Effect of pellicle on galvanic corrosion of amalgam

Abstract – Galvanic corrosion of amalgam, induced by contact with a type IV dental casting gold alloy, was determined under simulated oral conditions in an electrochemical cell. The effect of a pellicle layer formed by 1 h exposure to saliva in the oral cavity was determined. Pellicle on the amalgam had no effect on the maximum corrosion rate or the 2 h corrosion charge, whereas pellicle on the gold alloy substantially reduced both these parameters of the conventional low-copper amalgam; the corrosion of the high-copper amalgam was less and was not influenced by pellicle formation.     

Effect of oxygen on the corrosion of dental amalgam

The anodic and cathodic polarization of five dental amalgams were determined as a function of oxygen concentration in Ringer's solution and artificial saliva. The results indicate an anodic polarization behaviour dependent on Cl ion concentration and a corrosion potential determined primarily by oxygen concentration. At high oxygen concentrations the corrosion rates of traditional and copper enriched amalgams were equivalent.     

Unusual in vivo extensive corrosion of a low-silver amalgam restoration involving galvanic coupling: a case report.

The basic mechanism of dental amalgam corrosion has been thoroughly studied during the last 50 years by various experimental techniques, most often carried out in vitro. Electrochemical methods were extensively employed, and it was recognized that a gradual dealloying of the more electroactive components, Zn, Sn, and to a lesser extent Cu, contributed to change the surface composition. It is also well known that, in all circumstances, galvanic coupling threatens the longevity of the restoration. Among the patients examined at the hospital attached to the Department of Dental Surgery of the Paris 5 University, the authors observed, in a few exceptional circumstances of defective restorations, a release of liquid metal droplets in the surrounding tissue. These particles were identified as elemental mercury. This unexpected phenomenon was the result of the use of a low-silver-content alloy, combined with an enhanced corrosion process due to galvanic coupling with a silver alloy crown and brass screw-posts. In the described clinical case, the tooth, which had to be extracted, was axially sliced for observation by scanning electron microscopy coupled with elemental analysis by X-ray fluorescence energy dispersive spectroscopy. The observations showed a dramatic corrosion of the amalgam matrix with the formation of porosity even in the bulk of the material. Superposition of the element mappings proved (1) the electrochemical coupling of the amalgam with the silver-based crown, (2) the decalcification of the dentin, and (3) the formation of tin oxychloride precipitates in the matrix porosities and the interfacial gaps. These experimental results corroborate the corrosion mechanisms described hereupon. They support the current national and international recommendations for the dental amalgam formulation and for a rigorous professional practice.     

Effects of amalgam corrosion products on human cells

Using three independent criteria, we have found that 10⁻⁴-10⁻⁶M concentrations of ions presumably liberated from the corrosion of dental amalgam produce injurious effects on either human gingival fibroblasts or HeLa cells when the cells are grown in culture. Release of ⁵¹Cr and uptake of trypan blue dye were seen with 10⁻⁵M Hg⁺⁺ and Ag⁺. Inhibition of amino acid incorporation into protein-like material was seen with eluates of amalgam and with ionic solutions of most metals comprising dental amalgam. Stannous ion showed little if any cytotoxic potential. These results suggest that corrosion products of amalgam are capable of causing cellular injury or destruction.     

Crevice corrosion products of dental amalgam

The objective of this study was to determine the in vitro corrosion products that resulted from crevice corrosion of low- and high-copper dental amalgams. Specimens were potentiostatically polarized in a chloride-containing electrolyte while set against a PTFE surface to form a crevice. After 16 h, corrosion products were examined by light microscopy, SEM, EDS, and XRD. Analysis showed the presence of three previously reported products [Sn4(OH)6Cl2, SnO, and Cu2O] and a new product, CuCl, which formed on high-copper, gamma 2-free amalgams. Thermodynamic considerations show that CuCl is stable for the reported in vivo potentials of amalgam restorations and the high acidity and high chloride ion concentration associated with crevice corrosion.     

Mechanism of chloride corrosion of dental amalgam

Polarization of conventional dental amalgam in Ringer's solution produces a dissolution of the gamma 2 phase at--250 mV (SCE) and formation of Sn-Cl and Sn-O-Cl. This study was undertaken to determine the electrochemical nature of these reactions. Potentiostatic polarization scans were run at 4, 37 and 50 degrees C in several dilutions of aerated Ringer's solution. Manipulation of reactant concentrations and electrolyte temperature produced shifts in the potentiostatic profiles. At 50 degrees C, a gamma 2 dissolution peak shift to -300 mV was observed, and at 4 degrees C the peak shifted to -200 mV. Dilution of Ringer's solution with respect to [Cl-] over three orders of magnitude produced a linear shift in the noble direction of 500 mV. Cathodic polarization appears to be limited by the oxygen half cell reaction. Changes of [Cl] affect the shape of the peak, round out its approach to the current maximum. Shifts in peak potential due to temperature and concentration changes agreed with calculations based on critical pitting potential for gamma2 dissolution due to a chloride reaction. Polarization profiles in pooled saliva agreed with calculated potentials based on known saliva [Cl-].     

