T. KOSEC et al.: COMPARISON OF THE CORROSION PROPERTIES OF CoCrMo DENTAL ALLOYS ...
819–824
COMPARISON OF THE CORROSION PROPERTIES OF CoCrMo
DENTAL ALLOYS IN ARTIFICIAL SALIVA
PRIMERJAVA KOROZIJSKIH LASTNOSTI CoCrMo DENTALNIH
ZLITIN V UMETNI SLINI
Tadeja Kosec
1
, Mirjam Bajt Leban
1
, Matej Kurnik
2
, Igor Kopa~
2*
1
Slovenian National Building and Civil Engineering Institute, Dimi~eva ulica 12, Ljubljana, Slovenia
2
Medical Faculty, University of Ljubljana, Vrazov trg 22, Ljubljana, Slovenia
Prejem rokopisa – received: 2021-07-20; sprejem za objavo – accepted for publication: 2021-10-12
doi:10.17222/mit.2021.283
CoCrMo alloys are known for their biocompatible properties, which, together with their favorable mechanical properties, mean
they can be efficiently used in dentistry. With the development of selective laser melting for the fabrication of 3D-printed ob-
jects, interest in the corrosion properties of this alloy has risen in the field of prosthodontics. In the study, CoCrMoW dental al-
loys were studied in artificial saliva at body temperature, i.e., 37 °C. Different forms of CoCrMoW alloy were selected: a refer-
ence sample, i.e., original material as received from the supplier, a cast sample acquired from an ordinary procedure in a dental
laboratory, and two 3D-printed samples made from CoCrMoW powder using the selective laser melting (SLM) method. Electro-
chemical, spectroscopic and hardness measurements were conducted. It was shown that the reference and cast samples have
similar microstructural and electrochemical properties, while the electrochemical properties of the 3D-printed samples differ,
most probably due to the effect of the higher micro porosity and chemical composition of the alloys.
Keywords: CoCrMo dental alloys, artificial saliva, additive technology, precision casting, corrosion
CoCrMo-zlitine so znane po svojih biokompatibilnih lastnostih, ki jih zaradi ugodnih mehanskih lastnosti vse pogosteje
uporabljamo v zobozdravstvu. Z razvojem selektivnega laserskega taljenja (SLT) se je zanimanje za izdelavo 3D tiskanih
predmetov in preu~evanje korozijskih lastnosti teh zlitin pove~alo tudi v stomatolo{ki protetiki. V {tudiji je bila preu~evana
CoCrMoW-zlitina v umetni slini pri telesni temperaturi 37 °C. Izbrane so bile razli~ne oblike zlitin CoCrMoW: originalni
referen~ni vzorec dostavljen od dobavitelja, kovinski ulitek, pridobljen z ustaljenim postopkom v zobotehni~nem laboratoriju in
dva 3D vzorca iz prahu CoCrMoW, ki sta bila izdelana z uporabo metode selektivnega laserskega taljenja. Izvedene so bile
elektrokemijske in metalografske preiskave z merjenjem trdote. Ugotovljeno je bilo, da imata referen~ni in ulit material
podobne mikrostrukturne in elektrokemijske lastnosti, medtem ko imajo vzorci 3D tiska razli~ne elektrokemijske lastnosti,
najverjetneje zaradi u~inka ve~je mikro poroznosti in kemijske sestave zlitin.
Klju~ne besede: CoCrMo dentalne zlitine, umetna slina, aditivne tehnologije, precizilsko ulivanje, korozija
1 INTRODUCTION
CoCrMo alloys are known for their biocompatible
properties. They are resistant to wear and corrosion and
possess high values of hardness and compressive
strength.
1,2
Their use in orthopedics dates to the 1930s,
3
while their use in dentistry has increased over the last de-
cade, due to the use of CoCrMo powder for the produc-
tion of different dental assets through methods of novel
additive technology, namely, selective laser melting
(SLM).
4–8
CoCrMo alloys as a biomedical alloy have been ex-
tensively studied, especially regarding their characteris-
tics of corrosion.
4–13
Different physiological solutions
have been used as corrosion media, including Hank’s so-
lution, phosphate physiological solution, PBS, saline so-
lution (0.9 % NaCl), Minimum Essential Media (MEM),
and artificial saliva. The effect of complexing agents and
proteins, such as Bovine serum or albumin, has also been
thoroughly reported.
