Journal of Dental Research

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Electroetching of Dental Amalgam P.J. Staheli and J.A. Von Fraunhofer J DENT RES 1974 53: 468 DOI: 10.1177/00220345740530024901 The online version of this article can be found at: http://jdr.sagepub.com/content/53/2/468

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Electroetching of Dental Amalgam P. J. STAHELI and J. A. VON FRA UNHOFER Department of Dental Materials, Institute of Dental Surgery, University of London, London WCIX 8LD, England

Application of the potentiostatic polarization technique to the metallographic etching of set dental amalgam is described. Etching studies of amalgam prepared from a lathecut, a spherical, and a silver-copper dispersed-phase amalgam alloy are reported. The 72 phase in set dental amalgam can be characterized by this electrochemical technique. Metal and semiconductor surfaces are etched primarily to reveal their microstructures for metallographic examination and to permit the characterization of crystals by revealing dislocations and crystal orientations.1 Chemical etching techniques for characterization of the phases in dental amalgam are known,2-4 but electrolytic etching is a relatively new advance in dentistry.5 Identification of phases in an alloy through selective dissolution is performed conveniently by potentiostatic means. The principle of etching is that in a multiphase system, each phase should behave independently of the others present. Consequently, each phase will have a characteristic dissolution potential in a given electrolyte, and therefore, it can be etched by the correct choice of potential. Polarization curves that characterize the electrochemical behavior of a metal in a given electrolyte may be used to select the correct etch potential for a particular phase in a multiphase system. Etching at a given potential will be selective for a phase only if its dissolution rate is considerably greater than that of any other phases present. Potentiostatic etching is independent of the amount of the phases present and depends only on the electrode potential imposed on the metal. A potentiostat is an electronic instrument Received for publication January 26, 1973.

that can control the potential of an electrode at a required value, within closely defined limits, with respect to a standard or reference electrode. A reference electrode is an electrode or half cell that has a reversible electrode potential that is invariant under specified conditions; an example is the saturated calomel electrode (SCE). The controlled electrode, which is under investigation, is referred to as the working electrode. In addition to the working and reference electrodes, a third inert or auxiliary electrode, usually platinum, is required. The potentiostat controls the working electrode potential by comparing the set or required electrode potential with its actual potential with respect to the reference electrode. If the potential difference exceeds a few millivolts, a charge is passed by the potentiostat between the working and counter electrodes, and current flows until the electrode potential is restored to the required value. This output or cell current is measured and is used to plot polarization or E/i curves that present the pattern of current variation accompanying change in working electrode potential. Polarization currents may be measured either from the internal potentiostat ammeter or by potential drop across a resistance placed in the auxiliary electrode circuit. Because of the poor corrosion resistance and low strength of the Y2' tin mercury (SnHg), phase, there has been a great effort to reduce or even replace it in the set amalgam matrix. In an endeavor to improve the physical and electrochemical qualities of dental amalgam, a new copper (Cu)-silver (Ag), dispersed-phase alloy amalgam has been introduced,6 and other novel systems including gold (Au) alloy amalgam7 and gallium-palladium-tin alloy amalgams8 have been proposed. Furthermore, the incorporation of

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some Yi (Ag-Hg) phase into the components of an amalgam alloy before trituration has been advocated.9 Alternatively, it has been suggested that yj may be added to Hg before trituration with the alloy.9 Identification of the microstructure of the various phases of conventional and spherical alloy amalgams is difficult with the present technique of chemical etching, unless the results are qualified with accurate microhardness determinations and electron probe microanalysis. It is well-known, for example, that the Y2 phase is far softer (KHN 10 to 15) than the yj phase in a set dental amalgam (KHN 120).10 Jorgensenll considers the 72 phase to be continuously distributed throughout the Ag-Sn dental amalgam, whereas Wing3 considers it to be in discrete clusters, separated by the y and the yj phases; this indicates that metallographic interpretation of dental amalgam is difficult. An interesting aspect of the dispersed alloy amalgam is that adding an additional Ag-Cu eutectic phase into the -y matrix of the resulting amalgam could alter the physical properties without affecting the characteristic desirable manipulative behavior of dental amalgam. Mahler12 has indicated that there is no 72 phase in this amalgam. There is present, however, a reaction phase surrounding the Ag3Cu dispersant particles that has a composite intermediate between the intermetallic compounds of Cu3Sn and Cu6Sn5. This alloy as yet has not been fully characterized. The selective predominant dissolution of one phase of those present in the set amalgam at a discrete potential may assist in

AUXIUARY ELECTRODE

COMIARTMENT AND PLATINUM ELECTRODE STIRRER BAR

FIG 1.-Schematic diagram of electrochemical

cell.

