A Study of the Corrosion of Dental Amalgam Using the Ring-Disk Electrode LEAH GAL-OR* and STANLEY BRUCRENSTEIN,** Chemistry Department, and J. M. CARTER, Dental Materials Department, State University of h7ew York at Bu$alo, Bufalo, New York 14214
Summary The rotating ring-disk electrode technique has been applied t o the study of anodic dissolut,ion of dental amalgam in a simulated saline solution. The electroactive domains of the silver, tin, and mercury couples (the main constituents of the amalgam) were determined from current-potential curves obtained at a rotating gold-disk electrode in solutions containing salts of the respective metals. Subsequently, anodic currents were applied to a rotating amalgam-disk electrode and the soluble products produced were identified using a concentric gold ring electrode, i.e., using the rotating gold-ring, amalgam-disk electrode. Species generated a t the amalgam disk are transferred to the gold ring by convective diffusion. Tin ions were found to be the only soluble species generated a t the amalgam disk. No evidence for dissolution of other components was found. The selective dissolution of tin from the amalgam is also consistant with potential shifts observed in repetitive current-potential curves of an amalgam disk. This study provides a direct proof for the selective dissolution of tin during corrosion of dental amalgam in an in vitro environment.
INTRODUCTION The corrosion behavior of dental amalgams has been studied by numerous workers both in in vitro and in vivo environments. Metallurgical studies of the amalgam structure and composition'-6 have shown it to be a multiphase system composed of a silver-rich phase, y1 - AgZHg,, a tin-rich phase, yz - SnTP8 Hg, and unreacted Ag3Sn,
* Present address: Institute of
Metals, Israel Institute of Technology.
** Author to whom all correspondence shall be addressed. Journal of Biomedical Materials Research, Vol. 12,l-12 (1978) 0021-9304/78/0012-0001$01.00 0 1978 by John Wiley & Sons, Inc.
GAL-OR, BRUCKENSTEIN, AND CARTER
2
the y phase. This system is obtained during amalgam preparation by the reaction of mercury with Ag3Sn: Hg
+ Ag3Sn
+ y1
+ yz + unreacted Ag3Sn
dental amalgam Corrosion of amalgam in the oral environment can take place due to electrochemical microcells formed between the anodic yz phase and the cathodic y and y1 phases.' Oxygen concentration cells constitute an additional contributing factor t o the corrosion process. The following two complementary reactions are presumed t o take place in the oral fluid:7 1
+ 7-81
__
Sn7-8 Hg + Sn++
+Oz
+ 2e + H 2 0 + 2OH-
7-8
-
Hg
+e
anodic reaction cathodic reaction
If the saliva is acidic, ,due t o fermentation of carbohydrates and saccharides, a n additional cathodic reaction can take place :
2H+
+ 2e + Hz
The amalgam corrosion process releases, supposedly, the yz phase tin as tin ions; the mercury which is released is believed t o react with the previous unreacted y phase t o form fresh y1 and yz and thus the corrosion continues.R JGrgensen and Saitog have also shown that in bulk condensed amalgams, the yz phase forms a continuous network, and consequently the tin dissolution is not limited to the surface of the amalgam. Therefore, tin is believed to be the sole or main constituent released from dental amalgams during corrosion. Studies on the corrosion products of amalgams have also been reported. I n a n early work, Schoonover and Souder'O showed, by chemical analysis of corrosion products, that tin was their main cationic constituent. The composition of the corrosion products was studied later by x-ray diffraction techniques. Otani et al.7 identified SnO as a principal constituent of in vitro corrosion products formed in a NaCl solution. He showed that SnO +HzO constitutes the essential product in the early stages, with SnO appearing later. The ratio of SnO t o SnO . +HzO increased with time. These studies were performed by x-ray diffraction and electron microanalysis. Mateer and Iteitzl' found PSnOz and SnSz in in vivo cor-
-
CORROSION OF DENTAL AMALGAM
3
rosion products removed from amalgam restorations taken from human mouths. However, Swartz et a1.I2 noted the presence of silver and mercury sulfide in tarnished amalgam layers, whereas Guthrow et al.I3 noted deposition of AgCl on the y1 phase in anodic polarization experiments. It is the purpose of this work to contribute to the understanding of the initial stages of the amalgam dissolution mechanisms in a n in vitro environment by applying the rotating ring-disk technique. The ring-disk electrode technique is a powerful tool in the study of dissolution mechanisms and kinetics.14J5 This work is the first attempt to apply the technique to indentify the species generated by anodic dissolution of the amalgam disk. Soluble species generated by a n anodic current a t the amalgam disk are transferred by connective diffusion to a concentric gold-ring electrode where they are identified by their electrochemical properties. This application requires independent galvanostatic control of the disk current and potentiostatic control of the ring electrode so as to scan the ring potential and thus detect and identify each species by its response. The response characteristic of the species that might result during amalgam dissolution was determined, first, by scanning the potential of a rotating gold-disk electrode in solutions containing ions (salts) of the main metals that compose the amalgam. The advantage of the ring-disk method is its experimental ability to identify directly the minute amounts of the species produced a t the disk. The electrocheniical literature dealing with the properties of the ring-disk electrode is large, and an introduction to this technique is t o be found in the monograph by Albery and Hitchman.16 Applications to corrosion problems have also been reported by Pickering and V\iagnerl4 and by Miller and B e l l a ~ a n c e both , ~ ~ of whom have demonstrated the validity of this technique as a means of studying corrosion.
