Integrated currents between amalgams and a gold alloy in saline solutions and natural saliva with different chloride content
Berit I. Johansson and Foike Lagerlof NIOM, Scandinavian Institute of Dental Materials, Haslum, Norway
Johansson BI, Lageriof F: Integrated currents between amalgams and a gold alloy in saline solutions and natural saliva with different chioride content. Scand J Dent Res 1992; 100: 240-3.
The effect of different sodium chloride concentrations on the integrated currents (charge transfers) between dental amalgams and a gold alloy was studied in a bimetallic cell containing saliva or saline solutions. The integrated currents were only to a minor degree affected by increasing chloride concentrations for the high copper amalgams both in saliva and saline solutions. The integrated currents in the experiments involving conventional amalgam were several times higher than those for high copper amalgams. In saline solutions a charge transfer maximum was found at 0.2 M chloride. In saliva an increase of the charge transfer was found with increasing chloride content.
Electrical currents are created when amalgams come in contact with gold alloys in the presence of conductive solutions. The magnitudes of the currents are higher between conventional amalgam and gold alloy couples than between high copper amalgam and gold alloy couples in saliva, artificial saliva, and saline solutions (1-3). The saline solutions are a more aggressive environment than saliva, thus creating higher currents (1, 3). The corrosion of amalgams is influenced by the oxygen content of the solution (4, 5). The access to dissolved oxygen may be restricted by pellicle forming on the electrodes. HOLLAND (6) found that the galvanic currents between conventional amalgam and gold alloy decreased when the gold alloy electrode was covered by pellicle. This effect was not observed when gold alloy and high copper amalgam combinations were studied, nor when the amalgams were covered by pellicle. With increasing chloride content in sodium chloride solutions, the electric conductivity will increase and the corrosion will increase. The corrosion rate is highest around the composition of sea water; at even higher chloride concentrations the corrosion will be reduced due to a decrease of the solubility and diffusion rate of oxygen (7). Salivary secretion contains approximately 15
Key words: amalgam; bimetallic corrosion; gold alloy; saline solutions; saliva. B. I. Johansson, Department of Dental Materials and Technology, University of UmeS, S-901 87 UmeS, Sweden or F. Lageriof, Department of Carioiogy, Karolinska Institutet, S-141 04 Huddinge, Sweden Accepted for publication 31 August 1991
mM chloride (8). The concentration of chloride may increase to about 30 mM with increasing flow rate. In stimulated parotid saliva an approach to the chloride levels of plasma has been reported (9). In dental plaque the chloride concentrations are slightly higher than those reported for saliva (10). However, after contact with salty foodstuffs the concentration of chloride in the oral fluids may increase considerably. This may be especially likely in such oral areas where small volumes of saliva are entrapped, such as approximal spaces between teeth. Chloride ions from the oral fluids may also diffuse into the liquid in the gap between amalgam filling and tooth. The influence ofthe chloride content of saliva on bimetallic corrosion of amalgams has never been investigated. The aim of this study was to investigate the integrated currents between a gold alloy and conventional or high copper amalgams in natural saliva and saline solutions. Material and methods
Specimen preparation - Amalgam specimens (Table 1) were prepared according to ADA specification No. 1 (11). The specimens were allowed to set at 37°C for 1 wk. A gold alloy specimen (Table 1)
Corrosion in solutions with different chloride contents
241
Table 1 Amalgam and gold alloy specimens used in the stttdy Code Product R Revalloy (conventional alloy)
Manufacturer / distributor
D
Dispersalloy (high copper dispersed type alloy)
SS White Limited, Harrow, Middx, United Kingdom Johnson & Johnson Dental Products Company, East Windsor, NJ, USA
A
ANA 2000 (high copper single composition)
AB Nordiska Affineriet ANA, Helsingborg, Sweden
C Gold alloy JS C * Vivadent, Schaan, Lichtenstein.
