Materials Sciences and Applicatio ns, 2010, 1, 369-372
doi:10.4236/msa.2010.16053 Published Online December 2010 (http://www.scirp.org/journal/msa)
369
Influence of Ni and Cr Content on Corrosion
Resistance of Ni-Cr-Mo Alloys for Fixed Dental
Prostheses in 0.05% NaF Aqueous Solution
Nilo A. S. Sampaio, José W. J. Silva, Heloisa A. Acciari, Roberto Z. Nakazato, Eduardo N. Codaro,
Hamilton de Felipe
Chemistry and Physics Departament, Universidade Estadual Paulista, Guaratinguetá, Brazil.
Received August 13th, 2010; revised October 28th, 2010; accepted November 26th, 2010.
ABSTRACT
The Ni-Cr-Mo alloys have been used as dental prostheses because they own a good mechanical strength, high corro-
sion resistance and even to be econom ica lly viab le. Th ese alloys corro sion p ro tection again st in sa lt solution s typ ical o f
physiological med ia is due to passivatio n phenomenon with an oxide surface layer forma tion, mainly chromium oxid es.
This protective film, subjected to a mechanical stress in a corrosive environment, can partially dissolve by releasing
ions, which have deleterious effects in a human body. Fluoride ions, existing in hygiene products, change the buccal
environment and their presence may enable a localized corrosion process initiation. The aim of this work has been to
investigate the chemical composition influence of three alloys in corrosion resistance to: A (Ni-73% Cr-14% Mo-8.5%
Be-1.8% Al-1.8%), B (Ni-61% Cr-25% Mo-10.5% Si-1.5%) and C (Ni-65% Cr-22.5% Mo-9.5%) in media containing
fluorides that simulate mouthwashes solution. The study has been done in a 0.05% NaF solution, pH 6, at 37˚C using
electrochemical techniques. The alloy with the highest nickel and the lowest chromium content is not passive in the
middle studied, showing a continuous increase in current density with the potential increasing, while the other alloys
show passivation range of 600 mV and passive current density o f about 10-6 A/cm2.
Keywords: Ni-Cr-Mo Alloys, Dental Prostheses, Corrosion
1. Introduction
A large number of metal alloys are used in dentistry for
manufacturing of fixed and removable prostheses. In the
oral cavity, these structures are exposed to a chemically
adverse environment, with saliva being the most corro-
sive ag ent, due to th e high concentration of chloride ions
that are causing localized corrosion. Another important
factor is the pH, which can range from 2 to 11, depend-
ing on the ingested food. One of the most important fac-
tors determining the use of a metal alloy for making
prosthesis is its resistance to corrosion [1,2]. Non-pre-
cious metal alloys are being used due to their low cost
and adequate mechanical properties. In the '60s, basic
alloys such as nickel-chromium were developed. These
alloys are basically composed of Ni (68% to 80%) and
Cr (12% to 27%) [3], but it is necessary to add other
elements to ensure high mechanical strength achieve-
ment and adequate corrosion resistance, besides the
shaping facility and bonding with porcelains, molybde-
num, iron, aluminum, silicon, beryllium, cobalt, niobium,
titanium which are the most frequently used alloys ele-
ments [3,4]. Chromium presence improves the alloys
corrosion resistance in typical salt solutions of physio-
logical media with an oxide surface layer formation,
mainly chromium oxides [5,6]. This layer thickness is
typical of a few nanometers, which is sufficient to act as
a protective barrier between the metal surface and the
aggressive medium. The passive film slows the metals
dissolution rate, making difficult the ions transfer from
metal to solution. The human tissue reaction for dis-
solved ionic species can vary from a simple allergy to a
severe disruption in the region adjacent to the prosthesis
[7]. Since the chemical alloys composition can influence
the corrosion behavior, studies about corrosion resistance
in aggressive media, which simulate the oral environ-
ment, may provide information about metals or alloys to
be used in dental applications. Sodium Fluoride (NaF)
and other fluorides are often used as prophylactic agents

Influence of Ni and Cr Content on Corrosion Resistance of Ni-Cr-Mo Alloys for Fixed Dental 370 Prostheses in 0.05% NaF Aqueous Solution
in dental treatments to prevent dental plaque formation
and caries development, however, the prophylactic ac-
tion may have a side effect of metal materials corrosion
caused by this ion. Saliva infiltration containing fluoride
in the structure that holds implant and the contact with
crowns and bridges can be the corrosive attack cause.
