Journal of Oral Rehabilitation, \911, Volume 4, pages 261-267
The in vitro measurement of amalgam corrosion rates by the polarization resistance technique
p. J. S T A H E L I and}. A. VON F R A U N H O F E R Department of Dental Materials, Institute of Dental Surgery, University of London
Summary
Measurement of the corrosion rates of three distinctive amalgam alloys (lathe-cut, spherical and dispersed-phase) when immersed in three different electrolytes, including saliva, is reported. The results indicate that the dispersed phase alloy amalgam exhibits a lower corrosion rate than a conventional lathe-cut alloy amalgam but is somewhat inferior to a spherical alloy amalgam. Introduction
An accurate and very useful method for determining corrosion rates is the polarization resistance technique (von Fraunhofer, 1975). This technique originated from observations by Simmons (1955) and Skold & Larson (1957) that the slope AE/AI of the cathodic polarization curve near the corrosion potential bears a relationship to the corrosion rate of the polarized metal electrode. It should be noted that whenever an electrode passes a current, for example when it corrodes, its potential will change from its reversible value and the phenomenon is known as polarization. Furthermore, when a metal corrodes there are two concurrent reactions occurring, namely the metal dissolution or anodic reaction and a cathodic reaction involving the liberation of hydrogen gas (in acidic media) or oxygen reduction (in neutral or basic media). The corrosion potential of the metal is generally found to lie between the reversible potentials of the anodic and cathodic reactions, that is it exhibits a mixed potential, Ecorr (Fontana & Greene, 1967; von Fraunhofer, 1975). Later, Stern & Geary (1957) showed that AE/Al is inversely proportional to the corrosion rate within + 10 mV of the corrosion potential (Fig. 1). This term AE/Al has the dimensions of resistance and was called polarization resistance by Stern (1958). The basis of the technique is that the polarization resistance AE/AI is related to the corrosion current Icorr by the Stern-Geary equation:
AI
2-3(/3a+A.)Icorr
^^
where AE = observed potential shift AI=: external applied current density producing the potential shift Correspondence: Dr J. A. von Fraunhofer, Department of Dental Materials, Institute of Dental Surgery, University of London, Eastman Dental Hospital, 256 Gray's Inn Road, London WCIX 8LD.
261
262
P. J. Staheli and J. A. von Fraunhofer Polarisation resistance Overpotential
(nnV)
20 r
10
Anodic Applied current density ^
Cathodic
-10
-20'Fig. 1. Schematic diagram of the relationship between overpotential and current density.
and /3a, /3(. = anodic and cathodic Tafel slopes and provided the electrode potential is maintained within + 10 mV of the corrosion potential. This equation is used to determine the corrosion propensity of an electrode by measuring the current AI, required to polarize the electrode by ± 10 mV or less from its mixed or corrosion potential EcorrIf the j8 values (or Tafel constants) of the anodic and cathodic polarization are not known or cannot be determined readily from the curves, they can be taken to be 120 m V (the average value for most corrosion systems) and the equation will reduce to: AE_ 0-026 Al
Icorr
(2)
The slope of the linear polarization curve, viz. AE/AI is mainly determined by Icorr and will be relatively insensitive to the Tafel slopes (Fontana & Greene, 1967; von Fraunhofer, 1975). In this study the metal electrode is polarized 10 mV away from its rest potential, with respect to a reference electrode, by means of a potentiostat (Neufeld, 1964). Since the corrosion potential of some dental amalgams may be hundreds of millivolts from that of the reference electrode, it is difficult to accurately determine and maintain the required potential shift of 10 mV within the linear portion of the polarization curve. To overcome this difficulty it is possible to use as the reference electrode an unpolarized specimen of the same metal as that under investigation. Materials and methods
With regard to the above, the reference electrode, therefore, was prepared from the same amalgam as the test specimen. Samples of dental amalgams Ar, K and D as specified in Table 1 were prepared and condensed simultaneously into two cylindrical cavities 4 mm internal diamaterx 10 mm cut into cylindrical acrylic blocks. Insulated connectors ran from the base of each specimen (Fig. 2) so that twin identical amalgam electrodes were produced. The amalgams were allowed to age for 1 week and were then polished to a 17 /xm finish using water-cooled 600 grit carborundum paper on a rotating polishing wheel.