Alloy, mercury 1:1.1
rrituration in Silamat* 6s
1 :1
6s
1 : 1.05
5s
J Sjoding AB, Kista, Sweden
was cast according to the manufacturer's instructions. The specimens were mounted in the screwed part of polymer bolt shanks with cylindrical drilled holes in the center of the longitudinal axis (Fig. 1). The crevice between each sample and screw was sealed using a two-component glue (Araldit, Ciba Geigy AB, Vestra Frolunda, Sweden). Spring brass pins were threaded into the cylindrical holes of the cap bolts of the polymer screws, in order to achieve a metallic contact between the specimens and a measuring instrument. The specimen areas, 0.13 cm- and 0.28 cm- for the amalgam and gold alloy specimens, respectively, were ground wet with 1200 grit using a standard metallographic procedure before each test procedure. Electrolytes - Two types of electrolytes based on sodium chloride were used. One was made of whole saliva (A) and the other one of distilled water (B). A) sodium chloride was weighed into test tubes in such amounts that up to 1.6 M chloride electrolytes were made when 5 ml of pooled whole saliva collected from 10 people were added into each tube. The salivary flow was stimulated by letting subjects chew on a piece of plastic film (Parafilm M, American Can Company, Greenwich, CT, USA). The
chloride content of the saliva was ignored. The test tubes were sealed and stored frozen at — 20°C until used. Before the test occasion the test tubes were tempered for 15 min at 37°C in a water bath. B) sodium chloride solutions with chloride contents of up to 1.6 M were made with distilled water. The solutions were tempered before each test occasion in a water bath. Current registrations - A couple consisting of one of the amalgams and the gold alloy specimen was tested in an electrochemical cell at 37°C. The specimens were threaded into a mold made of plexiglas, which was equipped with taps allowing water to circulate between the double-walled mold and a water bath. The specimens were separated by 7 mm and the mold was filled with the electrolyte. The specimen-couple was connected to a multimeter (160B Digital multimeter, Keithley instruments, Cleveland, OH, USA) and the current was recorded for a period of 30 min. The integrated current (charge transfer) was determined by graphical integration of the recorder trace. The electrolytes made of sodium chloride and distilled water were air-bubbled, the saliva electrolytes were not. Airbubbling of saliva will remove the carbon dioxide and thus alter the carbonic acid-bicarbonate buffering system. Three parallel runs were carried out for each test condition. Results
Fig. 1. Part of experimental setup showing how specimens were mounted in polymer shanks, and how these were fixed in electrochemical cell. • .
The integrated current during 30 min between the amalgam and gold alloy specimens in saliva with different chloride contents is shown in Fig. 2. The alloy combination R/C had the highest currents compared with the A/C and D/C combinations. The charge transferred between R/C increased when the chloride content in the saliva electrolyte increased, whereas this phenomenon was not observed for the A/C and D/C combinations. Fig. 3 shows the integrated currents between amalgam and gold alloy specimens in air-bubbled sahne solutions. Again, the R/C combination had
242
Johansson & Lagerlof Discussion R/C
0.2 0.4 0.8 CHLORIDE CONCENTRATION IN SALIVA, M
1.6
Fig. 2. Charge tvansferi-ed between gold alloy and a) conventional amalgam R/C, b) dispersed high copper amalgam D/C, c) single composition high copper amalgam A/C in natural saliva with chlorides added. Standard deviations are indicated with screened areas.
the highest currents compared with A/C and D/ C. The charge transferred between R/C reached a peak around a chloride concentration of 0.2 M. The combinations A/C and D/C had approximately the same magnitudes of the currents regardless of the chloride content of the saline solutions. The integrated currents between the amalgam and gold alloy specimens were always higher in the saline solutions than in saliva with different chloride content.
