Journal of Oral Rehabilitation, 1975, Volume 2, pages 157-164
Electrochemical properties of copper and gold containing dental amalgams
N. K. S A R K A R * OT^/ E. H. G R E E N E R Illinois 606] \
Northwesiern Universitv, Chicago,
Summary
Alloying or admixing of a dental amalgam alloy with an element or alloy which has a higher affinity for tin ofifers a means for eliminating the corrosion prone yz phase. Accordingly, the corrosion behaviour of non-conventional amalgams containing Cu and Au were investigated by anodic polarization studies in Ringer's solution. The Cu containing amalgams were produced by admixing Cu-Hg (copper amalgam) with conventional amalgam in the ratios of 0-5 : I and I : 1. A 10 w/o Au in Ag3Sn was used as the gold amalgam alloy. The anodic polarization profile of the 0-5 : I Cu admixed was similar in shape to conventional amalgams although current densities were less. After ageing for I month the —250 mV peak associated with yi disappeared and the current density decreased by ten fold. After 2 months the current density decreased by one hundred fold. The anodic polarization of the I : 1 Cu admixed amalgams did not display the —250 mV (y2) peak after 1 week and current densities were 10^ below that of conventional amalgams. The behaviour of these interesting amalgams is rationalized on the basis of the ease of formation of Cu-Sn intermelallics where Cu/Sn ^ 3 . The anodic polarization profile of 10 w/o Au containing amalgam essentially is that of conventional amalgam and does not change drastically after 6 months. Although the Au may reduce or eliminate the y2 phase the resulting AuSn4 apparently corrodes in an analogous fashion. However, since distribution of AuSn4 may be different from 72, i.e. not continuous, the effect of corrosion on its mechanical properties may be different. Introdnction
Potentiostatic polarization measurements (Sarkar & Greener, 1975a, b; Sarkar, 1973) have indicated that at the in vitro corrosion potential of commercial dental amalgams in a physiological sahne solution, both SnTHg (72) and CueSn,'^ are passive whereas the other components, viz. Ag3Sn {y), Ag2Hg3+Sn (yi + Sn), Ag3Cu, CugSn, Cu-Hg are immune. Tin dissolved in the silver-mercury phase also apparently has little, if any, adverse effect on the corrosion behaviour of amalgams (Sarkar & Greener, 1974). It has also been established that the tin-mercury (72) phase, though passive, * Presently with Kerr Manufacturing Company, Romulus, Michigan 48174. Correspondence: Professor Evan H. Greener, Department of Biological Materials, Northwestern University, The Dental School, Ward Memorial Building, 3U E. Chicago Avenue, Chicago, HI. 60611, U.S.A. 157
158
A'. K. Sarkar and E. H. Greener
impairs the corrosion resistance of dental amalgams since the breakdown potential of this phase is not far away from the corrosion potential of the dental amalgam (Sarkar & Greener, 1974a, b). Moreover, the corrosion behaviour appears to be adversely affected by the presence of copper-tin intermetallics so long as the copper/tin atom ratio is less than three. At copper/tin ratio~3, the corrosion behaviour, however, does begin to improve. This is well demonstrated by the polarization behaviour of Dispersalloy* (Sarkar & Greener, 1974b; Sarkar, 1973), where the initially formed Cu-Sn complex (Cu/Sn < 3) undergoes solid state transformation with time, attaining a Cu/Sn > 3 with a considerable improvement in corrosion resistance. Thus, alloying (or admixing) of a dental amalgani alloy with an element (or alloy) which has a higher affinity for tin offers a means to eliminate the weak, corrosion-prone 72 phase, ln the Dispersalloy (lnnes & Youdelis, 1963), this has been achieved by the incorporation of a silver-copper etJtectic in the conventional silver-tin alloy. The addition of Pd in the dental amalgam alloy has been suggested earlier by Jorgensen & Saito (1970) to achieve this end. Recently, Johnson (1971) reported the virtual absence of the y-i phase in a 10 w/o gold containing silver-tin amalgam. The absence of the y-z phase in a high copper containing silver-tin amalgam is also reported by Sarkar & Greener (1972). The present study was undertaken to evaluate the corrosion resistance of several non-traditional dental amalgams in an efforl to understand the basic metallurgical factors in the design and development of a y'2'free dental amalgam. Materials and methods The fabrication of conventional! and gold containing amalgamj test samples was done by triturating 0-6 g of the respective amalgam alloys with 0-9 g of mercury§ in a mechanical amalgamatorll for a time period of 15 s. Cylindrical specimens of 4 mm diameter were obtained by condensing the triturated mass with a spring loaded condenser.!! The residual mercury content of the amalgams was about 50*0%. The nominal composition of the admixed copper amalgams used in this investigation is shown in Table 1. The admixed copper amalgam (I) was fabricated as follows: 0-2 g of copper amalgam** was heated in a test tube over a gas-air flame until the mercury started beading on the surface. It was then triturated for 15 s in a Wig-L-Bug. Table I. Nominal eomposilion* of admixed copper amalgams Amalgam
Silver
T.r,
Admixed copper amalgam (I) Admixed copper amalgam (2)
28 0 2}-34
100 8 33
Copper
Zinc
Mercury
n-6
0-4 0 33
50 0 50 0
18 0
• Composition by weight per cent. * Unitek Corporation, Monrovia, California. t New True Dentalloy, S. S. White Corporation, Philadelphia, Pa. X Developed by Dr L. B. Johnson, Jr, at the University of Virginia, Charlottesville, Virginia, alloy composition—Ag 64-0%, Sn 26-0%, Au 10-0%. ijD. F. Goldsmith Chemical and Metal Corporation, Chicago, Illinois. !• Wig-L-Bug, Crescent Dental Manufacturing Company, Chicago, Illinois. *l Presto amalgamator, McShirley Products, GiendaJe, California. *• Copper amalgam, J. Bird Moyer Co., Philadelphia, Pa., composition—Cu 48-68%, Hg 51-32%.
