Corrosion of dental alloys in vitro by differential oxygen concentration

Abstract — 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. Marled 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 SnG₂, 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.     

Accelerated corrosion analysis of dental amalgams

Abstract – The corrosion of powdered conventional and high Cu dental amalgam was studied in vitro under fixed conditions (100% oxygen, pH 4, and constant weak mechanical action). Results were retrieved from X-ray diffraction of samples of amalgam and solid corrosion products Formed, in combination with recording of the HC1 consumed to maintain the fixed pH. In the conventional amalgam no corrosion of γ₁ occurred until all γ₂ had corroded, whereas in the high Cu amalgam corrosion of γ₁ occurred from the beginning, concurrent with corrosion of ε and ń. Corrosion products found were AgCl, Hg₂Cl₂, CuCl₂-3Cu(OH)₂, and SnO₂. The results may be interpreted as follows: in the conventional amalgam the matrix phase γ₁ is anodically protected against corrosion as long as any γ₂ remians; in the high Gu amalgam the least noble phases ε and ń do not protect γ₁ in a similar way.     

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.     

Release of corrosion products from amalgam in phosphate containing solutions

Abstract— 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.     

Microstructural changes of dental amalgam by copper additions

The γ2 (Sn_7–8Hg) phase of the amalgam structure has been established as the most corrosion prone of conventional dental amalgam. Cu rich amalgams are currently of interest because some formulations appear to reduce or eliminate the γ2 phase. This investigation compares the microstructures of conventional amalgam to Cu rich amalgams prepared by various methods, i.e. a commercial alloy (Dispersalloy), an admixture of Cu amalgam with conventional amalgam, and admixtures of 30% Cu-Ag-Sn ternary alloy with conventional amalgam. These variations in amalgam formulation resulted in marked reduction or elimination of the γ2 phase in the Cu rich amalgam microstructures. The matrix or γ1 (Ag₂Hg₃) phase was common to all formulations. Other phases and their proportions changed dramatically with different methods of introducing the Cu. The saline corrosion resistance, determined by potentiostatic anodic polarization in Ringer's solution, was enhanced for the Cu rich amalgams as a result of the elimination of the γ2 phase. This study suggests there are a variety of methods to introduce copper into amalgams and that these methods generally serve to eliminate γ2 phase and increase saline corrosion resistance.     

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.     

Cytotoxicity of corroded and non-corroded dental silver amalgams

The cytotoxicity of one conventional and four non-gamma 2-amalgams was studied in a cell culture system, using the Millipore filter method. Before testing set amalgam specimens were kept in distilled water or in artificial saliva at pH 4, 5, or 7 for up to 28 wk to produce a corrosion layer on the test surface. Non-corroded set amalgam specimens was also tested. None of the noncorroded, set amalgams showed any sign of surface accumulation of cytotoxic products whereas the corroded amalgams showed varying degrees of cytotoxicity. Generally, the non-gamma 2-amalgams gave a more pronounced cytotoxic effect than the conventional amalgam. When the corrosion procedure was carried out at pH 7, the various non-gamma 2-amalgams showed different degrees of cytotoxicity. It appears that the difference in cytotoxic effect between the non-gamma 2-amalgams and the conventional amalgam as well as the differences among the various non-gamma 2-amalgams could be related to variation in the retention of corrosion products deposited on the amalgam surface.     

Effect of pellicle on galvanic corrosion of amalgam

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.     

In vitro polarization of dental amalgam in human saliva

Potentiostatic anodic polarization techniques in Ringer's solution are commonly used for the evaluation of the corrosion resistance of dental amalgam. This in vitro method strives to simulate the in vivo electrochemical behaviour of amalgam in an electrolyte of natural saliva. Several types of dental amalgam were anodically polarized in natural saliva solutions and supernatant saliva solutions. The anodic polarization profiles are presented, as well as scanning electron micrographs and x-ray analysis of the corroded amalgam surfaces. These reveal corrosion products consistent with results found with previous artificial salivas as well as deposits possibly related to the organic component of natural saliva.     

Corrosion current and pH rise around titanium coupled to dental alloys

Corrosion reactions around titanium, usually considered biologically inert, might be provoked by coupling it galvanically with more corrodible dental alloys. Experiments in vitro simulating the conditions of a titanium dental implant or root canal post coupled to an amalgam filling, demonstrated corrosion current densities up to 31 microA/cm2, anodic pH values around the amalgam down to 2, and cathodic pH values around the titanium up to 10. The amounts of tin released by the enhanced corrosion of amalgam might contribute measurably to the daily intake of this element; the corrosion current generated reached values known to cause taste sensations. If the buffer systems of adjacent tissues in vivo are not able to cope with the high pH generated around the titanium, local tissue damage may ensue; this relationship is liable to be overlooked, as it leaves no evidence in the form of corrosion products.     