70 O60.
ocso UJ
z QC h-
30
R/C
UJ
O 20 CC
X
10
o 0.05 0.2 0.4 0.8 1.6 CHLORIDE CONCENTRATION IN SALINE SOLUTION, M Fig. 3. Charge transferred between gold alloy and a) conventional amalgam R/C, b) dispersed high copper amalgam D/ C, c) single composition high copper amalgam A/C in saline solutions. Standard deviations are indicated with screened
The integrated currents between the gold alloy and the conventional amalgam in saline solutions were highly influenced by the chloride content and the currents were highest in the saline solutions with a chloride composition around the composition of sea water. Parameters such as the electric conductivity of the solution, solubility and diffusion rate of oxygen seem to play a major role in the corrosion of conventional amalgams in contact with a gold alloy in saline solutions. The integrated currents between the high copper amalgams and a gold alloy in saline solutions were not significantly influenced by the chloride content of the solutions. This is in agreement with the results by HOLLAND (6) who suggested that the corrosion of high copper amalgams in contact with gold alloys is limited by other factors than the rate of reduction of oxygen. FiNKELSTEiN & GREENER (12) found that the chloride content was of no importance for the corrosion of high copper amalgams. The integrated currents were considerably lower in the saliva than in the saline solutions. Surfaces exposed to saliva will be coated with a thin film called pellicle. This pellicle is made up of various macromolecules of mainly salivary origin but also by substances of bacterial origin (8). It seems likely that the pellicle may act as a diffusion barrier and thus restrict the corrosion. In an earlier study, the pellicle has been found to decrease the currents between conventional amalgam and a gold alloy when the gold alloy specimen was covered by the pellicle (6). Other substances in the saliva may also infiuence the corrosion: inorganic phosphate and bicarbonate have been found to inhibit in vitro corrosion of amalgam (13). In the present study an increased chloride content in saliva created higher currents between the conventional amalgam/gold alloy couple than between the high copper/gold alloy couples. The corrosion of the conventional amalgam/gold alloy couple in saliva does not seem to be restricted by low oxygen levels. Protective films of corrosion products occur on dental amalgams when exposed to de-aerated or aerated electrolytes (14). Chloride ions have been suggested to be part of the corrosion mechanism of conventional amalgams (14, 15). The high copper amalgams in contact with the gold alloy were not susceptible to high chloride contents in the electrolyte solutions in the same way as the conventional amalgam. High concentrations of chlorides in the gap between a conventional amalgam filling and tooth and thereby higher corrosions rates may be a reason for the increased marginal breakdown (16) observed for conventional amalgam filling compared to high copper amalgam filhngs.
Corrosion in solutions with different chloride contents The volume of saliva in the oral cavity is usually small. The volume after swallowing, the minimum level, has been estimated to 0.8 mL and the volume before swallowing, the physiologic maximum level, to 1.1 mL (17). It has been estimated that the total surface of the oral cavity is approximately 200 cm(18) from which the thickness of the salivary film covering the oral mucosa and the teeth could be calculated to average only 0.1 mm. Only a few milligrams of sodium chloride dissolved in this small volume may give rise to extremely high concentrations of chloride. This may be especially likely in local areas in direct contact with a salty foodstuff. For example, the small amount of 1 mg sodium chloride dissolved in a saliva film covering 1 cm^ will give the initial chloride concentrations in this area of the same magnitude as the highest chloride concentrations used in the present study. After the initial exposure of chloride to the oral fluids the chloride concentration of the saliva will decrease with time in the same manner as has been described for fluoride, a substance which also is a natural constituent of saliva (19). The rate of this process, salivary clearance of chloride, is dependent on several physiological factors, e.g. salivary flow rate and the volume in the mouth before and after swallowing (20, 21). Also, chloride may diffuse from the initially high concentrations of chloride in the saliva into the dental plaque fluid. The clearance of chloride may be considerably slower from plaque than from saliva, thus exposing plaque covered amalgam to chloride at high concentrations for prolonged times. In conclusion, the increased salivary content of chlorides after intake of salty food and its subsequent time-dependent elimination may be a factor in the corrosion of dental amalgam, and thus part of an explanation ofthe variation in corrosion rate found between individuals.