469

ELECTROETCHING OF DENTAL AMALGAM

Vol 53 No. 2

+ E 4

-4 z a

0

POTENTIAL, mV

+500

+

FIG 2.-E/i polarization curve for Aristaloy in 10% tartaric acid solution.

rapidly determining the microstructure of the newly developed dental amalgams. Materials and Methods The dental amalgams used in this study (Table) were packed into cylindrical cavities (4 mm internal diameter by 10 mm) cut into acrylic blocks that contained an insulated electric connector.13 These specimens (the working electrodes) were carved and allowed to set for seven days before polishing to a one micrometer (gm) finish with diamond paste. The working electrode was introduced into the electrolytic cell at a distance of 1 mm from the Luggin capillary (Fig 1) . The electrochemical cell contained 75 ml of 10% tartaric acid solution, a platinum auxiliary electrode, and a standard calomel reference electrode. All electrodes were connected to the potentiostata with short, low resistance leads. If a specimen in an electrolyte has its potential varied from, for example, -1,000 to + 1,500 mv (with respect to the SCE), the critical potentials of etching will be traversed.5 This was effected by changing by increments the controlled potential by an automatic sweep unit coupled to the potentiostat. The potential sweep rate was 500 mv/minute. The polarization (potential-time) curves were recorded using a two-pen potentiometric recorder.b One channel recorded the potential and the other recorded the corresponding current by potential drop across an internal resistor in the potentiostat. a Wenking Model 70HC3, G. Bank Electronic GmbH, Gottingen, W. Ger. b Telsec, T/T Recorder, Telsec Instruments Ltd., Oxford, Eng.


STAHELI AND VON FRAUNHOFER

470 +

+8 "2

4

E+4

2 z

:

-1000

+5O4

0

-500

POTENTIAL,

mV

+V

FIG 3.-E/i polarization curve for Kerr a-cap in 10% tartaric acid solution.

Spher-

As the potential is increased from -1,000 the current flow changes from nnegative (cathodic) in the region of hydrogen evolution to positive (anodic); this is bec; cause of anodic dissolution of the amalgann and, finally, oxygen evolution at noble pot entials. Each dental amalgam type was subje cted to a full potential traverse from -1, ,000 to +1,500 mv to determine the values of any dissolution potentials (anodic peaks,) that occurred. These anodic peaks are regions. of increased current flow associated with (etching at those potentials. Each anodic peak repre' sents a discrete phase within the set arn ialgam. Preliminary investigations reveale dh that 10% tartaric acid might be a suitable etchant. With fresh solutions, a highly ppolished specimen then was polarized from 600 mv to the potential of the first anodic (dissolution peak. Etching potential sweep 5s were started at -600 mv rather than -1,004 0 mv to avoid excessive hydrogen gas accumlulation .:-,I on the electrode surface. The potentt'al was maintained at this value, and the curryrent decay followed for five minutes while ietching proceeded. Under potentiostatic con trol of the working electrode, the current fol]lows an exponential decay law.' The specimLen was removed from the cell, rinsed with dlistilled water followed by methanol, and finally dried in warm air. The specimen th en was examined with a microscope.C Each specimen was repolished to a lutm diamond finish, and the same procediure was performed for each successive anodiic peak potential. mv,

J Dent Res March-April 1974 Phases representing 7yl 72' and Y1 + 72 in set dental amalgam matrix were prepared by reacting stoichiometric quantities of Hg with the Analar grade metal powders. These also were subjected to the same full potential traverse to determine the potentials for dissolution peak maximums. Copper amalgamd and as-cast dental alloy (Ag3Sn + Cu + Zn) as well as the pure metals, Ag, Sn, Cu, and Hg also were polarized in a 10% tartaric acid electrolyte. Ag3Cu eutectic wiree was polarized similarly. This work was done to relate the dissolution peak potentials that occurred in the various dental amalgams with the peak potentials of the matrix phases and constituent metals present in dental amalgam. Results The polarization curves for Aristaloy (Ar), Kerr's Spher-a-cap (K), and Dispersalloy (D) amalgams in 10% tartaric acid solution are shown in Figures 2 to 4. Typical microstructures produced by etching at the anodicpeak potentials in the polarization curves are shown in the photomicrographs (Figs 5 to 7)Etching at the anodic peak potentials yielded characteristic micrographs for the three alloys. The peak potentials for the three amalgams tested are given in Figure 8. The observed peak potentials of the various phases and pure metals present in dental amalgam after anodic polarization in tartaric acid also are shown.