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.     

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.     

Test of long-term corrosion of dental amalgams

Seven non-gamma 2-amalgams and one conventional amalgam were used in the study. Cylindrical specimens were kept in a test solution for 1 yr. The depth of corrosion was measured and leakage of copper, zinc and mercury was determined by atomic absorption spectrophotometry. EDX was used to analyze the corrosion products. The results showed that copper leakage from the non-gamma 2-amalgams was higher than from the conventional amalgam. The amount of copper in the solution was not related to the concentration in the alloy. Dissolution of this ion was low during the last 6 months of the experimental period. Mercury leakage was comparatively low for all tested amalgams but the rate seemed to be less time dependent and leakage probably had not stopped after 1 yr. The depth of corrosion in the amalgams was in the range 100-500 micron. Under the experimental conditions the non-gamma 2-amalgams were more corroded than the conventional amalgam.     

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.     

Corrosion current and pH rise around titanium coupled to dental alloys

Abstract – Corrosion reactions around titanium, usually considered biologically inert, might be provoked by coupling it galvanically with more corrodible dental alloys. Experiments in vitro simulating the conditions of a titanium dental implant or root canal post coupled to an amalgam filling, demonstrated corrosion current densities up to 31 μA/cm² anodic pH values around the amalgam down to 2, and cathodic pH values around the titanium up to 10. The amounts of tin released by the enhanced corrosion, of amalgam might contribute measurably to the daily intake of this element; the corrosion current generated reached values known to cause taste sensations. If the buffer systems of adjacent tissues in vivo are not able to cope with the high pH generated around the titanium, local tissue damage may ensue; this relationship is liable to be overlooked, as it leaves no evidence in the form of corrosion products.     

Cytotoxicity of corroded and non-corroded dental silver amalgams

The cytotoxicity of one conventional and four non-γ₂-amalgams was studied in a cell culture system, using the Millipore filter method. Before testing set amalgam specimens were kept in distilled water or in artificial saliva at pH 4, 5 or 7 for up to 28 wk to produce a corrosion layer on the test surface. Non-corroded set amalgam specimens were also tested. None of the non-corroded, set amalgams showed any sign of surface accumulation of cytotoxic products whereas the corroded amalgams showed varying degrees of cytotoxicity. Generally, the non-γ₂-amalgams gave a more pronounced cytotoxic effect than the conventional amalgam. When the corrosion procedure was carried out at pH 7, the various non-γ₂-amalgams showed different degrees of cytotoxicity. It appears that the difference in cytotoxic effect between the non-γ₂-amalgams and the conventional amalgam as well as the differences among the various non-γ₂-amalgams could be related to variation in the retention of corrosion products deposited on the amalgam surface.     

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.     

In vitro corrosion of dental amalgams with different Cu content.

Both Cu amalgam and Sn-containing Ag amalgam alloys with Cu contents ranging from 2 to 30% were used in the investigation. The amalgams were placed in artificial saliva and the content of the dissolved Cu was determined after 1 month. The amalgam remained in the saliva for another 5 months. After the amalgams were removed from the solution, metallographic and microprobe examinations were made. The Cu amalgam released about 10 times more Cu to the artificial saliva than any of the other amalgams. The dispersion type amalgams showed a relatively large dissolution of Cu, while a smaller release was found from amalgam made of powder with one composition. The phases that appeared to be corroded first were gamma2 and Cu-Sn phases. In dispersion type amalgams the gamma phase appeared to be attacked. Most of the corrosion products remained inside the amalgam and were identified to be tin oxide chlorides.     

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.     

In vitro corrosion of dental amalgams with different Cu content

ABSTRACT— Both Cu amalgam and Sn-containing Ag amalgam alloys with Cu contents ranging from 2 to 30% were used in the investigation. The amalgams were placed in artificial saliva and the content of the dissolved Cu was determined after 1 month. The amalgam remained in the saliva for another 5 months. After the amalgams were removed from the solution, metallographic and microprobe examinations were made. The Cu amalgam released about 10 times more Cu to the artificial saliva than any of the other amalgams. The dispersion type amalgams showed a relatively large dissolution of Cu, while a smaller release was found from amalgam made of powder with one composition. The phases that appeared to be corroded first were γ₂ and Cu-Sn phases. In dispersion type amalgams the γ phase appeared to be attacked. Most of the corrosion products remained inside the amalgam and were identified to be tin oxide chlorides.