243
dental alloys in vitro. Scand J Dent Res 1985; 93; 467-73. 3. JOHANSSON BI. An in vitro and in vivo study of galvanic currents between amalgam and gold alloy electrodes in saliva and saline solutions. Sccind J Dent Res 1986; 94; 562-8. 4. GREENER EH, MATSUDA K. Effect of oxygen on the corrosion of dental amalgam. J Orat Retuttvl 1985; 12; 123-33. 5. RAVNHOLT G . Corrosion of dental alloys in vitro by differential oxygen concentration. Scand J Dent Res 1986; 94; 370-6. 6. HOLLAND RI. Effect of pellicle on galvanic corrosion of amalgam. Scand J Dent Res 1984; 92; 93-6. 7. WRANGLEN G . An introduction to corrosion cind protection of metals. Frorne; Butler & Tanner Ltd, 1972; 92-3. 8. JENKINS G N . The ptiysiotogy and biociiemistry of ttie mouth. 4th ed. Oxford; Biackwell Scientific Publications, 1978; 284-413. 9. DAWES C. The approach to plasma levels of the chloride concentration in human parotid saliva at high flow rates. Arch Oral Biot 1970; 15: 97-9. 10. MARGOLIS HC. An assessment of recent advances in the study of the chemistry and biochemistry of dental plaque nuid. J Dent Res 1990; 69; 1337-42. 11. Guide to dental materials and devices. 7th ed. Chicago; American Dental Association, 1974-75; 171-2. 12. FiNKELSTEiN GF, GREENER EH. Mechanism of chloride corrosion of dental amalgam. J Orat Reliabit 1979; 6; 189-97. 13. PALAGHIAS G . The role of phosphate and carbonic acidbicarbonate buffers in the corrosion processes of the oral cavity. Dent Mater 1985; 1; 139^4. 14. MAREK M . The corrosion of dental materials. In; SCULLY, ed. Treatise on materiats science atid tectmotogy. Vol. 23. London; Academic Press, 1983; 331-94. 15. HAKANSSON B, YoNrcHEV E, VANNERBERG N - G , HEDEGARD
B. An examination of the surface corrosion state of dental fillings and constructions. I. A laboratory investigation of the corrosion behaviour of dental alloys in natural saliva and saline solutions. J Orat Retmbit 1986; 13; 235-46. 16. Quality evatiiation of dentat restorations. Criteria for ptacement and reptctcement. ANUSAVICE KJ, ed. Chicago; Quintessence Publishing Co., 1989. 17. LAGERLOF F, DAWES C. The volume of saliva in the mouth before and after swallowing. J Dent Res 1984; 63; 618-21. 18. DAWES C. Physiological factors affecting salivary flow rate, oral sugar clearance, and the sensation of dry mouth in man. / Dent Res 1987; 66; 648-53. 19. AASENDEN R, BRUDEVOLD F, RICHARDSON B. Clearance of
fluoride from the mouth after topical treatment or the use of a fluoride mouthrinse. Arch Oral Biol 1968; 13; 625-36.
References 1. FRAUNHOFER VON JA, STAHELI PJ. Gold-amalgam galvanic
cells. The measurements of corrosion currents. Br Dent J 1972, 132, 357-62. 2. ARVIDSON K , JOHANSSON EG. Galvanic currents between
20. LAGERLOE F, OLIVEBY A, EKSTRAND J. Physiological factors
influencing salivary clearence of sugar and fluoride. J Dent Res 1987; 66; 430-5. 21. LAGERLOE F. Salivary clearance and its effect on oral health. In; EDGAR W M , O'MULLANE DM, eds. Sattva and dentat
tieatth. 1st. ed. London; Brit Dent Journal, 1990; 69-80.
