Release of corrosion products from amalgam in phosphate containing solutions.

The effect of phosphate concentration on corrosion was compared for two types of amalgam: a conventional alloy (ANA 68) and a high-Cu admixed alloy (Dispersalloy). The test specimens were stored for 4 months in electrolytes containing 85 mM NaCl and 85 mM NaCl with 2.5, 10, or 100 mM phosphate buffer respectively. The solutions were renewed each month and analyzed for Cu, Zn, Sn, Hg, and Ag in an atomic absorption spectrophotometer. The surfaces and cross-sections of the specimens were studied in a scanning electron microscope (SEM) with an energy dispersive detector (EDAX). The corrosion products, mainly Sn-compounds, at the surface of the amalgams were less in the solutions containing high concentrations of phosphate. In cross-section subsurface corrosion of the high-Cu amalgam was observed especially in specimens immersed in the NaCl solution without phosphate. The conventional amalgam showed surface corrosion only. The decrease in release of elements with time from the conventional amalgam in all the experimental solutions might indicate passivation. For the high-Cu amalgam the release of elements increased with time, except for Cu and Sn in the solution with 100 mM phosphate, indicating that phosphate inhibits corrosion of the Cu-Sn-phases. Release of corrosion products from the high-Cu amalgam was more dependent on the presence of phosphate than the conventional amalgam.     

Area ratio effects on metal ion release from amalgam in contact with gold

Abstract – The effect of area ratio, chloride concentration and brushing on amalgam in contact with gold was studied by measuring the amounts of elements released in the electrolyte. A type III gold alloy was stored for 4 months in contact with a conventional amalgam, area relations 6/1 and 6/3, in an electrolyte containing 85 mM NaCl with 10 mM phosphate buffer. A specimen with area relation 6/1 was also stored in a 10 mM NaCl solution with 10 mM phosphate buffer. The solutions were renewed each month and analyzed for Cu, Zn, Sn, Hg, and Ag in an atomic absorption spectrophotometer. Cross-sections of the amalgams were studied in a scanning electron microscope. The tendency for Sn and Cu to be released from the amalgam was greater than for the other elements. The Sn-release probably mainly originated from surface corrosion and Cu-release from subsurface corrosion. A stronger galvanic influence enhanced only the release of Cu, and to a less extent Zn. The subsurface corrosion of the amalgam and increasing release of Cu was, in contrast to the other elements, largely dependent on a high chloride concentration in the electrolyte. Light brushing of the specimens had no effect on the amounts of elements released.     

Area ratio effects on metal ion release from amalgam in contact with gold

The effect of area ratio, chloride concentration and brushing on amalgam in contact with gold was studied by measuring the amounts of elements released in the electrolyte. A type III gold alloy was stored for 4 months in contact with a conventional amalgam, area relations 6/1 and 6/3, in an electrolyte containing 85 mM NaCl with 10 mM phosphate buffer. A specimen with area relation 6/1 was also stored in a 10 mM NaCl solution with 10 mM phosphate buffer. The solutions were renewed each month and analyzed for Cu, Zn, Sn, Hg, and Ag in an atomic absorption spectrophotometer. Cross-sections of the amalgams were studied in a scanning electron microscope. The tendency for Sn and Cu to be released from the amalgam was greater than for the other elements. The Sn-release probably mainly originated from surface corrosion and Cu-release from subsurface corrosion. A stronger galvanic influence enhanced only the release of Cu, and to a less extent Zn. The subsurface corrosion of the amalgam and increasing release of Cu was, in contrast to the other elements, largely dependent on a high chloride concentration in the electrolyte. Light brushing of the specimens had no effect on the amounts of elements released.     

Marginal microhardness of corroded amalgams: a comparative in vitro study

One conventional and two high-Cu amalgams were tested for marginal microhardness after 2 months' corrosion in an 85 mM NaCl solution. Amalgams immersed in 200 mM phosphate buffer solution were used as controls. The microhardness tests were conducted on cross-sections of the amalgams 50 microns from the surface edges. The microstructure of the amalgams was studied in SEM and the amounts of Sn, Cu, Zn, Ag, and Hg dissolved in the solutions were analyzed with an atomic absorption spectrophotometer. For the amalgams immersed in the NaCl solution the depth of corrosion after 2 months was between 50 and 400 microns. The specimens immersed in the phosphate solution showed no signs of subsurface corrosion. The marginal microhardness of all the amalgams was reduced after corrosion in the NaCl solution. The greatest microhardness in both the uncorroded and corroded states was shown in the two high-Cu amalgams. The reduction in marginal microhardness after corrosion can probably be attributed mainly to degradation of the gamma-2 phase for the conventional amalgam and to degradation of the eta' phase for the two high-Cu amalgams.     

