BIOMAT., MED. DEV., ART. ORG., 3(4), 411-427 (1975)

Electrochemical Behavior of Some Commercial Dental Amalgams in Artificial Saliva

J. R. CAHOON, Ph.D., and CARMELO REGALBUTO, B.S.

Metallurgical Science Laboratory Department of Mechanical Engineering University of Manitoba Winnipeg, Manitoba, Canada, R3T 2N2

ABSTRACT Cathodic, linear anodic, and anodic polarization studies conducted on three commercial dental amalgams, Caulk Fine Cut Alloy, Spheralloy, and Dispersalloy, showed that all amalgams were in a passive state a t the corrosion potential in synthetic saliva solution. The corrosion c u r r e n t s a t the corrosion potential were therefore small for a l l the amalgams, in the range 0.08 to 0.30 pA/cm2. However, the Caulk Fine Cut Alloy and Spheralloy amalgams exhibited a breakdown of passivity and high anodic c u r r e n t s a t potentials only % 100 mV m o r e noble than the corrosion potential whereas Dispersalloy amalgams maintained passivity a t potentials up to 700 mV m o r e noble than the corrosion potential. The breakdown of passivity in Caulk Alloy and Spheralloy amalgams is attributed to the presence of the y z phase (SnT-sHg) whereas the passive behavior of Dispersalloy amalgam is attributed to the absence of this phase. It is concluded that none of the amalgams will exhibit severe general corrosion in use, but

411 Copyright 0 1976 by Marcel Dekker, Inc All Rights Reserved Neither this work nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, rnicrofilrnmg, and recording, or by any information storage and retrieval system, without perrnlssion in writing from the publisher

412

CAHOONANDREGALBUTO

that both Caulk Alloy and Spheralloy amalgams will exhibit pitting corrosion whereas this type of corrosion should be minimal in Dispersalloy amalgams. I. I N T R O D U C T I O N

The corrosion of dental amalgams o r constituent phases there-in has been considered responsible for deleterious effects on the restoration such as l o s s of luster and thus aesthetic quality, pitting, and eventual weakening of the restoration [l-31. However, corrosion of the amalgam restoration is also considered to have beneficial effects such as sealing of leaky margins and increasing the mechanical anchoring [3]. Regardless of the beneficial o r deleterious effects of corrosion on amalgam restorations, electrochemical techniques such as potentiostatic ( o r potentiodynamic) and galvanostatic studies can provide considerable information concerning the corrosion behavior of amalgams. While the long-term, in-service corrosion r a t e s predicted by these rapid tests may be suspect, correlation of these t e s t s with clinical studies can provide a means for the rapid evaluation of new amalgams as well as provide a better understanding of the corrosion process in amalgams. Several electrochemical studies on various commercial dental amalgams have been reported [ 1, 2, 4, 51. Most of these studies present only anodic polarization curves which are valuable f o r understanding the corrosion p r o c e s s but which do not determine the corrosion rate although Guthrow et al. [l] do present a cathodic polarization curve from which they predict a r a t h e r large corrosion current of 9 pA/cm2. Unfortunately, there is considerable disagreement among the results of these investigations which limits the ability to draw conclusions concerning the corrosion behavior of dental amalgams. Therefore, the present study was initiated to determine the electrochemical behavior of three commercial amalgams, Caulk Fine Cut Alloy (L. D. Caulk Co., Melford, Delaware), Spheralloy (Kerr Manufacturing Co., Detroit, Michigan), and Dispersalloy (Western Metallurgical Co., Ltd., Edmonton, Alberta). Potentiodynamic and galvanostatic studies were conducted on the amalgams in an artificial saliva solution. Anodic polarization curves were obtained from the potentiodynamic studies for comparison with previous investigations. Cathodic and linear anodic polarization curves were obtained from the galvanostatic studies so that a

ELECTROCHEMICAL BEHAVIOR O F DENTAL AMALGAMS

4 13

value of the corrosion current (and thus the corrosion rate) could be predicted, 11. E X P E R I M E N T A L

