- Email: [email protected]
Corrosion and structure of a low-gold dental alloy
Corrosion and structure of a low-gold dental alloy
R. I. Holland, R. B. Jorgensen, H. Hero NIOM, Scandinavian Institute of Dental Materials, Oslo, Norway.
Holland RI, J0rgensen RB, Hero H. Corrosion and structure'of a low-gold dental alloy. Dent Mater 1986: 2: 143-146. Abstract - The relationship between structure and corrosion in vitro has been studied in a low-gold dental casting alloy. To obtain different structural conditions specimens were subjected to various heat treatments in evacuated quartz ampules. Four different structural states were tested, one of which was single-phased, the others were multi-phased. The specimens were corrosion tested both by potentiodynamic and potentiostatic polarization in artificial saliva. The single-phased specimen corroded the most, although it was the least discolored. The multi-phased structures exhibited less corrosion, but was more discolored. Pretreatment in 0.1 mM NazS for 21 h did not, for any of the samples, reduce the corrosion rate when tested by potentiodynamic polarization. This study indicates that discoloration due to corrosion (tarnish) and corrosion sensitivity are not related, i.e. tarnish testing cannot be used as a corrosion test, and vice versa.
The use of low-gold dental casting alloys, i.e. alloys with a content of metals of the platinum group lower than 75%, has increased during the last years. Generally, these alloys have a lower resistance to corrosion than the high-gold alloys. In the preparation-of cast dental restorations, heat treatment of the alloy is part of the procedure (casting, soldering, annealing, hardening). Such heat treatments may introduce various structural states and phases, some of which may have different corrosion potentials causing local galvanic corrosion effects and, thus, make the alloy particularly corrosion sensitive. The purpose of the present work was to study the effect of various heat treatments, known to introduce different structural states (1), on the corrosion of a low-gold dental casting alloy. Material and methods
Alloy. A low-gold dental casting alloy* was employed throughout this study. The spectrographically determined composition of the alloy is shown in Table 1. Casting conditions and equipment have previously been described (1). The heat treatments of the alloy and their resulting structures are summar* Midigold, BEGO, Bremer Goldschl~igerei, Wilh. Herbst GmbH.
Table 1. The composition of the applied alloy, Midigold (BEGO). Element Au Ag Cu Pd In
Weight-% 54.0 31.0 9.4 4.1 1.0
ized in Table 2. A detailed description has previously been published (1). Electrochemical cell. A closed glass flask fitted with an outer jacket for temperature control with water flow and with fittings for the electrodes, salt bridge, gas inlet/outlet and temperature measurement was used. The specimen electrode was mounted vertically 30 mm apart from the platinum counter electrode. The salt bridge connected the cell to a saturated calomel reference electrode (SCE)*. The electrolyte applied was an artificial saliva solution (Table 3) simulating the concentrations of ions in saliva. A fresh solution was made from stock solutions for each experiment. Stock solutions were stored in closed glass vessels at room temperature. Unless otherwise stated, the electrolyte was deaerated by Nz-bubbling, agitated with a magnetic stirrer, and kept at 37~ throughout the experiments. Beckman Instruments Inc., USA.
Key words: dental materials, corrosion, structure, alloy.
Roy I. Holland, NIOM, Scandinavian Institute of Dental Materials, Forskningsveien 1, 0371 Oslo 3, Norway.
ReceivedMay 30, 1985; accepted January 7, 1986.