Marginal microhardness of corroded amalgams: a comparative in vitro study

Abstract – One conventional and two high-Cu amalgams were tested for marginal microhardness after 2 months' corrosion in an 85 mM NaCl solution. Amalgams immersed in 200 mM phosphate buffer solution were used as controls. The microhardness tests were conducted on cross-sections of the amalgams 50μm from the surface edges. The microstructure of the amalgams was studied in SEM and the amounts of Sn, Cu, Zn, Ag, and Hg dissolved in the solutions were analyzed with an atomic absorption spectrophotometer. For the amalgams immersed in the NaCl solution the depth of corrosion after 2 months was between 50 and 400μm. The specimens immersed in the phosphate solution showed no signs of subsurface corrosion. The marginal microhardness of all the amalgams was reduced after corrosion in the NaCl solution. The greatest microhardness in both the uncorroded and corroded states was shown in the two high-Cu amalgams. The reduction in marginal microhardness after corrosion can probably be attributed mainly to degradation of the gamma-2 phase for the conventional amalgam and to degradation of the η' phase for the two high-Cu amalgams.     

Corrosion behavior and microhardness of three amalgams

The marginal microhardness of three different types of amalgam was tested after 2 months' immersion in an aqueous solution of NaCl (85 mM) and phosphates (Na2HPO4 100 mM and NaH2PO4 100 mM). Amalgams immersed in distilled water were used as controls. The microhardness tests were conducted at a distance of 50 micron from the margins and at the bulk of each specimen. The solutions were analyzed for Sn, Cu, Zn, Ag, and Hg by means of atomic absorption spectrophotometry (AAS). A statistically significant reduction in the marginal microhardness after immersion in the test solution was found for the conventional and the high-Cu single composition amalgam but not for the high-Cu blended amalgam. SEM-examination of cross-sections of the amalgams revealed small areas of subsurface grain boundary corrosion, no deeper than 10 micron for all the amalgams. The SEM-examination of the specimens and AAS analysis of the solutions indicated that the reduction in marginal microhardness was attributed mainly to corrosion of the Cu-rich phases for the high-Cu single composition amalgam and to corrosion of the gamma 2 phase for the conventional amalgam. The phosphates reduced the corrosion of the amalgams in the presence of NaCl. It is concluded that the marginal strength of dental amalgams in a corrosive environment is largely dependent upon their corrosion resistance.     

Corrosion behavior and microhardness of three amalgams

Abstract – The marginal microhardness of three different types of amalgam was tested after 2 months' immersion in an aqueous solution of NaCl (85 mM) and phosphates (Na₂HPO₄ 100 mM and NaH₂PO₄ 100 mM). Amalgams immersed in distilled water were used as controls. The microhardness tests were conducted at a distance of 50 μm from the margins and at the bulk of each specimen. The solutions were analyzed for Sn, Cu, Zn, Ag, and Hg by means of atomic absorption spectrophotometry (AAS). A statistically significant reduction in the marginal microhardness after immersion in the test solution was found for the conventional and the high-Cu single composition amalgam but not for the high-Cu blended amalgam. SEM-examination of cross-sections of the amalgams revealed small areas of subsurface grain boundary corrosion, no deeper than 10 μm for all the amalgams. The SEM-examination of the specimens and AAS analysis of the solutions indicated that the reduction in marginal microhardness was attributed mainly to corrosion of the Cu-rich phases for the high-Cu single composition amalgam and to corrosion of the γ₂ phase for the conventional amalgam. The phosphates reduced the corrosion of the amalgams in the presence of NaCl. It is concluded that the marginal strength of dental amalgams in a corrosive environment is largely dependent upon their corrosion resistance.     

