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materials
Article
Influence of Co Content and Chemical Nature of the Co Binder
on the Corrosion Resistance of Nanostructured WC-Co
Hardmetals in Acidic Solution
Tamara Aleksandrov Fabijani´c 1,* , Marin Kurtela 1, Matija Sakoman 1and Mateja Šnajdar Musa 2
Citation: Aleksandrov Fabijani´c, T.;
Kurtela, M.; Sakoman, M.; Šnajdar
Musa, M. Influence of Co Content
and Chemical Nature of the Co
Binder on the Corrosion Resistance of
Nanostructured WC-Co Hardmetals
in Acidic Solution. Materials 2021,14,
3933. https://doi.org/10.3390/
ma14143933
Academic Editor: Artur Czupry ´nski
Received: 30 April 2021
Accepted: 30 June 2021
Published: 14 July 2021
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Attribution (CC BY) license (https://
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4.0/).
1Faculty of Mechanical Engineering and Naval Architecture, University of Zagreb, Ivana Luˇci´ca 5,
10000 Zagreb, Croatia; marin.kurtela@fsb.hr (M.K.); matija.sakoman@fsb.hr (M.S.)
2Department of Polytechnics, University of Rijeka, Sveuˇcilišna avenija 4, 51000 Rijeka, Croatia;
mateja.snajdar@uniri.hr
*Correspondence: tamara.aleksandrov@fsb.hr; Tel.: +385-9845-3916
Abstract:
The electrochemical corrosion resistance of nanostructured hardmetals with grain sizes
d
WC
< 200 nm was researched concerning Co content and the chemical nature of the Co binder. Fully
dense nanostructured hardmetals with the addition of grain growth inhibitors GGIs, VC and Cr
3
C
2
,
and 5 wt.%Co, 10 wt.%Co, and 15 wt.%Co were developed by a one cycle sinter-HIP process. The
samples were detailly characterized in terms of microstructural characteristics and researched in the
solution of H
2
SO
4
+ CO
2
by direct and alternative current techniques, including electrochemical
impedance spectroscopy. Performed analysis revealed a homogeneous microstructure of equal
and uniform grain size for different Co contents. The importance of GGIs content adjustment was
established as a key factor of obtaining a homogeneous microstructure with WC grain size retained
at the same values as in starting mixtures of different Co binder content. From the conducted
research, Co content has shown to be the dominant influential factor governing electrochemical
corrosion resistance of nanostructured hardmetals compared to the chemical composition of the Co
binder and WC grain size. Negative values of E
corr
measured for 30 min in 96% H
2
SO
4
+ CO
2
were
obtained for all samples indicating material dissolution and instability in acidic solution. Higher
values of R
p
and lower values of i
corr
and v
corr
were obtained for samples with lower Co content.
In contrast, the anodic Tafel slope increases with increasing Co content which could be attributed
to more pronounced oxidation of the higher Co content samples. Previously researched samples
with the same composition but different chemical composition of the binder were introduced in
the analysis. The chemical composition of the Co binder showed an influence; samples with lower
relative magnetic saturation related to lower C content added to the starting mixtures and more W
dissolved in the Co binder during the sintering process showed better corrosion resistance. WC-
5Co sample with significantly lower magnetic saturation value showed approximately 30% lower
corrosion rate. WC-10Co sample with slightly lower relative magnetic saturation value and showed
approximately 10% lower corrosion rate. Higher content of Cr
3
C
2
dissolved in the binder contributed
to a lower corrosion rate. Slight VC increase did not contribute to corrosion resistance. Superior
corrosion resistance is attributed to W and C dissolved in the Co binder, lower magnetic saturation,
or WC grain size of the sintered sample.
Keywords:
nanostructured hardmetals; Co content; GGIs; chemical nature of Co binder; grain size;
electrochemical corrosion resistance; H2SO4+ CO2
1. Introduction
Hardmetals contain tungsten carbide WC particles joined by a binder, most commonly
cobalt Co, by a liquid phase sintering process. The properties of the obtained composite
derive directly from its constituents; hard and brittle carbides and softer and more ductile
binder [
1
]. By connecting these two components, superior mechanical, physical, and
Materials 2021,14, 3933. https://doi.org/10.3390/ma14143933 https://www.mdpi.com/journal/materials
Materials 2021,14, 3933 2 of 17
chemical properties are achieved. In recent years, the development of hardmetals is based
mainly on the application of ultrafine and nano WC particles (grain size less than 0.5
µ
m)
which require the addition of grain growth inhibitors GGIs to retain the WC grain size
in the sintered product. Consequently, a homogeneous microstructure and significantly
improved mechanical properties (hardness, wear-resistance, and strength) are achieved.
Furthermore, achieving a homogeneous microstructure with a WC grain size in the nano
area (<0.2
µ
m) allows application at higher cutting speeds, lower tolerance, and longer tool
life. Due to superior mechanical properties, hardmetals are used in various applications
with certain limitations in chemically aggressive environments because of relatively poor
corrosion resistance [1].
The corrosion mechanism of conventional WC-Co hardmetals in the neutral and
acidic solution is governed by the reduction of the Co binder. At the same time, WC
particles are not affected by the corrosion attack [
2
–
4
]. Zheng et al. found that the binder
dissolution started from the center of binder pools in the acid media, independent of binder
chemical nature, and spreads to the edges until the binder phase was consumed entirely [
4
].
Accordingly, it is expected that the corrosion rate will increase with increasing Co content
in the starting mixture.
Besides Co binder content, the corrosion mechanisms in hardmetals depend on many
other factors such as surface characteristics and integrity, corrosive environment, and
hardmetal microstructure [
5
–
7
]. Hardmetal microstructure, including WC grain size,
binder composition/binder chemical nature, grain growth inhibitors GGIs, and porosity,
influence the corrosion behavior of hardmetals [
3
–
10
]. Researchers have reported different
experimental variations concerning the relationship between microstructure and corrosion
resistance.
The chemical nature of the Co binder is represented by magnetic saturation. It depends
on the C content added to the starting hardmetal mixture and technological processes
of consolidation, among which the most important are sintering parameters and atmo-
sphere [
11
–
13
]. Co binder is advantageous because of the relatively large carbon contents
that give the preferred two-phase WC-Co composition, commonly called the carbon win-
dow [
14
,
15
]. During sintering, the Co binder is alloyed with tungsten (W) and carbon (C);
other constituents such as GGIs also add alloying elements to the binder [
14
]. If a higher
amount of tungsten is dissolved in the Co binder, lower relative magnetic saturation values
would be obtained [
3
], and the formation of the brittle
η
-phase carbides M
6
C and M
12
C
would occur in the microstructure of hardmetals. It was found from previous research that
hardmetals with lower relative magnetic saturation values show lower values of corrosion
current density (i
corr
) and critical current density (i
crit
) measured by potentiodynamic
polarization [2,6,7,16].
Regarding the influence of the WC grain size, different conclusions can be found in the
literature. Li Zhang et al. found pseudo-passivation behavior of conventional hardmetals
with the WC grain sizes ranging from 1.2
µ
m to 8.2
µ
m in sulfuric acid H
2
SO
4
and better
corrosion resistance of the coarse WC grain sizes [
3
]. On the other hand, Imasato et al.
found that the corrosion rate of the WC-Co alloy with a smaller WC grain size was lower
than coarse WC grain size both in acid and neutral solution. WC-Co alloy with a smaller
WC grain size showed a higher corrosion resistance in the polarization test because of
the low current densities of active dissolution and passivated region in the polarization
curve [
17
]. Also, they found that the amount of dissolved metals in neutral and acidic
solutions decreased with decreasing WC grain size and carbon content. Most published
papers refer to conventional hardmetals, while there is still a lack of results published on
nanostructured hardmetals with a WC grain size less than 200 nm.