11,12
Artificial saliva is the proper
medium for corrosion studies investigating the corrosion
of CoCrMo alloys for use in dental applications. Artifi-
cial saliva consisting of different chlorides, phosphates,
carbonates, tianocyanates and citric acid was chosen for
the present study.
14
CoCrMo alloys spontaneously oxidize in air. The
composition of the passive film is similar whether it is
formed in air or during potentiodynamic polarization.
The passive film consists of CrO
3
and/ or Cr(OH)
3
. Dur-
ing a potentiodynamic scan, Co and Mo enter the passive
film at potentials more positive than 0.3 V, as was stud-
ied with X-ray photoelectron spectroscopy at different
potentials in the anodic region. Co is present as CoO and
Mo as MoO
3
, with an increase in the thickness also re-
ported at anodic potentials of 0.4 V and 0.7 V.
13
A transpassive peak in the anodic region of the
potentiodynamic curve is sometimes observed; this is
due to the formation of Cr(VI) species, which is found in
oxide films formed in solutions containing phosphate.
10,11
The electrochemical/corrosion properties of CoCrMo
dental alloys are defined in our research. In order to
study the differences in corrosion and mechanical prop-
Materiali in tehnologije / Materials and technology 55 (2021) 6, 819–824 819
UDK 620.193:544.6:616.314:669.018.8:66.017 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(6)819(2021)
*Corresponding author's e-mail:
igor.kopac@mf.uni-lj.si (Igor Kopa~)

erties, different forms of the alloy were studied, namely a
reference sample (an original metal disc received from
the supplier, as incoming materials), a cast sample and
3D-printed samples made using CoCrMo powder as the
source material. The second goal of the study was to de-
fine the differences in SLM printing parameters: the first
sample was printed with optimal parameters and the sec-
ond one with non-optimal parameters, which led to
higher porosities.
The microstructural, physical and corrosion proper-
ties of the different forms of CoCrMo alloys are defined
and compared. The effects of the microstructure, poros-
ity, and hardness were sought in relation to the properties
of corrosion and performance.
2 MATERIAL AND METHODS
Electrochemical tests were conducted using a Gamry
ref 600 + Potentiostat/Galvanostat (USA, 2015). First,
the open-circuit potential (OCP) was measured for at
least 2 h, or until a steady state was achieved. Linear po-
larization measurements followed, using a scan rate of
0.1 mV/s in the potential range ± 20 mV vs E
corr
(results
not shown in this study). Electrochemical impedance
spectroscopy measurements were then taken by measur-
ing the impedance at different frequencies between
65 kHz and 1 mHz, using a perturbation signal of 20 mV
and measuring 7 points per decade. Finally, potentio-
dynamic measurements were executed starting at
–250 mV cathodically vs E
corr
, progressing in the anodic
direction up to 1.2 V or 1 mA/cm
2
at a scan rate of
1 mV/s. Different electrochemical parameters were ex-
tracted from the electrochemical measurements using
Echem Analyst Software.
The corrosion cell for the electrochemical tests con-
sisted of an assembly of three electrodes in a jacked cell
with a volume of a 350 cm
3
, using an Ag/AgCl reference
electrode and a graphite rod as a counter electrode. The
areas of the working electrodes were as follows:
3D-printed samples 0.64 cm
2
, the reference sample
0.278 cm
2
, and the cast sample 0.65 cm
2
. All the results
presented were normalized. The working electrodes and
the electrochemical cell for the corrosion testing are pre-
sented in Figure 1.
The electrolyte for the corrosion studies was simu-
lated saliva, containing 0.6 g/L NaCl, 0.72 g/L KCl,
0.22 g/L CaCl
2
·2 H
2
O, 0.68 g/L KH
2
PO
4
, 0.856 g/L
Na
2
HPO
4
·12 H
2
O, 0.060 g/L KSCN, 1.5 g/L KHCO
3
and
0.03 g/L citric acid and having a pH of 6.5.
14
All mea-
surements were conducted at body temperature, i.e.,
37 °C, using an external heating unit that sustained a
constant temperature.
All specimens were prepared using wet abrasion with
up to 1200-grit SiC emery paper and ultrasonically
cleaned in acetone for 3 min.
