An electrochemical and multispectroscopic study of corrosion of Ag-Pd-Cu-Au alloys

Corrosion of a multi-phase Ag-Pd-Cu-Au-based commercial dental casting alloy and a Cu-Pd-rich and Ag-rich single-phase alloy was studied by open-circuit potential measurements, atomic absorption spectrometry, and electron spectroscopy. The alloys were immersed in an artificial saliva solution for 24 hr while the open-circuit potentials of the alloys were measured. The potentials were found to stabilize at certain levels after a steep rise during the first hours of the experiment. Cu was found to dissolve considerably from the Cu-Pd-rich alloy, with simultaneous enrichment of Pd in the surface layer of the alloy. Ag dissolved slightly from the Ag-rich alloy, but both Cu and Ag were found to dissolve from the multi-phase alloy. Neither Pd nor Au dissolved from any of the alloys studied.     

Adhesion of adhesive resin to dental precious metal alloys. Part I. New precious metal alloys with base metals for resin bonding

New dental precious metal alloys for resin bonding without alloy surface modification were developed by adding base metals (In, Zn, or Sn). Before this, binary alloys of Au, Ag, Cu, or Pd containing In, Zn, or Sn were studied for water durability and bonding strength with 4-META resin. The adhesion ability of the binary alloys was improved by adding In equivalent to 15% of Au content, Zn equivalent to 20% of Ag content, and In, Zn, or Sn equivalent to 5% of Cu content. There was no addition effect of the base metals on Pd, however 15% of In addition improved adhesion with Pd-based alloys containing equi-atomic % of Cu and Pd. The alloy surfaces were analyzed by XPS and showed that oxides such as In2O3, ZnO, or SnO play an important role in improving the adhesive ability of the alloys.     

Electrochemical characteristics of high-Pd alloys in relation to Pd-allergy.

OBJECTIVE: The aim of this study was to determine whether the Pd-Cu-based dental ceramic alloys possess any electrochemical characteristics distinguishable from that of other Pd-containing alloys. Of all Pd-containing alloys, this particular alloy group has been linked to frequent incidence of allergy and hypersensitivity reactions. Electrochemical corrosion may instigate these reactions. METHODS: Four groups of alloys, Pd-Cu, Pd-Ga-(with and without Ag), Pd-Ag, and Au-Pd, were evaluated by traditional corrosion measurement techniques in a phosphated buffer saline solution at 20 degrees C. The electrochemical characteristics measured were: (1) 20 h open circuit potential (OCP); (2) 20 h corrosion rate (Icorr); and (3) anodic polarization (E-i) curves. RESULTS: The OCP values (232 +/- 25 mV) of the Ag-free Pd-Ga and Pd-Cu-based alloys were higher than and distinctly different from that (144 +/- 52 mV) of the Ag-containing alloys. The Icorr values of different alloys, despite varied compositions, were indistinguishable from one another. The E-i curves of all alloys were essentially similar, with the Ag-containing (> 5 wt%) alloys showing a subtle difference in their anodic slope within 100 mV above their corrosion potentials. SIGNIFICANCE: The OCP values of Pd-Cu alloys and the Ag-free Pd-Ga alloy are comparable to that reported for pure Pd (239 +/- 21 mV), which indicates that during corrosion these alloys undergo dealloying and consequent Pd-enrichment on their surface. Such a condition is conducive to the release of allergenic Pd++ ions and offers a plausible explanation for the frequent incidence of hypersensitivity reactions associated with the Pd-Cu alloys. The OCP values in other alloys are attributed to dealloying followed by surface enrichment with Ag and/or Au and the possible formation of an insoluble AgCl surface film on the respective alloy surfaces. These events have the potential to suppress or prevent Pd++ ion release. Alloys showing these characteristics have seldom been linked to allergic reactions.     

Dealloying and electroformation in high-Pd dental alloys.