The paper summarizes long-term research on the corrosion resistance of nanostruc-
tured hardmetals. From previous research it was found that the chemical nature of the
binder has a more substantial influence on the electrochemical corrosion resistance com-
pared to Co content in the starting mixture in neutral and acidic solution, which was
quite surprising and not in line with conventional WC-Co hardmetals [
6
,
7
]. The presented
Materials 2021,14, 3933 3 of 17
research was performed to bring more exact conclusions on the influence of Co content
and other microstructural characteristics on the corrosion resistance of nanostructured
hardmetals.
2. Materials and Methods
Different starting mixtures were prepared to investigate the influence of Co content
and chemical nature of the Co binder on the corrosion resistance of nanostructured hard-
metals with a grain size d
WC
< 200 nm. WC powder produced by H.C. Starck Tungsten
(Goslar, Germany) with an average grain size d
BET
of 95 nm and a specific surface area (BET)
of 3.92 m
2
/g, classified as real nanopowder with a grain size less than 100 nm, was used
as a carbide phase. Grain growth inhibitors GGIs, vanadium carbide VC and chromium
carbide Cr
3
C
2
were added to the starting mixtures. VC powder has an average grain size
d
BET
of 350 nm and a specific surface area (BET) of 3.0 m
2
/g, while Cr
3
C
2
has an average
grain size d
BET
of 450 nm and a specific surface area (BET) of 2.0 m
2
/g. Besides controlling
the WC grain growth, VC and Cr
3
C
2
increase hardness and reduce the rate of corrosion. At
the same time, Cr significantly lowers the initial melting point and broadens the melting
range, particularly at low carbon levels [
12
–
14
,
16
]. The amount of VC and Cr
3
C
2
differs
for each mixture; it was optimized to withhold WC powder size in the sintered samples
and increased Co binder content. Half micron cobalt HMP Co, produced by Umicore
(Brussels, Belgium), was used as a binder. Three mixtures with different Co content; 5, 10,
and 15 wt.%Co were prepared. The consolidation process consisted of powder mixture
homogenization in a horizontal ball mill (Zoz GmbH, Wenden, Germany). Compacting was
performed by uniaxial die press type CA-NCII 250 (Osterwalder AG, Lyss, Switzerland).
Final consolidation to total density was achieved by one cycle sinter-HIP process by furnace
FPW280/600-3-2200-100 PS (FCT Anlagenbau GmbH, Sonneberg, Germany) at 1350
◦
C
for 30 min, followed by 100 bars Argon 4.8 pressure for 45 min. The characteristics of the
starting mixtures are presented in Table 1.
Table 1. The characteristics of the starting mixtures.
Mixture Starting WC Powder Grain Size
dBET, nm
Specific
Surface, m2/g Co, wt.% GGI, wt.%
WC-5Co
WC DN 4-0 (H.C. Starck)
95 3.92
50.3% VC 160 (H. C. Starck)
0.5% Cr3C2160 (H. C. Starck)
WC-10Co 10 0.5% VC 160 (H. C. Starck)
0.75% Cr3C2160 (H. C. Starck)
WC-15Co 15 0.75% VC 160 (H. C. Starck)
1.13% Cr3C2160 (H. C. Starck)
The goal was to develop fully dense samples with optimal microstructural characteris-
tics with no irregularities such as
η
-phase in the structure. Previous research found that C
and GGIs content and WC grain size have a more substantial influence on the corrosion
resistance of nanostructured hardmetals than Co content [
6
,
7
]. For the mentioned reason,
special care was taken to obtain the optimal and comparable microstructural characteristics
of consolidated samples.
The samples were detailly characterized, especially in terms of microstructural char-
acteristics. The characterization of samples consisted of density measurements (Metler
Toledo) according to ISO 3369:2006, the specific saturation magnetization (Setaram Instru-
mentation, Sigmameter) according to D6025, and the coercive field strength measurement
(Foerster, Koerzimat 1.096) according to ISO 3326. Diamond disc and pastes were used
to grind and polish the samples’ surface before microstructural characterization and elec-
trochemical measurements. Microstructural characterization consisted of porosity, free
carbon, and
η
-phase evaluation. It was performed by comparing the sample’s surfaces
with photomicrographs from the standard ISO 4499-4:2016. For that purpose, an optical
microscope (Olympus, Shinjuku City, Tokyo, Japan) was used. A field emission scanning
Materials 2021,14, 3933 4 of 17
electron microscope FESEM (Zeiss, Oberkochen, Germany) was used for WC grain size
measurement by the linear intercept method and detection of irregularities such as ab-
normal growth and WC grains grouping or Co lakes. X-ray diffraction XRD analysis was
used to identify the phases present in consolidated samples and exclude the occurrence of
η-phase.
After detailed characterization of the samples and determination of optimal mi-
crostructural characteristics, the corrosion measurements were performed. The surface
of the samples was placed into the corrosion cell filled with H
2
SO
4
+ CO
2
(pH = 0.6). As
reference electrode, saturated calomel electrode SCE (SCHOTT Instruments GmbH, Mainz,
Germany) with a potential of + 0.242 V according to the standard hydrogen electrode
was selected. Graphite wires were used as a counter electrode. The samples were first
researched by direct current techniques DC, the open-circuit potential E
corr
, the linear
polarization resistance (LPR), and the Taffel extrapolation method. Corrosion potential
E
corr
versus SCE was recorded for 30 min. LPR was carried out in the potential range from
−
0.02 V vs. open circuit potential to 0.02 V vs. open circuit potential with a scan rate of
0.167 mV/s. Tafel extrapolation was conducted in the potential range from
−
0.25 V vs.
open circuit potential to 0.25 V vs. open circuit potential, total points 1001 with a scan
rate of 0.167 mV/s. Immediately after the DC techniques, the samples were researched
by alternating current (AC) techniques, more precisely by electrochemical impedance
spectroscopy (EIS).
The EIS start frequency was 100,000 Hz, and the end frequency was 0.001 Hz; the
amplitude was in the range of 10 mV root-mean-square (RMS). The recorded measurements
were analyzed by software SoftCorr III (AMETEK Scientific Instruments, Princeton applied
research, Berwyn, PA, USA). A convenient electrical equivalent circuit (EEC) was selected
by fitting the results of measurements and presented in Nyquist and Bode plots. At each
excitation frequency, an imaginary impedance component Zim is drawn according to the
actual impedance component Zre. The impedance and the phase shift curves were plotted
against the excitation frequency. Both DC and AC techniques were performed on the
potentiostat AMETEK, Princeton applied research, model VersaSTAT3, and the results were
recorded and analyzed by software SoftCorr III (AMETEK Scientific Instruments, Princeton
applied research, Berwyn, PA, USA).
3. Results
3.1. Microstructural Characteristics of Consolidated Samples
Characteristics of consolidated samples with different Co content are presented in Ta-
ble 2. This section may be divided by subheadings. It should provide a concise and precise
description of the experimental results, their interpretation, as well as the experimental
conclusions that can be drawn.
Table 2. Characteristics of consolidated samples.