Amalgams containing copper and gold
159
in a separate procedure, 0-4 g of a conventional amalgam alloy* with 0-6 g of mercury was triturated for 15 s. The resultant plastic masses obtained from the above two procedures were placed together in a single capsule, triturated for an additional 15 s and condensed as before. The fabrication of admixed copper amalgam (2) was done as described above, but the ratio of copper amalgam to conventional alloy was 1 : 1 , instead of 0-5 : 1, as in admixed copper amalgam (1). The corrosion study was carried out with amalgam samples aged at room temperature for different time periods between 1 day and 1 year. Potentiostatic anodic polarization measurement has been utilized to characterize the corrosion behaviour of the amalgams in a stagnant Ringer's solution at 37"C. The details of the technique have been reported elsewhere (Sarkar & Greener, 1973). The standard surface preparation consisted of a metallographic polish through 4/0 emery paper, ultrasonic cleaning and washing with distilled water immediately before electrochemical study. Results Admixed copper amalgams The corrosion potentials of the admixed copper amalgams aged between 1 day and I year are shown in Table 2. Al! potentials reported in this investigation are with respect to a saturated calomel electrode (SCE). Table 2 also includes the corrosion potentials Table 2. Corrosion potentials of cominercial and experimental amalgams in Ringer's solution
Amalgams
Age
Admixed copper amalgam (I)
1 day 1 month 2 months I day 1 week 2 weeks I month 1 year 1 day 1 month 2 months 6 months 1 day I year
Admixed copper amalgam (2)
Gold-containing amalgam
Conventional amalgam (New True Den tal loy)
J h Corrosion potential {mV/Vs. SCE) -450 -300 - 260 —425 -400 -275 —240 -200 -475 -460 -450 -450 -450 -430
of a conventional amalgam. It is obvious from Table 2 that the admixed copper amalgams became less active with ageing even though the I-day-oId admixed copper amalgams showed almost the same corrosion potential as that of a conventional amalgam. The potentiostatic anodic polarization of the l-day-o!d admixed copper amalgams are shown in Fig. 1 along with that of a conventional amalgam. The admixed copper amalgam (1) showed a current-potential profile similar to that of a conventional * Ultra Brand alloy (72 0% silver), Jalinek Alloys Division, Long Island, New York.
160
N. K. Sarkar and E. H. Greener
Fig. 1. Potentiostatic anodic polarization behaviour of the admixed copper amalgams. • Admixed copper amalgam (I); x — - — x admixed copper amalgam (2); O^ -O conventional amalgam (New True Dentalloy). amalgam. The current maximum for this amalgam at —250 mV was 1 -9 x 10"^ mA/cm^ compared to I-7m.A/cm'^ of a conventional amalgam at the same potential. The current density in the potential range of —200 and — 100 mV was ca. 6 x 10"^ mA/cm^ compared to 3 x 10 ' mA/cm'^ of a conventional amalgam in that zone. There was no similarity in the current potential profile of the I-day-old admixed copper amalgam (2) with that of a conventional amalgam. At all potentials between the corrosion potential and —50 mV, the anodic current density was lower than that observed in admixed copper amalgam (I). No current maximum at — 250mV was indicated. A small peak at — 150 mV was observed corresponding to a current density of 4-4x iO~^ mA/cm2. There was a reduction in current density at - 100 mV, above which current continued to increase with polarization. The anodic polarization behaviour of the admixed copper amalgam (1) as a function of ageing is shown in Fig. 2. The peak in current density at —250 mV observed in the I-day-old amalgam was absent in the amalgams aged for 1 month and longer. Also, the current density for the aged amalgams between the corrosion potential and — 100 mV was about one order of magnitude less than that of a 1-day-old sample. The conventional amalgam did not indicate any effect as a result of ageing. The anodic polarization behaviour of the admixed copper amalgam (2) as a function of ageing is shown in Fig. 3. This amalgam also showed a shift of the current potential curve to the left on the current axis with ageing. However, not much shift in the curve was observed when the amalgams were aged longer than 2 weeks. Gold containing amalgams The corrosion potential of the gold containing amalgam was close to that of a conventional amalgam (Table 2). Like the conveational amalgam, no significant change in the corrosion potential of the gold containing amalgam was noticed as a function of ageing. The anodic polarization behaviour of the experimental gold containing amalgam (Fig. 4) was very similar to that of a traditional dental amalgam (Fig. 1). During
..imatgams containing copper and gotd
161
Admix •d copper a ™Hom(ll
-400
V \
a. -200
0
o-C 1-200
Ig. 2. Potentiostatic anodic polarization behaviour of admixed copper amalgam (I) as a function of geing. Age; 1 day; x x 1 month; O O 2 months.
