E. AYGUL et al.: CORROSION CHARACTERISTICS OF AN ADDITIVE-MANUFACTURING ...
535–540
CORROSION CHARACTERISTICS OF AN
ADDITIVE-MANUFACTURING COBALT–CHROME–WOLFRAM
BIOMEDICAL ALLOY UNDER HEAT-TREATED AND
MOLYBDENUM-DOPED CONDITIONS
KOROZIJSKE LASTNOSTI TOPLOTNO OBDELANIH IN Z
MOLIBDENOM DOPIRANIH Co-Cr-W BIOMEDICINSKIH ZLITIN,
IZDELANIH Z DODAJNO TEHNOLOGIJO
Ebuzer Aygul
1*
, Senai Yalcinkaya
1
, Yusuf Sahin
2
1
Marmara University, Faculty of Technology, Department of Mechatronics Engineering, 34722, Istanbul, Turkey
2
Near East University, Faculty of Engineering, Department of Mechanical Engineering, Fahrettin Kerim Gokay Street, Kadikoy,
99138, Mersin, Turkey
Prejem rokopisa – received: 2019-11-07; sprejem za objavo – accepted for publication: 2020-03-15
doi:10.17222/mit. 2019.272
Two groups of cobalt-chrome-wolfram alloys, Co-Cr-W-Si and Co-Cr-W-Mo-Si, were selected for additive manufacturing. One
of these samples was subjected to heat treatment after production under the same conditions. Thus, it is aimed to investigate the
effects of heat treatment and molybdenum doping on the corrosion behaviour of cobalt-chrome alloys. The corrosion behaviour
of the samples was investigated by using an electrochemical corrosion technique. Ringer’s solution was used for the aqueous en-
vironment of the electrochemical corrosion technique.
Keywords: heat treatment, molybdenum doped, electrochemical corrosion, ringer solution
Avtorji ~lanka so izbrali dve skupini biomedicinskih zlitin Co-Cr-W-Si in Co-Cr-W-Mo-Si za izdelavo z dodajalno tehnologijo.
Eno skupino vzorcev so tudi termi~no obdelali in na ta na~in {tudirali vpliv termi~ne obdelave in dodatka Mo na njihovo
obna{anje. Korozijske lastnosti vzorcev so preiskovali z uporabo elektrokemijskih korozijskih tehnik. Uporabili so Ringerjevo
raztopino za izvajanje elektrokemijskih korozijskih tehnik v vodnem okolju.
Klju~ne besede: toplotna obdelava, dopiranje z molibdenom, elektrokemijska korozija, Ringerjeva raztopina
1 INTRODUCTION
Biometals are classified in terms of their dynamic
and mechanical properties, both against human body tis-
sue and against each other. Metals and alloys are de-
graded by electrochemical attacks when exposed to an
enemy tissue environment. The aggressive aqueous envi-
ronment in the human body contains various anions such
as chloride, phosphate and bicarbonate. Because of the
colodic and anodic reactions, the metallic components of
the alloy are oxidized to ions and dissolved oxygen is re-
duced to hydroxide ions. Corrosion-resistant metallic im-
plants are passive. Passive metals have a thin, compact
oxide layer that separates ion release on their surfaces.
This release may cause allergic and toxic reactions in the
body.
1,2
Co-Cr-based metallic bio-alloys are known for
their good corrosion rate, high temperature resistance
and good wear resistance. In general, the co-base alloys
obtained by the casting method were first introduced in
the medical field as a dental implant. Subsequently,
many mechanical and dynamic tests in the body environ-
ment have shown that Co-based alloys are biocompatible
and can be used as bioimplants.
3,4
Co-based alloys are
strengthened by the addition of Cr, and the addition of
Mo results in a fine-grained structure that improves the
mechanical properties. Co-Cr and Co-Cr-Mo are mainly
used in dentistry, newly developed artificial joints, hip
and knee joints as prosthetic stem material. The most im-
portant feature of co-base alloys is the high corrosion re-
sistance in a chlorine environment due to the high chro-
mium content. The chromium oxide layer (usually
Cr
2
O
3
) formed on the surface is the element which pro-
vides the basic corrosion resistance of Co-based alloys.
5
Then the content of this alloy is changed by cobalt-chro-
mium-molybdenum (Co-Cr-Mo) alloy, cobalt-nickel-
chromium-tungsten-iron (Co-Ni-Cr-W-Fe) alloy and co-
balt-nickel-chromium-molybdenum Co-Ni-Cr-Mo) alloy
was developed. The vitalium (Co-Cr-Mo) alloy has been
used for many years in dentistry and recently in the pro-
duction of artificial joints. The Co-Ni-Mo alloy is a
newer material and is used in joints under high load
(such as knee and hip) and prostheses.