Sample Density,
g/cm3Relative
Density, %
Magnetic Saturation,
µTm3/kg Rel. Magnetic
Saturation, %
Coercive
Force,
kA/m
ISO Porosity dWC,
nm
A B C
WC-5Co 14.91 100.0 8.4 92 52.0
A00
B00
C00
187
WC-10Co 14.31 100.0 14.8 79 40.0
A00
B00
C00
198
WC-15Co 13.84 100.0 22.3 79 37.0
A00
B00
C00
192
Full densification was achieved for all samples. The samples are characterized by
the lowest possible degree of porosity, A00, B00, and C00, meaning no uncombined/free
carbon or
η
-phase were revealed on the sample’s surface. A high density of samples is
related to Co liquid phase, which is spreading onto the surrounding WC particles. Binder
propagation is associated with Laplace forces acting along the wetting front between Co
binder and WC grains while rearranging the WC particles and reducing the mean distance
between neighboring particles, resulting in densification [
16
,
18
]. It may be concluded
Materials 2021,14, 3933 5 of 17
that the WC particles were rearranged, and Co binder filled the micropores between the
neighboring WC grains resulting in a theoretical/full density of the samples.
The amount of W dissolved in the Co binder phase can be assessed by measuring the
magnetic saturation. The saturation value of Co decreases linearly with the addition of
tungsten W and is not affected by the carbon content in the solution [
19
]. Typical relative
magnetic saturation/percentage saturation ranges from 80–100% [
19
], while percentage
saturation values higher than 70% indicate two-phase WC-Co microstructure. The highest
percentage of 91% was measured for the WC-5Co sample, while 79% was measured for WC-
10Co and WC-15Co samples. The two-phase WC-Co microstructure of researched samples
is confirmed by optical analysis. Based on coercive force measurement it was estimated
the WC grain size. Measured values indicate that all samples fall in the nano range. Two-
phase WC-Co microstructure was confirmed by optical analysis, XRD analysis where only
WC with the hexagonal crystal structure and Co with FCC cubic crystal structure were
identified. Investigation of microstructure revealed homogeneous, uniform distribution
of WC grains, without abnormal grain growth due to optimal GGIs content added to the
starting mixtures. It was necessary to adjust the content of GGIs for different Co content to
achieve a homogeneous and comparable microstructure with retained WC grain size of the
starting powders in the sintered samples. Co binder was uniformly distributed, and no
Co lakes occurred. Microstructure images and XRD patterns of samples are presented in
Figures 1–3.
Materials 2021, 14, x FOR PEER REVIEW 5 of 17
to Co liquid phase, which is spreading onto the surrounding WC particles. Binder propa-
gation is associated with Laplace forces acting along the wetting front between Co binder
and WC grains while rearranging the WC particles and reducing the mean distance be-
tween neighboring particles, resulting in densification [16,18]. It may be concluded that
the WC particles were rearranged, and Co binder filled the micropores between the neigh-
boring WC grains resulting in a theoretical/full density of the samples.
The amount of W dissolved in the Co binder phase can be assessed by measuring the
magnetic saturation. The saturation value of Co decreases linearly with the addition of
tungsten W and is not affected by the carbon content in the solution [19]. Typical relative
magnetic saturation/percentage saturation ranges from 80–100% [19], while percentage
saturation values higher than 70% indicate two-phase WC-Co microstructure. The highest
percentage of 91% was measured for the WC-5Co sample, while 79% was measured for
WC-10Co and WC-15Co samples. The two-phase WC-Co microstructure of researched
samples is confirmed by optical analysis. Based on coercive force measurement it was es-
timated the WC grain size. Measured values indicate that all samples fall in the nano
range. Two-phase WC-Co microstructure was confirmed by optical analysis, XRD analy-
sis where only WC with the hexagonal crystal structure and Co with FCC cubic crystal
structure were identified. Investigation of microstructure revealed homogeneous, uni-
form distribution of WC grains, without abnormal grain growth due to optimal GGIs con-
tent added to the starting mixtures. It was necessary to adjust the content of GGIs for
different Co content to achieve a homogeneous and comparable microstructure with re-
tained WC grain size of the starting powders in the sintered samples. Co binder was uni-
formly distributed, and no Co lakes occurred. Microstructure images and XRD patterns
of samples are presented in Figures 1–3.
(a)
Figure 1. Cont.
Materials 2021,14, 3933 6 of 17
Materials 2021, 14, x FOR PEER REVIEW 6 of 17
(b)
Figure 1. Microstructure and XRD pattern of WC-5Co sample [20]. (a) SEM image of microstructure; (b) XRD pattern.
(a)
Figure 1. Microstructure and XRD pattern of WC-5Co sample [20]. (a) SEM image of microstructure; (b) XRD pattern.
Materials 2021, 14, x FOR PEER REVIEW 6 of 17
(b)
Figure 1. Microstructure and XRD pattern of WC-5Co sample [20]. (a) SEM image of microstructure; (b) XRD pattern.
(a)
Figure 2. Cont.
Materials 2021,14, 3933 7 of 17
Materials 2021, 14, x FOR PEER REVIEW 7 of 17
(b)
Figure 2. Microstructure and XRD pattern of WC-10Co sample [21]: (a) SEM image of microstructure; (b) XRD pattern.
(a)
Figure 2. Microstructure and XRD pattern of WC-10Co sample [21]: (a) SEM image of microstructure; (b) XRD pattern.
Materials 2021, 14, x FOR PEER REVIEW 7 of 17
(b)
Figure 2. Microstructure and XRD pattern of WC-10Co sample [21]: (a) SEM image of microstructure; (b) XRD pattern.
(a)
Figure 3. Cont.
Materials 2021,14, 3933 8 of 17
Materials 2021, 14, x FOR PEER REVIEW 8 of 17
(b)
Figure 3. Microstructure and XRD pattern of WC-15Co sample [20]: (a) SEM image of microstructure; (b) XRD pattern.
3.2. Results of DC Techniques
The results of electrochemical DC techniques are presented in Table 3.
Table 3. Electrochemical DC techniques results.
Sample Ts [°C] E
corr
vs.
SCE [mV]
R
p
[Ωcm
2
]
β
a
[mV/dec]
β
c
[mV/dec]
i
corr
[µA/cm
2
]
v
corr
[mm/y]
WC-5Co 20 ± 2 −249 654.5 75.31 90.37 20.7 0.1748
WC-10Co 20 ± 2 −308 452.8 98.34 97.67 36.6 0.3888
WC-15Co 20 ± 2 −291 349.9 120.19 86.95 50.8 0.4162
Ts—measured temperature; E
corr
—corrosion potential; R
p
—polarization resistance; βa—a slope of anodic Tafel curve; βc—
a slope of cathodic Tafel curve; i
corr
—corrosion current density; v
corr
—corrosion rate.
3.3. Results of Electrochemical Impedance Spectroscopy EIS
EIS measurements aimed to investigate the corrosion behavior at the interface be-
tween the sample surface and electrolyte solution and determine the samples’ corrosion
rate. The results are presented in Table 4.
Table 4. Electrochemical impedance spectroscopy EIS technique results.
Sample
T
s
[°C]
R
s
[Ωcm
2
] Q n
1
R
p/
R
ct
[Ωcm
2
]
WC-5Co 20 ± 2 4.022 1.761·10
−3
0.745 1.101·10
−3
WC-10Co 20 ± 2 4.504 2.213·10
−3
0.725 8.068·10
−2
WC-15Co 20 ± 2 5.797 2.552·10
−3
0.683 4.657·10
−2
Ts—measured temperature; R
s
—solution resistance between the working electrode and the reference electrode in a three-
electrode cell; Q—Constant Phase Element (CPE); n
1
—constant; R
ct
—polarization resistance or resistance to charge transfer
on the electrode/electrolyte interface.
