Materials Sciences and Applicatio ns, 2011, 2, 42-48
doi:10.4236/msa.2011.21006 Published Online January 2011 (http://www.SciRP.org/journal/msa)
Electrochemical and Microstructural Study of
Ni-Cr-Mo Alloys Used in Dental Prostheses
José W. J. Silva*, Lucíola L. Sousa, Roberto Z. Nakazato, Eduardo N. Codaro, Hamilton de Felipe
Departamento de Física e Química, Universidade Estadual Paulista, Campus de Guaratinguetá, Guaratinguetá, Brazil.
Received September 24th, 2010; revised December 7th, 2010; accepted December 28th, 2010.
ABSTRACT
Ni-Cr-Mo alloys have been widely used as fixed dental prostheses. Recast process influence on corrosion behavior of
Ni-Cr-Mo dental alloy in simulated physiological serum has been investigated using chemical and electrochemical
techniques. Ni-Cr-Mo alloy recast by induction (induction) or by blowtorch (torch) has exhibited similar dendritic
structures with wide and precipitate grains in their boundaries. The torch alloy has presented good corrosion resis-
tance in physiological serum. Passivation process provides th is corrosion resistance. Passivation of Ni-Cr-Mo alloy is
often attributed to the formation of a thin and compact layer of chromium oxide (Cr2O3). This film is self-limiting be-
cause it acts as a barrier to the oxygen transport and metal ions. This film stability will depend on its solubility to the
working temperature. Different recast procedures change electrochemical parameters as stabilizing potential in open
circuit, current density and passivation interva l.
Keywords: Corrosion, Metal and Alloys, Microstructure, Nickel-Chrome-Molybdenum
1. Introduction
In dental restorations, the most decisive property of a
cast alloy in biocompatibility is corrosion [1]. Co and Ni
based alloys were widely used in dental skeletal struc-
tures and orthopedic implants such as screws, pins and
plates. And recently they have been applied for making
stents [1,2]. The advantages of these alloys include low
cost of casting, matching thermal expansion coefficient
with the ceramics of metal-ceramic restorations, and ac-
ceptable mechanical and tribological properties in vivo
[3,4]. However, the possible release of toxic metal ions
due to corrosion remains a major concern [3-5]. Casting
is one of the main methods of producing shaped metals
and alloys. Electrochemical corrosion behavior of Co-Cr
and Ni-Cr dental cast alloys depends primarily on the Cr
and Mo levels in an alloy [6]. In commercial alloys, the
compositions of Cr and Mo usually range from 11% to
25% and from 0 to 10% (mass fraction), respectively [7].
Both microstructure and casting defect have pronounced
effect on the ion release in actual practice. The defects of
dental cast alloys include mainly shrinkage porosity, in-
clusion, micro-crack and dendritic structure [8]. Only
few reported works were available on the influence of
casting procedures on the corrosion resistance of dental
alloys [9,10].
In Ni-Cr alloys, resistance to corrosion is relatively
high because of passivation effect of the oxide layer cov-
ering a surface [11]. During production of dental alloy,
trace elements are used due to playing an important role
in their properties. They improve the use and casting
qualities, and increase the porcelain-metal fusion as well
as corrosion resistance [12,13]. An early study high-
lighted that a variety of different microstructures could
be formed depending on such an alloy added [12]. For
example, Be has been added to improve both the alloy
castability and the adherence of veneering porcelain.
However, it decreases significantly the alloy corrosion
resistance owing to formation of a Cr-depleted Ni-Be
eutectic phase [12,14].
Phase structures (microstructure) of dental alloys af-
fect their clinical performance, especially biocompatibil-
ity [15,16]. Alloys can either be single-phase (homoge-
nous) or multiple-phase (heterogeneous). If all elements
are completely mutually soluble in solid state (gold, pal-
ladium and copper), then alloy will be single-phase; if
not (gold and platinum), then alloy may be multiple-
phase [17,18]. The relationship between microstructure
and corrosion behavior of Ni based dental casting alloys
has been studied. It has been reported that in regions

Electrochemical and Microstructural Study of Ni-Cr-Mo Alloys Used in Dental Prostheses43
where low levels of Cr and Mo have been observed, se-
lective dissolution has occurred in the microstructure of a
Ni alloy [19]. Increasing concentration of Cr and Mo in
the Ni-Cr alloys, may synergistically lower the dissolu-
tion rates of metals, which may subsequently reduce the
cytotoxicity of alloys [20].