CoCrMoW was supplied in powder form by Scheft-
ner, and had a composition in mass fraction as follows:
61 w/% Co, 27.5 w/% Cr, 8.5 w/% W, 3.5 w/% Mo,
1.6 w/% Si, C, Fe and Mn 1 w/%. The reference sam-
ple, a disc supplied by Scheftner, had the same chemical
composition. For the cast sample the composition was as
follows: 63.3 w/% Co, 24.8 w/% Cr, 5.3 w/% W, 5.1 w/%
Mo and 1 % Si, Ce < 1 w/%.
The SLM samples were printed at two different en-
ergy densities – low (78.1 J/mm
3
) and high
(153.8 J/mm
3
) – in order to study the difference between
the optimal and non-optimal printing parameters chosen.
The latter resulted in a porous structure, while the former
resulted in the samples having minimal porosity.
Light microscopy on the metallographically prepared
samples at different magnifications was executed using a
Carl Zeiss metallographic microscope (Germany, 2009).
3 RESULTS AND DISCUSSION
3.1 Microstructure, porosity and hardness
First, the samples were metallographically prepared
(Figure 2) and investigated using light microscopy at
different magnifications. The porosity of each sample
was defined at 12.5-times magnification with image
analysis using ImageJ software according to the standard
T. KOSEC et al.: COMPARISON OF THE CORROSION PROPERTIES OF CoCrMo DENTAL ALLOYS ...
820 Materiali in tehnologije / Materials and technology 55 (2021) 6, 819–824
Figure 1: Photographic images: a) electrochemical cell, b) working electrode, fabricated for 3D-printed cube in epoxy resin with an electrical
contact

ASTM E2109-01.
15
Hardness measurements were then
executed with a minimum of 6 measurements taken for
each sample. The results are presented in Table 1.
Table 1: Vickers hardness HV and porosity values
sample E/(J/mm
3
)
Porosity
(%)
HV 0.3 HV 10
CoCrMo 55 805 78.1 4.8 417 ± 28 356 ± 21
CoCrMo 70 520 153.8 0.1 409 ± 27 390 ± 5.3
CoCrMo ref / 0 361 ± 19 302 ± 9.4
CoCrMo cast / 0 312 ± 13 305 ± 6.3
It can be observed that the CoCrMo samples printed
at a low energy density (78.1 J/mm
3
) were highly porous
(4.8 %), while the sample printed with optimal laser pa-
rameters had an acceptable porosity as low as 0.1 %. Po-
rosity also had a significant impact on the mechanical
properties – namely, the higher porosity, the lower the
hardness of the material. From the hardness measure-
ments in Table 1 the printed specimens exhibited a
higher hardness compared to the reference and cast sam-
ples. The main reason for this is the finer microstructure,
cellular dendrites and elongated participates, which acted
as obstacles to dislocation movement.
16–18
By comparing
the printed specimens it can be seen that there is no sig-
nificant difference between the specimens printed with
low and optimal parameters when a low load is used for
the hardness measurements (HV 0.3, 2.93 N). On the
other hand, hardness measurements performed at a
higher load (HV 10, 98 N) showed that the highly porous
specimen had lower hardness, as a greater average sur-
face area was considered, which is a more representative
property than HV 0.3.
Comparing the hardness of the reference and cast
samples, the HV 10 hardness is similar, whereas the
HV 0.3 hardness is significantly higher in the reference
material. The reference material is also cast, but it is very
possible that it contained a larger amount of Mo than the
cast specimen after casting in the dental shop. Casting in
a dental laboratory is performed in atmospheric condi-
tions, and not in a vacuum or protective atmosphere. Due
to this, Mo, which (besides W and C), contributes the
most to the final hardness, also forms MoO
3
, which eas-
ily evaporates above 450–500 °C. The vapour pressure of
the MoO
3
becomes significant, and at higher tempera-
tures, depending on the total partial pressure of oxygen,
the rate of evaporation of the MoO
3
equals its rate of for-
mation.
19,20
An additional EDS analysis was performed
on the samples in order to compare the amount of Mo
and W in the samples. It was found that the reference,
cast, and 3D samples contained similar amounts of Mo
(1.88–2.39 w/%), while the amount of W was
2.76–3.38 w/% in the cast sample and 5.69–5.94 w/% in
the 3D-printed samples.
If we compare the microstructure of the reference
sample with the sample cast from the reference material
at a higher magnification (100 ) we can observe a finer
dendritic microstructure in the reference materials,
which should lead to a higher hardness,
17
as was stated
earlier in this article.