Berit I. Johansson and Foike Lagerlof NIOM, Scandinavian Institute of Dental Materials, Haslum, Norway
Johansson BI, Lageriof F: Integrated currents between amalgams and a gold alloy in saline solutions and natural saliva with different chioride content. Scand J Dent Res 1992; 100: 240-3.
The effect of different sodium chloride concentrations on the integrated currents (charge transfers) between dental amalgams and a gold alloy was studied in a bimetallic cell containing saliva or saline solutions. The integrated currents were only to a minor degree affected by increasing chloride concentrations for the high copper amalgams both in saliva and saline solutions. The integrated currents in the experiments involving conventional amalgam were several times higher than those for high copper amalgams. In saline solutions a charge transfer maximum was found at 0.2 M chloride. In saliva an increase of the charge transfer was found with increasing chloride content.
Electrical currents are created when amalgams come in contact with gold alloys in the presence of conductive solutions. The magnitudes of the currents are higher between conventional amalgam and gold alloy couples than between high copper amalgam and gold alloy couples in saliva, artificial saliva, and saline solutions (1-3). The saline solutions are a more aggressive environment than saliva, thus creating higher currents (1, 3). The corrosion of amalgams is influenced by the oxygen content of the solution (4, 5). The access to dissolved oxygen may be restricted by pellicle forming on the electrodes. HOLLAND (6) found that the galvanic currents between conventional amalgam and gold alloy decreased when the gold alloy electrode was covered by pellicle. This effect was not observed when gold alloy and high copper amalgam combinations were studied, nor when the amalgams were covered by pellicle. With increasing chloride content in sodium chloride solutions, the electric conductivity will increase and the corrosion will increase. The corrosion rate is highest around the composition of sea water; at even higher chloride concentrations the corrosion will be reduced due to a decrease of the solubility and diffusion rate of oxygen (7). Salivary secretion contains approximately 15
Key words: amalgam; bimetallic corrosion; gold alloy; saline solutions; saliva. B. I. Johansson, Department of Dental Materials and Technology, University of UmeS, S-901 87 UmeS, Sweden or F. Lageriof, Department of Carioiogy, Karolinska Institutet, S-141 04 Huddinge, Sweden Accepted for publication 31 August 1991
mM chloride (8). The concentration of chloride may increase to about 30 mM with increasing flow rate. In stimulated parotid saliva an approach to the chloride levels of plasma has been reported (9). In dental plaque the chloride concentrations are slightly higher than those reported for saliva (10). However, after contact with salty foodstuffs the concentration of chloride in the oral fluids may increase considerably. This may be especially likely in such oral areas where small volumes of saliva are entrapped, such as approximal spaces between teeth. Chloride ions from the oral fluids may also diffuse into the liquid in the gap between amalgam filling and tooth. The influence ofthe chloride content of saliva on bimetallic corrosion of amalgams has never been investigated. The aim of this study was to investigate the integrated currents between a gold alloy and conventional or high copper amalgams in natural saliva and saline solutions. Material and methods
Specimen preparation - Amalgam specimens (Table 1) were prepared according to ADA specification No. 1 (11). The specimens were allowed to set at 37°C for 1 wk. A gold alloy specimen (Table 1)
Corrosion in solutions with different chloride contents
241
Table 1 Amalgam and gold alloy specimens used in the stttdy Code Product R Revalloy (conventional alloy)
Manufacturer / distributor
D
Dispersalloy (high copper dispersed type alloy)
SS White Limited, Harrow, Middx, United Kingdom Johnson & Johnson Dental Products Company, East Windsor, NJ, USA
A
ANA 2000 (high copper single composition)
AB Nordiska Affineriet ANA, Helsingborg, Sweden
C Gold alloy JS C * Vivadent, Schaan, Lichtenstein.