Electrochemical methods also allow investigation of
uniform and localized corrosion susceptibility and its
relation to material microstructure. Degradation of most
used metallic materials may not be generally uniform
like corrosion in implants materials. Localized corrosion
is observed because of its heterogeneous microstructural
features [21]. Pitting is a form of extremely localized
attack that results in holes on surface. Among all corro-
sion types, pitting corrosion is extremely dangerous in
dental applications. Because the initial formation of pits
is difficult to detect due to the small size, it requires a
prolonged time for visual detection. Ions migrate towards
the bottom of pits and molecules react with water mole-
cules on metal surface [22]. Therefore, metal chloride
and hydroxyl ions are produced. This is an oxidation
process known as metal dissolution. For this reason, pit-
ting potentials of a higher value of dental alloy are pre-
ferred and a useful guide to alloy behavior in clinical or
other use.
Alloys coalition and recast still represent the labora-
tory procedures more thoroughly used for the dental res-
torations production [12,23]. As those procedures are
accomplished with a little or no atmospheric and tem-
perature control, changes occur in microstructure being
necessary a recast material study. In this work, it is pur-
posed to compare the electrochemical behavior and the
microstructure of two Ni-Cr-Mo alloys used in fixed
dental prostheses. The study is accomplished in vitro in
NaCl 0.9% in mass, pH 6 to 37˚C simulating the buccal
environment aggressiveness.
2. Material and Methods
Two alloys, Ni-Cr-Mo commercially available in the
market for dental casting: Wironia and Wiron 99 (BEGO,
Germany) were studied. Table 1 exhibits their chemical
compositions provided by the manufacturer. It was also
used commercially pure chromium, nickel and molybde-
num metals dowels for comparative effect on corrosion
resistance.
The work electrodes were mounted from an as-received
Table 1. Chemical composition of alloys (mass%).
Alloys Ni Cr Mo Nb Fe Ce Si C
Wiron 9965.0 22.5 9.5 1.0 0.5 0.5 1.0 Max. 0.02
Wironia 59.6 24.0 9.8 - - - - -
material (as-cast). Ingots were recast in a cylindrical
form using the lost wax method. Thus, wax cylinders
measuring 0.5 cm2 cross-section and 3 cm length, were
put in a ceramic cast. This cast was heated in an electric
furnace at high temperature (800˚C) for 1 hour to dis-
solve and evaporate the wax. The two recasting proce-
dures that were employed in this study are: remelting by
high frequency induction (induction) and remelting by
acetylene/oxygen blowtorch (flame)/(torch). In both
procedures the molds were cooled naturally without at-
mospheric control. The samples were removed from the
mold, cut in cylinders of 1 cm length and machined in
cylinders form and, in this condition, used for the respec-
tive analyses.
2.1. Surface Analysis
In microstructural analysis, the surface specimens were
mechanically polished with 220, 400, 600, 1200 and
1500 grade emery papers, finished with 0.3 m diamond
dust. The specimens were electrolytic etched in an aque-
ous solution containing 20% HCl, and application of a
constant potential of 2 V, 0.5 A for 5s through a Hewlett
Packard E 3610 A Potentiostat as current source contin-
ues. Surface analysis was performed on each specimen
by means of scanning electron microscopy (SEM) and
energy dispersion spectrometry (EDS) was accomplished
with a Leica Stereoscan 440 microscope and an Oxford
Link Exl II spectrometer.
2. 2. Electrochemical Measurements
For electrochemical measurements, test specimens were
embedded in epoxy resin leaving an exposed area of 1
cm2 to form the working electrode. Prior to the experi-
ments, the electrode surface was polished with 220 to
1500 grade emery papers, degreased with acetone and
rinsed in distilled water and finally air-dried. Measure-
ments were made at 37.0 ± 0.5˚C using a conventional
three-compartment double wall glass cell containing
0.9% NaCl in naturally aerated solution (pH 6.0). The
electrode potentials were determined against a saturated
calomel electrode (SCE) connected to the solution th-
rough a Luggin-Habber capillary. A platinum sheet was
used as counter electrode.