Investigation of surface corrosion in amalgam.

In this study, surface corrosion of carved and polished specimens of conventional and high-copper amalgams was investigated. The specimens were studied to detect surface corrosion after immersion in artificial saliva for 48 h or 1 year, and later investigated by SEM. Corrosion was greatest in 1-year-immersed carved conventional amalgam specimens. The surface was very rough with the contours of the particles visible and deep holes opening at the surface. High-copper amalgam specimens showed less corrosion than conventional amalgam specimens.     

Corrosion by galvanic coupling between amalgam and different chromium-based alloys.

OBJECTIVES: In recent years there has been an increase in the use of dental casting alloys in prosthodontic treatment. Many patients have metals or alloys, as well as amalgam fillings, in their mouth, and will have them for many years. The aim of this study was to evaluate and compare, in vitro, the galvanic corrosion behavior of chromium-cobalt alloy (Remanium GM 380) and chromium-nickel alloy (Remanium CS) when bound together or coupled with silver-based amalgam (Amalcap plus). METHODS: An electrochemical characterization of the alloys was performed by potentiostatic and potentiodynamic methods, i.e. the open circuit potential (OCP), the corrosion potential (E(CORR)), corrosion current density (i(CORR)) and corrosion resistance (R(P)). The electromotive force (EMF) of the bimetallic cells was also tested. Electroanalytical techniques were used to estimate the release of any respective element from the dental alloys under study into the artificial saliva solution. RESULTS: It was found that a bimetallic cell consisting of Remanium CS and Remanium GM 380 alloys has a very low EMF (a few mV) and is not a potential source of galvanic currents in the oral cavity. However, galvanic cells prepared from Amalcap plus and Remanium CS or Remanium GM 380 showed a much greater EMF: 104 and 109mV, respectively. This clearly indicates that in these latter cases it is possible to expect some metal ions in the saliva solution as a result of the work of galvanic currents. It was found, by adsorptive stripping voltammetry analysis, that nickel or cobalt, depending on the alloy used, appeared in the saliva solution and increased in concentration over time. SIGNIFICANCE: The results indicate that the correct design and use of dental alloys are important when determining the appropriate treatment for a specific patient.     

Effect of manifacturing technology on the resistance to corrosion of amalgam fillings and speed of mercury emission

The study investigates the influence of the various manufacturing technological alternatives of a given alloy on the corrosion properties, as well as it evaluates the quantity of mercury release from the amalgam filling having the same alloy. The resistance of dental amalgams to corrosion is the function of their precious metal (mercury and silver) content. The release speed of the mercury depends on the original condition of the alloy. The highest values were given for filing amalgam. When investigating the composition of amalgam mainly in spherical form, a significant decrease could be here determined.     

An application of impedance techniques to corrosion research on dental amalgam

AC impedance techniques are finding increasing application in corrosion research. It was our aim to introduce them as a method for the evaluation of the electrochemical behaviour of dental amalgam. The complex plane plots are large depressed semicircles representing the frequency dependence of the impedance and measured at the corrosion potential. Conventional dental amalgams, in a chloride medium, exhibit a current peak in the proximity of -250 mV (vs. SCE). The corresponding impedance spectra are rather complex and difficult to interpret. The high frequency part, however, can be fitted to a semicircle with its centre lying below the real axis. An evaluation of the corresponding 'charge transfer resistance' shows that it is inversely proportional to the corrosion current density.     

Correlation of dental amalgam crevice corrosion with clinical ratings

In vitro corrosion behavior of gamma 2-containing and gamma 2-free dental amalgams was examined under crevice conditions which simulated the amalgam-tooth interface. By means of potentiostatic testing, current-density/time behavior and integrated anodic current were measured for 16 hr. All of the amalgams demonstrated crevice corrosion susceptibility. Crevice corrosion propagation for gamma 2-free vs. gamma 2-containing amalgams was characterized by lower acceleration and maximum rates during the most dynamic period. Integrated anodic current results were correlated with marginal breakdown ratings reported in two clinical investigations. Strong and significant coefficients were found between integrated current and marginal breakdown ratings when gamma 2-containing and gamma 2-free amalgams were correlated as a single group, but not when correlated separately.     