OBJECTIVE: The corrosion of high-Pd dental alloys, depending on their composition, is postulated to be associated with dealloying and electroformation. The aim of this study was to obtain additional information to support these postulations. METHODS: The corrosion characteristics of two commercial high-Pd alloys, Naturelle (79Pd-10Cu-2Au-9Ga wt%) and Rx 91 (54Pd-37Ag-9Sn), and their elemental components were evaluated in a phosphated buffer saline (PBS) solution. Indium, a common element in high-Pd alloys, was also included. The corrosion characteristics measured for each material were the 24 h open circuit potential (OCP) and the potentiodynamic anodic polarization curve. Additionally, the surface composition of the two alloys, before and after immersion corrosion in PBS for 2 months, was analyzed by X-ray photo electron spectroscopy (XPS). RESULTS: Of the pure metals, Ga had the most electroactive OCP followed in order by In, Sn, Cu, Ag, Au, and Pd. The anodic polarization data showed all base metals to be unstable in PBS. The electroformation of AgCl was evidenced in the polarization curve of pure Ag. Both electrochemical characteristics of the PdCu-based alloy were very similar to that of pure Pd. The PdAg-based alloy displayed corrosion behavior resembling that of Ag. XPS data showed that the corrosion of the PdCu-based alloy was associated with a decrease in surface content of Cu and Ga but an increase in Pd and Au. The PdAg-based alloy surface during corrosion showed a decrease in Sn, an increase in Ag, and an unaltered Pd content. The behavior of the PdCu-based alloy is attributed to the operation of a galvanic interaction that causes dissolution of base metals and surface enrichment with primarily Pd. Dealloying, Ag-enrichment, and AgCl formation are thought to have contributed to the observed behavior of the PdAg-based alloy. These mechanisms are consistent with data from published ion release studies. SIGNIFICANCE: The allergenic potential of any Pd alloy is dependent on its propensity to develop a Pd-rich surface and thus release Pd+2 ions. The present study, though limited, has shown that electrochemical characteristics, namely OCP and polarization curves, can be used to identify such alloys. Further studies are warranted to evaluate the widespread applicability of these characteristics in distinguishing between Pd alloys that are biologically safe and those that are not.     

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.     

In vitro cytotoxicity of metallic ions released from dental alloys

The cytotoxicity of a dental alloy depends on, but is not limited to, the extent of its corrosion behavior. Individual ions may have effects on cell viability that are different from metals interacting within the alloy structure. We aimed to investigate the cytotoxicity of individual metal ions in concentrations similar to those reported to be released from Pd-based dental alloys on mouse fibroblast cells. Metal salts were used to prepare seven solutions (concentration range 100 ppm–1 ppb) of the transition metals, such as Ni(II), Pd(II), Cu(II), and Ag(I), and the metals, such as Ga(III), In(III), and Sn(II). Cytotoxicity on mouse fibroblasts L929 was evaluated using the MTT assay. Ni, Cu, and Ag are cytotoxic at 10 ppm, Pd and Ga at 100 ppm. Sn and In were not able to induce cytotoxicity at the tested concentrations. Transition metals were able to induce cytotoxic effects in concentrations similar to those reported to be released from Pd-based dental alloys. Ni, Cu, and Ag were the most cytotoxic followed by Pd and Ga; Sn and In were not cytotoxic. Cytotoxic reactions might be considered in the etiopathogenesis of clinically observed local adverse reactions.     

Corrosion of three experimental AgMn-based casting alloys

OBJECTIVE: Alloys based on AgMn are being evaluated in our laboratory for their possible use as an alternative to Type III dental alloys. They respond to heat treatment and develop hardness values comparable to that of Type III alloys. The objective of the present research was to evaluate their corrosion characteristics. METHODS: The three experimental silver-based alloys of the following composition (at%): (1) 63Ag37Mn, (2) 60Ag35Mn5Au and (3) 60Ag35Mn5Pd, were tested in their peak-hardened condition. Following 0.5 h open-circuit potential (OCP) measurement of each alloy in a phosphated buffer saline (PBS) solution, its current-potential profile was generated by the cyclic voltammetry technique within -1300 and +200 mV (SCE) at 1 mV s-1. In a separate test, the OCP of each alloy was monitored over a 24 h period. RESULTS: Each of the three alloys showed ennoblement of their OCP over time due to dissolution of Mn and consequent enrichment with Ag (Au or Pd). At 24 h, the two ternary alloys were the most noble followed by the binary alloy. With respect to the cyclic voltammetry, oxidation of Ag was noted during forward scans at around 0 mV. The reverse scan was associated with a reduction current peak between -37 and -128 mV. The values for this peak, which is a measure of Ag oxidation, were highest (11.5 mA cm-2) for the binary alloy followed by the Au- (2.9 mA cm-2) and Pd-containing (0.04 mA cm-2) alloys, respectively. This indicates that, for equivalent concentration, Pd is more effective in reducing Ag corrosion than Au. SIGNIFICANCE: Alloys based on the AgMn system are as hard as Type III dental alloys. Information on the corrosion characteristics of the AgMn-based alloys presented here is of value in further development of this alloy system.     