Table 2. Heat treatments and resulting structures. As-cast a. Segregations of Cu and Au in the grain boundaries and between dendritic arms. b. Eutectic particles, 0.05 ~tm lamellae. c. A matrix split into 0.02 ~tmlamellae parallel with 100 planes. d. High density of very small fct ordered AuCuI particles. 700~ 10 min a. Segregations mainly as for the as-cast structure, but without eutectic particles. b. One fcc phase. 700~ 10 min/500~ 11 wk a. No segregations. b. Matrix split into three fcc phases (Ag, Cu and Pd-rich, respectively). 700~ 10 min/500~ 11 wk/350~ 8 h a. No segregations. b. Matrix split into three fcc phases (Ag, Cu and Pd-rich, respectively). c. Plates of the AuCuI phase 0.02 ~tm thick and with 0.2 p,m sides in the Au-Cu-rich grains, parallel to the 100 planes of the matrix (formed at 350~
Table 3. The basic constituents of the artificial saliva (pH 5.2-5.7). Compound
Amount
NaCI KCI CaCI2 - 2H20 NaHzPO4 - 2H20 Na2S 9xH20 (x=7-9) Urea H20
0.4 g 0.4 g 0.795 g 0.78 g 0.005 g 1g to 1 litre
144
Holland et al.
Cast specimens were embedded in plastics, polished with 1 Ixm diamond paste and washed in ethanol and distilled water less than 2 h before each experiment. The metal/plastic interface was insulated with a lacquer ~. Polarization testing. The specimens were subjected to in vitro potentiodynamic (continuous scan) polarization corrosion test scanned at a rate of 1 mV/s, usually from - 7 0 0 mV to approx. +800 mV (SCE). In addition, potentiostatic (step-scan) polarization testing, both anodic and cathodic, were done. The cathodic polarization was performed in air-bubbled electrolyte. Unless otherwise stated, the samples were equilibrated open circuit for 1 h before start of polarization. The polarization steps were 50 mV and the current recorded when an apparent steady level was reached, usually after 10-20 min. The polarization and the current measurements were done with a polarographic analyzer II and the potential and current recorded on a x-y recorder ~.
880mY
~
=
OmV
f
w
|
..................................................................................................................
-700mV -0.30
-0.15
|
|
0.15
0.30
mAcm -2
Fig. 1. Potentiodynamic polarization curves for the as cast condition of the alloy in artificial
saliva.
830mV
,
=
if -
Results
From the potentiodynamic polarization curves (Figs. 1--4) the structural state resulting from annealing only (700~ for 10 min) was discernable due to markedly increased anodic current density. However, it was the least discolored (tarnished) after the corrosion test. The microscopic examinations (both light and SEM) of the corroded surfaces did not indicate that any particular phase was imore prone to corrosion than the others. To give a numerical value for the difference in corrosion sensitivity of the different structural states in artificial saliva, their anodic charge was calculated by integrating the potentiodynamic polarization curve from the zero current potential (ZCP) to +200 mV SCE. These are shown in Table 4 together with the ZCP. The single-phased structure, made by heat treatment at 700~ only, is far the most corrosion sensitive, whereas the age-hardened are the least. In the potentiostatic polarization * Epofix, Struers, Copenhagen, Denmark. Plastik 70, Kontakt Chemie, Rastatt, Western Germany. tl Model 174A, Princeton Applied Research, NJ, USA. Model 7035B, Hewlett-Packard, CA, USA.
OmV
.........................................................
-700mY 20,30
-0.15
0.15
0.30
mAcm -=
Fig. 2. Potentiodynamic polarization curves for the single-phased structural state (heat
treated by 700~ for 10 rain before quenching) of the alloy in artificial saliva.
810mY
0mV
=
=
............................................................
] -700mV
I
=
-0.30
-0.15
~
I
I
O. 15
0,30
mAcm -2
Fig. 3. Potentiodynamic polarization curves for the structural state of the alloy resulting from
heat treatment at 500~ for 11 wk after homogenization at 700~
Corrosion and structure of low-gOld
145
780mV 100 0 -10O
LU
0mY
........................................................1 .........................................................
+
-200 -300 -400
CURRENT
-700mY
' -0.30
= -0.15
~2~
I 0.15
I 0.30
mAcro-2
Fig. 4. Potentiodynamic polarization curves for the structural state of the alloy resulting from
heat treatment after homogenization at 700~ at 500~ for 11 wk and 350~ for 8 h.