1 Adrr^xed copper oma gar r > ( 2 )
-400
• \
>-200 E
a
0
V >
9
+ 200
1 'Z.
1
1 1
1
1
1
1 1 1
III
1
I I
1
1 1 M 1
Fig, 3. Potentiostatic anodic polarization behaviour of admixed copper amalgam (2) as a function o! ageing. Age, I day; x x 1 week; O O 2 weeks; 0 0 1 month; 7 7 1 year.
anodic polarization the current density reached a maximum of 2 5 mA/cm^ at —250 mV where a transition occurred. With further polarization there was a slight increase in current density followed by a reduction in current density. With ageing for 6 months, the peak current density at —250 mV was reduced from 2-5 x IO^mA/cm^toS-S x ]0~i mA/cm^. This effect of ageing was quite negligible compared to that observed in admixed copper amalgams (Figs. 2 and 3),
162
A'. K. .Sarkar and E. H. Greener
Fig. 4. Potentiostatic anodic polarization behaviour of the gold containing amalgam as a function of ageing. Age. . , / 1 day; y, —OJ 1 month; x ~-x 6 months.
Discussion Admixed copper amalgams The shift of the anodic polarization curve of the admixed copper amalgams with the increasing amount of copper in a conventional dental amalgam (Fig. I) can best be rationalized in the context of the microstructural constituents. On the basis of the theory of the electrochemical dissolution of a multiphase alloy, it has been shown elsewhere (Sarkar & Greener, 1975a; Sarkar, 1973; Steigerwald & Greene, 1962) that the upper part of the anodic pofarization curve (Fig. !) of a conventional dental amalgam is associated with the dissolution behaviour of Sn7Hg {y^) and CugSn^, whereas the lower part is related to the presence of Ag2Hg3 (yi) and Ag3Sn (y). The lower part of the curve can also be related to Cu3Sn, Cu-Hg if they are present in the amalgam. X-ray diffraction analysis by Sarkar & Greener (1972) has indicated the absence of the y-^ phase and the presence of a copper-tin intermetallic (CugSn) in admixed copper amalgam (2). Thus, the absence of the current peak at around — 250 mV in the anodic polarization curve of this amalgam corroborates the fact that neither y-2 nor CugSn^ is present. The peak in current density observed at —250 mV in admixed copper amalgam (1) (Fig. I) appears to be due to both yo and CueSns. The magnitude of the peak current density of l-9x 10"^ mA/cm- compared with 1-7 x 10^^ mA/cm^ of a conventional amalgam suggests that the amount of 72 and CUSSHS in the former amalgam is much less. The peak at - 150 mV in admixed copper amalgam (2) is perhaps associated with the formation of a film of Cu^O (Sarkar & Greener. 1975a; Sarkar. 1973). The increase in current for all these amalgams above -100 mV may be due to the formation of AgCI, Hg2Ci3. CuCt and CuO (Sarkar & Greener, 1975a; Sarkar, (973). Since anodic current is a measure of the dissolution tendency of an alloy, a comparison of the anodic polarization behaviour of conventional and high copper amalgams (Fig. 1) suggests that the incorporation of copper in a conventional amalgam in increasing amount does result in improved amalgams from the corrosion standpoint.
Amalgams containing copper and gold
163
fhis improvement clearly results from the partial or complete elimination of 72 and Cu6Sn5, depending on the amount of copper. The improved clinical performance of admixed copper amalgams has been reported (Marker, 1969). Ageing. The ennoblement of the corrosion potential of admixed copper amalgam (I) (Table 2) as well as the shift of the anodic polarization curve (Fig. 2) indicates that the corrosion resistance of this amalgam is improved with ageing. No attempt was made to identify the phases appearing during ageing. However, on the basis of the behaviour of the dispersed phase amalgam (Sarkar & Greener, 1975a, b; Sarkar. 1973). it can be reasonably stated that the solid state reaction involves the complete or partial elimination of the 72 phase and gradual conversion of the corrosion-prone Cu-Sn (Cu/Sn < 3) phase to a more noble one (Cu/Sn > 3). The improvement in corrosion resistance of the admixed copper amalgam (2) with ageing is indicated in the corrosion potentials (Table 2) as well as in the anodic polarization curves (Fig. 3). Since it is established before that there is no yo or CuoSns phase in this amalgam, it does appear that perhaps the initially formed Cu-Sn intermetallic (Cu/Sn > 3) is undergoing compositional change, enriching itself with more copper available from Cu-Hg incorporated during fabrication. However, this compositional change is almost complete within 2 weeks, as is evident from the corrosion potentials and the anodic polarization curves of the amalgam aged between 2 weeks and 1 year. Gold containing amalgam The presence of 72 and AuSn4, along with y and yi, has been identified in the l-day-old gold-containing amalgam (Johnson, 1971; Waterstrat, 1972). It has also been claimed that the amount of the yt present is small, although the quantitative estimation is not available in the literature. Besides 72, Ag3Sn, the other tin containing phase present in the gold-containing amalgam is AuSn.i, On the basis of previous hypotheses (Sarkar &Greener, 