Rectangular specimens of amalgam ( 2 c m X 1.3 cm X 0.3 cm) weighing approximately 10 g were prepared by titrating equal weights of alloy and m e r c u r y for 20 s e c and mechanically condensing the amalgam in a dye. The specimens were aged for 3 weeks, sanded with 600 grit paper, and bolted to a stainless steel rod. A 1-cm2 area on each side of the specimen was covered with masking tape and the entire assembly was coated with Quelspray air d r y vinyl spray (Quelcor Inc., Medio, Pennsylvania) and allowed to d r y for 24 hr. After drying, the tape was removed, and the specimen w a s cleaned with ethyl alcohol and immersed in synthetic saliva solution contained in an electrochemical cell of standard design [6]. The composition of the synthetic saliva (Table 1) was very simiIar to saliva #1 used by Merek and Hochman [5]. T e s t s were conducted in as-prepared (aerated) solution with a p02 of = 70 mm Hg and in deaerated solution (deaerated by continuously bubbling argon through the solution). The pH of the solution was maintained a t 6.7 by additions of 1 M HC1 and 1 M NaOH. The amounts of HC1 and NaOH added were never sufficient toappreciably a l t e r the Na o r C1 concentrations of the solution. The pH and p 0 of ~ the solution were measured with a Radiometer analyzer model #PHM 71b. All t e s t s were conducted a t a temperature of 37°C maintained by placing the electrochemical cell in a circulating water bath. After allowing each specimen to stabilize in the solution for 20 hr, three types of electrochemical t e s t s for each amalgam specimen were conducted with duplicate specimens for each of the two test solutions (aerated and deaerated). A.

Cathodic Polarization

T h i s test was conducted by passing a cathodic current from a Kiethly Constant Current Source, model #225, between the specimen and the platinum counter electrodes. The current densities used were in the range 0.01 to 10 pA/cm2, and the system was allowed to stabilize for 5 min a t each value of c u r r e n t density before the potential was recorded. The potential was measured with reference to a saturated calomel electrode (SCE) using a Keithly Digital Multimeter model #160.

414

CAHOON AND REGALBUTO TABLE 1. Composition of Synthetic Saliva

Compound

Amount (g)

Dipotassium hydrogen phosphate ( K z H P O ~ )

0.20

Disodium hydrogen phosphate (NazHP04)

0.26

Potassium t hioc yanate (KCN S)

0.33

Sodium bicarbonate (NaHCO,)

1.50

Sodium chloride (NaC1)

0.70

Potassium chloride (KC1)

1.20

Urea

0.13

(NHZ

CONH~)

1,000

Distilled water

B.

L i n e a r Anodic P o l a r i z a t i o n

This test was conducted similarly to those for cathodic polarization except that small anodic c u r r e n t s were used (0 to 0.05 pA/cm2) so that the potential change from the r e s t potential was linear and in the o r d e r of 10 mV. C.

Anodic P o l a r i z a t i o n

Potentiodynamic anodic polarization curves a t a scan r a t e of 600 rnV/hr were obtained using a Wenking potentiostat, model #68TS3, and an Erwin Halstrup motorpotentiometer, model #MP 165. The c u r r e n t was recorded continuously via the potentiostat r e c o r d e r terminals using a s t r i p chart recorder. Anodic polarization curves were obtained for a third test solution saturated with COZ f o r comparison with the r e s u l t s of Merek and Hochman [ 51 who maintained the pH of their solutions by bubbling with COZ. The pH of the C02 saturated solution was allowed to stabilize a t pH = 5.8. 111. R E S U L T S

The cathodic polarization curves for the three amalgams tested in aerated and deaerated saliva are shown in Figs. 1 and 2, respectively. The corrosion current density can be obtained by extrapolation (Tafel

ELECTROCHEMICAL BEHAVIOR O F DENTAL AMALGAMS -

3

CAULK 0

ALLOY 0

4 15

7

E R = -290 m.v

c

,,j

v

-300

SPHERALLOY

CURRENT DENSITY (pA/cm*)

FIG. 1. Cathodic polarization results in aerated synthetic saliva.

extrapolation) of the linear portion of the curve to the corrosion potential (ER) since at this point the rate of hydrogen evolution is equal to the rate of metal dissolution ["I. The corrosion currents obtained via Tafel extrapolation are included in Table 2. The linear anodic polarization curves for the three amalgams

CAHOONANDREGALBUTO

4 16

CAULK

ALLOY

-

-600

h

-700

SPHERALLOY 7 - - -

- 5 0 0 - E ~ = - 4 1 2my.