MATERIALS AND METHODS
Electrodes The amalgam-disk, gold-ring electrode is pictured schematically in Figure 1. The electrode consists of two major components: a) the disk assembly, which comprises a stainless steel shaft brazed to a
4
GAL-OR, BRUCKENSTEIN, AND CARTER Shank and Disc Electrode Contact
Ring Electrode Contoct
I
Outer Sheath
Resin Insulating Gap L--Teflon
Insulating Gap
Fig. 1. Rotating amalgam-disk, gold-ring electrode.
shank made to fit into a collet of a high-speed rotator, and the disk material fastened to the end of the shaft; b) the ring assembly, which consists of a ring electrode tube made of brass with a gold ring silversoldered to its end. The disk and ring assemblies are separated by an insulating gap made of shrinkable Teflon tubing. Due to instability of the amalgam a t the elevated temperature required to shrink Teflon tubing, the gap between the amalgam disk a n d the ring tube had to be filled with a room-temperature curing resin. The ring tube was then surrounded by an outer Teflon sheath. The amalgam disk was prepared from a commercial dental alloy (“Caulk Fine Cut,” L. D. Caulk Co.) which was mechanically triturated with triple-distilled mercury following manufacturer’s instructions. The resulting amalgam had a composition of 34% Ag, 12% Sn, 53% Hg, 0.5% Cu, and
Summary The rotating ring-disk electrode technique has been applied t o the study of anodic dissolut,ion of dental amalgam in a simulated saline solution. The electroactive domains of the silver, tin, and mercury couples (the main constituents of the amalgam) were determined from current-potential curves obtained at a rotating gold-disk electrode in solutions containing salts of the respective metals. Subsequently, anodic currents were applied to a rotating amalgam-disk electrode and the soluble products produced were identified using a concentric gold ring electrode, i.e., using the rotating gold-ring, amalgam-disk electrode. Species generated a t the amalgam disk are transferred to the gold ring by convective diffusion. Tin ions were found to be the only soluble species generated a t the amalgam disk. No evidence for dissolution of other components was found. The selective dissolution of tin from the amalgam is also consistant with potential shifts observed in repetitive current-potential curves of an amalgam disk. This study provides a direct proof for the selective dissolution of tin during corrosion of dental amalgam in an in vitro environment.
INTRODUCTION The corrosion behavior of dental amalgams has been studied by numerous workers both in in vitro and in vivo environments. Metallurgical studies of the amalgam structure and composition'-6 have shown it to be a multiphase system composed of a silver-rich phase, y1 - AgZHg,, a tin-rich phase, yz - SnTP8 Hg, and unreacted Ag3Sn,
* Present address: Institute of
Metals, Israel Institute of Technology.
** Author to whom all correspondence shall be addressed. Journal of Biomedical Materials Research, Vol. 12,l-12 (1978) 0021-9304/78/0012-0001$01.00 0 1978 by John Wiley & Sons, Inc.