As already mentioned in Section 2, a convenient and optimal electrical equivalent
circuit (EEC) was selected using software SoftCorr III by fitting the measurements’ results
and presented in Nyquist and Bode plots. At each excitation frequency, an imaginary im-
pedance component Zim is drawn according to the actual impedance component Zre. The
Figure 3. Microstructure and XRD pattern of WC-15Co sample [20]: (a) SEM image of microstructure; (b) XRD pattern.
3.2. Results of DC Techniques
The results of electrochemical DC techniques are presented in Table 3.
Table 3. Electrochemical DC techniques results.
Sample Ts [◦C]
Ecorr vs.
SCE
[mV]
Rp
[Ωcm2]
βa
[mV/dec]
βc
[mV/dec]
icorr
[µA/cm2]
vcorr
[mm/y]
WC-5Co 20 ±2−249 654.5 75.31 90.37 20.7 0.1748
WC-
10Co 20 ±2−308 452.8 98.34 97.67 36.6 0.3888
WC-
15Co 20 ±2−291 349.9 120.19 86.95 50.8 0.4162
Ts—measured temperature; E
corr
—corrosion potential; R
p
—polarization resistance;
β
a—a slope of anodic Tafel
curve; βc—a slope of cathodic Tafel curve; icorr—corrosion current density; vcorr—corrosion rate.
3.3. Results of Electrochemical Impedance Spectroscopy EIS
EIS measurements aimed to investigate the corrosion behavior at the interface between
the sample surface and electrolyte solution and determine the samples’ corrosion rate. The
results are presented in Table 4.
Table 4. Electrochemical impedance spectroscopy EIS technique results.
Sample Ts
[◦C]
Rs
[Ωcm2]Q n1Rp/Rct
[Ωcm2]
WC-5Co 20 ±2 4.022 1.761·10−30.745 1.101·10−3
WC-10Co 20 ±2 4.504 2.213·10−30.725 8.068·10−2
WC-15Co 20 ±2 5.797 2.552·10−30.683 4.657·10−2
Ts—measured temperature; R
s
—solution resistance between the working electrode and the reference electrode in
a three-electrode cell; Q—Constant Phase Element (CPE); n
1
—constant; R
ct
—polarization resistance or resistance
to charge transfer on the electrode/electrolyte interface.
As already mentioned in Section 2, a convenient and optimal electrical equivalent
circuit (EEC) was selected using software SoftCorr III by fitting the measurements’ results
and presented in Nyquist and Bode plots. At each excitation frequency, an imaginary
Materials 2021,14, 3933 9 of 17
impedance component Zim is drawn according to the actual impedance component Zre.
The impedance and the phase shift curves were plotted against the excitation frequency.
The selected ECC model is shown in Figure 4.
Materials 2021, 14, x FOR PEER REVIEW 9 of 17
impedance and the phase shift curves were plotted against the excitation frequency. The
selected ECC model is shown in Figure 4.
Figure 4. Selected EEC [22].
The same model R(QR), which best corresponds to the processes and reactions on the
sample’s surface, was selected for all samples. It is essential to mention that the same EEC
model was established in previous research performed on near nanostructured WC-11Co
and WC-11Ni samples [22]. The mentioned indicates the repeatability of the corrosion
process between nanostructured hardmetal and acidic solutions. The impedance of a con-
stant phase element is defined as:
()
1
n
QYj
ω
−
=
(1)
where Y and n (− 1 ≤ n ≤ 1) are constants independent of the angular frequency (ω) and
temperature. For the value in the range 0.6 < n ≤ 1, CPE has the physical meaning of ca-
pacitance, an ideal inductor for n = −1, and an ideal resistor for n = 0.
4. Analysis and Discussion
4.1. Influence of Co Content on the Corrosion Resistance of Nanostructured WC-Co Hardmetals
From conducted research, it can be concluded that the corrosion potential E
corr
of
samples changes depending on the Co content. E
corr
of WC-5Co and WC-10Co samples
changed from more negative to more positive values indicating that the surfaces of the
samples are getting passivated, and a reduction occurred. The corrosion potential curves
are unstable and show random fluctuations. A drop from more positive to more negative
values was detected for the WC-15Co sample, indicating sample oxidation in contact with
the acidic electrolyte and reducing protons at the surface. The changes of E
corr
are not sig-
nificant, and the corrosion potential variations of each sample occurred in a narrow range.
Negative values of E
corr
measured for 30 min in 96% H
2
SO
4
+ CO
2
were obtained for all
samples indicating material dissolution and instability in acidic solution. The E
corr
vs. time
curves are presented in Figure 5, and sample Tafel extrapolation curves in Figure 6.
Figure 5. Corrosion potential E
corr
of samples.
Figure 4. Selected EEC [22].
The same model R(QR), which best corresponds to the processes and reactions on the
sample’s surface, was selected for all samples. It is essential to mention that the same EEC
model was established in previous research performed on near nanostructured WC-11Co
and WC-11Ni samples [
22
]. The mentioned indicates the repeatability of the corrosion
process between nanostructured hardmetal and acidic solutions. The impedance of a
constant phase element is defined as:
Q=Y(jω)n−1(1)
where Yand n(
−
1
≤
n
≤
1) are constants independent of the angular frequency (
ω
) and
temperature. For the value in the range 0.6 < n
≤
1, CPE has the physical meaning of
capacitance, an ideal inductor for n=−1, and an ideal resistor for n= 0.
4. Analysis and Discussion
4.1. Influence of Co Content on the Corrosion Resistance of Nanostructured WC-Co Hardmetals
From conducted research, it can be concluded that the corrosion potential E
corr
of
samples changes depending on the Co content. E
corr
of WC-5Co and WC-10Co samples
changed from more negative to more positive values indicating that the surfaces of the
samples are getting passivated, and a reduction occurred. The corrosion potential curves
are unstable and show random fluctuations. A drop from more positive to more negative
values was detected for the WC-15Co sample, indicating sample oxidation in contact with
the acidic electrolyte and reducing protons at the surface. The changes of E
corr
are not
significant, and the corrosion potential variations of each sample occurred in a narrow
range. Negative values of E
corr
measured for 30 min in 96% H
2
SO
4
+ CO
2
were obtained
for all samples indicating material dissolution and instability in acidic solution. The E
corr
vs. time curves are presented in Figure 5, and sample Tafel extrapolation curves in
Figure 6
.
Materials 2021, 14, x FOR PEER REVIEW 9 of 17
impedance and the phase shift curves were plotted against the excitation frequency. The
selected ECC model is shown in Figure 4.
Figure 4. Selected EEC [22].
The same model R(QR), which best corresponds to the processes and reactions on the
sample’s surface, was selected for all samples. It is essential to mention that the same EEC
model was established in previous research performed on near nanostructured WC-11Co
and WC-11Ni samples [22]. The mentioned indicates the repeatability of the corrosion
process between nanostructured hardmetal and acidic solutions. The impedance of a con-
stant phase element is defined as:
()
1
n
QYj
ω
−
=
(1)
where Y and n (− 1 ≤ n ≤ 1) are constants independent of the angular frequency (ω) and
temperature. For the value in the range 0.6 < n ≤ 1, CPE has the physical meaning of ca-
pacitance, an ideal inductor for n = −1, and an ideal resistor for n = 0.