Alloy, mercury 1:1.1
rrituration in Silamat* 6s
1 :1
6s
1 : 1.05
5s
J Sjoding AB, Kista, Sweden
was cast according to the manufacturer's instructions. The specimens were mounted in the screwed part of polymer bolt shanks with cylindrical drilled holes in the center of the longitudinal axis (Fig. 1). The crevice between each sample and screw was sealed using a two-component glue (Araldit, Ciba Geigy AB, Vestra Frolunda, Sweden). Spring brass pins were threaded into the cylindrical holes of the cap bolts of the polymer screws, in order to achieve a metallic contact between the specimens and a measuring instrument. The specimen areas, 0.13 cm- and 0.28 cm- for the amalgam and gold alloy specimens, respectively, were ground wet with 1200 grit using a standard metallographic procedure before each test procedure. Electrolytes - Two types of electrolytes based on sodium chloride were used. One was made of whole saliva (A) and the other one of distilled water (B). A) sodium chloride was weighed into test tubes in such amounts that up to 1.6 M chloride electrolytes were made when 5 ml of pooled whole saliva collected from 10 people were added into each tube. The salivary flow was stimulated by letting subjects chew on a piece of plastic film (Parafilm M, American Can Company, Greenwich, CT, USA). The
chloride content of the saliva was ignored. The test tubes were sealed and stored frozen at — 20°C until used. Before the test occasion the test tubes were tempered for 15 min at 37°C in a water bath. B) sodium chloride solutions with chloride contents of up to 1.6 M were made with distilled water. The solutions were tempered before each test occasion in a water bath. Current registrations - A couple consisting of one of the amalgams and the gold alloy specimen was tested in an electrochemical cell at 37°C. The specimens were threaded into a mold made of plexiglas, which was equipped with taps allowing water to circulate between the double-walled mold and a water bath. The specimens were separated by 7 mm and the mold was filled with the electrolyte. The specimen-couple was connected to a multimeter (160B Digital multimeter, Keithley instruments, Cleveland, OH, USA) and the current was recorded for a period of 30 min. The integrated current (charge transfer) was determined by graphical integration of the recorder trace. The electrolytes made of sodium chloride and distilled water were air-bubbled, the saliva electrolytes were not. Airbubbling of saliva will remove the carbon dioxide and thus alter the carbonic acid-bicarbonate buffering system. Three parallel runs were carried out for each test condition. Results
Fig. 1. Part of experimental setup showing how specimens were mounted in polymer shanks, and how these were fixed in electrochemical cell. • .
The integrated current during 30 min between the amalgam and gold alloy specimens in saliva with different chloride contents is shown in Fig. 2. The alloy combination R/C had the highest currents compared with the A/C and D/C combinations. The charge transferred between R/C increased when the chloride content in the saliva electrolyte increased, whereas this phenomenon was not observed for the A/C and D/C combinations. Fig. 3 shows the integrated currents between amalgam and gold alloy specimens in air-bubbled sahne solutions. Again, the R/C combination had
242
Johansson & Lagerlof Discussion R/C
0.2 0.4 0.8 CHLORIDE CONCENTRATION IN SALIVA, M
1.6
Fig. 2. Charge tvansferi-ed between gold alloy and a) conventional amalgam R/C, b) dispersed high copper amalgam D/C, c) single composition high copper amalgam A/C in natural saliva with chlorides added. Standard deviations are indicated with screened areas.
the highest currents compared with A/C and D/ C. The charge transferred between R/C reached a peak around a chloride concentration of 0.2 M. The combinations A/C and D/C had approximately the same magnitudes of the currents regardless of the chloride content of the saline solutions. The integrated currents between the amalgam and gold alloy specimens were always higher in the saline solutions than in saliva with different chloride content.