Table 4. Anodic charge (AC) from zero current potential to +200 mV (SCE) for the different structural states of the alloy in A): artificial saliva, and B): artificial saliva after pretreatment in 0.1 mM sodium sulfide for 21 hr,
A) B)
As cast
700~
1.1+0.3 (4)* 2.7+0.8 (3)
11.5+4.0 (4) 9.4_+4,2 (3)
700~176 0.6+0.2 (6) 1.5+0.5 (3)
9
t l A c m -e
100
700~176176 1.9• (5) 2.1+1.2 (3)
DENSITY
Fig. 6. Potentiostatic (cathodic) polarization curves for the different structural states of the alloy in artificial saliva, after pretreatment of the samples in 0.1 mM sodium sulfide for 21 h. The structural states are as cast (El), 700~ (0), 700~176 (A), and 700~176176 (V).
"---. -200
9 Mean + S.D. (no. of observations). CURRENT
testing (Fig. 5), both the anodic and the cathodic curves of the single-phased sample were different from those of the other 3 structural states. Based on the crossing points for the anodic and the cathodic polarization curves, the maximum current density at that potential can be at least twice as high (approx. 3 ~A/cm 2) for the single-phased sample
200
100 0 03
~
o
~
O
/ Z'"
~,',-2.%.._~
ku - 1 0 0 -200 -300
CURRENT
Fig. 5.
DENSITY
p A c m -2
Potentiostatic polarization curves (anodic and cathodic) for the different structural states of the alloy in artificial saliva. The structural states are as east (liD), 700~ (OO), 700~176 (A A), and 700~176176 (T ~). Filled symbols: anodic polarization, open symbols: cathodic polarization.
than for the other structural states (0.5-1.7 ~A/cm2). It was considered possible that the corrosion of the different structural states of the alloy in sulfide solution could reduce further corrosion by providing inhibitory layers of corrosion products on the surface and/or selective leaching leaving the surface more noble and, thereby, less corrosion prone (2). In order to check this possibility, samples were immersed in 0.1 mM sodium sulfide solution for 21 h prior to potentiodynamic polarization testing. No reduction in anodic charge from ZCP to +200 mV SCE could be detected for the pretreated samples (Table4). In the potentiostatic cathodic polarization testing, however, a substantial effect of pretreatment in 0.1 mM sulfide for 21 h was found (Fig. 6). The effect was greatest for the multi-phased structure, which also exhibited most tarnish due to the pretreatment, and least for the single-phased sample, which was the least tarnished. Based on the crossing points for the anodic and the cathodic polarization curves, the pretreatment reduced the maximum current density with approximately 50%, but with only 20% for the single-phased sample. The cathodic polarization curves of n o n -
D E N S I T Y p A c m -z
Fig. 7. Potentiostatic (cathodic) polarization curves for the different structural states of the alloy in artificial saliva, without the 1-h open circuit equilibration period, The structural states are as cast (C~), 700~ (9 700~176 (A), and 700~176176 (V).
pretreated samples and without the 1 h open circuit equilibration period, showed the least difference between the single- and the multi-phased states (Fig. 7).
Discussion It is a generally accepted concept that "single-phased (solid solution) alloys are more corrosion resistant than alloys with 2 (or more) phases, since the galvanic coupling effects are not present" (3). However, the results of the present study show that the alloy investigated did not behave accordingly. The singlephased structure exhibited the greatest corrosion, whereas all the multi-phased structural states presently tested showed far better corrosion resistance. In contrast, recording macroscopical discoloration of the alloy after the corrosion testing gave directly opposing results: the single-phased structural
146
H o l l a n d et al.