Fluorides usually attack reactive metals such as titanium,
especially in acidic medium, causing corrosion due to
their passivity destruction and mechanical properties
decrease. To date, a few researches have been done to
analyze the fluorine-containing composites influence in
dental alloys corrosion resistance. N. Chiff et al. 2004 [8]
studied the fluoridated mouthwashes influence on tita-
nium alloys corrosion resistance used in orthodontic
wires manufacture, using electrochemical techniques that
indicated correlation between corrosion and mouth-
washes composition. Mayont, A. M. et al. 2004 [9] ana-
lyzing the titanium alloys corrosion behavior found high
corrosion rates in neutral solutions with lesser fluoride
contents and in acid solutions with low fluoride levels.
According to the authors, the fluoride concentration in-
crease leads to a decrease in thickness and increase in
porosity on oxide layer, which reduces its corrosion pro-
tection. Rezende, A, B, C. and other [10] evaluated effect
of three commercial mouthwashes on corrosion resis-
tance of a Ti-10 Mo alloy developed in their lab, com-
paring it with the commercially pure titanium behavior.
Through electrochemical techniques, they concluded that
corrosion resistance depends predominantly on the
mouthwash composition. With respect to Ni-Cr, it has
only been found in literature one non-conclusive quote
that prophylactic fluoride solution (pH = 6.5) is a reac-
tive agent and can cause damage to metal restorations
[11]. Theref ore, it is impor tant to probe know ledge abou t
the Ni-Cr-Mo reactivity in media containing fluoride,
which is this work aim.
2. Materials and Methods
The three Ni-Cr-Mo alloys corrosion resistance has been
determined in naturally aerated 0.05% NaF solution, pH
6.0 at 37˚C using electrochemical techniques. The alloys
have been acquired in the national market, being com-
mercialized in a billet form in the as-cast condition. Ta-
ble 1 shows the chemical compositions provided by the
manufacturers, being named in this work by the letters A,
B and C.
For electrochemical measurements, the alloys, as re-
ceived, have been embedded in polyester resin, leaving
an exposed area in the flat disk form of about 1 cm2. Be-
fore each experiment, the electrode surfaces have been
mechanically polished, degreased with acetone, washed
with distilled water and air-dried. Measures have been
Table 1. Alloys mass percentage composition.
AlloyNi Cr Mo Be Si Al
A 73 14 8.5 1.8 --------- 1.8
B 61 25 10.5 ------- 1.5 --------
C 65 22.5 9.5 ------- ------------------
taken at 37˚C using a conventional thermostated electro-
chemical cell containing 0.05% NaF solution used as elec-
trolyte and prepared using NaF and distilled water.
The electrochemical measurements have been carried
out using a potentiostat/galvanostat EG & PAR 283 and
a Frequency Response Analyzer, EG & PAR 1025
(Perkin Elmer Instruments Inc., USA), both interfaced to
a computer for collecting and analyzing data. The work-
ing electrodes potentials have been determined using a
saturated calomel electrode as reference, and used a
platinum ring as counter electrode. Op en circuit potential
(OCP) values have been recorded during 400 minutes of
immersion. Potentiodynamic polarization curves have
been obtained immediately after obtaining the OCP
curves, with a scan rate of 20 mV min-1 towards electro-
positive. The experiments have been performed in tripli-
cate to observe repeatability. The cyclic voltammetry
tests have been carried out with a 10 mV/s sp eed starting
at –600 mV, advancing towards the cathode up to the
potential value where the metals dissolution process
starts and returning to the initial poten tial.