In vitro measurement of amalgam corrosion rates
263
Table 1. Amalgam alloys
Name
Code
Aristaloy
Ar
Kerr Spher-a-cap Dispersalloy
Hg/Alloy ratio
Trituration time(s)
6:5
10
K
4-8:5
7
D
5:4
20
Manufacturer Engelhard Industries, Baker Platinum Div., England Kerr Dental Mfg. Co., U.S.A. Western Metallurgical Ltd, Canada
(3)
Fig. 2. The electrodes used in polarization resistance measurements. Fig. 3. Circuit diagram for polarization resistance studies.
A glass sleeve was placed around the specimen holder to act as a reservoir for the electrolyte. The cell was connected to a DC millivolt source (Type 2003, Time Electronics Ltd, England) such that one amalgam specimen was held 10 mV anodic to the other. The circuit diagram is shown in Fig. 3. The external current flow corresponding to this 10 mV potential, viz. I was measured by means of the iR drop across a 1 MO resistor using a Solartron high impedancedigital voltmeter. The current flow was determined at 5 min intervals for 1 h, and then at quarter or half hourly intervals for a further period of 2 h. The following electrolytes were used in this study: (a) 1 % KCl solution, pH 6-2; (b) Natural saliva produced by wax stimulated secretion, pH 7-7-4; (c) A mixture of equal volumes of 1 % KCl (pH 6-2) and fresh wax stimulated natural saliva. The saliva was collected from a single person at the same time each day (at 16.00 hours) prior to the experimental study and the person involved was a non-smoker with good oral hygiene and general health with a mixture of gold, amalgam and silicate restorations. The volume of electrolyte used in all studies was 5 ml and the natural saliva
264
P, J. Staheli and J. A. von Fraunhofer 6) Saliva + KOI solution
( 5 ) Sahvo l-7r-
l-lr
*
•5 F \
0-9
0-7
10 20 30 40 50 60 90150
10 20 30 40 50 60 90 150 Time (mm)
10 20 30 40 50 60 90 150
Fig. 4. The corrosion current-time behaviour of three dental amalgams in 1 ','„ KCl (pH 6 2) at 20 C. Fig. 5. The corrosion current-time behaviour of three dental amalgams in natural saliva (pH 7-0) at 20 C. Fig. 6. The corrosion current-time behaviour of three dental amalgams in a mixture of natural saliva and 1 % KCl solution at 20 C.
electrolyte was always covered by a layer of liquid paraffin to obviate pH changes due to loss of dissolved carbon dioxide. In all cases static solutions and ambient temperature (23+1 C) conditions prevailed. Each experiment was performed in duplicate and was found to be highly reproducible. The corrosion current Icorr was obtained by calculation, viz. substituting the external current and potential shift (10 mV) into Equation 2. Results Typical corrosion current vs time plots for the amalgams in the three electrolytes are given in Figs 4-6. The initial corrosion currents for all three amalgams in 1 % KCl solution are similar. Fig. 4. There is, however, an initial decrease in Icorr for 10 min followed by an increase in Icorr to a quasi-limiting value after I^IA min for Ar and D. The spherical alloy K, however, exhibited a progressive but slow decrease in corrosion current throughout the test. In contrast, the initial corrosion currents of D and K were similar in saliva (Fig. 5) whilst Ar exhibited a 40",, greater Icorr value. Ar showed an increasing corrosion current over the first 25 min followed by a slow decrease in Icorr up to 60 min and a rapid decrease was found after this time. K exhibited a virtually unchanged corrosion current throughout the test period until the later stages {viz. after 90 min). In contrast, D showed a small initial decrease over 5-10 min followed by a progressive increase in corrosion rate throughout the remainder of the test period. Polarization of the corrosion reactions of all three amalgams appears to occur in the later stages of the test. The behaviour of the three amalgams in the saliva-KCl mixture is intermediate between that in the other two electrolytes. Ar and K exhibited behaviour similar in this solution to that in KCl but D was markedly different (Fig. 6). It can be seen that Icorr for Dispersalloy decreased by some 60 % from its initial value over the first 20 min
In vitro measurement of amalgam corrosion rates
265
