Electrochemical and Microstructural Study of Ni-Cr-Mo Alloys Used in Dental Prostheses

doi:10.4236/msa.2011.21006

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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.

Email: [email protected]

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

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

(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

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

(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

9-10

~0.60

~0.98

~0.86

~0.90

~0.20

~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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