Evaluation of the long-term corrosion behavior of dental amalgams: influence of palladium addition and particle morphology
Introduction
Dental amalgams are discussed according to their mercury content and release [1], [2], [3]. However, amalgam is a filling material which is still widely used because of its cost, durability and ease of manipulation [4], [5]. In the US, approximately, 96 million amalgam restorations were placed in 1991 [6].
Corrosion is the primary degradation process dental amalgam undergoes [7], [8]. Since the longevity of the restoration is put forth as an argument in its defense [5], [9], [10], a consideration of its corrosion behavior as a function of time could be of interest.
The setting reaction of high copper amalgam leads to the formation of two new phases: [11] the silver–mercury gamma 1 phase, and the copper–tin eta-prime phase, the most corrodable phase in high copper amalgam. Some transformation from the gamma 1 phase to the more thermodynamically stable beta 1 phase [12] can occur. The composition of tin in the different phases is modified as a function of time [13].
Evidence exists that low rates of mercury are released from aged dental amalgam restorations [14], [15], [16], [17], [18], [19], [20]. Powder morphology, especially the use of spherical particles, is well known as a way of reducing the mercury ratio.
Efforts have been made to reduce this mercury emission by altering the alloy ratio [21], [22], and by trituration of the alloy with binary mercury/indium [23], [24]. Mahler et al. studied the influence of tin associated with the gamma 1 phase on the vapor pressure of gamma 1 and on the release of mercury [25]. More recently, addition of palladium into the amalgam powder has been shown to be an effective way to reduce mercury release [26].
Mercury release from dental amalgam decreases as a function of amalgam age during the first 30 days. [27]. This result could occur in relation to microstructural changes associated with the solid state transformation reported by Marshall and Marshall [28]. It would, therefore, be of interest to consider the preservation of some clinically acceptable older restorations, in view of the low mercury release rates in such restorations.
The addition of palladium has been reported to have a positive influence on the corrosion behavior of dental amalgams [29], [30].
Corrosion behavior is often evaluated on samples of amalgams without information about their age. Studies concerning the improvement of corrosion resistance obtained by palladium addition are also short-term studies.
The aim of this electrochemical study is to evaluate, in vitro, based on an experimental single formulation of a high copper amalgam powder:
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the influence of particle morphology
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the influence of addition of 0.5 and 1% palladium on the corrosion behavior
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the evolution of this corrosion behavior on a long-term clinically significant period extending over 10 years.
Section snippets
Amalgams
The four experimental powders were prepared by a French manufacturer, Dentoria, in order to obtain the same chemical composition, but differing in spherical or lathe cut morphology, and to obtain the same spherical particles with differing ratios of palladium. Their chemical compositions are represented in Table 1.
The amalgams were triturated in an amalgamator (Silamat, Ivoclar, Schann, Liechtenstein) for 8 s and mechanically condensed into cylindrical specimens (4.0 mm diameter X 5.0 mm height)
Results
The open circuit potentials at 5 months and 10 years are shown on Table 3. For each specimen open circuit potential increases after 10 years. When comparing the values between the different amalgams, the data range decreases after 10 years.
Potentiokinetic curves of different amalgams for specimens 5 months old are shown on Fig. 2.
The current density increases for the LCP sample at a lower potential value than for all the other spherical amalgams.
With regard to the current density of the first
Discussion
The current density at the first anodic peak appears to be a good method for studying the corrosion behavior of dental amalgams. High rates of degradation occur for this potential value but the range of potential is beyond the open circuit potential data. However, in oral situations, galvanic situation with other metallic restorations are able to reach this range of potential values (approximately +40 mV/SCE) as demonstrated by previous studies [31], [32]. Otherwise, the range of potential
Acknowledgements
The authors are grateful to the manufacturer, Dentoria, for the elaboration of experimental amalgam powders.
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