-600-

DISPERSALLOY

-500. ER 8 -400 m.v.

-600 -700

.oI CURRENT

.I

0

DENSITY (pA/cm*)

FIG. 2. Cathodic polarization results in deaerated synthetic saliva. tested in aerated and deaerated solutions are shown in Figs. 3 and 4, respectively. The corrosion current density can a l s o be obtained from the slope (polarization resistance) of the linear anodic polarization curves using the relation [8]

0.24 0.4

18

.09

.19

.30

0.59

0.4

.08 .ll

Deaerated .253

Dispersalloy Deaerated .280

.25

.18

Spheralloy

a

.12

.23

Deaerated .230

.172

Dispersalloy Aerated

0.59

.24

B

c1

i

>

r

v

0

8

5

s

X

M

a

Corrosion current g (from Eq. 2) i; > (~A/cm') r

Caulk Alloy

.168

Aerated

Spheralloy

0.4

.ll

Aerated

Caulk Alloy

.2 11

Solution

Amalgam

Corrosion current Polarization (Tafel extrapolation) resistance (pA/cm2) (v/~A/cm')

Cathodic Tafel slope (V/decade of current)

TABLE 2. Cathodic and Linear Anodic Polarization Results

M

CAHOON AND REGALBUTO

418

-278

-284 -286

-290

- 304- 306 -308 -310-312-314-316

i i i

SPHERALLOY

I/,

,

I

,

-274 -2761 -278

i

,

i

-282

-286

.01 .02 03 .04 .05

CURRENT DENSITY ( p A / c m 2 )

FIG. 3. Linear anodic polarization results in aerated synthetic

saliva.

ELECTROCHEMICAL BEHAVIOR O F DENTAL AMALGAMS

419

418420.

1

422-

/

424-

7

426428

CAULK ALLOY

./’

j/

422-

424426.

SPHERALLOY

. . .

42f.

,

430

426r

4341

/

.i

ALLOY

436L

438

.01 .02 .03 .04 0 5

CURRENT DENSITY (pA/cm2)

FIG. 4. Linear anodic polarization r e s u l t s in deaerated synthetic saliva.

CAHOON AND REGALBUTO

420

where

hE A1 Ba BC

=

slope of the linear anodic polarization curve

=

anodic Tafel slope (volts/decade of current)

=

cathodic Tafel slope (volts/decade of current)

= corrosion current density corr The corrosion potential of all the amalgams in both aerated and deaerated solutions appears to occur in a passive region where the anodic Tafel slope (B,) approaches infinity (see the anodic polarization

i

curves in Figs. 5 and 6 ) and therefore Eq. (1) reduces to

The cathodic Tafel slopes obtained from the cathodic polarization curves (Figs. 1 and 2), the polarization resistance (AE/AI), and the corrosion current density (i ) calculated from Eq. (2) are included corr in Table 2. The anodic polarization curves f o r the three amalgams in aerated, deaerated, and COZ saturated solutions are shown in Figs. 5, 6, and 7, respectively. IV. D I S C U S S I O N

The corrosion current densities for all the amalgams a t the corrosion potential calculated from both Tafel extrapolation and linear anodic polarization are quite small, 0.1 to 0.3 pA/cm2 (Table 2). The corrosion currents obtained by Tafel extrapolation for the amalgams tested in aerated solutions tend to be slightly s m a l l e r than those obtained for amalgams tested in deaerated solutions which is reasonable since the presence of oxygen should accelerate the corrosion process. The corrosion c u r r e n t s densities obtained from linear anodic polarization curves (Eq. 2) tend to be somewhat higher than the values obtained by Tafel extrapolation but this may result from the assumption that the anodic Tafel slope (B ) i s infinite. Inspection of Eq. (1) shows that a

a

ELECTROCHEMICAL BEHAVIOR O F DENTAL AMALGAMS

I

I '

422

CAHOON AND REGALBUTO

pH * 5.8 1 = SIOC.

- DISPERSALWY --- SPMERALLOY --- CAULK ALLOY

CURRENT

DENSITY (pA/cm2)

\

I

FIG. 7. Anodic polarization r e s u l t s in COa-saturated synthetic saliva.