Electrochemical measurements were carried out with
an EG&G PAR Potentiostat/Galvanostat Model 283 and
an EG&G PAR Frequency Response Analyzer Model
1025 (Perkin-Elmer Instruments Inc., USA), both inter-
faced with a microcomputer for data acquisition and
analysis. Open circuit potential measurements were re-
corded during an immersion time of 720 min. Potentio-
dynamic polarization curves were recorded in electro-
positive direction starting from –1.00 V up to 1.00 V at
sweep rates of 0.02 V min–1. In Cyclic Voltammetry Te-

Electrochemical and Microstructural Study of Ni-Cr-Mo Alloys Used in Dental Prostheses 44
chnique, a scanning was made in the –1.0 V to 0.8 V
interval with a 33.3 mV/s speed. The experiments were
performed three times for each recast alloy.
3. Results and Discussion
3.1. Surface Analysis
The surface metallographic analysis of alloys Wiron 99
as-cast and Wironia as-cast are characterized by a solid
solution array in dendritic disposition of as-cast state
(primary phase), Figures 1(a,b) and an interdendritic
phase (secondary) regularly distributed. In remelted
samples, Figures 2 and 3, it is observed two phases:
primary and grain boundary. The principal, also known
as homogeneous, still keeps a dendritic character; in the
grain boundary clusters are observed, probably consisting
of carbides of Cr and Mo. The as-cast structure is pre-
dominantly dendritic and it is known that in this structure
the dendrites have concentration of present metals, dif-
ferent of metals concentration in the inter-dendritic
spaces. This is due to solidification process, where den-
drites solidify firstly and, consequently, have a resistance
to different chemical and electrochemical attack.
Wiron 99 induction presents smaller precipitate con-
centration, Figure 2(b). Those precipitates are probably
Figure 1. SEM micrographs of as-cast (a) Wironia and (b)
Wiron 99 alloy surfaces V: Ni-Cr-Mo matrix; P: Precipitate
rich in Cr and Mo; P1: Black dots.
(a)
(b)
Figure 2. SEM micrographs of induction (a) Wir onia and (b)
Wiron 99 alloy surfaces V: Ni-Cr-Mo matrix; P: Precipitate
rich in Cr and Mo; P1: Black dots.
(a)
constituted of Cr and Mo carbides, mainly Mo. This
structure is similar to that obtained by several authors
who have analyzed alloys with composition close to this
work [14,24].
The EDS analysis result of as-cast alloy Wironia is dif-
ferent from chemical composition of manufacturer, as
seen in Table 1. The Ni content in region V of EDS has
increased 4% compared to Ni of as-cast alloy, Table 2.
In results of EDS analysis for alloys Wiron 99 induc-
tion in P and P1 there is Si presence, being in P1 greater
than in P. Si presence in the alloy induction may be due
to mold contamination in the process of recasting, Table
3.
(b)
The main difference in behavior between the alloy
Wironia and Wiron 99 may be related to the fact that
Wiron 99 presents a very low carbon amount (< 0.02% C)
and the presence of Nb (1.0% Nb), which has a greater
affinity for carbon leading to Nb carbide formation,
while Cr is kept in solution to maintain corrosion resis-
tance.
The corrosion resistance of Ni-Cr alloys varies with
their chemical compositions and the homogeneity of the
passive film formed [25]. Difference in microstructure
can influence the initial growth, the compactness and the

Electrochemical and Microstructural Study of Ni-Cr-Mo Alloys Used in Dental Prostheses 45
Mo is less important than Cr; however, alloy with less
Mo was more susceptible to pitting [24].