Release of elements from some gold alloys and amalgams in corrosion

The release of Au, Ag, Cu and Zn elements from six commercially available gold alloys and three amalgam alloys was studied. The polarizing electropotential system with modified Fusayama solution was used. The cycle process was repeated 500 times between +/- 1000 mV during a period of 5 h 22 min. During the process samples of solution were analyzed in order to follow the release of elements. Results revealed rapid release of Zn from most alloys, release of Ag from most alloys and release of Au from two alloys. Also, rapid release of Cu was found from amalgams but not from gold alloys. Release of Ag from gold alloys showed very good passivation up to 3 h, after which it was released in varying amounts.     

Evaluation of the amalgamation reaction of experimental Ag-Sn-Cu alloys containing Pd using a mercury plating technique

A mercury plating technique was used to determine the phases forming on experimental Ag-Sn-Cu alloy powders (with and without Pd) exposed to electrolytically deposited mercury. Four series of alloy powders were made: a) 1.5% Pd with 10-14% Cu (CU series); b) 1.0% Pd with 10-14% Cu (1PD series); c) 1.5% Pd with different ratios of Ag3Sn (gamma) to Ag4Sn (beta) with 12% Cu (AGSN series); and d) 9-13% Cu with no Pd (NOPD series). Each powder was pressed on a freshly prepared amalgam specimen made from the same powder and metallographically polished until cross sections appeared; mercury was electroplated on the alloy particles. Alloy powders, amalgams and electroplated specimens were examined using XRD and SEM/EDS. XRD confirmed the presence of gamma2 in amalgams made from alloys with Cu < 13% or with Ag3Sn/Ag4Sn > 0.8. Specimens with moderately plated Hg showed gamma1 (Ag2Hg3) polyhedra and eta' Cu6Sn5, but not gamma2. This method effectively identifies alloys prone to forming gamma2.     

Microstructures of admixed amalgams produced from Pd-containing dispersants.

Blended Pd-containing dental amalgams were developed by substituting Pd for up to 20 w/o Ag or Cu in the Ag-Cu eutectic alloy. Melted ingots were lathe-comminuted to a particle size distribution of 1-45 microns. Alloy blends were created from two parts of a traditional amalgam and one part of experimental Ag-Cu-Pd particles. Amalgams with from 0.42 to 1.67 w/o Pd were fabricated by trituration of alloys and mercury at a Hg/alloy ratio of 1:1 with a mechanical triturator at 5000 rpm for 10 s and hand-condensed. XRD studies of these amalgams revealed the suppression of eta' (Cu6Sn5) phase with Pd addition and no gamma 2 (Sn8Hg) phase found. At 3.3 w/o Pd, the eta' (Cu6Sn5) concentration of the amalgam was below the detection limit of the instrumentation (1%). SEM micrographs revealed that reaction zones around eutectic particles decreased with increased Pd concentration.     

Au, Cu, Sn, Zn and Ag ion release from some Cd-free gold solders

The aim of this study was to determine the release of Au, Cu, Sn, Zn, and Ag from five commercially available Cd-free gold solders. The electropotential system in modified Fusayma solution was used. The cycle process was repeated between +/- 250 mV for 24 hours. To study the ion release, samples of solution were analysed during the process. Rapid release of Zn from most alloys was found. Cu and Ag ions were also released, but only small amounts of the latter. Au showed very good passivation up to 24 hours. The use of soldered joints should be reduced in order to minimize corrosion.     