-

c Reichert MeF2, Vienna, Austria.

Reichert

Optische Werke

AG,

d Ash Globe, Amalgamated Dental Trade Distributors,

London, Eng. e Johnson Matthey Metals Ltd., London, Eng. 4 +

n

FIG 4.-E/i polarization curve for Dispersalloy in 10% tartaric acid solution.



Vol 53 No. 2

471

-5*- , ~ '0''

0

*~~~~~~~~~~~~~~~~~~~~~~*A

FIG 7.-Dispersalloy etched to peak 4 (orig

FIG 5.-Kerr Spher-a-cap etched to peak 2

(orig mag x 640, reproduced at 71%)

mag x 480, reproduced at

.

Discussion

The polarization curves show that Ar and K have one major anodic dissolution peak (Fig 2, peak 1 and Fig 3, peak 1) at a similar potential, -70 to -60 mv in 10% tartaric acid solution. The current density at this potential is greater with the Ar amalgam. Mercury, 72 and -y + 72 dissolution peaks occur within this range (Fig 8), which suggests that the reaction associated with peak 1 of these two amalgams is attack on the HgSn 72 phase. Areas of attack within the matrix, presumably on the 72 regions, as well as some initial attack within the y particle, were detected in the etched amalgam surface. The etching of the y phase becomes slightly more pronounced at the nobler potential of peak 2 and corresponds to attack on Sn in the y phase. The differences in etch potential arise because certain metals ap-

71%) .

pear to be more active when amalgamated with Hg than when present as either pure metals or as alloys.14 The polarization behavior of D amalgam, when compared with the data in Figure 8, indicates that differences exist between it and the Ar and K amalgams. Peak 1 in Figure 4 occurred at a potential of -120 mv that coincides, approximately, with the anodic dissolution potential of Hgrich Sn. However, from the magnitude of current density, there appears to be little SnHg 72 phase present. This supports the evidence of Mahler.12 An inner reaction ring within the Ag-Cu eutectic particle was detected in the microstructure of the etched surface and suggests that a complex reaction is occurring. Figure 4 shows that the peak 2 potential for D amalgam coincides with the peak 2

14' K

D *,,

iSf N

K1

-0 goe;: ~ ~ ~ ~ ~~~~~~~~~~~~~~~~~~~~

... tS

1

Hg

-4400

a,

0300

-200

-100

0

*+gg*v

n-ba *400 * 1 200 +300 1400 +500 1100 +200

iSCE

(MV)

H

61 + sZ21 Ag3Sn- Cu -Zn Ag, Cth Sn

Cu

An r

Ag

if

FIG 6.-Dispersalloy etched to peak 2 (orig mag x 480, reproduced at 71%).

K--

FIG 8.-Potentiostatic polarization in 10% tartaric acid solution. Top, peak potentials of amalgam; bottom, constituent phases and pure metals.


472

J Dent Res March-April 1974

STAHELI AND VON FRAUNHOFER TABLE AMALGAM ALLOYS

Alloy

Code

Hg-Alloy Ratio

Aristaloy

Ar

6:5

Trituration Time (sec)