GAL-OR, BRUCKENSTEIN, AND CARTER
2
the y phase. This system is obtained during amalgam preparation by the reaction of mercury with Ag3Sn: Hg
+ Ag3Sn
+ y1
+ yz + unreacted Ag3Sn
dental amalgam Corrosion of amalgam in the oral environment can take place due to electrochemical microcells formed between the anodic yz phase and the cathodic y and y1 phases.' Oxygen concentration cells constitute an additional contributing factor t o the corrosion process. The following two complementary reactions are presumed t o take place in the oral fluid:7 1
+ 7-81
__
Sn7-8 Hg + Sn++
+Oz
+ 2e + H 2 0 + 2OH-
7-8
-
Hg
+e
anodic reaction cathodic reaction
If the saliva is acidic, ,due t o fermentation of carbohydrates and saccharides, a n additional cathodic reaction can take place :
2H+
+ 2e + Hz
The amalgam corrosion process releases, supposedly, the yz phase tin as tin ions; the mercury which is released is believed t o react with the previous unreacted y phase t o form fresh y1 and yz and thus the corrosion continues.R JGrgensen and Saitog have also shown that in bulk condensed amalgams, the yz phase forms a continuous network, and consequently the tin dissolution is not limited to the surface of the amalgam. Therefore, tin is believed to be the sole or main constituent released from dental amalgams during corrosion. Studies on the corrosion products of amalgams have also been reported. I n a n early work, Schoonover and Souder'O showed, by chemical analysis of corrosion products, that tin was their main cationic constituent. The composition of the corrosion products was studied later by x-ray diffraction techniques. Otani et al.7 identified SnO as a principal constituent of in vitro corrosion products formed in a NaCl solution. He showed that SnO +HzO constitutes the essential product in the early stages, with SnO appearing later. The ratio of SnO t o SnO . +HzO increased with time. These studies were performed by x-ray diffraction and electron microanalysis. Mateer and Iteitzl' found PSnOz and SnSz in in vivo cor-
-
CORROSION OF DENTAL AMALGAM
3
rosion products removed from amalgam restorations taken from human mouths. However, Swartz et a1.I2 noted the presence of silver and mercury sulfide in tarnished amalgam layers, whereas Guthrow et al.I3 noted deposition of AgCl on the y1 phase in anodic polarization experiments. It is the purpose of this work to contribute to the understanding of the initial stages of the amalgam dissolution mechanisms in a n in vitro environment by applying the rotating ring-disk technique. The ring-disk electrode technique is a powerful tool in the study of dissolution mechanisms and kinetics.14J5 This work is the first attempt to apply the technique to indentify the species generated by anodic dissolution of the amalgam disk. Soluble species generated by a n anodic current a t the amalgam disk are transferred by connective diffusion to a concentric gold-ring electrode where they are identified by their electrochemical properties. This application requires independent galvanostatic control of the disk current and potentiostatic control of the ring electrode so as to scan the ring potential and thus detect and identify each species by its response. The response characteristic of the species that might result during amalgam dissolution was determined, first, by scanning the potential of a rotating gold-disk electrode in solutions containing ions (salts) of the main metals that compose the amalgam. The advantage of the ring-disk method is its experimental ability to identify directly the minute amounts of the species produced a t the disk. The electrocheniical literature dealing with the properties of the ring-disk electrode is large, and an introduction to this technique is t o be found in the monograph by Albery and Hitchman.16 Applications to corrosion problems have also been reported by Pickering and V\iagnerl4 and by Miller and B e l l a ~ a n c e both , ~ ~ of whom have demonstrated the validity of this technique as a means of studying corrosion.
MATERIALS AND METHODS
Electrodes The amalgam-disk, gold-ring electrode is pictured schematically in Figure 1. The electrode consists of two major components: a) the disk assembly, which comprises a stainless steel shaft brazed to a
4
GAL-OR, BRUCKENSTEIN, AND CARTER Shank and Disc Electrode Contact
Ring Electrode Contoct
I
Outer Sheath
Resin Insulating Gap L--Teflon
Insulating Gap
Fig. 1. Rotating amalgam-disk, gold-ring electrode.
shank made to fit into a collet of a high-speed rotator, and the disk material fastened to the end of the shaft; b) the ring assembly, which consists of a ring electrode tube made of brass with a gold ring silversoldered to its end. The disk and ring assemblies are separated by an insulating gap made of shrinkable Teflon tubing. Due to instability of the amalgam a t the elevated temperature required to shrink Teflon tubing, the gap between the amalgam disk a n d the ring tube had to be filled with a room-temperature curing resin. The ring tube was then surrounded by an outer Teflon sheath. The amalgam disk was prepared from a commercial dental alloy (“Caulk Fine Cut,” L. D. Caulk Co.) which was mechanically triturated with triple-distilled mercury following manufacturer’s instructions. The resulting amalgam had a composition of 34% Ag, 12% Sn, 53% Hg, 0.5% Cu, and