6,7
The use of co-
balt-chromium alloys in the biomedical field dates back
to the 1940s. The materials used are mainly cobalt-chro-
mium-molybdenum based, but are preferred in load bear-
ing hip and joint implants due to their high static and dy-
Materiali in tehnologije / Materials and technology 54 (2020) 4, 535–540 535
UDK 620.1:620.194: 621.86/.87 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(4)535(2020)
*Corresponding author's e-mail:
ebuzer.aygul@marmara.edu.tr (Ebuzer Aygul)

namic strength. While cobalt and chromium form solid
melts up to 65 % cobalt, the added chromium makes the
structure more resistant and increases its corrosion resis-
tance. Molybdenum, on the other hand, makes an extra
contribution to the strength of the structure by showing a
grain-reducing effect.
8
The international American Soci-
ety for Testing and Materials (ASTM) F75 cast Co-
Cr-Mo alloy is generally known as Vitallium and Haynes
21 and has long been used in the biomedical field. The
main feature of this alloy is its high corrosion resistance
in a chlorine environment due to its high chromium con-
tent. The chromium oxide layer (usually Cr
2
O
3
) formed
on the surface provides this corrosion resistance.
9
Among the rapid and layered production techniques, se-
lective laser melting SLM is known to be the most com-
monly used method for the restoration of dental restora-
tions.
10,11
SLM, which is supported by CAD/CAM
technology, is a production process that melts metal
powders with a focused laser beam and produces
3D-CAD data layered.
12
SLM can produce metal parts
with high density precision and good density quality.
13
Compared to the CAST and CNC technique, the SLM
technique provides a smoother surface, shorter machin-
ing cycles, fewer internal defects and better compliance
with patients. Therefore, SLM for dental restorations ap-
pears to be the most suitable manufacturing technology
to date.
14,15
2 EXPERIMENTAL PART
In this study, the SLM production method was cho-
sen because of its innovative approach to produce
Co-Cr-W-Si and Co-Cr-W-Mo-Si alloys. The SLM man-
ufacturing method is known as the most suitable method
for producing dental restorations among the addi-
tive-manufacturing production methods. The chemical
compositions of the alloys produced in the study are
given in Table 1.
In biomedical applications, W, Si and Mo additives to
cobalt-chrome alloys have been reported to improve the
wear and corrosion properties of the alloy, and the Mo
additive is known to make the alloy extra fine. The SLM
method was used to produce W, Si and Mo-doped Co-Cr
alloys. The samples were produced in the SISMA com-
pany’s MYSINT 100 Dual Laser machine in a nitrogen
gas environment. The metal powders produced (from
BEGO and Scheftner Dental Alloys) were supplied. Af-
ter the alloys were produced by SLM, one sample was
heat treated at 1000 °C for 1 hour. For the heat treatment,
a chamber oven of the protherm company was used.
Samples produced with SLM are given in Figure 1.
Figure 1a Co-Cr-W-Si alloy produced by SLM
method, Figure 1b the annealed Co-Cr-W-Si alloy after
it is produced with SLM method, Figure 1c the annealed
Co-Cr-W-Mo-Si alloy after it is produced with SLM
method,and Figure 1d Co-Cr-W-Mo-Si alloy produced
by SLM method. In order to understand the mechanical
and corrosive properties of the alloys, the density values
were measured and compared with the Archimedes prin-
ciple, then the microhardness values were measured, and
finally the electrochemical corrosion test was performed.
2 1. Materials and sample preparation
The initial sizes of the Co-Cr-W-Mo-Si powders sup-
plied by Scheftner Dental Alloys are 30 microns, density
of 8.8 g/cm
3
and microhardness values of 345/490 HV.
The chemical compositions of the powders are given in
Table 1. SLM is the technique chosen to produce these
powders. The initial size of the Co-Cr-W-Si alloy was 30
microns, density 8.6 g/cm
3
and micro hardness 470/430
HV.