1975a, b; Sarkar, 1971)itcan be reasonably assumed that the upper half of the polarization curve (Fig. 4) can be linked with the dissolution characteristics of both AuSnii and y-y, the major contribtition being from the phase AuSnj, since the amount of 72 is very small. The anodic polarization behaviour, in general and in particular, the peak current density at -250mV when compared to that of a conventional amalgam (Fig. I), indicates they are almost identical suggesting that no improvement in corrosion resistance is obtained with the addition of gold. Considering the amount of 72 which is negligibly small, it appears that the tin in tin-rich AuSn3 is afl"ected in a saline solution the same way as does tin in the tinmercury (72) phase of the conventional amalgam. However, because AuSn^i is not a continuous phase the role of its corrosion upon the mechanical properties of Au containing amalgam (particularly marginal integrity) may be greatly different than the effect of corrosion of ys. Ageing. Solid state transformation in the gold-containing amalgam is not markedly displayed by the change in corrosion potentials (Table 2). However, the shift of the peak current density at -250 mV from 2'5 x 10" to 8-8 x lO-i mA/cm^ with ageing for 6 months (Fig. 4) indicates that such a process is operative. This is in agreement with the work of Johnson (1971) where it is reported that the negligible amount of 72 phase initially formed continuously decreases for about 2 weeks at body temperature or nearly a month at room temperature. However, the AuSn4 intermetallic being equally corrosion prone, the elimination of the yi phase does not result in appreciable reduction in current density and, conse-
164
N. K. Sarkar and E. H. Greener
quently, corrosion. The results of the present work are in agreement with the recei study of Young, Finger & Ross (1973) that both conventional and gold-containin;i amalgani perform in the same way in the oral environment. Conclusions
It has been established through potentiostatic anodic polarization measurements that the addition of copper in the conventional dental amalgam improves its chloride corrosion resistance to a great extent by the complete or partial elimination of the y-i phase and the formation of a copper-tin intermetallic which has a Cu/Sn ratio of greater than 3. Such systems have another advantage from the viewpoint of galvanic interaction between different phases in a composite amalgam. The presence of a large potential gradient (Sarkar & Greener, 1975a; Sarkar, 1973) between different phases in a conventional amalgam leads to higher current flow and rapid dissolution of the yz phase. In admixed copper amalgam systems containing no y^ and a copper-tin complex (Cu/Sn > 3) the amalgam surface is about equipotential since the potential of the amalgam is around —200 mV and the potentials of the component phases (Sarkar & Greener, 1975a; Sarkar, 1973) are close to -200 mV. Consequently, deterioration from galvanic interaction is considerably reduced. It has also been shown that the chloride corrosion resistance of a conventional silver-tin amalgam does not appear to be improved by alloying with gold. This is explained to be due to the presence of a corrosion-prone, tin-rich gold-tin intermetallic (AuSn4) even though the presence of tin-mercury (72) is suppressed or eliminated. References INNLS, D.B.K. & YoimtiLis, W.V. (19(..^J Dispersion strengthened amalgams. Journal of the Canadian Dental Association, 29, 587. JOHNSON, L . B . JR (1971) A new dental alloy. lADR Program ami Abstracts of papers No. 23. J0RGENSEN, K.D. & SAITO, T . (1970) Structure and corrosion of dental amalgams. Acta odontologica scandinewica, 28, 129. MARKER, B . D . (1969) Northwestern University Dental School. Personal communication. SAHKAR, N.K. (1973) Electrochemical behavior of dental amalgams and their component phases. Ph.D. iliesix. Northwestern University. SARKAR, N.K. & GREENER, E.H. (1972) Absence of the ya phase in amaJgams with high copper concentrations. Jowrw/Z^j/Dcw/o/^MfarrA, 51, 1511. SARKAR. N.K. & GREENER, E.H. (1973) In iliro corrosion resistance of new dental alloys. Biomaterials. Medical Devices and Artificial Organs. 1, 121. SARKAR. N.K. & GREENER, E.H. (1974) Corrosion behavior of the tin containing silver-mercury phase of denta) amalgam. Journal of Dental Research, 53, 925. SARKAR, N.K. & GREENER, E.H. (1975a) Electrochemistry of the .saline corrosion of conventional dental amalgams. Journal of Oral Rekahililation, 2, 49. SARKAR. N.K. & GREENER, E.H. (1975b) In vitro chloride corrosion behyviour of Dispersalloy. Journal of Oral Rehahilitation, 2, 139. STEIGERWALD, R.F. & GREENE, N . D . (J962) The anodic dissolution of binary allays. Journal of the Electrochemical Society, 109, 1026. WATERSTRAT, R . M . (1972) X-ray and neutron diffraction study of amalgams with gold additions. I ADR Program ami Abstracts of Papers, No. 53. YOUNG, F.A., FINGER, W.W. & Ross, S.A. (1973) A clinical study of dental amalgam containing gold. lADR Program attd Abstracts of Papers, N o . 205.