and then it increased again to a quasi-limiting value over the next 40 min. This behaviour was not observed with Ar or K. Discussion The detailed mechanism of amalgam corrosion has been postulated and verified elsewhere (von Fraunhofer & Staheli, 1971. 1972a) and need not be considered here. It is apparent, however, that the three amalgams considered here differ in their corrosion behaviour in the three test electrolytes. The lathe-cut alloy, Ar, exhibited the greatest corrosion rate in all media, and this finding supports other work on amalgam galvanic corrosion cells (von Fraunhofer & Staheli, 1972b). It is also interesting to note that natural saliva appears to be more aggressive than 1 % KCl although it should be noted that saliva can vary markedly from person to person and consequently its corrosiveness will also vary. Furthermore it should be noted that the corrosion currents are of a very low order and are akin to those found in passive metal systems, e.g. stainless steel in aerated acid media. The Icorr-time behaviour observed here for the different amalgams is similar to that found for the inter-reactions of the constituent phases of the matrix material. Other work has shown that 72 is anodic to both y and yi as well as to mercury whilst it has been demonstrated that yi is anodic to mercury-rich yi and to mercury (von Fraunhofer & Staheli, 1972a). Furthermore it was observed that a mixture of yi + yo was anodic to yi (von Fraunhofer & Staheli, 1972b). It is clear from the above, therefore, that the corrosion behaviour of a set amalgam will be determined largely by the matrix rather than the residual y phase present. Since the Icorr-time behaviour of the set amalgams was found to parallel the current decay in galvanic couples composed of the constituent phases in the matrix, the observed differences in the three amalgams must be due to the proportions of the different phases in the matrices. The behaviour of Ar amalgam corresponds closely to the behaviour of a yi vs mercury galvanic couple whilst that of K amalgam resembles that of a yi + y2/yi couple. It would appear then that the lower corrosion susceptibility of amalgam K when compared with Ar amalgam is due to a greater juxtaposition of y2 to yi in the matrix in the set Kerr amalgam. There would be a more random distribution of y2 in the Ar matrix. These findings are supported by electrochemical etching studies (Staheli & von Fraunhofer, 1974). Consequently, the formation of yi and y2 phases in close proximity within the matrix would appear to be conducive to enhanced corrosion resistance. The atypical behaviour of D must be ascribed to the complex nature of the set amalgam. There would appear to be a minimal y2 content in the matrix whilst a novel eutectic alloy phase, together with its amalgamation products, is present (Johnson, 1972; Staheli & von Fraunhofer, 1974; Mahler, Adey & van Eysden, 1975). At this time, the nature of the set D amalgam is not yet fully eludicated and a detailed explanation of the observed corrosion behaviour must await the characterization of both the setting reaction and the amalgam matrix. It can be stated, however, that D amalgam appears to be superior to Ar amalgam but slightly inferior to K amalgam in corrosion resistance. The results presented here indicate that amalgam behaves as a quasipassive metal in aggressive media, e.g. KCl solution, and exhibits very low corrosion rates. Differences in corrosion rates will arise through variations in the matrix composition (and the initial mercury-alloy ratio, e.g. K amalgam which has a low ratio). Other variations 18
266
P. J. Staheli and J. A. von Fraunhofer
will arise in the measured corrosion behaviour of amalgam when the composition, pH and conductivity of natural saliva change. Comparison of this data with previous work on amalgam galvanic cells (von Fraunhofer & Staheli, 1972c) indicates that there was a greater difference in the corrosion of K and Ar when coupled to gold than the 'free' corrosion studies reported here would indicate. This difference can only be ascribed to the fact that galvanic coupling of dissimilar metals will accelerate any corrosion tendencies due to depolarization of the cathodic reaction. Greater corrosion rates will, therefore, be found in dissimilar metal cells than in non-perturbated systems as reported here. It should also be noted from the work reported here that natural saliva appears to be rather more corrosive than KCl solution and gives no indication of the inhibitive capability proposed previously (von Fraunhofer & Staheli, 1972c). Several factors enter into this; firstly, the volume of electrolyte and the amount of dissolved oxygen, secondly, the pH and conductivity of the saliva, thirdly, the time of day when saliva is collected and the method of collection, and fourthly, various physiological and dietary factors. Furthermore, the previous dissimilar metal studies were performed on freshly prepared amalgam surfaces (which have a greater initial corrosion susceptibility) and the saliva was not covered with liquid paraffin. Accordingly it would be inadvisable to conclude that saliva necessarily is more corrosive than 1 % KCl since it is known to vary markedly with the individual although the inhibitive properties of saliva have not been conclusively established. Other workers have shown, for example, that ex-hog gastric mucin has no effect on the corrosiveness of artificial saliva (Carter, Ross & Smith, 1967).