2.2 Mechanical property characterization
The density of the samples was measured using Ar-
chimedes’ principle. The Vickers hardness (HV) of the
samples was performed using a universal microhardness
tester (FM-310e) with a load of 100 g and a residence
time of 10 s. Finally, the Mo additive, production tech-
nique and the effects of heat treatment on alloys were in-
vestigated. By means of Archimedes’ principle, the den-
sities of the samples were determined from the verious
E. AYGUL et al.: CORROSION CHARACTERISTICS OF AN ADDITIVE-MANUFACTURING ...
536 Materiali in tehnologije / Materials and technology 54 (2020) 4, 535–540
Table 1: The chemical compositions of alloys
Element Cobalt Chrome Wolfram Silicon Molybdenum alloys
Content (w/%)
Content (w/%)
59
66–67
25
26–28
9.5
8–9
1
1,5
3,5
–
Co-25Cr-9,5W-3,5Mo-1Si
Co-29Cr-9W-1Si
Figure 1: Alloys produced by additive manufacturing (SLM)

parts of each sample in different atmospheres. In the
density-calculation stage, the following formula was
used:
D
W
WW
D
g
a
a b
HO
2
=
−
⎡ ⎣ ⎢ ⎤ ⎦ ⎥ ⋅ (1)
Where D
g
,W
a,
W
b
, and D
H2O
are visible density of the
sample, weight of the dry sample in air, hanging weight
of the sample in water, and water density, respectively.
2 3. Electrochemical test
Corrosion is the degradation of metals and their al-
loys by interacting with their environment. This deterio-
ration may be electrochemical or chemical. Nowadays,
many corrosion measurement techniques are available.
Among these techniques, the most prominent one is the
electrochemical corrosion measurement method accord-
ing to the others. Electrochemical corrosion is based on
the creation and use of anodic and cathodic polarization
curves. Our alloys were analysed in a ringer solution of
artificial body fluid. One ringer tablet was calcined in
500 mL of pure water at 121 °C for 15 min. Typical che-
mical composition of the tablet used is ammonium chlo-
ride 0.00525; sodium hydrogen carbonate 0.005; calcium
chloride-2 hydrate 0.04; potassium chloride 0.00525; so-
dium chloride 1,125. The Bakelite-coated metal alloys
are placed in the corrosion apparatus by connecting them
as electrodes. Tafel curves and open-circuit voltages
were obtained in the potential range determined by using
an electrode and a graphite cathode. Reference elec-
trodes used in response to the working electrodes
(Co-based alloys prepared) are calomel. Gamry’s inter-
face 1000 serial electrochemical analyser is used for all
corrosion measurements. Graphite was used as the cath-
ode. The samples were allowed to reach the open-loop
potential (Ecorr) within 40 minutes prior to testing. Elec-
trochemical Tafel curve analyses were obtained using
Gamry Echem Analyst software. Nowadays, many corro-
sion measurement techniques are available. Among these
techniques, the most prominent one is the electrochemi-
cal corrosion measurement method according to the oth-
ers. In our study, the corrosion rate was determined by
the curve drawn on semi-logarithmic scale (Tafel extrap-
olation method) using current-potential curves. Metallic
biomaterials undergo electrochemical degradation as a
result of environmental interaction. This negative condi-
tion causes corrosion tissue and organ poisoning, but
also changes the physical or chemical properties of the
material. The electrochemical corrosion test is carried
out in an aqueous environment. Ringer’s solution, which
is close to body fluid, was preferred for aqueous media.
3 RESULTS
3.1 Density and microhardness study
The calculated density values for as-cast and an-
nealed W-, Si- and Mo-doped Co-Cr alloys were found
to be 8.769 g/cm
3
and 8.691 g/cm
3
, respectively. It is
clear that the production process results in an improve-
ment of 0.4% g/cm
3
in the Co-Cr-W-Mo-Si alloy. This
ratio is 1.3% g/cm
3
in the annealed sample. It is seen that
annealing results in an improvement of 0.89% g/cm
3
in
the sample. Density values of as-cast and annealed sam-
ples in the Co-Cr W-Si alloy were measured as
8.56 g/cm
3
and 8.35 g/cm
3
, respectively. It is clear that
the production process produces an improvement of
0.5 % g/cm
3
in the Co Cr-W-Si alloy. This ratio appears
to be 3 % g/cm
3
in the annealed sample. It is seen that
annealing produces an improvement of 2.5 % g/cm
3
in
the sample. In general, Mo-doped increases the density
of the sample and annealing decreases the density of the
sample.