70 O60.
ocso UJ
z QC h-
30
R/C
UJ
O 20 CC
X
10
o 0.05 0.2 0.4 0.8 1.6 CHLORIDE CONCENTRATION IN SALINE SOLUTION, M Fig. 3. Charge transferred between gold alloy and a) conventional amalgam R/C, b) dispersed high copper amalgam D/ C, c) single composition high copper amalgam A/C in saline solutions. Standard deviations are indicated with screened
The integrated currents between the gold alloy and the conventional amalgam in saline solutions were highly influenced by the chloride content and the currents were highest in the saline solutions with a chloride composition around the composition of sea water. Parameters such as the electric conductivity of the solution, solubility and diffusion rate of oxygen seem to play a major role in the corrosion of conventional amalgams in contact with a gold alloy in saline solutions. The integrated currents between the high copper amalgams and a gold alloy in saline solutions were not significantly influenced by the chloride content of the solutions. This is in agreement with the results by HOLLAND (6) who suggested that the corrosion of high copper amalgams in contact with gold alloys is limited by other factors than the rate of reduction of oxygen. FiNKELSTEiN & GREENER (12) found that the chloride content was of no importance for the corrosion of high copper amalgams. The integrated currents were considerably lower in the saliva than in the saline solutions. Surfaces exposed to saliva will be coated with a thin film called pellicle. This pellicle is made up of various macromolecules of mainly salivary origin but also by substances of bacterial origin (8). It seems likely that the pellicle may act as a diffusion barrier and thus restrict the corrosion. In an earlier study, the pellicle has been found to decrease the currents between conventional amalgam and a gold alloy when the gold alloy specimen was covered by the pellicle (6). Other substances in the saliva may also infiuence the corrosion: inorganic phosphate and bicarbonate have been found to inhibit in vitro corrosion of amalgam (13). In the present study an increased chloride content in saliva created higher currents between the conventional amalgam/gold alloy couple than between the high copper/gold alloy couples. The corrosion of the conventional amalgam/gold alloy couple in saliva does not seem to be restricted by low oxygen levels. Protective films of corrosion products occur on dental amalgams when exposed to de-aerated or aerated electrolytes (14). Chloride ions have been suggested to be part of the corrosion mechanism of conventional amalgams (14, 15). The high copper amalgams in contact with the gold alloy were not susceptible to high chloride contents in the electrolyte solutions in the same way as the conventional amalgam. High concentrations of chlorides in the gap between a conventional amalgam filling and tooth and thereby higher corrosions rates may be a reason for the increased marginal breakdown (16) observed for conventional amalgam filling compared to high copper amalgam filhngs.
Corrosion in solutions with different chloride contents The volume of saliva in the oral cavity is usually small. The volume after swallowing, the minimum level, has been estimated to 0.8 mL and the volume before swallowing, the physiologic maximum level, to 1.1 mL (17). It has been estimated that the total surface of the oral cavity is approximately 200 cm(18) from which the thickness of the salivary film covering the oral mucosa and the teeth could be calculated to average only 0.1 mm. Only a few milligrams of sodium chloride dissolved in this small volume may give rise to extremely high concentrations of chloride. This may be especially likely in local areas in direct contact with a salty foodstuff. For example, the small amount of 1 mg sodium chloride dissolved in a saliva film covering 1 cm^ will give the initial chloride concentrations in this area of the same magnitude as the highest chloride concentrations used in the present study. After the initial exposure of chloride to the oral fluids the chloride concentration of the saliva will decrease with time in the same manner as has been described for fluoride, a substance which also is a natural constituent of saliva (19). The rate of this process, salivary clearance of chloride, is dependent on several physiological factors, e.g. salivary flow rate and the volume in the mouth before and after swallowing (20, 21). Also, chloride may diffuse from the initially high concentrations of chloride in the saliva into the dental plaque fluid. The clearance of chloride may be considerably slower from plaque than from saliva, thus exposing plaque covered amalgam to chloride at high concentrations for prolonged times. In conclusion, the increased salivary content of chlorides after intake of salty food and its subsequent time-dependent elimination may be a factor in the corrosion of dental amalgam, and thus part of an explanation ofthe variation in corrosion rate found between individuals.