Conclusions
The technique for corrosion monitoring reported here shows great promise as a method for assessing the corrodibility of dental amalgam. The ease and speed of measurement should facilitate routine assessment of the corrosivement of patients' saliva and the suitability of amalgam restorations for certain types of patient. Polarization resistance measurements are also particularly useful in detecting (and assessing) the effects of changes in oral conditions, diet, surface finish and oral hygiene on metallic restoration corrosion. It should be noted that although the measured corrosion currents are low, e.g. akin to that of passive stainless steel in many media, an Icorr value of 1 fiA/cm- represents a metal loss of 5 mg/dm-^/day (5 mdd). This is a low to moderate corrosion rate and would be sufficient to plug or seal small crevices at the marginal walls of amalgam restorations in a relatively short period, particularly with freshly condensed amalgams which have a higher initial corrosion susceptibility. References D.A., Ro.ss, T.K. & SMITH, D.C. (1967) Some corrosion studies on silver-tin amalgam. British Journal of Corrosion, 1, 199. FONTANA, M . G . & GREENE, N.D. (1967) Corrosion Engineering. McGraw Hill, New York. JOHNSON, L . N . (1972) Phase discrimination using scanning electron microscopy techniques. Journal of Dental Research, 51, 789. MAHLER, D.B., At:)EY, J.D. & VAN EYSDEN, J. (\915) Journal of Dental Research, 54, 218. NEUFELD, P. (1964) Application of the polarization resistance technique to corrosion monitoring. Corrosion Science, 4, 245. CARTER,
In vitro measurement of amalgam corrosion rates SIMMONS, E.J.
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(1955) Use of the Pearson bridge in corrosion inhibitor evaluation. Corrosion, I I ,
225. & LARSON, T . E . (1957) Measurement of instantaneous corrosion rate by means of polarization data. Corrosion, 13, 139. STAHELI, P.J. & VON FRAUNHOFER, J.A. (1974) Electroetching of Dental Amalgam. Journal of Dental Research, 53, 468. STERN, M . (1958) A method for determining corrosion rates from linear polarization data. Corrosion, 14, 440. STERN, M . & GEARY, A.L. (1957) Electrochemical polarization I. A theoretical analysis, the shape of polarization curves. Journal of the Electrochemical Society, 104, 56. VON FRAUNHOFER, J.A. (1975) Concise Corrosion Science. Portcullis Press, London. VON FRAUNHOFER, J.A. &STAHEL], P.J. (1971) Corrosion of amalgam restorations—a new explanation. British Dental Journal, 130, 522. VON FRAUNHOFER, J.A. & STAHELI, P.J. (1972a) Corrosion of dental amalgam. Nature, 240, 304. VON FRAUNHOFER, J.A. & STAHELI, P.J. (1972b) The measurement of galvanic corrosion currents in dental amalgam. Corrosion Science, 12, 767. VON FRAUNHOFER, J.A. & STAHELI, P.J. (1972c) Gold-amalgam galvanic cells. The measurement of corrosion currents. British Dental Journal, 132, 357. SKOLD, R . N .
Manuscript accepted 12 July 1976