In vivo galvanic currents of intermittently contacting dental amalgam and other metallic restorations.

OBJECTIVE: It was the objective of this study to determine the magnitude of in vivo galvanic currents produced by simulating electrical contact between occluding metallic restorations, and to examine the influence of restoration age, difference in pre-contact corrosion potentials, and surface area. METHODS: A convenience sample of 106 human subjects was studied. A Ag/AgCl micro-reference electrode and a high impedance voltmeter were used to measure the pre-contact corrosion potentials. Galvanic couples (n = 194) were next formed by simultaneously contacting occluding restorations with gold-plated probe tips for 15 s. The resulting current-time transients were measured with a zero-resistance ammeter and recorded with an electronic data acquisition program. The vast majority of couples measured contained at least one dental amalgam restoration. RESULTS: Galvanic current-time transients were typically characterized by an immediate and rapid rise to a peak current, followed by an exponential decay to a much lower value at 15 s. For couples with a restoration < or = 12 mo old, median peak current and median current at 15 s were 2.26 microA (range 0.24-13.06 microA) and 0.57 microA (range 0.03-6.47 microA), respectively. For couples with a restoration >12 mo old, median peak current and median current at 15 s were 1.40 microA (range 0.24-12.09 microA) and 0.37 microA (range 0.00-3.05 microA), respectively. Couples with fresh amalgams (< or = 6 mo) generally had elevated currents (range 2.56-102.54 microA). SIGNIFICANCE: A wide range of galvanic currents resulted from electrical contact of restorations in vivo. These currents were influenced by restoration age and total surface area of the galvanic couple. For amalgam-amalgam couples, the difference in the pre-contact corrosion potentials may be useful in predicting galvanic currents, when the difference is at least 24 mV.     

The ultrastructural localization of the corrosion products of dental amalgam

The ultrastructural localization of corrosion products released from dental amalgam implants in rat subcutaneous tissue was studied by means of a sulphide-silver technique. Implants stimulated a prolonged inflammatory response with delayed granulation tissue formation and slow and faulty collagen formation. The metallic corrosion products were found both in cells and in association with matrix components. Intracellular metal deposits were predominantly in the cytoplasmic matrix and in vacuoles in the early (19 day) lesions, but at 60 days there was metal in many nuclei, both viable and dead. Collagen fibrils had metal on the surface and vascular basement membranes had granules of bound metals.     

Time-dependent corrosion potential of newly-placed admixed dental amalgam restorations.

OBJECTIVE: It was the objective of this study to measure the corrosion potential over time of newly-placed admixed dental amalgam restorations as a possible indicator of corrosion activity. METHODS: The corrosion potentials of 271 amalgam restorations, 4-min to 24-month old were measured in a convenience sample of 81 subjects. The selected restorations had no occlusal or interproximal contact with other metallic restorations. Eighty-one percent of the restorations aged 4 min to 7 months were made from zinc-containing admixed high-copper amalgam. The remaining amalgams were an unknown mixture of admixed high-copper and single-composition-alloy amalgams. RESULTS: The data were fit to a mixed-effects model with random patient effects to allow for the correlation, using the maximum likelihood method. The early data best fit a quadratic model with an initially rapidly rising corrosion potential that leveled off to a constant mean value of -146 (60)mV (versus Ag/AgCl, 3M KCl at 35 degrees C) by approximately 7 months. Most of the ennoblement occurred within the first 4 months after restoration placement. SIGNIFICANCE: This in vivo study observed a slower rate of corrosion potential ennoblement for admixed amalgam restorations than would be predicted from in vitro studies. The slower rate is believed due to the combination of mechanical, chemical and biological forces in the mouth that are generally absent using simulated conditions. The slower rate of ennoblement suggests a slower rate of achieving maximum corrosion resistance than would be predicted using in vitro studies.     

Marginal degradation and corrosion of a dispersed high copper amalgam

The marginal degradation and corrosion of restorations made of a high copper dispersed amalgam were studied as a function of time for up to 5-yr observation periods. The effect of minor defects present after finishing the restorations was included in the assessment of the initial marginal degradation. A slow progressive marginal degradation was noted and it was enhanced by initial defects. Subsurface corrosion including attacks of the phase and grain boundaries, corrosion within the bulk, and layers of corrosion products along the tooth/restoration interfaces were observed in restorations after some years in clinical service. The subsurface corrosion is considered to weaken the amalgam and it might enhance the marginal degradation, i.e. a stress corrosion type of degradation.