Manuscript accepted 13 May 1974
Electrochemical properties of copper and gold containing dental amalgams
N. K. S A R K A R * OT^/ E. H. G R E E N E R Illinois 606] \
Northwesiern Universitv, Chicago,
Summary
Alloying or admixing of a dental amalgam alloy with an element or alloy which has a higher affinity for tin ofifers a means for eliminating the corrosion prone yz phase. Accordingly, the corrosion behaviour of non-conventional amalgams containing Cu and Au were investigated by anodic polarization studies in Ringer's solution. The Cu containing amalgams were produced by admixing Cu-Hg (copper amalgam) with conventional amalgam in the ratios of 0-5 : I and I : 1. A 10 w/o Au in Ag3Sn was used as the gold amalgam alloy. The anodic polarization profile of the 0-5 : I Cu admixed was similar in shape to conventional amalgams although current densities were less. After ageing for I month the —250 mV peak associated with yi disappeared and the current density decreased by ten fold. After 2 months the current density decreased by one hundred fold. The anodic polarization of the I : 1 Cu admixed amalgams did not display the —250 mV (y2) peak after 1 week and current densities were 10^ below that of conventional amalgams. The behaviour of these interesting amalgams is rationalized on the basis of the ease of formation of Cu-Sn intermelallics where Cu/Sn ^ 3 . The anodic polarization profile of 10 w/o Au containing amalgam essentially is that of conventional amalgam and does not change drastically after 6 months. Although the Au may reduce or eliminate the y2 phase the resulting AuSn4 apparently corrodes in an analogous fashion. However, since distribution of AuSn4 may be different from 72, i.e. not continuous, the effect of corrosion on its mechanical properties may be different. Introdnction
Potentiostatic polarization measurements (Sarkar & Greener, 1975a, b; Sarkar, 1973) have indicated that at the in vitro corrosion potential of commercial dental amalgams in a physiological sahne solution, both SnTHg (72) and CueSn,'^ are passive whereas the other components, viz. Ag3Sn {y), Ag2Hg3+Sn (yi + Sn), Ag3Cu, CugSn, Cu-Hg are immune. Tin dissolved in the silver-mercury phase also apparently has little, if any, adverse effect on the corrosion behaviour of amalgams (Sarkar & Greener, 1974). It has also been established that the tin-mercury (72) phase, though passive, * Presently with Kerr Manufacturing Company, Romulus, Michigan 48174. Correspondence: Professor Evan H. Greener, Department of Biological Materials, Northwestern University, The Dental School, Ward Memorial Building, 3U E. Chicago Avenue, Chicago, HI. 60611, U.S.A. 157
158
A'. K. Sarkar and E. H. Greener
impairs the corrosion resistance of dental amalgams since the breakdown potential of this phase is not far away from the corrosion potential of the dental amalgam (Sarkar & Greener, 1974a, b). Moreover, the corrosion behaviour appears to be adversely affected by the presence of copper-tin intermetallics so long as the copper/tin atom ratio is less than three. At copper/tin ratio~3, the corrosion behaviour, however, does begin to improve. This is well demonstrated by the polarization behaviour of Dispersalloy* (Sarkar & Greener, 1974b; Sarkar, 1973), where the initially formed Cu-Sn complex (Cu/Sn < 3) undergoes solid state transformation with time, attaining a Cu/Sn > 3 with a considerable improvement in corrosion resistance. Thus, alloying (or admixing) of a dental amalgani alloy with an element (or alloy) which has a higher affinity for tin offers a means to eliminate the weak, corrosion-prone 72 phase, ln the Dispersalloy (lnnes & Youdelis, 1963), this has been achieved by the incorporation of a silver-copper etJtectic in the conventional silver-tin alloy. The addition of Pd in the dental amalgam alloy has been suggested earlier by Jorgensen & Saito (1970) to achieve this end. Recently, Johnson (1971) reported the virtual absence of the y-i phase in a 10 w/o gold containing silver-tin amalgam. The absence of the y-z phase in a high copper containing silver-tin amalgam is also reported by Sarkar & Greener (1972). The present study was undertaken to evaluate the corrosion resistance of several non-traditional dental amalgams in an efforl to understand the basic metallurgical factors in the design and development of a y'2'free dental amalgam. Materials and methods The fabrication of conventional! and gold containing amalgamj test samples was done by triturating 0-6 g of the respective amalgam alloys with 0-9 g of mercury§ in a mechanical amalgamatorll for a time period of 15 s. Cylindrical specimens of 4 mm diameter were obtained by condensing the triturated mass with a spring loaded condenser.!! The residual mercury content of the amalgams was about 50*0%. The nominal composition of the admixed copper amalgams used in this investigation is shown in Table 1. The admixed copper amalgam (I) was fabricated as follows: 0-2 g of copper amalgam** was heated in a test tube over a gas-air flame until the mercury started beading on the surface. It was then triturated for 15 s in a Wig-L-Bug. Table I. Nominal eomposilion* of admixed copper amalgams Amalgam
Silver
T.r,
Admixed copper amalgam (I) Admixed copper amalgam (2)
28 0 2}-34
100 8 33
Copper
Zinc
Mercury
n-6
0-4 0 33
50 0 50 0
18 0
• Composition by weight per cent. * Unitek Corporation, Monrovia, California. t New True Dentalloy, S. S. White Corporation, Philadelphia, Pa. X Developed by Dr L. B. Johnson, Jr, at the University of Virginia, Charlottesville, Virginia, alloy composition—Ag 64-0%, Sn 26-0%, Au 10-0%. ijD. F. Goldsmith Chemical and Metal Corporation, Chicago, Illinois. !• Wig-L-Bug, Crescent Dental Manufacturing Company, Chicago, Illinois. *l Presto amalgamator, McShirley Products, GiendaJe, California. *• Copper amalgam, J. Bird Moyer Co., Philadelphia, Pa., composition—Cu 48-68%, Hg 51-32%.