4. Analysis and Discussion
4.1. Influence of Co Content on the Corrosion Resistance of Nanostructured WC-Co Hardmetals
From conducted research, it can be concluded that the corrosion potential E
corr
of
samples changes depending on the Co content. E
corr
of WC-5Co and WC-10Co samples
changed from more negative to more positive values indicating that the surfaces of the
samples are getting passivated, and a reduction occurred. The corrosion potential curves
are unstable and show random fluctuations. A drop from more positive to more negative
values was detected for the WC-15Co sample, indicating sample oxidation in contact with
the acidic electrolyte and reducing protons at the surface. The changes of E
corr
are not sig-
nificant, and the corrosion potential variations of each sample occurred in a narrow range.
Negative values of E
corr
measured for 30 min in 96% H
2
SO
4
+ CO
2
were obtained for all
samples indicating material dissolution and instability in acidic solution. The E
corr
vs. time
curves are presented in Figure 5, and sample Tafel extrapolation curves in Figure 6.
Figure 5. Corrosion potential E
corr
of samples.
Figure 5. Corrosion potential Ecorr of samples.
Materials 2021,14, 3933 10 of 17
Materials 2021, 14, x FOR PEER REVIEW 10 of 17
Figure 6. Tafel extrapolation curves.
Higher values of R
p
and lower values of i
corr
were obtained for samples with lower
Co content. Accordingly, the corrosion rate in acidic solution increases with increasing Co
content due to selective dissolution of the Co matrix. The cathodic Tafel slopes of samples
show a similar trend. In contrast, the anodic Tafel slope increases with increasing Co con-
tent which could be attributed to more pronounced oxidation of the higher Co content
samples. The dependence of polarization resistance R
p
and corrosion rate v
corr
for different
Co contents is presented graphically in Figure 7.
Figure 7. The dependence of R
p
and v
corr
concerning Co content.
Even though the addition of the refractory metal carbides, VC, Cr
3
C
2
in the starting
mixtures was increased with increasing Co content to maintain the WC powder size, Co
content showed stronger influence on the electrochemical corrosion resistance of
nanostructured hardmetal samples with optimal microstructural characteristics. The low-
est v
corr
of 0.1748 mm/y was obtained for the WC-5Co sample, while the highest v
corr
of
0.4162 mm/y was measured for the WC-15Co sample.
Figures 8–10 present the Nyquist and Bode diagrams for the WC-5Co, WC-10Co, and
WC-15Co samples, obtained using the corresponding EEC simulation model of EIS re-
sults. The highest Rp of 1.101·103 Ωcm
2
was measured for the WC-5Co sample, followed
by Rp of 8.068·102 Ωcm
2
measured for the WC-10Co sample, and Rp of 4.657·102 Ωcm
2
measured for the WC-15Co sample. Higher Rp values were detected for the samples with
lower Co content, indicating better corrosion resistance (Figure 11) which corresponds to
the results obtained by DC linear polarization techniques. It can be seen from Figures 8–
0.0
0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
0.5
0
100
200
300
400
500
600
700
WC-5Co WC-10Co WC-15Co
Corrosion rate vcorr, mm/year
Polarisation resistance Rp, Ωcm2
Rp vcorr
Figure 6. Tafel extrapolation curves.
Higher values of R
p
and lower values of i
corr
were obtained for samples with lower
Co content. Accordingly, the corrosion rate in acidic solution increases with increasing Co
content due to selective dissolution of the Co matrix. The cathodic Tafel slopes of samples
show a similar trend. In contrast, the anodic Tafel slope increases with increasing Co
content which could be attributed to more pronounced oxidation of the higher Co content
samples. The dependence of polarization resistance R
p
and corrosion rate v
corr
for different
Co contents is presented graphically in Figure 7.
Materials 2021, 14, x FOR PEER REVIEW 10 of 17
Figure 6. Tafel extrapolation curves.
Higher values of R
p
and lower values of i
corr
were obtained for samples with lower
Co content. Accordingly, the corrosion rate in acidic solution increases with increasing Co
content due to selective dissolution of the Co matrix. The cathodic Tafel slopes of samples
show a similar trend. In contrast, the anodic Tafel slope increases with increasing Co con-
tent which could be attributed to more pronounced oxidation of the higher Co content
samples. The dependence of polarization resistance R
p
and corrosion rate v
corr
for different
Co contents is presented graphically in Figure 7.
Figure 7. The dependence of R
p
and v
corr
concerning Co content.
Even though the addition of the refractory metal carbides, VC, Cr
3
C
2
in the starting
mixtures was increased with increasing Co content to maintain the WC powder size, Co
content showed stronger influence on the electrochemical corrosion resistance of
nanostructured hardmetal samples with optimal microstructural characteristics. The low-
est v
corr
of 0.1748 mm/y was obtained for the WC-5Co sample, while the highest v
corr
of
0.4162 mm/y was measured for the WC-15Co sample.
Figures 8–10 present the Nyquist and Bode diagrams for the WC-5Co, WC-10Co, and
WC-15Co samples, obtained using the corresponding EEC simulation model of EIS re-
sults. The highest Rp of 1.101·103 Ωcm
2
was measured for the WC-5Co sample, followed
by Rp of 8.068·102 Ωcm
2
measured for the WC-10Co sample, and Rp of 4.657·102 Ωcm
2
measured for the WC-15Co sample. Higher Rp values were detected for the samples with
lower Co content, indicating better corrosion resistance (Figure 11) which corresponds to
the results obtained by DC linear polarization techniques. It can be seen from Figures 8–
0.0
0.1
0.1
0.2
0.2
0.3
0.3
0.4
0.4
0.5
0
100
200
300
400
500
600
700
WC-5Co WC-10Co WC-15Co
Corrosion rate vcorr, mm/year
Polarisation resistance Rp, Ωcm2
Rp vcorr
Figure 7. The dependence of Rpand vcorr concerning Co content.
Even though the addition of the refractory metal carbides, VC, Cr
3
C
2
in the starting
mixtures was increased with increasing Co content to maintain the WC powder size, Co
content showed stronger influence on the electrochemical corrosion resistance of nanos-
tructured hardmetal samples with optimal microstructural characteristics. The lowest v
corr
of 0.1748 mm/y was obtained for the WC-5Co sample, while the highest v
corr
of 0.4162
mm/y was measured for the WC-15Co sample.
Figures 8–10 present the Nyquist and Bode diagrams for the WC-5Co, WC-10Co, and
WC-15Co samples, obtained using the corresponding EEC simulation model of EIS results.
The highest Rp of 1.101
·
103
Ω
cm
2
was measured for the WC-5Co sample, followed by Rp
of 8.068
·
102
Ω
cm
2
measured for the WC-10Co sample, and Rp of 4.657
·
102
Ω
cm
2
measured
for the WC-15Co sample. Higher Rp values were detected for the samples with lower
Co content, indicating better corrosion resistance (Figure 11) which corresponds to the
Materials 2021,14, 3933 11 of 17
results obtained by DC linear polarization techniques. It can be seen from Figures 8–10
that the radius r of the capacitive semi-circles in the Nyquist plots differ for samples with
different Co content. The diameter of the capacitive loop decreased with increasing Co
content, indicating better charge transfer resistance on the electrode/electrolyte interface.
Subsequently, the decrease in the diameter of the capacitance loop may be ascribed to the
weaker protective ability of the sample surface.
Materials 2021, 14, x FOR PEER REVIEW 11 of 17
10 that the radius r of the capacitive semi-circles in the Nyquist plots differ for samples
with different Co content. The diameter of the capacitive loop decreased with increasing
Co content, indicating better charge transfer resistance on the electrode/electrolyte inter-
face. Subsequently, the decrease in the diameter of the capacitance loop may be ascribed
to the weaker protective ability of the sample surface.