Electrochemical properties of corroded amalgams

Three types of amalgam, one conventional, ANA 68, and two with high copper content, Dispersalloy (dispersed type) and ANA 2000 (single composition type), were investigated. The amalgams were immersed for periods of 7 wk at a time, up to 35 wk, in 23 ml (37 degrees C) of 0.9% NaCl aqueous solutions and in 0.9% NaCl solution buffered with NaH2PO4 (8.8 mM) and Na2HPO4 (1.2 mM). The amalgam specimens were embedded in epoxy resin. The surface area of amalgam exposed to the solutions was 0.2 cm2 for each specimen. Every 7 wk the corrosion potential was measured, the amalgam specimens lightly brushed with a soft toothbrush, and the solutions renewed. After 14-21 wk and 35 wk the currents during anodic polarization sweeps over the amalgams were recorded. The corrosion potential for the high-Cu amalgams was somewhat more positive (noble) in the phosphate buffered solution than in the non-buffered solution during the 35 wk of corrosion. The phosphate buffer reduced the reactivity of the amalgams during anodic polarization. Corrosion made the amalgams more passive during the anodic polarization. However, all the elements leached from the amalgams into the solutions throughout the entire experiment.     

Long-term corrosion studies in vitro of amalgams and casting alloys in contact

One conventional amalgam and two amalgams with higher copper content were stored in contact with various dental casting alloys in phosphate-buffered 0.9% NaCl solution, pH 6, at 37 degrees C during 35 weeks. Every 7 weeks the solutions were changed, and the corrosion was measured by analyzing the solutions for the amount of Cu, Zn, Sn, Hg, Ag, Co, Cr, and Ni. In the first 7 weeks the amalgams immersed in contact with acrylic or Co-Cr alloys released small amounts of elements, whereas the release was larger when the amalgams were in contact with the gold alloys. During the rest of the experiment the release of elements tended towards the same level, independent of the casting alloy in contact. In general, the high copper amalgams released more corrosion products into the solutions than the conventional one. The release of corrosion products was strongly pH-dependent. Changing the solutions more often and saturating the solutions with oxygen decreased the amounts of Hg and Ag released.     

Recent developments in alloys for dental amalgams: their properties and proper use.

Since the development more than ten years ago of a new type of amalgam alloy that will react with mercury without forming the unfavourable gamma2 (Sn7-8Hg) phase, several new alloy brands with a similar reaction pattern have been presented. This new generation of amalgam alloy is called non-gamma2 alloy as distinct from the conventional alloys. Three of the new alloys studied contain approximately three volume percent gamma2 and should be classified as modified concentional amalgams. The non-gamma2 amalgams may be characterized in the following way: (1) instead of gamma2 they contain a reaction product of copper and tin (Cu6Sn5), the so-called eta1 phase. (2) They are significantly more resistant to corrosion than the conventional amalgams but do in time produce sufficient solid corrosion products to seal the micro-fissures between filling and cavity walls. (3) When corroding, they do not release metallic mercury, and the solid corrosion products formed and precipitated on the free surface of the fillings seem relatively little resistant to the organic acids of the plaque. (4) Most of them have relatively low creep and tensile strength, but high compressive strength. (5) Clinically the non-gamma2 amalgams are remarkable for superior marginal integrity and, seemingly, also for improved persistence of surface lustre.     

The microstructure of corroded amalgams

One conventional amalgam and two amalgams with a high copper content were stored in 0.9% NaCl solution buffered with phosphate to pH 6. In one experimental series the amalgams were placed in contact with a gold alloy. Every 7 weeks the solutions were changed and analyzed with regard to elements released from the amalgams. The microstructure of the specimens was studied in a scanning electron microscope before immersion and after 7 and 35 weeks in the solution. All the amalgams corroded along the grain boundaries in the gamma 1 phase. Corrosion was greatest in the gamma 2 phase of ANA 68, in the eta phase of ANA 2000 and in the reaction zone (eta + gamma 1) surrounding the Ag-Cu-eutectic particles of Dispersalloy. The microstructure of the corroded amalgams showed similarities to amalgams corroded in vivo. The change in microstructure observed in cross-sections of the corroded specimens was related to the amounts of corrosion products released into the saline solution.     

Electrochemical properties of copper and gold containing dental amalgams

Alloying or admixing of a dental amalgam alloy with an element or alloy which has a higher affinity for tin offers a means for eliminating the corrosion prone γ₂ phase. Accordingly, the corrosion behaviour of non-conventional amalgams containing Cu and Au were investigated by anodic polarization studies in Ringer's solution. The Cu containing amalgams were produced by admixing Cu-Hg (copper amalgam) with conventional amalgam in the ratios of 0.5:1 and 1:1. A 10 w/o Au in Ag₃Sn was used as the gold amalgam alloy. The anodic polarization profile of the 0.5:1 Cu admixed was similar in shape to conventional amalgams although current densities were less. After ageing for I month the – 250 mV peak associated with γ₂ disappeared and the current density decreased by ten fold. After 2 months the current density decreased by one hundred fold. The anodic polarization of the 1: 1 Cu admixed amalgams did not display the -250 mV (γ₂) peak after 1 week and current densities were I0³ below that of conventional amalgams. The behaviour of these interesting amalgams is rationalized on the basis of the ease of formation of Cu-Sn intermetallics where Cu/Sn > 3. The anodic polarization profile of 10 w/o Au containing amalgam essentially is that of conventional amalgam and does not change drastically after 6 months. Although the Au may reduce or eliminate the γ₂ phase the resulting AuSn₄ apparently corrodes in an analogous fashion. However, since distribution of AuSn₄ may be different from γ₂, i.e. not continuous, the effect of corrosion on its mechanical properties may be different.     