Ni-Cr alloys with higher level of Cr (about 25%) have
exhibited superior corrosion resistance due to the more
uniform distribution of Cr in the microstructure of alloy
[7]. A higher content of Cr2O3 and MoO3 in the passive
film could lead to higher resistance to metal ion transfer
through the passive film. The homogeneous distribution
of Cr is critical especially in low-Cr nickel-based alloys
for better corrosion resistance. Compared with Cr2O3, the
oxide of nickel is more porous and has less protective
ability to corrosion. Hence, the passive film zones, which
are rich in NiO, will act as weak regions for localized
corrosion, which can cause localized dissolution of
Ni-rich phases.
3.2. Open Circuit Potential Measurements
Tests of open circuit potential over time provide infor-
mation about the stability of material when immersed in
the aggressive environment studied and have been con-
ducted to evaluate the electrochemical behavior of alloys
studied in the middle, without electrical current. The po-
tential ennoblement in the first minutes for Ni-Cr alloy is
due to formation and thickening of a film that has protec-
tion characteristics. The potential stabilization, which is
most evident in the induction, shows that the film is
really formed. Despite the potential stabilization are dif-
ferent, the curves are similar to that observed for Cr. So,
this behavior can be attributed to the presence of chro-
mium oxides and probably Mo in the alloy surface. Al-
loys Wiron 99 and Wironia as-cast have presented a po-
tential ennoblement slightly higher than the other alloys
restructured. The alloy Wiron 99 induction and the alloy
Wironia torch appear to be relatively less resistant to
corrosion than the other ones.
Figure 3. SEM micrographs of torch (a) Wironia and (b)
Wiron 99 alloy surfaces V: Ni-Cr-Mo matrix; P: Precipitate
rich in Cr and Mo; P1: Black dots.
Table 2. Analysis by EDS of alloy Wironia as-received and
recast in different processes (% mass).
Wironia V P
Elements Ni Cr Mo Ni Cr Mo
As-cast 64.03 25.93 10.0564.49 26.00 9.50
Torch 65.85 25.07 9.8 57.63 22.26 20.12
Induction 64.73 25.29 9.98 57.56 22.84 19.593.3. Potentiodynamic Polarization Curves
compositional homogeneity of a passive film [26]. As
well known, the main component of the passive oxide
film is Cr (about 90% Cr oxides) [27,28]. The minor con-
stituents of the passive layer are oxides of Co, Mo and Ni.
In the passive region, Cr is present mainly as Cr (III)
oxide and in smaller amount as Cr (III) hydroxide [27]
Polarization curves for alloys Ni-Cr-Mo in NaCl 0.9%
m/m as shown in Figures 5 (a,b) had a cathode region,
where at pH 6.0 reduction of H+ and/or oxygen can occur.
In anode region, there is a large passive region (~1 V)
ranging from –0.3 V to about +0.7 V. For higher poten-
tials (~0.7 V) there is a progressive increase in current
Table 3. Analysis by EDS of Wiron 99 alloy as received and recast in different processes (% mass).
W99 V P P1
Elements Ni Cr Mo Ni Cr Mo Si Ni Cr Mo Si
As-cast 67.93 24.03 8.05 66.31 24.45 9.24 _ 65.91 23.85 10.24 _
Torch 66.37 24.04 9.60 66.01 23.86 10.13 _ 66.01 23.77 10.22 _
Induction 67.80 23.38 8.81 55.19 19.31 20.69 4.80 53.03 21.64 19.63 5.69
(b)
(a)

Electrochemical and Microstructural Study of Ni-Cr-Mo Alloys Used in Dental Prostheses 46
(a)
(b)
Figure 4. Open circuit potential versus time curves for (a)
Alloy Wironia and pure metals and (b) Alloy Wiron 99 and
pure metals.
(a)
(b)
Figure 5. Polarization curves for (a) Alloy Wironia in NaCl
0.9%, pH 6.0 (b) Alloy Wiron 99 in NaCl 0.9%, pH 6.0.