(a) (b)
Figure 8. Nyquist (a) and Bode (b) plots of WC-5Co.
(a) (b)
Figure 9. Nyquist (a) and Bode (b) plots of WC-10Co.
(a) (b)
Figure 10. Nyquist (a) and Bode (b) plots of WC-15Co.
Figure 8. Nyquist (a) and Bode (b) plots of WC-5Co.
Materials 2021, 14, x FOR PEER REVIEW 11 of 17
10 that the radius r of the capacitive semi-circles in the Nyquist plots differ for samples
with different Co content. The diameter of the capacitive loop decreased with increasing
Co content, indicating better charge transfer resistance on the electrode/electrolyte inter-
face. Subsequently, the decrease in the diameter of the capacitance loop may be ascribed
to the weaker protective ability of the sample surface.
(a) (b)
Figure 8. Nyquist (a) and Bode (b) plots of WC-5Co.
(a) (b)
Figure 9. Nyquist (a) and Bode (b) plots of WC-10Co.
(a) (b)
Figure 10. Nyquist (a) and Bode (b) plots of WC-15Co.
Figure 9. Nyquist (a) and Bode (b) plots of WC-10Co.
Materials 2021, 14, x FOR PEER REVIEW 11 of 17
10 that the radius r of the capacitive semi-circles in the Nyquist plots differ for samples
with different Co content. The diameter of the capacitive loop decreased with increasing
Co content, indicating better charge transfer resistance on the electrode/electrolyte inter-
face. Subsequently, the decrease in the diameter of the capacitance loop may be ascribed
to the weaker protective ability of the sample surface.
(a) (b)
Figure 8. Nyquist (a) and Bode (b) plots of WC-5Co.
(a) (b)
Figure 9. Nyquist (a) and Bode (b) plots of WC-10Co.
(a) (b)
Figure 10. Nyquist (a) and Bode (b) plots of WC-15Co.
Figure 10. Nyquist (a) and Bode (b) plots of WC-15Co.
Materials 2021,14, 3933 12 of 17
Materials 2021, 14, x FOR PEER REVIEW 12 of 17
Figure 11. The dependence of Rp concerning Co content.
Nanostructured hardmetals with optimal microstructural characteristics exhibited
behavior similar to previously researched conventional hardmetals with coarser WC grain
size. Electrochemical corrosion resistance decreases with increasing Co content in a corro-
sive, acidic environment due to predominant active Co binder reduction, also known as
Co leaching.
4.2. Influence of Co Binder Chemical Nature on the Corrosion Resistance of Nanostructured WC-
Co Hardmetals
The chemical nature of the Co binder can be characterized by magnetic saturation. It
depends on the C content added to the starting hardmetal mixture and consolidation pro-
cedure, where sintering parameters and atmosphere have a crucial influence. As referred
in the Introduction, it was found from previous research that C content added to the start-
ing mixture can significantly influence the electrochemical corrosion resistance in both
neutral (3.5% NaCl with pH = 6.6) and acidic (96% H2SO4 + CO2 with pH = 0.6) environ-
ments [6,7]. Comparing WC-5Co samples of the same composition, better corrosion re-
sistance was observed for samples with lower C-added content, lower magnetic satura-
tion, and coarser WC grain size. The opposite behavior governed by the C content and
magnetic saturation was noted for WC-15Co samples [7]. Compared to C content, GGIs
content, and grain size, the Co content showed less impact on the electrochemical corro-
sion resistance in both acid and neutral solutions. There was no clear trend of increasing
corrosion current densities icorr and decreasing polarization resistance Rp with increasing
Co content typical for conventional hardmetals. Microstructural characteristics, in this
case WC grain size, has shown to have a greater influence on the sintered samples corro-
sion resistance [6,7]. To obtain better insight into the electrochemical corrosion resistance
of nanostructured hardmetals, previously researched samples designated as WC-5Co-1
and WC-10Co-1 were introduced in the analysis. WC-5Co-1 and WC-10Co-1 samples were
consolidated using same production procedure and characterized by the same methods
described in Section 2. The only alteration was the use of lower C and GGIs content added
to the starting mixture, which caused different chemical nature of the Co binder and lower
0
5
10
15
20
0
200
400
600
800
1000
1200
123
Rp 1.10E+03 8.07E+02 4.66E+02
Co 51015
Co, wt.%
Polarisation resistance Rp[Ωcm2]
Samples
Figure 11. The dependence of Rpconcerning Co content.
Nanostructured hardmetals with optimal microstructural characteristics exhibited
behavior similar to previously researched conventional hardmetals with coarser WC grain
size. Electrochemical corrosion resistance decreases with increasing Co content in a corro-
sive, acidic environment due to predominant active Co binder reduction, also known as Co
leaching.
4.2. Influence of Co Binder Chemical Nature on the Corrosion Resistance of Nanostructured
WC-Co Hardmetals
The chemical nature of the Co binder can be characterized by magnetic saturation.
It depends on the C content added to the starting hardmetal mixture and consolidation
procedure, where sintering parameters and atmosphere have a crucial influence. As
referred in the Introduction, it was found from previous research that C content added to
the starting mixture can significantly influence the electrochemical corrosion resistance
in both neutral (3.5% NaCl with pH = 6.6) and acidic (96% H
2
SO
4
+ CO
2
with pH =
0.6) environments [
6
,
7
]. Comparing WC-5Co samples of the same composition, better
corrosion resistance was observed for samples with lower C-added content, lower magnetic
saturation, and coarser WC grain size. The opposite behavior governed by the C content
and magnetic saturation was noted for WC-15Co samples [
7
]. Compared to C content, GGIs
content, and grain size, the Co content showed less impact on the electrochemical corrosion
resistance in both acid and neutral solutions. There was no clear trend of increasing
corrosion current densities i
corr
and decreasing polarization resistance Rp with increasing
Co content typical for conventional hardmetals. Microstructural characteristics, in this case
WC grain size, has shown to have a greater influence on the sintered samples corrosion
resistance [
6
,
7
]. To obtain better insight into the electrochemical corrosion resistance of
nanostructured hardmetals, previously researched samples designated as WC-5Co-1 and
WC-10Co-1 were introduced in the analysis. WC-5Co-1 and WC-10Co-1 samples were
consolidated using same production procedure and characterized by the same methods
described in Section 2. The only alteration was the use of lower C and GGIs content
added to the starting mixture, which caused different chemical nature of the Co binder and
Materials 2021,14, 3933 13 of 17
lower values of relative magnetic saturation. Characteristics of the additionally introduced
samples and comparison with previously characterized nanostructured hardmetals with
dWC < 200 nm are presented in Tables 5and 6.
Table 5. Comparison of WC-5Co samples with different characteristics.
Sample GGI,
wt.%
Added C,
wt.%
Density,
g/cm3ρ,
%
Relative
Magnetic
Saturation,
%
Coercive
Force,
kA/m
vcorr
[mm/y]
WC-5Co-1 0.41%VC,
0.80% Cr3C20.150 14.96 100 48.0 44.9 0.1181
WC-5Co 0.30%VC,
0.50% Cr3C20.275 14.91 100 92.0 52.0 0.1748
Table 6. Comparison of WC-10Co samples with different microstructural characteristics and magnetic saturation.