New conversion method of metal surfaces for resin bonding. Conversion effects for pure metals in dental precious metal alloys

Excellent adhesion of adhesives to metals can be realized by simply applying liquid Ga-Sn alloy (Adlloy) on the adherend metal surface. This method is only effective on dental precious metal alloys. Five metals, Au, Pt, Pd, Ag, and Cu, included in dental precious metal alloys were converted by this method to determine the most effective pure metal from bonding strength measurements, water durability at the adhesion interface, and ESCA measurements. All metals converted by Adlloy showed excellent bonding strength and water durability, whereas nonconverted metals showed poor water durability. ESCA measurements showed that metal surfaces converted by Adlloy are covered with a 3-6 nm thick Ga and Sn oxide film and that the diffusibilities of Ga in the metals are in the order Ag greater than Au greater than Pt greater than Cu greater than Pd. From the viewpoint of handling, Ag is the most effective metal.     

Studies on Pd base ternary alloys for dental amalgam. (Part 1) Effect of size and shape of particle on dimensional change and mechanical properties (author's transl)]

Although much efforts have been paid to improve the properties of silver-tin amalgam, some clinical problems are not resolved yet. Palladium alloys have good properties for dental material, and they react on mercury well. So they are considered to be good amalgam alloys. In this paper, palladium base ternary alloys are studied for dental amalgam. The effect of size and shape of particle on dimensional change, compressive strength and hardness is studied. Main results obtained are as follows. (1) The dimensional change of spherical particle amalgam is smaller than that of fine cut particle. The finer the particle size is, the smaller the dimensional change is both for spherical and fine cut particles. (2) The dimensional change of amalgam of 60 wt% Pd-40 wt% Ag alloy decreased by addition of Sn or Cu, and these values under 20 micrometer/cm by addition of more than 5 wt% Cu or 10 wt% Sn. (3) Compressive strength of Pd alloy amalgam scatter so widely that the effects of alloying element are not able to be evaluated. Both maximum compressive strengths are 22 kg/mm2 and 28 kg/mm2 for the alloys containing Cu and Sn respectively. (4) Vicker's hardness of palladium alloy amalgam increased by addition of Sn and ranged 80 approximately 120 for spherical amalgam, which is higher than that of commercial alloy. Significant difference is not found in Cu added alloys. (5) From the analysis by XMA, it is found that matrix is composed of one phase and there is compositional difference between center of matrix and near the particle. Silver is rich in the center of matrix and palladium is rich near the alloy particle. Cu and Sn are seemed to distribute uniformly.     

An application of potentiostatic current--time transients to study the corrosion of dental amalgams.

Potentiostatic current-time transients obtained during anodic polarization of a conventional dental amalgam and its component phases have been analysed to characterize the possible electrochemical reactions involved in amalgam corrosion. These reactions involve such surface phenomena as adsorption, anodic film formation, film dissolution, etc. which are well reflected in current-time transients. The analysis of these transients from the standpoint of thermodynamics, suggests that the saline corrosion of dental amalgam may occur through the dissolution of the passive anodic films on Sn7Hg(gamma2) and Cu6Sn5 (eta') and the dissolution reaction appears to be related to the formation of tin-oxychloride.     

Electrochemical behavior of cast Ti-Ag alloys

Anodic polarization tests were performed in 0.9% NaCl and 1% lactic acid solutions to characterize the relationship between the corrosion behavior and the microstructures of cast Ti-Ag (5-40% Ag) alloys. The anodic polarization curves for the Ti-Ag alloys up to 17.5% Ag were similar to those for pure titanium in both solutions. On the other hand, an abrupt increase in the current density was observed for the alloys with more than 20% Ag in the NaCl solution and with more than 27.5% Ag in the lactic acid solution. The microstructures of the corroded alloy surfaces indicated the deterioration of precipitated intermetallic compounds along the grain boundaries. The Ti-Ag alloys up to 17.5% Ag had excellent corrosion resistance similar to that of pure titanium. The alloys with 20-25% Ag may be also used as dental alloys, since they passivated again immediately after preferential dissolution in the NaCl solution.     