state was clearly less discolored (tarnished) than any of the others, which is in agreement with other reports (4, 5). This indicates that discoloration due to corrosion (tarnish) and corrosion sensitivity are at least not positively correlated, and that no conclusion should be drawn from one to the other of these parameters, which means that tarnish testing cannot be used as a corrosion test, and vice versa. More research is needed to establish whether there is an inverse relationship between tarnish and corrosion, as appears from the present study. Previous studies indicate both possibilities: A brass gold substitute** is reported to show very little discoloration, but high corrosion (6). In contrast, reports showing good relationship between corrosion rate and discoloration have also been published for another group of alloys, those consisting mainly of Ag and Pd (7). The possibility exits, that there is an inverse relationship between tarnish and corrosion sensitivity for those alloys and for those potential ranges where the corrosion rate is limited by the rate of the cathodic reactions. A hypothetical explanation for the lack of correlation between tarnish and corrosion in the present study may be as follows: In a solid-solution (singlephased) alloy the anodic and cathodic reactions occur uniformly spread throughout the corroding surface. This situation will disfavor any precipitation of corrosion products onto the surface, due to the continuous release of material anywhere on the surface. In a multi-phase alloy, the anodic and the cathodic reactions may be separated. It is likely that the different phases will have different nobility, i.e. different
** Progold, Birdsall Enterprises, Ballwin, MO, USA.
Ltd.,
corrosion potentials, and that a multitude of microscopical bimetal galvanic cells will be formed. In that case, anodic reactions will preferably take place on the least noble phase (lowest corrosion potential) and cathodic reactions on the most noble. This system will favor possible precipitation of corrosion products on the most noble phase(s), because they represent a stable surface, in that no or only very little material is released from it. The film so formed will constitute the tarnish of the alloy surface. Pretreatment of the samples in sulfide solution did not affect subsequent corrosion as tested by potentiodynamic polarization, i.e. no surface enoblement by dissolution of Cu or Ag seemed to have occurred (2). The pretreatment resulted in tarnish, particularly on the multi-phased structural states. Such a tarnish layer did not, however, reduce further anodic reaction (8). The single-phased structural state behaved differently from the multiphased states not only in anodic, but also in cathodic polarization. Also after pretreatment in sulfide, that difference persisted. The pretreatment affected the cathodic polarization curves of the multi-phased specimens, but not of the single-phased one. This is in agreement with the tarnish result; the surfaces of the multi-phased specimens are changed, i.e. a tarnish film is developed, whereas the surface of the singlephased specimen is unchanged and, thus, allows the cathodic reactions to continue undisturbed. However, this cannot be taken as a proof of corrosion resistance of the single-phased structure. Its appearantly unchanged surface
can be the result of a corrosion process continuously removing material from the surface while nothing is precipitated onto it. The tarnish film probably decreases the access of 02 to the metal surface, thereby diminishing the overall rate of the main cathodic reaction taking place; the reduction of 02. The difference among the non-pretreated samples is, therefore, probably caused by difference in tarnish film, made in the l h equilibration period prior to start of polarization. When this equilibration period was omitted, the difference in the cathodic polarization curves between the single- and the multi-phased structures was the least detected.
References 1. Hero H, J0rgensen R, S0rbr0den E. A low-gold dental alloy: Structure and segregations. J Dent Res 1982: 61: 1292-8. 2. Hultquist G, Hero H. Surface enoblement by dissolution of Cu, Ag and Zn from single phase gold alloys. Corros Sci 1984: 24: 789-805. 3. Fontana MG, Greene ND. Corrosion engineering. New York: McGraw-Hill Book Company, 1967: 26. 4. German RM. The role of microstructure in the tarnish of low-gold alloys. Metallography 1981: 14: 253-66. 5. Hero H, J0rgensen RB. Tarnishing of a low-gold dental alloy in different structural states. J Dent Res 1983: 62: 371-6. 6. Sarkar NK, Fuys RA, Stanford JW. Corrosion and microstructure of Progold. J Prosthet Dent 1978: 40: 50-5. 7. Ishizaki N. Corrosion resistance of Ag-Pd alloy system in artificial saliva: an electrochemical study. J Osaka Dent Univ 1969: 3: 121-33. 8. Holland RI. Effect of pellicle on galvanic corrosion of amalgam. Scand J Dent Res 1984: 92: 93-6.