The Vickers test for all alloys was carried out at a
constant 100 g load and for 10 s. For as-cast and heat-
treated samples, 5 hardness measurements were per-
formed on random surfaces. The arithmetic means of the
5 microhardness values were obtained and exhibited in
Table 2. As can be seen in Table 3, the microhardness
values varied slightly, but the average hardness for the
cast samples and heat-treated samples (Co-Cr-W-Si)
were about 531, 566 HV, respectively. Improvements in
heat-treated samples were about 6.2 % HV in compari-
son to the same cast alloy. We figured out from the re-
sults of Table 2 that the structure in both samples was
sufficient. The Vickers test for Co-Cr-W-Mo-Si alloy
was carried out at a constant 100 g load and 10 seconds
time. For as-cast and heat-treated samples, 5 hardness
measurements were performed on random surfaces. The
arithmetic means of the 5 microhardness values were ob-
tained and exhibited in Table 2. As can be seen in Table
2, the microhardness values varied slightly, but the aver-
age hardnesses for the cast samples and heat-treated
E. AYGUL et al.: CORROSION CHARACTERISTICS OF AN ADDITIVE-MANUFACTURING ...
Materiali in tehnologije / Materials and technology 54 (2020) 4, 535–540 537
Table 2: Microhardness values of heat-treated and as-cast samples produced by SLM
The micro hardness test Co-Cr-W-Mo-Si
Heat-treated
Co-Cr-W-Mo-Si
Co-Cr-W-Si
Heat-treated
Co-Cr-W-Si
kgf/mm
2
, (HV)
371.3 330.1 513.4 590.9
348 440.9 515.7 579.1
386 449.6 575.9 565.8
398.8 452.7 533.2 545.6
432.2 485.5 517.5 551.7
Average value 387.26 431.76 531.14 566.62

samples were about 387, 431 HV, respectively. Improve-
ments in the heat-treated samples were about 10.3 % HV
in comparison to the same cast alloy. We figured out
from the results of Table 2 that the structure in both sam-
ples was sufficient.
3.2 Electrochemical test study
Figure 2 shows the open-circuit potential measure-
ments of the samples in the ringer solution for 40 min.
The annealed Co-Cr W-Si alloy was thermodynamically
stable during the measurement period. A relatively stable
thermodynamic behaviour is also observed in the an-
nealed Co-Cr-W-Mo-Si alloy. Open-circuit potentials are
approximately 8 mV for Co-Cr-W-Mo-Si alloy, –65 mV
for annealed Co Cr-W-Mo-Si alloy, 41 mV for
Co-Cr-W-Si alloy and annealed Co-Cr. It was measured
as 60 mV for the W-Si alloy. The Co-Cr-W Mo-Si alloy
deviated in a positive direction and completed the mea-
surement at higher values than it started. Samples that
exhibit such behaviour are caused by a protective oxide
layer formed on their surface. The Co-Cr-W-Si alloy
sample exhibits a different behaviour than other alloys.
Potential measurements show a steady behaviour in the
first 150 sec, while the potential increases rapidly with
time. It can be emphasized that a thin layer formed over
time dissolves rapidly. As a result of the measurement,
the thermodynamic stability of the equilibrium with the
solution was not fully achieved.
Figure 3 shows the graphical data resulting from the
Tafel analysis. The corrosion potential values of the
Co-Cr-W-Si and annealed Co-Cr-W-Mo-Si coded sam-
ples were very close to each other, while the annealed
Co-Cr-W-Si and Co-Cr-W-Mo-Si coded samples had
corrosion potential values of each other. In this case, the
most important parameter determining the rate of corro-
sion is beta A and beta C values. Unstable data values of
Co-Cr-W-Si and annealed Co Cr-W-Mo-Si coded sam-
ples are seen in the anodic part. Since the anodic part in
most samples is opposed to dissolution, surface dissolu-
tion is observed due to the rapid occurrence. When com-
pared according to corrosion rates, the highest ratio is
4.809 mpy annealed Co-Cr-W-Si alloy and the smallest
value is 11.11e-3 mpy Co-Cr-W-Mo-Si alloy. The
Co-Cr-W-Si alloy has a value of 2.003 mpy and annealed
Co-Cr-W Mo-Si 410.5e-3 mpy. The most important val-
ues determining corrosion rates are; corrosion potential
(E
CORR
), corrosion current (I
CORR
) and corrosion rates
(mpy) are given in Table 3.