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.     

Elements released from dental casting alloys and their cytotoxic effects

PURPOSE: This in vitro study investigated the element release from seven commercially available dental casting alloys and tested their cytotoxic effects. MATERIALS AND METHODS: The casting alloys tested were one high-noble alloy (Bioherador N) and six base-metal alloys, including four Ni-Cr alloys (Remanium CS, Heranium NA, Wiron 99, CB Soft), one Co-Cr alloy (Wirobond C), and one Cu-based alloy (Thermobond). Ten specimens from each alloy were prepared in the form of disks, and each of the seven dental casting alloys (10 disks per group) were conditioned in distilled water at 37 degrees C for either 72 or 168 hours. The conditioning media were analyzed for element release, and the cytotoxic effects were assessed on Balb C fibroblasts using MTT assay. RESULTS: Element release was greater at 168 hours of conditioning than at 72 hours. The extract from the high-noble alloy showed the least amount of element release (only Zn), with no cytotoxic effects. The greatest amount of element release was detected in the Cu-based alloy Thermobond and the Ni-Cr alloy CB Soft; their extracts were significantly more toxic than all the other alloy extracts. The cytotoxic effects of the other Ni-Cr alloy extracts were not statistically significantly different from the high-noble alloy extract. However, the Co-Cr alloy (Wirobond C) extract was significantly more cytotoxic than the high-noble alloy extract. CONCLUSION: Element release from casting alloys is proportional to the conditioning time. The content of Cr and Mo in the alloy protects the alloy from dissolution, while the Cu content makes it more susceptible to corrosion and dissolution, rendering it more cytotoxic.     

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.     

Electrochemical characterization of novel Ag-based brazing alloys for dental applications

ObjectiveTaking into account that clinical data have proven the decomposition of Ag brazing alloys used in the production of orthodontic appliances the aim of this study was to develop new Ag based soldering alloys free of Cu and Zn.MethodsFour commercially available Ag brazing alloys were selected and their electrochemical properties were compared to the following experimental alloys: Ag12Ga, Ag10Ga5Sn, Ag20In and Ag7Sn. 112 disk shape specimens were prepared for each alloy and their electrochemical properties were evaluated by Open Circuit Potential (OCP), linear sweep voltametry (LSV), cyclic polarization (CP) and electrochemical impedance spectroscopy (EIS) in a NaCl 0.9% and a Ringer’s electrolyte solution.ResultsThe experimental alloys combined higher OCP and E_corr with lower I_corr values. The impedance values of the commercial alloys were lower showing that any surface layers formed are not protective and steady compared to those of the novel ones. In conclusion experimental alloys demonstrated enhanced electrochemical properties.SignificanceIn and Sn showed a more beneficial effect on electrochemical properties compared to Ga and thus can be considered as a promising option for the development of a new family of Ag brazing alloys with increased biocompatibility.     

Electrochemistry of the saline corrosion of conventional dental amalgams

The electrochemical behaviour of the component phases of dental amalgam, viz. Ag₃Sn (γ), Ag₂Hg₃ (γ1), Sn_7–8Hg (γ2), Cu₃Sn, Cu₆Sn₅ and a composite of γ1 +γ2 phases were evaluated in saline solution by standard anodic polarization techniques. Using multi-electrode theory a theoretical polarization diagram was constructed from the composite phases and compared with an anodic polarization profile attained from conventional dental amalgam. A strong agreement was obtained. From analysis of the electrochemical reactions it was found that the passivity of γ2 is due to the formation of stannous and/or stannic oxide (hydroxide) and that the corrosion failure of dental amalgam occurs through the dissolution of this passive oxide in γ2 at potentials of ?250 mV Vs. SCE with the formation of tin oxychloride. The corrosion behaviour of Cu₆Sn₅ is similar to that of γ2 phase and its presence impairs the corrosion resistance of dental amalgam. If the copper containing phase is Cu₃Sn, it will be passive in Ringer's solution, possibly as a result of Tamman's ‘multiple rule of eight’.