R. I. Holland, R. B. Jorgensen, H. Hero NIOM, Scandinavian Institute of Dental Materials, Oslo, Norway.
Holland RI, J0rgensen RB, Hero H. Corrosion and structure'of a low-gold dental alloy. Dent Mater 1986: 2: 143-146. Abstract - The relationship between structure and corrosion in vitro has been studied in a low-gold dental casting alloy. To obtain different structural conditions specimens were subjected to various heat treatments in evacuated quartz ampules. Four different structural states were tested, one of which was single-phased, the others were multi-phased. The specimens were corrosion tested both by potentiodynamic and potentiostatic polarization in artificial saliva. The single-phased specimen corroded the most, although it was the least discolored. The multi-phased structures exhibited less corrosion, but was more discolored. Pretreatment in 0.1 mM NazS for 21 h did not, for any of the samples, reduce the corrosion rate when tested by potentiodynamic polarization. This study indicates that discoloration due to corrosion (tarnish) and corrosion sensitivity are not related, i.e. tarnish testing cannot be used as a corrosion test, and vice versa.
The use of low-gold dental casting alloys, i.e. alloys with a content of metals of the platinum group lower than 75%, has increased during the last years. Generally, these alloys have a lower resistance to corrosion than the high-gold alloys. In the preparation-of cast dental restorations, heat treatment of the alloy is part of the procedure (casting, soldering, annealing, hardening). Such heat treatments may introduce various structural states and phases, some of which may have different corrosion potentials causing local galvanic corrosion effects and, thus, make the alloy particularly corrosion sensitive. The purpose of the present work was to study the effect of various heat treatments, known to introduce different structural states (1), on the corrosion of a low-gold dental casting alloy. Material and methods
Alloy. A low-gold dental casting alloy* was employed throughout this study. The spectrographically determined composition of the alloy is shown in Table 1. Casting conditions and equipment have previously been described (1). The heat treatments of the alloy and their resulting structures are summar* Midigold, BEGO, Bremer Goldschl~igerei, Wilh. Herbst GmbH.
Table 1. The composition of the applied alloy, Midigold (BEGO). Element Au Ag Cu Pd In
Weight-% 54.0 31.0 9.4 4.1 1.0
ized in Table 2. A detailed description has previously been published (1). Electrochemical cell. A closed glass flask fitted with an outer jacket for temperature control with water flow and with fittings for the electrodes, salt bridge, gas inlet/outlet and temperature measurement was used. The specimen electrode was mounted vertically 30 mm apart from the platinum counter electrode. The salt bridge connected the cell to a saturated calomel reference electrode (SCE)*. The electrolyte applied was an artificial saliva solution (Table 3) simulating the concentrations of ions in saliva. A fresh solution was made from stock solutions for each experiment. Stock solutions were stored in closed glass vessels at room temperature. Unless otherwise stated, the electrolyte was deaerated by Nz-bubbling, agitated with a magnetic stirrer, and kept at 37~ throughout the experiments. Beckman Instruments Inc., USA.
Key words: dental materials, corrosion, structure, alloy.
Roy I. Holland, NIOM, Scandinavian Institute of Dental Materials, Forskningsveien 1, 0371 Oslo 3, Norway.
ReceivedMay 30, 1985; accepted January 7, 1986.
