposure (Fig. 4, a). Afterwards, its dissolution rate sharply rises
becoming similar to this of the reference NieCo coating D, at the initial
exposure period (Fig. 4, d). Current arrest is observed for all investigated NieCoeP coatings, when sufficiently positive anodic potentials are reached. This effect is observed until the 504th. hour of
exposure for coatings B and C (Fig. 4b and c) and until the 168th hour
(Fig. 4, a) of exposure for the coating A, respectively.
These conditions promote metallic ions and phosphorus heaping, as
well as acidification of the electrolyte near the surface layer, as a result
of the hydroxyl ions oxidation, following reaction (1) [31]:
4OH− - 4e− → O2 + 2H2O; E0 = 0.817 V (SHE) for pH = 7
(1)
These conditions predetermine corrosion products formation (most
probably metal phosphates), which acts as a barrier against the further
film dissolution, similar to that established for NieP coating [5,25].
Any current retention was not registered in the case of coating D, in the
entire anodic potentials range. These data correlate very well with the
EIS results, confirming the lowest durability of coating D and the inferior protective properties possessed by coating A, in comparison with
compositions B and C.
The anodic curves for coating A after 336 h of exposure and for
compositions B and C after 504 h of exposure respectively, achieve a
current density maximum at about −0.04 V (Ag/AgCl), followed by
Fig. 3. SEM image (a) and X-Ray diffractogram (b) of NieCoeP coatings, deposited from electrolyte with composition C from Table 1; j = 62.5 mA cm−2.
179
Materials Chemistry and Physics 219 (2018) 175–181
K. Ignatova et al.
Fig. 4. Аnodic polarization curves, acquired for NieCoeP coatings А-С (a-с) and for the reference NieCo coating D (d) acquired after different times of exposure to
the MCM (3.5% NaCl).
weak decrease and subsequent plateau (Fig. 4, а-с). For the referent
NieCo coating (D), such current density maximum was registered even
at the initial 24 h of exposure, at almost the same potential as for the
coatings А-С. (Fig. 4, d). This fact gives the reason to suppose that for all
coatings, the observed maxima are related to the formation of the same
type of corrosion products. Furthermore, these products are not metallic phosphates, because in the case of the combined NieCo coating
there is not any reason for their existence. The potentials of all possible
anodic reactions with the MCM components were calculated, in respect
to Ag/AgCl reference electrode, in order to determine the nature of the
anodic processes, related to the appearance of this peak, and to define
whether the corrosion products are metallic oxides, or consequence of
the interactions between the copper substrate and the 3.5% NaCl,
(pH = 7), corrosive solution. The standard potential values in respect to
standard hydrogen electrode (SHE) are taken from literature data
[31,32]. Among all the possible processes on the corroding metallic
surface, the reactions represented below (equations (2) and (3)) possess
the most closed values to those of observed maxima:
CuCl + e− → Cu° + Cl−, E° = 0.137 V
CuCl2 + 2e
−
→ Cu° + 2Cl , E° = 0.190 V
-
value determined after the initial 24 h of exposure is owed by the reference NieCo coating D (Rp = 372.0 kΩ cm2). However, it sharply
decays reaching the minimal 16.3 kΩ cm2 value after the 504th hour of
exposure (Тable 4). A weaker Rp decrease was registered for coating A.
For comparison, coating C possesses rather high Rp values and keeps
them during the entire exposure period (Rp = 231 kΩ cm2 after 24 h;
601.0 kΩ cm2 after 336 h, and 267.8 kΩ cm2 after 672 h of exposure,
respectively).
High and even further increasing Rp values after the 168th and the
504th hour of exposure were registered for the composition B. Similar
tendency has been observed for the Rct values determined by the EIS
measurements, due to the described above reasons. Because the Rp
values comprise both the Rct and the resistance owed by the oxide or
salt film deposits, the anomalous Rp increment for coatings B and C,
evinces the appearance of protective superficial layers composed by
corrosion products accumulated during the exposure tests. These products are probably metal phosphates, and this assumption is in agreement with the reports of other authors [5–7,25].
The numerical data obtained from the LVA curve analysis completely correlate with the statements done for EIS measurements about
the corrosion protective properties of the investigated coatings. On the
basis of both the electrochemical analytical methods, it can be concluded that regarding their corrosion protective abilities, the investigated coatings can be ordered as follows: from the best one C,
followed subsequently by B, and A, to the worst one-composition D.
(2)
(3)
The potentials change their values to −0.070 V and −0.020 V, after
recalculations in respect to the used Ag/AgCl reference electrode (EAg/
AgCl = 0.194 V, according to SHE). These values are rather similar to the
potential values of the observed anodic peaks. Therefore, a conclusion
could be done that these peaks are namely result of the substrate metallic surface uncovering. Consequently, copper dissolution appears and
it causes a sharp current rise, followed by subsequent current decrease,
due to formation of insoluble compounds like CuCl and CuCl2. The
processes of formation of these compounds serve as an indication for
the initiation of barrier properties loss due to the coating dissolution.
The peak and plateau appearance happens at different periods of exposure to MCM, which completely correlate with the EIS data, appointing exactly the same periods necessary for coating failure. The
LVA curves were submitted to Tafel plot analysis and the resulting
corrosion potential Ecorr and polarization resistance Rp are shown in
Table 4.
The polarization resistance Rp values determined for the investigated coatings (Таble 4) reveal identical to the Rct values evolution
trends defined by the EIS analysis (Table 2). The highest average Rp
4. Conclusions
The following conclusions can be drown from this study: The
highest barrier ability and durability for the entire 672 h period of exposure in 3.5% NaCl was registered for the NieCoeP coating, obtained
from the electrolyte with simultaneous occurrence of content of both
H3PO3 and NaH2PO2. The high phosphorus content (14÷31%) is not
the unique reason for the superior ability of NieCoeP coatings in
comparison with NieCo coating. The occurrence of an amorphous
phase in the structure of NieCoeP coatings, combined with the increase of cobalt content and reducing that of nickel, predetermines the
best registered protective properties of the alloys.
It was established anomalous Rct and Rp increment during the exposure of the NieCoeP coatings, wich could be related to corrosion
products accumulation, most probably metal phosphates. The resulting
180
Materials Chemistry and Physics 219 (2018) 175–181
K. Ignatova et al.
Table 4
Data regarding the corrosion potential Еcorr, mV (versus Ag/AgCl) and polarization resistance, Rp, kΩ cm2 from the Tafel anodic polarization curves, acquired after
different times of exposure to the MCM (3.5% NaCl).
Electrolyte→ (coating)
A
B
C
Hours of exposure↓
Еcorr, mV
Rp, kΩ cm2
Еcorr, mV
Rp, kΩ cm2 <
Еcorr, mV
Rp, kΩ cm2
Еcorr, mV
Rp, kΩ cm2
24
168
336
504
672
−241
−202
–
−221
−244
205.6
148.8
–
135.2
125.9
−0.238
−0.231
−0.217
−0.177
−0.208
200.60
352.60
160.60
309.40
220.40
−246
−321
−213
−186
−173
231.00
378.80
601.00
223.20
267.80
−0.211
−0.177
−0.175
−0.165
−0.168
120.64
16.10
9.58
7.58
4.20
NieCoeP alloys can be applied as protective coatings and as electrodes
in electrochemical energy storage devices.
[14]
Acknowledgements
[15]
The authors acknowledge the Scientific Research Section to UCTM
–Sofia for the financial support via Project 2FHN_К_I – 2017 (internal
№11642).
[16]
[17]
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