Electrochemical stability of dental materials. Part 4. Immersion test of amalgam

Specimens prepared with three commercial dental amalgams were immersed in three kinds of solutions: Ringer's, 1% lactic acid and 0.05% hydrochloric acid, at 37 degrees C, and subjected to a repeated corrosion test for fifteen weeks. Quantitative analysis was performed by inductively coupled plasma emission spectrometry for released metal elements to determine the composition of these amalgam alloy powders and immersed specimens. The immersion test for one week was not sufficient to prove the corrosion tendency of amalgam and a longer period of immersion was considered necessary. The kinds and amounts of released elements in each solution differed, and the corrosion tendency of high copper amalgam differed from that of conventional amalgam.     

Long-term corrosion studies in vitro of amalgams in contact

Three types of amalgams, one conventional and two with a high copper content, were stored in phosphate-buffered 0.9% NaCl solution, at pH 6, for 35 weeks. Every 7 weeks the solutions were changed and analyzed with regard to Cu, Zn, Sn, Hg, and Ag. In one of the amalgam combinations, the conventional amalgam and one of the copper-rich amalgams in an area ratio of 2:1, contact between the amalgams clearly increased the amounts of Cu, Hg, and Ag released the first 14 weeks compared with when immersed in separate solutions. With the reversed area relation, Cu, Hg, and Ag decreased when they were in contact. The conventional amalgam in contact with the other copper-rich amalgam, in an area ratio of 2:1, reduced the amount of Cu but increased the Zn released. Polishing initially decreased the amounts of Cu and Zn released compared with the unpolished amalgams.     

In vitro chloride corrosion behaviour of Dispersalloy

The in vitro chloride corrosion behaviour of Dispersalloy has been investigated by anodic polarization as a function of time after trituration (ageing) and compared with conventional dental amalgam. Dispersalloy is a commercial alloy containing a mechanical mixture of Ag₃Sn and Ag–Cu eutectic in the ratio 2:1. From measurement of 1 h corrosion potentials it was observed that over a time period of 1 year Dispersalloy became appreciably more noble (potential shift from –500mV to – 350 mV Vs. SCE), whereas the corrosion potential of conventional amalgam remains unchanged. Initial anodic polarization profiles for Dispersalloy indicated presence of y₂ and Cu₆Sn₅ phases as evidenced by presence of the –250 mV peak. However, the current density associated with this peak was about three times less than that of conventional amalgam. As a function of ageing time the polarization profile is shifted in the direction of decreasing current density and after 1 year the –250 mV peak is absent. The anodic polarization profile of conventional amalgam is unchanged after 1 year. The improved corrosion resistance of Dispersalloy is attributed to the formation of Cu₃Sn as a function of time due to solid state reaction between Ag–Cu eutectic and y₂.     

Effect of free mercury on the strengths of dental amalgams

A previous study of several different Ag?Sn, Ag?Cu, and Ag?Sn?Cu alloys revealed that some of these alloys were susceptible to mercury embrittlement. The present study was undertaken to determine the extent to which embrittlement of alloy particles in amalgam affects the strength of the material. Specimens of low copper amalgam (O), high copper admixed amalgam (D), and high copper single composition amalgams (S and T) were condensed into 2 mm×4 mm×15 mm molds using a pressure of 14 MPa. After aging the specimens for 7 days at 37°C, mercury was coated on each specimen either by electro-plating mercury or by immersion in mercury. In each method, mercury was applied for 15 s or 1 min. At 30 s after coating, each specimen was placed in a 3-point fracture fixture and immediately loaded at 0.25 and 2.54 mm/min until fracture occurred. As controls, specimens of each amalgam without mercury coating were also tested. Ten specimens per experimental condition were tested. Fractured surfaces were examined using a scanning electron microscope. Strength reductions as high as 60% for O amalgam, 44% for D amalgam, and 31% for T amalgam were found. Two observations suggest that embrittlement of unconsumed alloy particles is primarily responsible for the strength reduction: 1) the amalgams showing susceptibility to mercury embrittlement in the present study contain alloy particles of alloys that were shown to be susceptible to embrittlement in an earlier study, and 2) the amalgam S, which showed the least susceptibility to mercury embrittlement, contains unconsumed alloy particles of an alloy that showed no significant embrittlement in the earlier study. Experimental evidence supports the hypothesis that free mercury released during corrosion or aging of amalgam restorations can contribute to the weakening of these restorations.     