Table 2. Heat treatments and resulting structures. As-cast a. Segregations of Cu and Au in the grain boundaries and between dendritic arms. b. Eutectic particles, 0.05 ~tm lamellae. c. A matrix split into 0.02 ~tmlamellae parallel with 100 planes. d. High density of very small fct ordered AuCuI particles. 700~ 10 min a. Segregations mainly as for the as-cast structure, but without eutectic particles. b. One fcc phase. 700~ 10 min/500~ 11 wk a. No segregations. b. Matrix split into three fcc phases (Ag, Cu and Pd-rich, respectively). 700~ 10 min/500~ 11 wk/350~ 8 h a. No segregations. b. Matrix split into three fcc phases (Ag, Cu and Pd-rich, respectively). c. Plates of the AuCuI phase 0.02 ~tm thick and with 0.2 p,m sides in the Au-Cu-rich grains, parallel to the 100 planes of the matrix (formed at 350~
Table 3. The basic constituents of the artificial saliva (pH 5.2-5.7). Compound
Amount
NaCI KCI CaCI2 - 2H20 NaHzPO4 - 2H20 Na2S 9xH20 (x=7-9) Urea H20
0.4 g 0.4 g 0.795 g 0.78 g 0.005 g 1g to 1 litre
144
Holland et al.
Cast specimens were embedded in plastics, polished with 1 Ixm diamond paste and washed in ethanol and distilled water less than 2 h before each experiment. The metal/plastic interface was insulated with a lacquer ~. Polarization testing. The specimens were subjected to in vitro potentiodynamic (continuous scan) polarization corrosion test scanned at a rate of 1 mV/s, usually from - 7 0 0 mV to approx. +800 mV (SCE). In addition, potentiostatic (step-scan) polarization testing, both anodic and cathodic, were done. The cathodic polarization was performed in air-bubbled electrolyte. Unless otherwise stated, the samples were equilibrated open circuit for 1 h before start of polarization. The polarization steps were 50 mV and the current recorded when an apparent steady level was reached, usually after 10-20 min. The polarization and the current measurements were done with a polarographic analyzer II and the potential and current recorded on a x-y recorder ~.
880mY
~
=
OmV
f
w
|
..................................................................................................................
-700mV -0.30
-0.15
|
|
0.15
0.30
mAcm -2
Fig. 1. Potentiodynamic polarization curves for the as cast condition of the alloy in artificial
saliva.
830mV
,
=
if -
Results
From the potentiodynamic polarization curves (Figs. 1--4) the structural state resulting from annealing only (700~ for 10 min) was discernable due to markedly increased anodic current density. However, it was the least discolored (tarnished) after the corrosion test. The microscopic examinations (both light and SEM) of the corroded surfaces did not indicate that any particular phase was imore prone to corrosion than the others. To give a numerical value for the difference in corrosion sensitivity of the different structural states in artificial saliva, their anodic charge was calculated by integrating the potentiodynamic polarization curve from the zero current potential (ZCP) to +200 mV SCE. These are shown in Table 4 together with the ZCP. The single-phased structure, made by heat treatment at 700~ only, is far the most corrosion sensitive, whereas the age-hardened are the least. In the potentiostatic polarization * Epofix, Struers, Copenhagen, Denmark. Plastik 70, Kontakt Chemie, Rastatt, Western Germany. tl Model 174A, Princeton Applied Research, NJ, USA. Model 7035B, Hewlett-Packard, CA, USA.
OmV
.........................................................
-700mY 20,30
-0.15
0.15
0.30
mAcm -=
Fig. 2. Potentiodynamic polarization curves for the single-phased structural state (heat
treated by 700~ for 10 rain before quenching) of the alloy in artificial saliva.
810mY
0mV
=
=
............................................................
] -700mV
I
=
-0.30
-0.15
~
I
I
O. 15
0,30
mAcm -2
Fig. 3. Potentiodynamic polarization curves for the structural state of the alloy resulting from
heat treatment at 500~ for 11 wk after homogenization at 700~
Corrosion and structure of low-gOld
145
780mV 100 0 -10O
LU
0mY
........................................................1 .........................................................
+
-200 -300 -400
CURRENT
-700mY
' -0.30
= -0.15
~2~
I 0.15
I 0.30
mAcro-2
Fig. 4. Potentiodynamic polarization curves for the structural state of the alloy resulting from
heat treatment after homogenization at 700~ at 500~ for 11 wk and 350~ for 8 h.