Amalgams containing copper and gold
159
in a separate procedure, 0-4 g of a conventional amalgam alloy* with 0-6 g of mercury was triturated for 15 s. The resultant plastic masses obtained from the above two procedures were placed together in a single capsule, triturated for an additional 15 s and condensed as before. The fabrication of admixed copper amalgam (2) was done as described above, but the ratio of copper amalgam to conventional alloy was 1 : 1 , instead of 0-5 : 1, as in admixed copper amalgam (1). The corrosion study was carried out with amalgam samples aged at room temperature for different time periods between 1 day and 1 year. Potentiostatic anodic polarization measurement has been utilized to characterize the corrosion behaviour of the amalgams in a stagnant Ringer's solution at 37"C. The details of the technique have been reported elsewhere (Sarkar & Greener, 1973). The standard surface preparation consisted of a metallographic polish through 4/0 emery paper, ultrasonic cleaning and washing with distilled water immediately before electrochemical study. Results Admixed copper amalgams The corrosion potentials of the admixed copper amalgams aged between 1 day and I year are shown in Table 2. Al! potentials reported in this investigation are with respect to a saturated calomel electrode (SCE). Table 2 also includes the corrosion potentials Table 2. Corrosion potentials of cominercial and experimental amalgams in Ringer's solution
Amalgams
Age
Admixed copper amalgam (I)
1 day 1 month 2 months I day 1 week 2 weeks I month 1 year 1 day 1 month 2 months 6 months 1 day I year
Admixed copper amalgam (2)
Gold-containing amalgam
Conventional amalgam (New True Den tal loy)
J h Corrosion potential {mV/Vs. SCE) -450 -300 - 260 —425 -400 -275 —240 -200 -475 -460 -450 -450 -450 -430
of a conventional amalgam. It is obvious from Table 2 that the admixed copper amalgams became less active with ageing even though the I-day-oId admixed copper amalgams showed almost the same corrosion potential as that of a conventional amalgam. The potentiostatic anodic polarization of the l-day-o!d admixed copper amalgams are shown in Fig. 1 along with that of a conventional amalgam. The admixed copper amalgam (1) showed a current-potential profile similar to that of a conventional * Ultra Brand alloy (72 0% silver), Jalinek Alloys Division, Long Island, New York.
160
N. K. Sarkar and E. H. Greener
Fig. 1. Potentiostatic anodic polarization behaviour of the admixed copper amalgams. • Admixed copper amalgam (I); x — - — x admixed copper amalgam (2); O^ -O conventional amalgam (New True Dentalloy). amalgam. The current maximum for this amalgam at —250 mV was 1 -9 x 10"^ mA/cm^ compared to I-7m.A/cm'^ of a conventional amalgam at the same potential. The current density in the potential range of —200 and — 100 mV was ca. 6 x 10"^ mA/cm^ compared to 3 x 10 ' mA/cm'^ of a conventional amalgam in that zone. There was no similarity in the current potential profile of the I-day-old admixed copper amalgam (2) with that of a conventional amalgam. At all potentials between the corrosion potential and —50 mV, the anodic current density was lower than that observed in admixed copper amalgam (I). No current maximum at — 250mV was indicated. A small peak at — 150 mV was observed corresponding to a current density of 4-4x iO~^ mA/cm2. There was a reduction in current density at - 100 mV, above which current continued to increase with polarization. The anodic polarization behaviour of the admixed copper amalgam (1) as a function of ageing is shown in Fig. 2. The peak in current density at —250 mV observed in the I-day-old amalgam was absent in the amalgams aged for 1 month and longer. Also, the current density for the aged amalgams between the corrosion potential and — 100 mV was about one order of magnitude less than that of a 1-day-old sample. The conventional amalgam did not indicate any effect as a result of ageing. The anodic polarization behaviour of the admixed copper amalgam (2) as a function of ageing is shown in Fig. 3. This amalgam also showed a shift of the current potential curve to the left on the current axis with ageing. However, not much shift in the curve was observed when the amalgams were aged longer than 2 weeks. Gold containing amalgams The corrosion potential of the gold containing amalgam was close to that of a conventional amalgam (Table 2). Like the conveational amalgam, no significant change in the corrosion potential of the gold containing amalgam was noticed as a function of ageing. The anodic polarization behaviour of the experimental gold containing amalgam (Fig. 4) was very similar to that of a traditional dental amalgam (Fig. 1). During
..imatgams containing copper and gotd
161
Admix •d copper a ™Hom(ll
-400
V \
a. -200
0
o-C 1-200
Ig. 2. Potentiostatic anodic polarization behaviour of admixed copper amalgam (I) as a function of geing. Age; 1 day; x x 1 month; O O 2 months.