Sample GGI,
wt.%
Added C,
wt.%
Density,
g/cm3ρ,
%
Relative
Magnetic
Saturation,
%
Coercive
Force,
kA/m
vcorr
[mm/y]
WC-10Co-1 0.37%VC,
0.72% Cr3C20.225 14.35 100 74.7 35.1 0.3463
WC-10Co 0.5%VC,
0.75% Cr3C20.250 14.32 100 79.0 40.0 0.3888
As indicated in Table 5, the WC-5Co-1 sample has a significantly lower relative mag-
netic saturation of 48.0%, attributed to the lower added C content and the different chemical
nature of the Co binder. As mentioned, typical relative magnetic saturation/percentage
saturation ranges from 80–100%. Saturation percentage values lower than 70% indicate the
presence of microstructural irregularity
η
-phase, confirmed by optical, FESEM, and XRD
analysis [
6
,
7
]. Previous research found that
η
-phase most likely enhances the passive layer
formation on the sample surfaces, thereby reducing the tendency of sample dissolution
and increasing the stability of oxides forming in addition to the existing passive layer on
the surface [
2
,
6
,
7
]. The slightly higher measured density of the WC-5Co-1 sample is also
associated with
η
-phase presence in the structure since W
6
Co
6
C or W
3
Co
3
C has higher
density when compared to a two-phase WC-Co hardmetal. The coercive force obtained for
the WC-5Co-1 sample amounts to 44.9 kA/m and is lower than that of WC-5Co, suggesting
a coarser grain size of the WC-5Co-1 sample. Thus, its microstructure can be classified as
near nano, in the ultrafine range from 200 to 500 nm. Tafel extrapolation curves of WC-5Co
samples are presented in Figure 12.
Materials 2021, 14, x FOR PEER REVIEW 13 of 17
values of relative magnetic saturation. Characteristics of the additionally introduced sam-
ples and comparison with previously characterized nanostructured hardmetals with d
WC
< 200 nm are presented in Tables 5 and 6.
Table 5. Comparison of WC-5Co samples with different characteristics.
Sample GGI,
wt.%
Added C,
wt.%
Density,
g/cm
3
ρ,
%
Relative
magnetic
saturation, %
Coercive force,
kA/m
v
corr
[mm/y]
WC-5Co-1 0.41%VC,
0.80% Cr
3
C
2
0.150 14.96 100 48.0 44.9 0.1181
WC-5Co 0.30%VC,
0.50% Cr
3
C
2
0.275 14.91 100 92.0 52.0 0.1748
As indicated in Table 5, the WC-5Co-1 sample has a significantly lower relative mag-
netic saturation of 48.0%, attributed to the lower added C content and the different chem-
ical nature of the Co binder. As mentioned, typical relative magnetic saturation/percent-
age saturation ranges from 80–100%. Saturation percentage values lower than 70% indi-
cate the presence of microstructural irregularity η-phase, confirmed by optical, FESEM,
and XRD analysis [6,7]. Previous research found that η-phase most likely enhances the
passive layer formation on the sample surfaces, thereby reducing the tendency of sample
dissolution and increasing the stability of oxides forming in addition to the existing pas-
sive layer on the surface [2,6,7]. The slightly higher measured density of the WC-5Co-1
sample is also associated with η-phase presence in the structure since W
6
Co
6
C or W
3
Co
3
C
has higher density when compared to a two-phase WC-Co hardmetal. The coercive force
obtained for the WC-5Co-1 sample amounts to 44.9 kA/m and is lower than that of WC-
5Co, suggesting a coarser grain size of the WC-5Co-1 sample. Thus, its microstructure can
be classified as near nano, in the ultrafine range from 200 to 500 nm. Tafel extrapolation
curves of WC-5Co samples are presented in Figure 12.
Figure 12. Tafel extrapolation curves of WC-5Co samples with different microstructural characteristics.
The WC-5Co-1 sample with lower magnetic saturation value and consequently
higher W and C content in the Co binder showed approximately 30% lower corrosion rate,
which is in line with previous research. Sutthiruangwong and Mori found that the mag-
netic saturation related to Co binder composition plays an essential role in the corrosion
properties of hardmetals [15,23,24]. F.J.J. Kellner et al. found that electrochemical corro-
sion resistance of hardmetals is influenced by the W and C diffusion in the Co binder
amount which is increased by decreasing the WC grain size [25]. They concluded that W
and C dissolved in Co binder during the sintering process stabilize the thermodynami-
cally unstable FCC Co crystal structure at room temperature, the amount of which is in-
creased by an increase of W and C content in the binder. FCC Co is characterized by better
Figure 12. Tafel extrapolation curves of WC-5Co samples with different microstructural characteristics.
Materials 2021,14, 3933 14 of 17
The WC-5Co-1 sample with lower magnetic saturation value and consequently higher
W and C content in the Co binder showed approximately 30% lower corrosion rate, which
is in line with previous research. Sutthiruangwong and Mori found that the magnetic
saturation related to Co binder composition plays an essential role in the corrosion prop-
erties of hardmetals [
15
,
23
,
24
]. F.J.J. Kellner et al. found that electrochemical corrosion
resistance of hardmetals is influenced by the W and C diffusion in the Co binder amount
which is increased by decreasing the WC grain size [
25
]. They concluded that W and
C dissolved in Co binder during the sintering process stabilize the thermodynamically
unstable FCC Co crystal structure at room temperature, the amount of which is increased
by an increase of W and C content in the binder. FCC Co is characterized by better corrosion
resistance compared to the HCP crystal structure of Co, thermodynamically stable at room
temperature [
25
], when a HCP+FCC Co layer around the HCP Co binder is formed [
25
].
Their research was performed in an alkaline medium. Still, since in this study the tests
were performed in an acidic solution where the lower corrosion resistance of hardmetals is
attributed to the dissolution of the HCP Co matrix, these claims could be expected to be
more pronounced.
Besides lower magnetic saturation, the WC-5Co-1 sample has a higher content of GGIs,
VC, Cr
3
C
2
in the starting mixture, as presented in Table 5. It is well-known that GGIs are
dissolved and distributed among the WC phase and binder during sintering and influence
characteristics of hardmetals [
26
]. Sutthiruangwong and Mori have found that higher
corrosion resistance can be assigned to binders that experience higher chromium dissolution
rates during sintering [
23
,
24
]. Tomlinson and Ayerst found that small additions of Cr
3
C
2
improve the electrochemical corrosion resistance of hardmetals due to the formation of
Cr
2
O
3
film on the Co binder surface [
26
]. On the other hand, a small addition of VC in
combination with Cr
3
C
2
decreases the positive Cr
3
C
2
influence in acidic solution [
26
].
The WC-5Co-1 sample has a 0.3 wt.% higher content of Cr
3
C
2
, which dissolved in the
binder and contributed to a 30% lower corrosion rate than the WC-5Co sample. In this
research, it is hard to distinguish which factor has the most substantial influence on
electrochemical corrosion resistance. To specify more clearly, WC-10Co samples with
different microstructural characteristics and magnetic saturation were compared in Table 6.
The WC-10Co-1 sample has a slightly lower relative magnetic saturation of 74.7% than
the WC-10Co sample, which is related to marginally lower C content in the amount of
0.025 wt.% added to the starting mixture. Lower C content resulted in more W dissolved in
the Co binder, which, as in the case of the WC-5Co sample, most probably stabilized Co’s
thermodynamically unstable FCC crystal structure. Consequently, the WC-10Co-1 sample
is characterized by a marginally higher measured density. Density values vary within the
two-phase region of the WC-Co phase, and is increased with an increasing amount of W
which remains in the Co binder [
27
]. Sample WC-10Co-1 is located at the lower end of the
two-phase WC-Co region in the isothermal part of the WC-Co phase diagram. Therefore,
the measured density value is slightly higher than the theoretical density, despite
η
-phase
not detected. The coercive force of WC-10Co-1 amounts to 35.1 kA/m, i.e., lower when
compared to WC-10Co, which indicates a coarser grain size of the WC-10Co-1 sample. The
same was noted for WC-5Co samples. Tafel extrapolation curves of WC-10Co samples are
presented in Figure 13.