Corrosion of dental alloys in vitro by differential oxygen concentration

The corrosion of a conventional and a high Cu dental amalgam, a Cu-Zn dental alloy, and pure silver (control) were studied in vitro under conditions of differential oxygenation, resembling a partially covered dental filling. The experimental method enabled the simultaneous recording of separate pH changes at anordic and cathodic areas and of the corrosion current generated; the corrosion products formed and the corroded alloys were examined by powder X-ray diffraction, optical and scanning electron microscopy, and EDAX elemental analysis. Marked anodic pH drops and cathodic pH rises were seen which, with the total electric change integrated from the recordings, formed the basis for a qualitative evaluation (electrochemical reaction schemes) and a quantitative evaluation (amounts of metal corroded) of the progress of corrosion. The corrosion products found were SnO2, ZnSn(OH)6, and Cu2O of the amalgams, and Zn5(OH)8Cl2 of the Cu-Zn alloy; twice as much tin corroded from the conventional as from the high Cu amalgam, and zinc corroded selectively from the Cu-Zn alloy. These findings were substantiated by the microscopic and EDAX observations.     

Long-term corrosion studies in vitro of gold, cobalt-chromium, and nickel-chromium alloys in contact

Specimens of various types of dental casting alloys were stored in phosphate-buffered 0.9% NaCl solution for 35 weeks. Every 7 weeks the solutions were changed and analyzed with regard to elements released from the alloys. The release of Cu from type III gold alloy increased in contact with gold alloys for metallo-ceramic use in a 1:2 area relation. However, when the area relation was reversed, no difference in the amounts of elements released was observed. Crevice corrosion was initiated with one of the Co-Cr alloys in contact with type III gold alloy. The crevice corrosion increased the amounts of Co and Cr released into the solutions. The release of Ni and Cr from the Ni-Cr alloys was prominent. These alloys were very susceptible to crevice corrosion. With one of the Ni-Cr alloys the release of elements increased in contact with type III gold alloy.     

The tooth-amalgam interaction

The interaction between tooth and amalgam during in vitro corrosion of dental amalgams has been studied in this investigation. Extracted teeth have been restored with five commercial amalgams, one of which was γ₂-free and the others contained the γ₂-phase. The restored teeth were immersed in a 1% NaCl solution for 9 months. Post-corrosion restorations have been examined by optical microscopy, scanning electron microscopy, and X-ray microanalysis. The results are: (1)γ₂-containing amalgam surfaces were covered with Ca-Sn-P-rich corrosion products of various morphology which occasionally contained relatively low concentrations of Cl and/or Zn; (2) the corrosion products on the γ₂-free amalgam surface indicated relatively high concentrations of Hg in addition to Ca, P, Sn, Cu, and Zn. These results agree with the past observations that corrosion of amalgam restorations is not an isolated process. Rather it may involve reactions of the restoration and the surrounding oral environment including tooth and oral fluids in which interactions of Sn, Zn, Hg, Ca and P take place.     

Microscopy and X-ray microanalysis of subcutaneously implanted conventional and high-copper dental amalgam powders in the guinea pig

With each material, there was only a mild early inflammatory response; particles were taken up by macrophages and giant cells. By 1.5 to 3 months, chronic granulomas had developed. With conventional amalgam, there were early changes in the intracellular material, associated with the rapid degradation of Sn-Hg particles, corresponding to the gamma 2 phase. Thereafter, intracellular particles from both types of amalgam underwent progressive degradation, producing halos of secondary material containing Ag and S. Apart from an initial loss of Cu and Sn from some high-Cu amalgam particles, there were no qualitative differences in the later changes between the two materials, although conventional amalgam particles appeared to degrade faster. With both, vast numbers of fine secondary particles containing Ag and S became widely distributed throughout the lesions and gave rise to macroscopic tattooing of the skin. Secondary material and small degrading primary particles from both types of amalgam were detected in the submandibular lymph nodes where they caused localized staining.