1 Adrr^xed copper oma gar r > ( 2 )
-400
• \
>-200 E
a
0
V >
9
+ 200
1 'Z.
1
1 1
1
1
1
1 1 1
III
1
I I
1
1 1 M 1
Fig, 3. Potentiostatic anodic polarization behaviour of admixed copper amalgam (2) as a function o! ageing. Age, I day; x x 1 week; O O 2 weeks; 0 0 1 month; 7 7 1 year.
anodic polarization the current density reached a maximum of 2 5 mA/cm^ at —250 mV where a transition occurred. With further polarization there was a slight increase in current density followed by a reduction in current density. With ageing for 6 months, the peak current density at —250 mV was reduced from 2-5 x IO^mA/cm^toS-S x ]0~i mA/cm^. This effect of ageing was quite negligible compared to that observed in admixed copper amalgams (Figs. 2 and 3),
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Fig. 4. Potentiostatic anodic polarization behaviour of the gold containing amalgam as a function of ageing. Age. . , / 1 day; y, —OJ 1 month; x ~-x 6 months.
Discussion Admixed copper amalgams The shift of the anodic polarization curve of the admixed copper amalgams with the increasing amount of copper in a conventional dental amalgam (Fig. I) can best be rationalized in the context of the microstructural constituents. On the basis of the theory of the electrochemical dissolution of a multiphase alloy, it has been shown elsewhere (Sarkar & Greener, 1975a; Sarkar, 1973; Steigerwald & Greene, 1962) that the upper part of the anodic pofarization curve (Fig. !) of a conventional dental amalgam is associated with the dissolution behaviour of Sn7Hg {y^) and CugSn^, whereas the lower part is related to the presence of Ag2Hg3 (yi) and Ag3Sn (y). The lower part of the curve can also be related to Cu3Sn, Cu-Hg if they are present in the amalgam. X-ray diffraction analysis by Sarkar & Greener (1972) has indicated the absence of the y-^ phase and the presence of a copper-tin intermetallic (CugSn) in admixed copper amalgam (2). Thus, the absence of the current peak at around — 250 mV in the anodic polarization curve of this amalgam corroborates the fact that neither y-2 nor CugSn^ is present. The peak in current density observed at —250 mV in admixed copper amalgam (1) (Fig. I) appears to be due to both yo and CueSns. The magnitude of the peak current density of l-9x 10"^ mA/cm- compared with 1-7 x 10^^ mA/cm^ of a conventional amalgam suggests that the amount of 72 and CUSSHS in the former amalgam is much less. The peak at - 150 mV in admixed copper amalgam (2) is perhaps associated with the formation of a film of Cu^O (Sarkar & Greener. 1975a; Sarkar. 1973). The increase in current for all these amalgams above -100 mV may be due to the formation of AgCI, Hg2Ci3. CuCt and CuO (Sarkar & Greener, 1975a; Sarkar, (973). Since anodic current is a measure of the dissolution tendency of an alloy, a comparison of the anodic polarization behaviour of conventional and high copper amalgams (Fig. 1) suggests that the incorporation of copper in a conventional amalgam in increasing amount does result in improved amalgams from the corrosion standpoint.
Amalgams containing copper and gold
163
fhis improvement clearly results from the partial or complete elimination of 72 and Cu6Sn5, depending on the amount of copper. The improved clinical performance of admixed copper amalgams has been reported (Marker, 1969). Ageing. The ennoblement of the corrosion potential of admixed copper amalgam (I) (Table 2) as well as the shift of the anodic polarization curve (Fig. 2) indicates that the corrosion resistance of this amalgam is improved with ageing. No attempt was made to identify the phases appearing during ageing. However, on the basis of the behaviour of the dispersed phase amalgam (Sarkar & Greener, 1975a, b; Sarkar. 1973). it can be reasonably stated that the solid state reaction involves the complete or partial elimination of the 72 phase and gradual conversion of the corrosion-prone Cu-Sn (Cu/Sn < 3) phase to a more noble one (Cu/Sn > 3). The improvement in corrosion resistance of the admixed copper amalgam (2) with ageing is indicated in the corrosion potentials (Table 2) as well as in the anodic polarization curves (Fig. 3). Since it is established before that there is no yo or CuoSns phase in this amalgam, it does appear that perhaps the initially formed Cu-Sn intermetallic (Cu/Sn > 3) is undergoing compositional change, enriching itself with more copper available from Cu-Hg incorporated during fabrication. However, this compositional change is almost complete within 2 weeks, as is evident from the corrosion potentials and the anodic polarization curves of the amalgam aged between 2 weeks and 1 year. Gold containing amalgam The presence of 72 and AuSn4, along with y and yi, has been identified in the l-day-old gold-containing amalgam (Johnson, 1971; Waterstrat, 1972). It has also been claimed that the amount of the yt present is small, although the quantitative estimation is not available in the literature. Besides 72, Ag3Sn, the other tin containing phase present in the gold-containing amalgam is AuSn.i, On the basis of previous hypotheses (Sarkar &Greener, 1975a, b; Sarkar, 1971)itcan