Materials 2021,14, 3933 15 of 17
Materials 2021, 14, x FOR PEER REVIEW 15 of 17
Figure 13. Tafel extrapolation curves of WC-10Co samples with different microstructural characteristics.
The WC-10Co-1 sample with a lower relative magnetic saturation value showed ap-
proximately 10% lower corrosion rate. Both microstructures consist of two phases, WC
and Co, with no η-phase detected in the microstructure. The difference in GGIs content is
relatively small and amounts to an extra 0.13 wt.%VC and 0.03 wt.% Cr
3
C
2
added to the
WC-10Co mixture. It can be concluded that the slight increase of VC wt.% did not contrib-
ute to corrosion resistance which corresponds to previously published research. Machio
et al. found that small VC addition of 0.4 wt.% increase i
corr
and make hardmetals more
sensitive to pitting corrosion due to VC influence on the W dissolution in the Co matrix
and formation of (V,W)C layer around the WC grains. VC decreases the dissolution of W
atoms in the Co binder during the sintering process and increases the magnetic saturation
compared to pure WC-Co hardmetal without GGIs [1,28]. D.S. Konadu et al. found that
WC-Co hardmetal possesses nobler corrosion resistance compared to 0.4 wt.%VC contain-
ing hardmetal in both HCl and H
2
SO
4
[1]. Accordingly, better corrosion resistance in this
research may be related to W and C dissolution in the Co binder, magnetic saturation, or
WC grain size in the sintered sample.
5. Conclusions
The following conclusions can be drawn from the conducted research:
(1) Fully dense nanostructured hardmetals with a WC grain size d
WC
≤ 200 nm were de-
veloped utilizing the single-cycle sinter-HIP process. For different Co contents, a ho-
mogeneous microstructure of equal and uniform grain size without microstructural
defects in the form of carbide agglomerates, abnormal grain growth, or Co lakes was
successfully obtained.
(2) The importance of GGIs content adjustment was established as a key factor of obtain-
ing a homogeneous microstructure with WC grain size retained at the same values as
in starting mixtures of different Co binder content.
(3) The Co content in the starting mixture proved to have a significant influence on the
electrochemical corrosion resistance of nanostructured hardmetals in acidic solution.
A noticeable trend of polarization resistance R
p
decrease, and current density i
corr
and
corrosion rate v
corr
increase has been established with increasing Co content.
Nanostructured hardmetals with the grain size d
WC
˂ 200 nm showed the same corro-
sion behavior as coarser grain-size conventional WC hardmetals depending on the Co
content.
(4) The chemical composition of the Co binder showed a significant influence. Samples
with lower relative magnetic saturation related to lower added C content and more
W dissolved in the Co binder showed better corrosion resistance. Significant differ-
ences in magnetic saturation for samples with the same Co content lead to more pro-
nounced differences in the corrosion rates. A slight difference in magnetic saturation
and WC grain size changed the Taffel curves.
(5) Co content was shown to be the dominant influential factor governing electrochemi-
cal corrosion resistance of nanostructured hardmetals when compared to the chemical
Figure 13. Tafel extrapolation curves of WC-10Co samples with different microstructural characteristics.
The WC-10Co-1 sample with a lower relative magnetic saturation value showed
approximately 10% lower corrosion rate. Both microstructures consist of two phases,
WC and Co, with no
η
-phase detected in the microstructure. The difference in GGIs
content is relatively small and amounts to an extra 0.13 wt.%VC and 0.03 wt.% Cr
3
C
2
added to the WC-10Co mixture. It can be concluded that the slight increase of VC wt.%
did not contribute to corrosion resistance which corresponds to previously published
research. Machio et al. found that small VC addition of 0.4 wt.% increase i
corr
and make
hardmetals more sensitive to pitting corrosion due to VC influence on the W dissolution
in the Co matrix and formation of (V,W)C layer around the WC grains. VC decreases the
dissolution of W atoms in the Co binder during the sintering process and increases the
magnetic saturation compared to pure WC-Co hardmetal without GGIs [
1
,
28
]. D.S. Konadu
et al. found that WC-Co hardmetal possesses nobler corrosion resistance compared to 0.4
wt.%VC containing hardmetal in both HCl and H
2
SO
4
[
1
]. Accordingly, better corrosion
resistance in this research may be related to W and C dissolution in the Co binder, magnetic
saturation, or WC grain size in the sintered sample.
5. Conclusions
The following conclusions can be drawn from the conducted research:
(1)
Fully dense nanostructured hardmetals with a WC grain size d
WC ≤
200 nm were
developed utilizing the single-cycle sinter-HIP process. For different Co contents, a
homogeneous microstructure of equal and uniform grain size without microstructural
defects in the form of carbide agglomerates, abnormal grain growth, or Co lakes was
successfully obtained.
(2)
The importance of GGIs content adjustment was established as a key factor of obtain-
ing a homogeneous microstructure with WC grain size retained at the same values as
in starting mixtures of different Co binder content.
(3)
The Co content in the starting mixture proved to have a significant influence on the
electrochemical corrosion resistance of nanostructured hardmetals in acidic solution.
A noticeable trend of polarization resistance R
p
decrease, and current density i
corr
and corrosion rate v
corr
increase has been established with increasing Co content.
Nanostructured hardmetals with the grain size d
WC
< 200 nm showed the same
corrosion behavior as coarser grain-size conventional WC hardmetals depending on
the Co content.
(4)
The chemical composition of the Co binder showed a significant influence. Samples
with lower relative magnetic saturation related to lower added C content and more W
dissolved in the Co binder showed better corrosion resistance. Significant differences
in magnetic saturation for samples with the same Co content lead to more pronounced
differences in the corrosion rates. A slight difference in magnetic saturation and WC
grain size changed the Taffel curves.
(5) Co content was shown to be the dominant influential factor governing electrochemical
corrosion resistance of nanostructured hardmetals when compared to the chemical
composition of the Co binder and WC grain size. Samples with lower Co content
exhibited lower corrosion rates.
Materials 2021,14, 3933 16 of 17
(6)
The slight increase of GGIs content, Cr
3
C
2
, and VC did not improved the corrosion
resistance significantly for the samples with the same Co content. Higher content
of Cr
3
C
2
dissolved in the binder contributed to a lower corrosion rate. Slight VC
increase did not contribute to corrosion resistance. Superior corrosion resistance is
attributed to W and C dissolved in the Co binder, lower magnetic saturation, or WC
grain size of the sintered sample.
Author Contributions:
Conceptualization, T.A.F.; formal analysis, T.A.F., M.K., M.S., and M.Š.M.;
investigation, T.A.F., M.K., and M.S.; writing—original draft, T.A.F.; writing—review & editing,
M.Š.M. All authors have read and agreed to the published version of the manuscript.
Funding:
This work is supported in part by the Croatian Science Foundation under Project Number
UIP-2017-05-6538 Nanostructured hardmetals–New challenges for Powder Metallurgy.
Acknowledgments:
The authors acknowledge the infrastructure and research support of Fraunhofer
IKTS, Dresden, Germany, group Hardmetal and Cermet.
Conflicts of Interest: The authors declare no conflict of interest.
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