Table 4. Anodic charge (AC) from zero current potential to +200 mV (SCE) for the different structural states of the alloy in A): artificial saliva, and B): artificial saliva after pretreatment in 0.1 mM sodium sulfide for 21 hr,
A) B)
As cast
700~
1.1+0.3 (4)* 2.7+0.8 (3)
11.5+4.0 (4) 9.4_+4,2 (3)
700~176 0.6+0.2 (6) 1.5+0.5 (3)
9
t l A c m -e
100
700~176176 1.9• (5) 2.1+1.2 (3)
DENSITY
Fig. 6. Potentiostatic (cathodic) polarization curves for the different structural states of the alloy in artificial saliva, after pretreatment of the samples in 0.1 mM sodium sulfide for 21 h. The structural states are as cast (El), 700~ (0), 700~176 (A), and 700~176176 (V).
"---. -200
9 Mean + S.D. (no. of observations). CURRENT
testing (Fig. 5), both the anodic and the cathodic curves of the single-phased sample were different from those of the other 3 structural states. Based on the crossing points for the anodic and the cathodic polarization curves, the maximum current density at that potential can be at least twice as high (approx. 3 ~A/cm 2) for the single-phased sample
200
100 0 03
~
o
~
O
/ Z'"
~,',-2.%.._~
ku - 1 0 0 -200 -300
CURRENT
Fig. 5.
DENSITY
p A c m -2
Potentiostatic polarization curves (anodic and cathodic) for the different structural states of the alloy in artificial saliva. The structural states are as east (liD), 700~ (OO), 700~176 (A A), and 700~176176 (T ~). Filled symbols: anodic polarization, open symbols: cathodic polarization.
than for the other structural states (0.5-1.7 ~A/cm2). It was considered possible that the corrosion of the different structural states of the alloy in sulfide solution could reduce further corrosion by providing inhibitory layers of corrosion products on the surface and/or selective leaching leaving the surface more noble and, thereby, less corrosion prone (2). In order to check this possibility, samples were immersed in 0.1 mM sodium sulfide solution for 21 h prior to potentiodynamic polarization testing. No reduction in anodic charge from ZCP to +200 mV SCE could be detected for the pretreated samples (Table4). In the potentiostatic cathodic polarization testing, however, a substantial effect of pretreatment in 0.1 mM sulfide for 21 h was found (Fig. 6). The effect was greatest for the multi-phased structure, which also exhibited most tarnish due to the pretreatment, and least for the single-phased sample, which was the least tarnished. Based on the crossing points for the anodic and the cathodic polarization curves, the pretreatment reduced the maximum current density with approximately 50%, but with only 20% for the single-phased sample. The cathodic polarization curves of n o n -
D E N S I T Y p A c m -z
Fig. 7. Potentiostatic (cathodic) polarization curves for the different structural states of the alloy in artificial saliva, without the 1-h open circuit equilibration period, The structural states are as cast (C~), 700~ (9 700~176 (A), and 700~176176 (V).
pretreated samples and without the 1 h open circuit equilibration period, showed the least difference between the single- and the multi-phased states (Fig. 7).
Discussion It is a generally accepted concept that "single-phased (solid solution) alloys are more corrosion resistant than alloys with 2 (or more) phases, since the galvanic coupling effects are not present" (3). However, the results of the present study show that the alloy investigated did not behave accordingly. The singlephased structure exhibited the greatest corrosion, whereas all the multi-phased structural states presently tested showed far better corrosion resistance. In contrast, recording macroscopical discoloration of the alloy after the corrosion testing gave directly opposing results: the single-phased structural
146
H o l l a n d et al.