be reasonably assumed that the upper half of the polarization curve (Fig. 4) can be linked with the dissolution characteristics of both AuSnii and y-y, the major contribtition being from the phase AuSnj, since the amount of 72 is very small. The anodic polarization behaviour, in general and in particular, the peak current density at -250mV when compared to that of a conventional amalgam (Fig. I), indicates they are almost identical suggesting that no improvement in corrosion resistance is obtained with the addition of gold. Considering the amount of 72 which is negligibly small, it appears that the tin in tin-rich AuSn3 is afl"ected in a saline solution the same way as does tin in the tinmercury (72) phase of the conventional amalgam. However, because AuSn^i is not a continuous phase the role of its corrosion upon the mechanical properties of Au containing amalgam (particularly marginal integrity) may be greatly different than the effect of corrosion of ys. Ageing. Solid state transformation in the gold-containing amalgam is not markedly displayed by the change in corrosion potentials (Table 2). However, the shift of the peak current density at -250 mV from 2'5 x 10" to 8-8 x lO-i mA/cm^ with ageing for 6 months (Fig. 4) indicates that such a process is operative. This is in agreement with the work of Johnson (1971) where it is reported that the negligible amount of 72 phase initially formed continuously decreases for about 2 weeks at body temperature or nearly a month at room temperature. However, the AuSn4 intermetallic being equally corrosion prone, the elimination of the yi phase does not result in appreciable reduction in current density and, conse-
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N. K. Sarkar and E. H. Greener
quently, corrosion. The results of the present work are in agreement with the recei study of Young, Finger & Ross (1973) that both conventional and gold-containin;i amalgani perform in the same way in the oral environment. Conclusions
It has been established through potentiostatic anodic polarization measurements that the addition of copper in the conventional dental amalgam improves its chloride corrosion resistance to a great extent by the complete or partial elimination of the y-i phase and the formation of a copper-tin intermetallic which has a Cu/Sn ratio of greater than 3. Such systems have another advantage from the viewpoint of galvanic interaction between different phases in a composite amalgam. The presence of a large potential gradient (Sarkar & Greener, 1975a; Sarkar, 1973) between different phases in a conventional amalgam leads to higher current flow and rapid dissolution of the yz phase. In admixed copper amalgam systems containing no y^ and a copper-tin complex (Cu/Sn > 3) the amalgam surface is about equipotential since the potential of the amalgam is around —200 mV and the potentials of the component phases (Sarkar & Greener, 1975a; Sarkar, 1973) are close to -200 mV. Consequently, deterioration from galvanic interaction is considerably reduced. It has also been shown that the chloride corrosion resistance of a conventional silver-tin amalgam does not appear to be improved by alloying with gold. This is explained to be due to the presence of a corrosion-prone, tin-rich gold-tin intermetallic (AuSn4) even though the presence of tin-mercury (72) is suppressed or eliminated. References INNLS, D.B.K. & YoimtiLis, W.V. (19(..^J Dispersion strengthened amalgams. Journal of the Canadian Dental Association, 29, 587. JOHNSON, L . B . JR (1971) A new dental alloy. lADR Program ami Abstracts of papers No. 23. J0RGENSEN, K.D. & SAITO, T . (1970) Structure and corrosion of dental amalgams. Acta odontologica scandinewica, 28, 129. MARKER, B . D . (1969) Northwestern University Dental School. Personal communication. SAHKAR, N.K. (1973) Electrochemical behavior of dental amalgams and their component phases. Ph.D. iliesix. Northwestern University. SARKAR, N.K. & GREENER, E.H. (1972) Absence of the ya phase in amaJgams with high copper concentrations. Jowrw/Z^j/Dcw/o/^MfarrA, 51, 1511. SARKAR. N.K. & GREENER, E.H. (1973) In iliro corrosion resistance of new dental alloys. Biomaterials. Medical Devices and Artificial Organs. 1, 121. SARKAR. N.K. & GREENER, E.H. (1974) Corrosion behavior of the tin containing silver-mercury phase of denta) amalgam. Journal of Dental Research, 53, 925. SARKAR, N.K. & GREENER, E.H. (1975a) Electrochemistry of the .saline corrosion of conventional dental amalgams. Journal of Oral Rekahililation, 2, 49. SARKAR. N.K. & GREENER, E.H. (1975b) In vitro chloride corrosion behyviour of Dispersalloy. Journal of Oral Rehahilitation, 2, 139. STEIGERWALD, R.F. & GREENE, N . D . (J962) The anodic dissolution of binary allays. Journal of the Electrochemical Society, 109, 1026. WATERSTRAT, R . M . (1972) X-ray and neutron diffraction study of amalgams with gold additions. I ADR Program ami Abstracts of Papers, No. 53. YOUNG, F.A., FINGER, W.W. & Ross, S.A. (1973) A clinical study of dental amalgam containing gold. lADR Program attd Abstracts of Papers, N o . 205.
Manuscript accepted 13 May 1974