-‘OL1

-2a

0

20(

4oc

60C

800

424

CAHOON AND REGALBUTO

finite value f o r B will tend to lower the calculated value of the corroa sion current density, In any event, the corrosion current densities for all amalgams a t the corrosion potential is small, much l e s s than the value of 9 pA/cm2 obtained by Guthrow et al. [l] using the Tafel extrapolation technique. Assuming a value of 0.2 pA/cm2 for the corrosion current density, a corroding a r e a of 0.2 cm2 f o r a typical amalgam restoration, and further assuming that Sn (atomic weight = 119) with a valence of 2 is the corroding species, the corrosion rate of an amalgam restoration is estimated to be 2 8 X g/year which is an acceptable value considering that a typical amalgam restoration weighs in the o r d e r of 0.5 g. The value of 9 pA/cm2 for the corrosion current density obtained by Guthrow e t al. [l] converts to a corrosion r a t e of 3.6 X lo-’ g/year for a typical amalgam restoration which clearly cannot be c o r r e c t since this suggests virtual consumption of the restoration within 10 to 20 years, a process not clinically observed. The anodic polarization curves shown in Figs. 5 - 7 indicate that the polarization behavior of any particular amalgam is essentially independent of the 0 2 o r COZ concentration of the solution. All amalgams in all solutions exhibited passive behavior a t the corrosion (or r e s t ) potential, which must be the case because of the low corrosion current densities obtained f r o m the cathodic and linear anodic polarization tests. Caulk Fine Cut Alloy and Spheralloy amalgams exhibited almost identical anodic polarization behavior with initial low passive current densities ( 51 pA/cm2) near the corrosion potential followed by breakdown and a large current densities a t potentials m o r e noble than = -200 mV v s SCE. The breakdown potential of Spheralloy amalgam was slightly but consistently about 50 mV m o r e noble than that for Caulk Fine Cut Alloy. The anodic polarization behavior of Dispersalloy amalgam was significantly different from that of the Caulk Alloy o r Spheralloy amalgams in that no breakdown potential was observed f o r Dispersalloy amalgams over the range of voltage utilized. The breakdown for the Caulk Alloy and Spheralloy amalgams a t voltages m o r e noble than -200 mV v s SCE is almost certainly due to the presence of yz-phase (Sn7 sHg) in these amalgams since both Guthrow e t al. [l] and Merek and Hochman [ 5 ] show that the yz-phase becomes active a t a potential of 2 -200 mV v s SCE. Dispersalloy amalgam, however, contains very little yz-phase, presumably due to the high copper concentration [8], and therefore the continuing passivity and absence of breakdown of Dispersalloy amalgam a t noble potentials can be attributed to the absence of the yz-phase. In aerated solutions the corrosion ( r e s t ) potential is about -300 mV v s SCE f o r all the amalgams (Figs. 1, 3, and 5). The effect of removing

ELECTROCHEMICAL BEHAVIOR OF DENTAL AMALGAMS

425

oxygen from the solution by bubbling with argon o r COz is to lower the corrosion potential to 2 -400 mV v s SCE (Figs. 2, 4, and 6) when the solution is deaerated with argon o r = -500 mV v s SCE when the solution is saturated with COZ (Fig. 7). The lowering of the corrosion potential of amalgams with removal of oxygen is in agreement with the r e s u l t s of Merek and Hochman [5] for deaerated solutions although they obtain lower corrosion potentials, = -700 mV v s SCE. Merek and Hochman [5] recorded corrosion potentials a t times up to 200 h r after immersion (vs 20 hr for the present investigation) and this may account for the discrepancy. The removal of oxygen from the solution, however, h a s no great effect on the corrosion current density of the amalgams which is in the range 0.08 to 0.30 kA/cm2 (Table 2) nor does it affect the breakdown potential of Caulk Alloy o r Spheralloy amalgams which remains constant at = -200 mV v s SCE (Figs. 5-7). Therefore it can be concluded that the diffusion of oxygen to the surface of the amalgam is not the rate-controlling process in the corrosion of dental amalgam. As indicated previously, the corrosion potential of a l l three amalgams l i e s in the passive region (= -300 mV v s SCE for aerated solutions), giving r i s e to low corrosion current densities. Therefore, g r o s s general corrosion of these amalgams will not occur, a fact which has been substantiated by many y e a r s of clinical experience. However, the breakdown potential of both Caulk Fine Cut Alloy and Spheralloy amalgams is = -200 mV v s SCE, only 100 mV more noble than the corrosion potential. Therefore, pitting corrosion of these two amalgams is expected to occur, and since the yz-phase is considered responsible for the breakdown of these amalgams, it is the yz-phase which is expected to corrode. This has also been substantiated by clinical and laboratory experiments 193. On the other hand, Dispersalloy amalgam retains passive behavior to potentials a t least 700 mV more noble than the corrosion potential (Figs. 5-7), and therefore pitting corrosion of this amalgam is not expected to occur. V. C O N C L U S I O N S 1. Electrochemical tests on Caulk Fine Cut Alloy, Spheralloy, and Dispersalloy amalgams indicate that all these amalgams are in a passive state a t the corrosion potential in artificial saliva solution. As a result, corrosion current densities a r e low and g r o s s general corrosion of these amalgams is not expected to occur. 2. Both Caulk Fine Cut Alloy and Spheralloy amalgams exhibit a breakdown of passivity a t potentials only = 100 mV more noble