T. KOSEC et al.: COMPARISON OF THE CORROSION PROPERTIES OF CoCrMo DENTAL ALLOYS ...
Materiali in tehnologije / Materials and technology 55 (2021) 6, 819–824 821
Figure 2: Photographic images of samples observed by optical mi-
croscopy at 12.5× magnification to observe the porosity and
microstructure
Figure 3: Microstructure of Co-Cr alloy, magnification 100 , electrolytically etched (100 mL H
2
O + 4 mL HCl, 5 V, 10 s): a) reference sample,
b) cast sample

3.2 Open-circuit potential measurements
The corrosion studies included a measurement of the
open-circuit potential, linear polarization measurement,
electrochemical impedance spectroscopy and potentio-
dynamic measurements. The results of the linear polar-
ization measurements are not presented in this study.
When CoCrMo samples were immersed in artificial sa-
liva at 37 °C, the potential slowly started to increase
(Figure 4). The increase in potential upon exposure to
the saliva indicates the growth of a passive layer. The po-
tential of the reference sample of CoCrMo was –0.399 V
at2hofimmersion. The potential of the cast sample was
slightly more positive, at –0.391 V. The most positive
OCP potential of the SLM fabricated CoCrMo alloys, at
–0.223 V, occurred in the sample printed at 55 W and
805 mm/s. The samples with a higher power (70 mm/s
and 520 mm/s) and slower scanning speed had a slightly
more negative potential (–0.244 V) after 2 h of exposure
to artificial saliva at 37 °C.
3.3 EIS
Electrochemical impedance measurements were
taken once a steady state was reached. It can be observed
that the impedance responses for the reference, cast and
3D-printed samples are different (Figure 5). The highest
impedance response was measured on the reference sam-
ple and the 3D-printed sample with a lower power and
high scan rate (P = 55 W, laser scan speed 805 mm/s)
and the highest measured porosity. The absolute imped-
ance of these two samples at the lowest measured fre-
quency was very high, with values of 1.42 M ·cm
2
and
2.37 M ·cm
2
, respectively. The high value for the 3D
printed sample might be due to its high porosity, mean-
ing it has a higher specific area and high impedance due
to the formation of a passive layer at places where an ox-
ide layer had not formed, and the presence of a thick ox-
ide layer resulting from the oxidative porosity. It has pre-
viously been reported that higher polarization resistance
values occur in samples which have a passive film in the
anodic region,
11
as could be the case in the porous
3D-printed CoCrMo alloy sample. In contrast, the
3D-printed sample (P = 70 W, 520 mm/s) had the small-
est absolute impedance at the lowest measured frequency
(475 k ·cm
2
).
The cast CoCrMo sample had an impedance response
value of 653 k ·cm
2
.
The SLM specimens contained a far higher concen-
tration of W (9.5 w/%) than the cast specimens
(5.1 w/%). The addition of W is known to improve the
corrosion resistance of Co-Cr alloys.
21
3.3 Potentiodynamic polarization measurement
Potentiodynamic curves for the cast sample, the ref-
erence material and the 3D-printed alloys are given in
Figure 6. There are some important observations which
T. KOSEC et al.: COMPARISON OF THE CORROSION PROPERTIES OF CoCrMo DENTAL ALLOYS ...
822 Materiali in tehnologije / Materials and technology 55 (2021) 6, 819–824
Figure 5: Nyquist and Bode plots of CoCrMo (ref., cast and SLM samples, printed with different parameters) in simulated saliva at 37 °C
Figure 4: Evolution of E
corr
for the CoCrMo samples (SLM method,
reference and cast sample) in artificial saliva at 37 °C

allow differentiation between the various CoCrMo sam-
ples. Namely, the corrosion potential, E
corr
, is more posi-
tive in the SLM fabricated samples, and they have simi-
lar values. The corrosion potentials of the reference and
cast samples, however, were more negative, with the ref-
erence sample having the lowest value, at –0.533 V (Ta-
ble 2). The values of the corrosion current density, j
corr
,
are similar for all the samples, with values between
13.6–22.1 nA/cm
2
.