Figure 6. Polarization curves for pure metals in NaCl 0.9%,
pH6.0.
density due to the alloy components dissolution, a phe-
nomenon known as transpassivation. This transpassive
region is characterized by film rupture, electrooxidation
of constituent elements of alloy and/or film, and oxygen
evolution reaction. These processes are revealed by a
yellowish coloration of the test solution appearance (oc-
currence probably due to Cr (VI) species) and gas bub-
bles on the electrode surface, respectively. This behavior
is similar to that obtained with Cr, but the current density
for the alloy in passivity region is higher. In this poten-
tials region, the alloy Wiron 99 induction presents a slow
increase in current density in a wide potential range (~
1.0 V), which can be interpreted as a pseudo-passivation
and afterwards transpassivation occurs. This behavior
difference may be associated with greater heterogeneity
in the structure, which makes more difficult a protective
film formation. In this sense, Torch alloy is more corro-
sion resistant than Induction alloy. In Ni and Mo curves,
Figure 6, it is not observed passivation, the process is
cathodically controlled mainly by oxygen reduction rate
and corrosion rate is approximately 100 times higher
than in Cr. Passivation of Ni-Cr-Mo alloys is often at-
tributed to formation of a thin and compact chromium
oxide layer (Cr2O3). Table 4 shows the folowing pa-
rameters: passivation current density (jpass), passivation
range (Erupture-Ecorr) and disruption potential (Erupture), all
obtained from potentiodynamic polarization curves of
alloys Wiron 99 and Wironia recast by different proc-
esses and from Cr.
3.4. Cyclic Voltammetry Measurements
Potentiodynamic tests were carried out by initiating
scanning in –1.0 V, in the hydrogen detachment area.
Soon afterwards, the scanning is proceeding in the posi-
tive potential sense. The scanning inversion was made in
0.8V, in the material dissolution area.
Figures 7(a,b) display, respectively, the cyclical
voltammograms for alloys Wiron 99 and Wironia as-cast
and submitted to different recast procedures. In all cases

Electrochemical and Microstructural Study of Ni-Cr-Mo Alloys Used in Dental Prostheses47
Table 4. Parameters obtained from potentiodynamic po-
larization curves from the following elements: Cr and alloys
Wironia and Wiron 99.
Elements jpass (μA cm-2)E
rupture-Ecorr (V) Erupture (V)
Wironia as-cast 1-2 ~0.90 ~0.60
Wironia torch 1-2 ~0.84 ~0.60
Wironia induction
Wiron 99 as-cast
Wiron 99 torch
Wiron 99 induction
8-9
2-3
2-3
9-10
~0.60
~0.98
~0.86
~0.90
~0.20
~0.60
~0.60
~0.60
Cr 0.7-0.8 ~1.12 ~0.70
(a)
(b)
Figure 7. Cyclic voltammograms, v = 33.3 mV/s in NaCl
0.9%, for alloy (a) Wironia (b) Wiron 99.
a wide area of stability is observed between –0.6 and 0.6 V.
Alloys Wiron 99, obtained by slow cooling, reveals an
increase in the current density in passive area compared
to the open fire recast process. Whereas, alloy as-cast
presents a current peak between 0.2 and 0.5 V, which
was attributed to the nickel oxidation present in larger
concentration in the material (Figure 7 (b)).
Torch alloys behave similarly to Cr, Figure 8, even as
current density in the passivity region. Alloys induction
show a current density about three times greater than that
observed for other alloys, indicating a higher reactivity.
The recast effect with slow cooling for Wironia is
Figure 8. Cyclic voltammograms, v = 33.3 mV/s of pure
metals in NaCl 0.9%.
similar to the one observed for Wiron 99. The main dif-
ference, however, consists in the fact that the alloy
as-cast has identical behavior to the alloy treated in open
fire, without showing oxidation current peaks presence.
4. Conclusions
The microstructure of the alloys has restructured partly
dendritic character with the appearance of clusters,
probably consisting of carbides of Cr and Mo. The in-
duction recast procedure produces microstructure with-
smaller precipitate amount, a larger current on passivity
area and hardness larger than the open fire recast.
Different recast procedures change electrochemical
parameters such as stabilization potential in open-circuit,
current density and passivation interval. This being so,
the alloys recast by induction are less corrosion resistant
in 0.9% NaCl at 37˚C for they do not passivate in this
medium and due to their high charge density, indicating
the superficial protective film formation less resistant.
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