3. Results and Discussion
3.1. OCP Measurements
At Figure 1, OCP curves for alloys A, B and C in 0.05%
NaF are illustrated. An upward trend at the very initial
immersion stage has been common for the three studied
alloys, but a significant decrease in the OCP values has
been observed, specifically for curve A and potential val-
ues obtained have been around 200 to 250 mV more nega-
tive than those observed for B and C. Fluoride ions pres-
ence in solution can alter the oxide la yer structure making
it more porous, thus leading to a decrease in corrosion
resistance in the considered medium. These observations
seem to be consistent with the differences in the alloys
compositions, since alloy A, by having a smaller chro-
mium amount in its composition, becomes more suscepti-
ble to corrosion in me di um cont ai ni ng fluoride. Chromi um
is responsible for corrosion resistance and for an oxide
film formation commonly called passive film [11]. It takes
at least 12% by weight for thi s film formation.
In laboratory studies, concen trations of 16%-27% tend
to form a more uniform oxide layer of Cr2O3 and
Cr(OH)3 with higher corrosion resistance [3]. The chro-
mium oxide layer is associated with the aluminum from

Influence of Ni and Cr Content on Corrosion Resistance of Ni-Cr-Mo Alloys for Fixed Dental 371
Prostheses in 0.05% NaF Aqueous Solution
Figure 1. Open circuit potential for alloys A, B and C im-
mersed in 0.05% NaF solution.
the liaison agent between the metal substrate and porce-
lain [14]. In order to obtain more information that can
characterize the corrosion behavior type and allow a
comparative analysis of corrosion resistance for the three
alloys, potentiodynamic polarization curves have been
obtained, as illustrated in Figure 2.
3.2. Potentiodynamic Polarization Curves
The different behavior observed for the alloy A in open
circuit potential appears again when comparing the po-
tentiodynamic profiles (Figure 2) indicating that a
smaller chromium content in the chemical alloy compo-
sition may lead to corrosion potentials of about 250 mV
more negative. Moreover, the passivity limit becomes so
narrow that it appears almost imperceptible in potentio-
dynamic profile analyze of curve A. While B and C pre-
sent virtually the same passivation interval, approxi-
mately 500 mV and the same passive current density of
about 10-6 A cm-2, th e alloy A has curr ent density 10,000
Figure 2. Polarization curves for alloys A, B and C immersed
in 0.05% NaF solution.
Figure 3. Cyclic voltammograms of alloys A, B and C in
0.05% NaF solution.
times greater, thus showing a low corrosion resistance.
3.3. Cyclic Voltammetry
Figure 3 shows the cyclic voltammograms of alloys A, B
and C in 0.05% NaF, pH 6 and 37˚C in the interval be-
tween –600 mV and 600 mV where it can be observed the
alloy A corrosion and alloys B and C passivation. The
alloy C shows the largest passivity range (approximately
800 mV). Alloys B and C have showed the greatest dis-
ruption potential in terms of passive films to be more re-
sistant. Another interesting parameter is the current den-
sity, which indicates alloy reactivity in direct order,
namely, the higher the current density, the higher the alloy
reactivity, where alloy A owns the highest current density.
The potential interval registered for the alloy A has been
lesser due to its generalized dissolution in potential values
higher than –200 mV. When the scan has been reversed, a
hysteresis loop appeared, denoting an acti ve state.
4. Conclusion
A distinct behavior has been verified when the potentio-
dynamic profiles of three different compositions used in
the dental market in NaF 0.05% medium are compared,
showing that the highest Ni content to the alloy, coupled
with the lowest Cr content, results in the greatest corro-
sion susceptibility.
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