243
dental alloys in vitro. Scand J Dent Res 1985; 93; 467-73. 3. JOHANSSON BI. An in vitro and in vivo study of galvanic currents between amalgam and gold alloy electrodes in saliva and saline solutions. Sccind J Dent Res 1986; 94; 562-8. 4. GREENER EH, MATSUDA K. Effect of oxygen on the corrosion of dental amalgam. J Orat Retuttvl 1985; 12; 123-33. 5. RAVNHOLT G . Corrosion of dental alloys in vitro by differential oxygen concentration. Scand J Dent Res 1986; 94; 370-6. 6. HOLLAND RI. Effect of pellicle on galvanic corrosion of amalgam. Scand J Dent Res 1984; 92; 93-6. 7. WRANGLEN G . An introduction to corrosion cind protection of metals. Frorne; Butler & Tanner Ltd, 1972; 92-3. 8. JENKINS G N . The ptiysiotogy and biociiemistry of ttie mouth. 4th ed. Oxford; Biackwell Scientific Publications, 1978; 284-413. 9. DAWES C. The approach to plasma levels of the chloride concentration in human parotid saliva at high flow rates. Arch Oral Biot 1970; 15: 97-9. 10. MARGOLIS HC. An assessment of recent advances in the study of the chemistry and biochemistry of dental plaque nuid. J Dent Res 1990; 69; 1337-42. 11. Guide to dental materials and devices. 7th ed. Chicago; American Dental Association, 1974-75; 171-2. 12. FiNKELSTEiN GF, GREENER EH. Mechanism of chloride corrosion of dental amalgam. J Orat Reliabit 1979; 6; 189-97. 13. PALAGHIAS G . The role of phosphate and carbonic acidbicarbonate buffers in the corrosion processes of the oral cavity. Dent Mater 1985; 1; 139^4. 14. MAREK M . The corrosion of dental materials. In; SCULLY, ed. Treatise on materiats science atid tectmotogy. Vol. 23. London; Academic Press, 1983; 331-94. 15. HAKANSSON B, YoNrcHEV E, VANNERBERG N - G , HEDEGARD
B. An examination of the surface corrosion state of dental fillings and constructions. I. A laboratory investigation of the corrosion behaviour of dental alloys in natural saliva and saline solutions. J Orat Retmbit 1986; 13; 235-46. 16. Quality evatiiation of dentat restorations. Criteria for ptacement and reptctcement. ANUSAVICE KJ, ed. Chicago; Quintessence Publishing Co., 1989. 17. LAGERLOF F, DAWES C. The volume of saliva in the mouth before and after swallowing. J Dent Res 1984; 63; 618-21. 18. DAWES C. Physiological factors affecting salivary flow rate, oral sugar clearance, and the sensation of dry mouth in man. / Dent Res 1987; 66; 648-53. 19. AASENDEN R, BRUDEVOLD F, RICHARDSON B. Clearance of
fluoride from the mouth after topical treatment or the use of a fluoride mouthrinse. Arch Oral Biol 1968; 13; 625-36.
References 1. FRAUNHOFER VON JA, STAHELI PJ. Gold-amalgam galvanic
cells. The measurements of corrosion currents. Br Dent J 1972, 132, 357-62. 2. ARVIDSON K , JOHANSSON EG. Galvanic currents between
20. LAGERLOE F, OLIVEBY A, EKSTRAND J. Physiological factors
influencing salivary clearence of sugar and fluoride. J Dent Res 1987; 66; 430-5. 21. LAGERLOE F. Salivary clearance and its effect on oral health. In; EDGAR W M , O'MULLANE DM, eds. Sattva and dentat
tieatth. 1st. ed. London; Brit Dent Journal, 1990; 69-80.