state was clearly less discolored (tarnished) than any of the others, which is in agreement with other reports (4, 5). This indicates that discoloration due to corrosion (tarnish) and corrosion sensitivity are at least not positively correlated, and that no conclusion should be drawn from one to the other of these parameters, which means that tarnish testing cannot be used as a corrosion test, and vice versa. More research is needed to establish whether there is an inverse relationship between tarnish and corrosion, as appears from the present study. Previous studies indicate both possibilities: A brass gold substitute** is reported to show very little discoloration, but high corrosion (6). In contrast, reports showing good relationship between corrosion rate and discoloration have also been published for another group of alloys, those consisting mainly of Ag and Pd (7). The possibility exits, that there is an inverse relationship between tarnish and corrosion sensitivity for those alloys and for those potential ranges where the corrosion rate is limited by the rate of the cathodic reactions. A hypothetical explanation for the lack of correlation between tarnish and corrosion in the present study may be as follows: In a solid-solution (singlephased) alloy the anodic and cathodic reactions occur uniformly spread throughout the corroding surface. This situation will disfavor any precipitation of corrosion products onto the surface, due to the continuous release of material anywhere on the surface. In a multi-phase alloy, the anodic and the cathodic reactions may be separated. It is likely that the different phases will have different nobility, i.e. different
** Progold, Birdsall Enterprises, Ballwin, MO, USA.
Ltd.,
corrosion potentials, and that a multitude of microscopical bimetal galvanic cells will be formed. In that case, anodic reactions will preferably take place on the least noble phase (lowest corrosion potential) and cathodic reactions on the most noble. This system will favor possible precipitation of corrosion products on the most noble phase(s), because they represent a stable surface, in that no or only very little material is released from it. The film so formed will constitute the tarnish of the alloy surface. Pretreatment of the samples in sulfide solution did not affect subsequent corrosion as tested by potentiodynamic polarization, i.e. no surface enoblement by dissolution of Cu or Ag seemed to have occurred (2). The pretreatment resulted in tarnish, particularly on the multi-phased structural states. Such a tarnish layer did not, however, reduce further anodic reaction (8). The single-phased structural state behaved differently from the multiphased states not only in anodic, but also in cathodic polarization. Also after pretreatment in sulfide, that difference persisted. The pretreatment affected the cathodic polarization curves of the multi-phased specimens, but not of the single-phased one. This is in agreement with the tarnish result; the surfaces of the multi-phased specimens are changed, i.e. a tarnish film is developed, whereas the surface of the singlephased specimen is unchanged and, thus, allows the cathodic reactions to continue undisturbed. However, this cannot be taken as a proof of corrosion resistance of the single-phased structure. Its appearantly unchanged surface
can be the result of a corrosion process continuously removing material from the surface while nothing is precipitated onto it. The tarnish film probably decreases the access of 02 to the metal surface, thereby diminishing the overall rate of the main cathodic reaction taking place; the reduction of 02. The difference among the non-pretreated samples is, therefore, probably caused by difference in tarnish film, made in the l h equilibration period prior to start of polarization. When this equilibration period was omitted, the difference in the cathodic polarization curves between the single- and the multi-phased structures was the least detected.
References 1. Hero H, J0rgensen R, S0rbr0den E. A low-gold dental alloy: Structure and segregations. J Dent Res 1982: 61: 1292-8. 2. Hultquist G, Hero H. Surface enoblement by dissolution of Cu, Ag and Zn from single phase gold alloys. Corros Sci 1984: 24: 789-805. 3. Fontana MG, Greene ND. Corrosion engineering. New York: McGraw-Hill Book Company, 1967: 26. 4. German RM. The role of microstructure in the tarnish of low-gold alloys. Metallography 1981: 14: 253-66. 5. Hero H, J0rgensen RB. Tarnishing of a low-gold dental alloy in different structural states. J Dent Res 1983: 62: 371-6. 6. Sarkar NK, Fuys RA, Stanford JW. Corrosion and microstructure of Progold. J Prosthet Dent 1978: 40: 50-5. 7. Ishizaki N. Corrosion resistance of Ag-Pd alloy system in artificial saliva: an electrochemical study. J Osaka Dent Univ 1969: 3: 121-33. 8. Holland RI. Effect of pellicle on galvanic corrosion of amalgam. Scand J Dent Res 1984: 92: 93-6.