CAHOON AND REGALBUTO

426

than the corrosion potential, likely because of the presence of y2-phase. Therefore, these two amalgams are likely to exhibit pitting corrosion because of dissolution of the yz-phase. 3. Dispersalloy amalgam exhibits no breakdown of passivity to potentials 700 mV m o r e noble than the corrosion potential. Therefore, pitting corrosion in this amalgam is expected to b e minimal. 4. The corrosion r e s i s t a n c e of Dispersalloy amalgam is expected to be considerably superior to either Caulk Fine Cut Alloy o r Spheralloy a m a l g a m s because of its resistance to pitting corrosion. ACKNOWLEDGMENTS The authors are grateful to the Medical R e s e a r c h Council of Canada for financial support in the f o r m of a n operating grant (MA 3607) and to the University of Manitoba Graduate Research Board f o r an equipment grant. The assistance of Dr. P e t e r Williams and Dr. Y. Galindo of the Department of Dental Materials, Faculty of Dentistry, University of Manitoba and the technical support of Ms. Laura Tennese are a l s o greatly appreciated. RE FERENCES

[l] C. E. Guthrow, L. B. Johnson, and K. R. Lawless, "Corrosion of Dental Amalgams and Its Component Phases," J. Dent. Res.,

46, 1372-1380 (1967). [2] [3]

[4] [5]

[6] [7] [8]

E. Stoner, K. R. Lawless, and R. Wawner, "Corrosion Resistant Silver T i n Amalgams," _ bid., __ 50, 519 (1971). R. S. Mateer and C. D. Reitz, "Corrosion of Amalgam Restorations," _ Ibid., __ 49, 399-407 (1970). G. E. Stoner, S. E. Senti, and E. Gileadi, "Effect of Sodium Fluoride and Stannous Fluoride on the Rate of Corrosion of Dental Amalgams," bid., -50, 1647-1653 (1971). M. Merek and R. F. Hochman, Paper presented to the 50th Session of IADR, Las Vegas, 1972. Annual Book of ASTM Standards, Part 31, American Society for Testing and Materials, Philadelphia, 1972, pp. 1094-1103. M. G. Fontana and N. D. Greene, Corrosion Engineering, McGraw-Hill, New York, 1967, p. 342. N. K. Sarkar and E. H. Greener, "Absence of the y2-Phase

ELECTROCHEMICAL BEHAVIOR O F DENTAL AMAWAMS

427

in Amalgams with High Copper Concentrations," J. Dent. Res., 51, 1511 (1972). 191 % Otani, W. A. Jesser, and H. G. F. Wilsdorf, "The in vivo and in vitro Corrosion Products of Dental Amalgams," J. Biomed. Mat. Res., 7, 523-539 (1973). Received by editor September 17, 1974

Electrochemical behavior of some commercial dental amalgams in artificial saliva.

Cathodic, linear anodic, and anodic polarization studies conducted on three commercial dental amalgams, Caulk Fine Cut Alloy, Spheralloy, and Dispersa...
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