There is a slight difference in the behavior of the var-
ious CoCrMo samples in the anodic region of the
potentiodynamic curve. Lower anodic current densities
were observed in the 3D-printed samples in a "pseudo
passive region" of the potentiodynamic curve. Since E
corr
was more positive, the potential range where the current
density is constant, is narrower in comparison to the cast
and reference samples ( E in Table 2). The more posi-
tive corrosion potential and narrower passive region
might be related to the increased effective surface result-
ing from the open porosities of the SLM-fabricated sam-
ples. The reference sample and the SLM-fabricated sam-
ple (55 W, 805 mm/s) exhibited transpassive behavior
with a decrease in current in the transpassive region,
which is related to the increased action of Cr (VI) in the
passive film.
13
3.4 Optical microscopy
Following the potentiodynamic (PD) scans, the sur-
faces exposed to artificial saliva were inspected. As ob-
served from the images after the PD scans, defects in the
form of spots can be observed. These are not pits but
rather the state of the surface after the experiments,
where the surface change is observed at the sites of the
pores in the form of round spots. These are circled in
Figure 7.
The 3D-printed sample with a higher porosity (50 W,
805 mm/s) showed a larger number of spots with a dis-
coloured surface, as indicated by the black circles in Fig-
ure 7f. A similar, but less frequent, change in colour and
texture appeared in the SLM fabricated sample printed at
70 W and 520 mm/s (Figure 7b). No indication of a
modified surface was found on the cast sample (Figure
7e), while the reference sample had a small, elongated
spot in the middle of the sample, although this did not af-
fect its electrochemical properties (Figure 7h).
T. KOSEC et al.: COMPARISON OF THE CORROSION PROPERTIES OF CoCrMo DENTAL ALLOYS ...
Materiali in tehnologije / Materials and technology 55 (2021) 6, 819–824 823
Table 2: Corrosion potential, corrosion current and corrosion rate for all samples obtained from cyclic polarization
Sample E
OCP/V E
corr
/V jcorr
/(nA cm
–2
) E
b/V  E (E
b–E
corr
)/V
CoCrMo SLM 55_805 –0.223 –0.265 13.6 0.436 0.701
CoCrMo SLM 70_520 –0.244 –0.217 22.1 0.417 0.634
CoCrMo Reference –0.399 –0.533 16.9 0.749 1.28
CoCrMo Cast –0.310 –0.391 21.5 0.742 1.13
Figure 7: Optical microscopy and SEM examination of the type and extent of corrosion damage on the samples following the potentiodynamic
experiments
Figure 6: Potentiodynamic curvesfor the SLM fabricated samples and
the cast and reference samples made from CoCrMo alloys in artificial
saliva at 37 °C

4 CONCLUSIONS
This research investigated CoCrMoW dental alloys in
artificial saliva at body temperature i.e., 37 °C. Different
forms of CoCrMoW alloy were studied, i.e., a reference
sample (material as received from the supplier), a cast
sample (using an ordinary procedure in a dental cast
shop), and two samples made from CoCrMoW powder
using the selective laser melting method.
Microstructural, physical and corrosion studies
showed that:
1. SLM-printed samples had a higher porosity than
the reference and cast samples, with the high porosity
being related to the printing parameters. The higher en-
ergy density obtained using optimal printing parameters
resulted in a lower micro-porosity.
2. After 2 h of immersion in artificial saliva the cor-
rosion potential was more positive in the SLM-printed
CoCrMoW samples, due to the presence of oxides in
open pores at the surface, while the corrosion potential
of the reference and cast samples was more negative.
3. Electrochemical impedance spectroscopy measure-
ments and potentiodynamic measurements showed simi-
lar electrochemical properties; the reference and cast
samples had very similar properties while the SLM fabri-
cated sample had diverse properties due to different lev-
els of porosity.
4. The SLM specimens exhibited higher hardness
than the reference and cast samples, which is due to the
finer microstructure of the SLM specimens, and the pres-
ence of cellular dendrites and elongated participates,
which acted as dislocation movement obstacles.
5. Due to the characteristics of the printed samples
found it can be concluded that metal frameworks fabri-
cated by SLM technology in clinical practice are suitable
for long span bridges in fixed prosthodontics.
Acknowledgments
The financial support of the Slovenian Research
Agency (SRA), under grant No L2-1831, is hereby grate-
fully acknowledged.
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T. KOSEC et al.: COMPARISON OF THE CORROSION PROPERTIES OF CoCrMo DENTAL ALLOYS ...
824 Materiali in tehnologije / Materials and technology 55 (2021) 6, 819–824