4 DISCUSSION
Although cobalt-chrome alloys were discovered as
biomaterials before titanium and their alloys, their use as
biomaterials after the discovery of titanium and its alloys
was pushed to the background. However, the use of co-
balt-chromium alloys has become widespread due to the
development of production techniques. Especially co-
balt-chromium-based biomaterials called Coarse ship,
which consists of a total of 5 metals Co-Cr-W-Mo Si al-
loy and including many cobalt-chromium alloys has
come to be easily produced by additive manufacturing
techniques. Cobalt-chromium alloys are being used in
E. AYGUL et al.: CORROSION CHARACTERISTICS OF AN ADDITIVE-MANUFACTURING ...
538 Materiali in tehnologije / Materials and technology 54 (2020) 4, 535–540
Figure 3: The polarization curves of selective laser melted Co-Cr-
W-Mo-Si and Co-Cr-W-Mo specimens in Ringer solution
Figure 2: Open-circuit voltage curves of selective laser melted
Co-Cr-W-Mo-Si and Co-Cr-W-Mo alloys
Table 3: The corrosions values of samples
Samples Co-Cr-W-Mo-Si
Heat-treated
Co-Cr-W-Mo-Si
Co-Cr-W-Si
Heat-treated
Co-Cr-W-Si
I
CORR
, (nA) 29.30 12.80 69.20 150.0
E
CORR
, (mV) 50.70 –145.0 –139.0 55.90
Corrosion rate, (mpy) 4.503 7.33e-02 1.125 2.35e-01

biomaterials science at a very high rate nowadays thanks
to the ease of production with additive-manufacturing
production technique. The variability of the production
technique, secondary heating processes or the addition of
alloys of different metal types can lead to serious
changes in the mechanical properties of the material. Es-
pecially for biomaterials which are in constant contact
with tissue and body fluids, good corrosion resistance is
the most important parameter. In this study, Mo-doped
Co-Cr-W-Mo-Si alloy stands out with suitable mechani-
cal properties in both tested alloys. Mo-doped was found
to improve the corrosion and microhardness values of the
alloy. Lu et al., in their corrosion study for Co-Cr-W-Si
alloy, it is very important that the apparent thin plate
degradation seen in the Tafel extrapolation curves does
not occur in ours, but unstable data values were observed
in our study.
16
L. Zeng et al.
17
suggested that the Co-Cr-
Mo alloy exhibits similar corrosion behaviour in differ-
ent production techniques.
17
In addition, secondary heat-
ing process applied after production increased the micro-
hardness and corrosion rate value just like our study and
decreased the corrosion current I
CORR
value.
5 CONCLUSIONS
Cobalt-chromium alloys are known in the medical
field for their good wear and corrosion resistance.
Co-Cr-W-Si and Co-Cr-W-Mo-Si alloys are the two most
popular members of the cobalt-chromium alloy family.
The Mo additive has been reported to render the alloy
fine. Additive-manufacturing production techniques are
highly skilled in producing cobalt chromium alloys. The
findings obtained in this study are listed as follows:
• The SLM production technique improved the density
of Co-Cr-W-Mo-Si alloy by 0.4 % g/cm
3
. This ratio
was found to be 1.3 % g/cm
3
for the annealed sample
of the Co-Cr-W-Mo-Si alloy.
• Heat-treated Co-Cr-W-Mo-Si alloy shows an im-
provement of 0.89 % g/cm
3
.
• The SLM production technique showed an improve-
ment of 0.5 % g/cm
3
in the density ratio of the
Co-Cr-W-Si alloy. This ratio was found to be 3 %
g/cm
3
for the annealed sample of the Co-Cr-W-Si al-
loy.
• Heat-treated Co-Cr-W-Si alloy shows an improve-
ment of 2.5 % g/cm
3
.
• It has been found that annealing has a density-reduc-
ing effect on both alloys. The Mo additive appears to
increase the density value.
• The annealing process increased the microhardness
value in both alloys.
• It is seen that the microhardness value of Mo-doped
alloy is lower.
• Annealing Co-Cr-W-Mo-Si while increasing the
microhardness value of the alloy by 10.3 % HV. This
ratio is 6.2 % HV in the Co-Cr-W-Si alloy.
• When the corrosion behaviour of the alloys is exam-
ined, the annealed Co-Cr-W Si alloy’s thermodynam-
ically stable behaviour during the measurement pe-
riod of the open-circuit voltage, the potential
measurements of the Co-Cr-W-Si alloy behave al-
most steady in the first 150 sec while the potential in-
creases rapidly with time.
The annealing process increases the corrosion rate
(mpy) in both samples.
When compared according to corrosion rates (mpy),
the highest ratio is 4.809 mpy annealed Co-Cr-W-Si al-
loy and the smallest value belongs to Co-Cr-W-Mo-Si al-
loy with 11.11e-3 mpy. The Co-Cr-W-Si alloy has a
value of 2.003 mpy and annealed Co-Cr-W-Mo-Si
410.5e-3 mpy.
Cobalt-chromium alloys attract attention with their
suitability to the additive-manufacturing production pro-
cess. Especially, the molybdenum-doped Co-Cr-W-Si al-
loy has a good static and dynamic properties. With the
advantages of the manufacturing process, it is obvious
that this alloy will attract more attention in biomedical
science.
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