Manufacturer

10

Engelhardt Industries,

Baker Platinum Div., Sutton, Surrey, Eng

Kerr Spher-a-cap

K

4.8:5

7

Dispersalloy

D

5:4

20

Kerr Dental Mfg. Co., Romulus, Mich, USA Western Metallurgical Ltd.,

Canada

potential for Ar and K amalgams. The reaction is that of attack on Sn in the -y phase with some contribution from attack on Sn in the quasi-Cu-Sn-Hg phase (Figs 6, 7). A greater amount of this constituent is present in D amalgam than in Ar and K amalgams and the current density at this potential is correspondingly greater. Peak 3 for D amalgam appears to correspond to attack on pure Cu (Fig 8), whereas peak 4, which is unique, may indicate attack on pure Ag or on the yl, phase. This peak is not found for the other two amalgams which both contain the Fy, phase. Therefore, this peak must be associated with a major phase present in D amalgam, but this phase is of only minor importance, if present at all, in Ar and K amalgams. The microstructure of D amalgam after etching at peak 4 potential (Fig 7) shows evidence of attack on the matrix and the eutectic particles. This peak would appear to be associated with attack on Ag but not on the 7y phase. Consequently, the set amalgam might contain some residual Ag particles, the surfaces of which have reacted previously to form the yl phase. This would account for the low 72 phase content of the set amalgam. It should, however, be noted that the CuSn-Hg phase mentioned here is not a true phase per se. Electron microprobe analysis indicated that there is segregation of Cu and Sn at the periphery but slightly within the particle of an aged Ar amalgam.'5 The Hg is associated with this segregated Cu-Sn mixture but an Ly line scan for Hg clearly indicated that the Hg is "lining" the segregated metals and is not actually amalgamated with them, as occurs, for example, with the yl and 72 phases. Similar electron microprobe analysis evi-

dence has been presented, although not commented upon, by Johnson16 whose work revealed a pattern of Sn-Cu-Hg segregation in the Ag3Cu particles of D amalgam. Futhermore, the presence of two Sn-etching peaks for D amalgam, the second greater than the first, is additional evidence for the presence of Sn in some form of alloy or mixture with Cu but which is not directly associated with Hg.

Conclusions Preliminary studies using potentiostatic polarization and a tartaric acid electrolyte showed that the 72 phase in Ag-Sn dental amalgam may be characterized by an electrochemical technique. The potentiostatic polarization technique as a method of controlled etching is valuable regardless of the number of phases present. By this method both the rate and degree of etching can be controlled. Although chemically etched specimens may reveal a differential rate of etching across the diameter of the grain, this will not occur with electrolytic etching because of the exponential decay law followed by the etching current. Potentiostatic etching may be regarded as a useful and rapid technique for characterizing the constituent phases of dental amalgams. The use of electrolytes other than tartaric acid should permit the elucidation of even greater structural information. References 1. VON FRAUNHOFER, J.A., and BANKS, C.H.: Potentiostat and Its Applications, London:

Butterworths, 1972. 2. SMITH, D.L.; FERGUSON, G.W.; and SCHOONOVER, I.C.: Microstructure of Dental Amalgams, JADA 47: 305, 1953. 3. WING, G.: The Metallography of Dental


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4. 5.

6.

7. 8.

9.


Amalgam, DDSc thesis, University of Sydney, 1961. ALLAN, F.C.; AsGAR, K.; and PEYTON, F.: Microstructure of Dental Amalgams, J Dent Res 44:1002, 1965. WILLIAMS, D.W., and VON FRAUNHOFER, J.A.: An Interim Report on an Electrochemical Method of Investigating Stainless Steel Orthodontic Wires, Dent Pract Dent Rec 22: 150, 1971. YONDELIS, W.V., and INNES, D.B.K.: Dispersion Strengthened Amalgams, J Can Dent Assoc 22: 587, 1963. JOHNSON, L.B., JR.: A New Dental Alloy, IADR Program and Abstracts of Papers, No. 23, 1971. WATERSTRAT, R.M., and LONGTON, R.W.: Gallium-palladium Alloys as Dental Filling Materials, Public Health Rep 79: 638, 1964. PAFFENBARGER, G.C.: Recent Advances in USA Research on Dental Amalgams and Possible Applications, Int Dent J 16: 450, 1966.

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10. WING, G., and RYGE, G.: Reaction of Ag-Sn Alloys and Mercury, J Dent Res 44: 701, 1965. 11. JORGENSEN, K.D.: The Mechanism of Marginal Fracture of Amalgam Fillings, Acta Odontol Scand 23: 347, 1965. 12. MAHLER, D.: Microprobe Analysis of a Dispersant Amalgam, IADR Program and Abstracts of Papers, No. 14, 1971. 13. VON FRAUNHOFER, J.A., and STAHELI, P.J.: Gold-Amalgam Galvanic Cells, Br Dent J 132: 357, 1972. 14. VON FRAUNHOFER, J.A., and STAHELI, P.J.: Corrosion of Dental Amalgam, Nature (Lond) 240: 304, 1972. 15. STAHELI, P.J.: The Physical and Electrochemical Properties of Dental Amalgam Surfaces in Natural and Artificial Saliva, PhD thesis, University of London, 1973. 16. JOHNSON, L.N.: Phase Discrimination Using Scanning Electron Microscopy Techniques, J Dent Res 51: 789, 1972.
