ORIGINAL PAPER

Wolf-Dieter Mueller Æ C. Schoepf Æ M. L. Nascimento

A. C. Carvalho Æ M. Moisel Æ A. Schenk Æ F. ScholzK. P. Lange

Electrochemical characterisation of dental alloys: its possibilitiesand limitations

Received: 19 November 2004 / Revised: 13 January 2005 / Accepted: 19 January 2005 / Published online: 13 April 2005

� Springer-Verlag 2005

Abstract Dental alloys are metallic biomaterials whichhave a broad variation of composition compared totechnical alloys. It is therefore in the interest of patientsand technicians to conduct a good assessment of theelectrochemical behaviour of dental alloys in order tocollect information about their corrosion resistance. Thepurpose of this work was to demonstrate possibilitiesand limitations of two electrochemical techniques: thevoltammetry of immobilised microparticles (ViMP) ontolead, and cyclic voltammetry measurements with thehelp of the mini-cell system (MCS). Based on fingerprintsobtained from ViMP it was possible to analyse anddifferentiate the dental alloys. The results obtained byMCS were comparable with ViMP, but give a betterunderstanding of the corrosion behaviour of the mate-rials.

Keywords Biomaterials Æ Dental alloys Æ Corrosion ÆSurface characterisation

Introduction

Dental alloys are mainly used for crown, bridges, pros-theses, supra-constructions and implants. They need tofulfil important requirements such as ease and reliabilityof handling and treatment, toughness appropriate to thesituation of application, good biocompatibility and

aesthetic properties. These materials are confronted withextreme environmental conditions in the mouth, as thetemperature can vary between 5 and 55�C and thecomposition and the pH of the saliva varies dependingon the nutrition. Moreover, the severe loading condi-tions, which depend on the force of muscles and onthe age of the patient, have to be considered as well.These factors imply that dental materials need to haveexceedingly good properties to cope with such adverseconditions [1–3]. One important requirement is achievedwith a good corrosion stability, which can be determinedby the study of the electrochemical behaviour of thematerial [4–6]. The interface between biomaterial andtissue can be described as an electrochemical system andsupports the application of electrochemical techniquesfor an assessment of metallic biomaterial surfaces [7].

Another problem involving the choice of the materialto be used in dental applications is inherent in the hugeamount of alloys with a great range of compositionwhich are available on the market [3]. On the Germanmarket alone there are more than 1,000 various dentalalloys available [8]; concerning the joint techniques, likesoldering and welding, more than 500,000 different di-rect and permanent combinations of alloys are possible.Furthermore, secondary connections, like temporarycontacts between different alloys, are also possible [9].This can represent a problem when combining materialswhose corrosion behaviour is not clearly known, par-ticularly in situations where galvanic or couplingcorrosion might occur [10, 11].

Therefore important questions have to be answeredsuch as which kind of alloy is in the mouth, whichcombinations are possible or how is the biocompatibilityeffected after time or after combination with variousalloys. In this respect, electrochemical experimentsmight help to choose certain combinations because whena material or a material combination has low corrosionresistance in experiments in 1% NaCl, the corrosion inthe human body will be much more intense.

Metallic dental and medical biomaterials have beenwell studied in terms of their corrosion behaviour [12–

W.-D. Mueller (&) Æ C. Schoepf Æ M. L. NascimentoA. C. Carvalho Æ M. Moisel Æ A. Schenk Æ K. P. LangeDental Materials and Biomaterial Research, Dental School,Charite Medical University of Berlin, Schuhmannstr. 20/21,10117 Berlin, GermanyE-mail: wolf-dieter.mueller@charite.de

M. L. Nascimento Æ A. C. CarvalhoChemistry Department, University of Madeira,9000-081 Funchal, Madeira, Portugal

F. ScholzDepartment of Analytical Chemistry, University of Greifswald,Soldmannstr. 23, 17489 Greifswald, Germany

Anal Bioanal Chem (2005) 381: 1520–1525DOI 10.1007/s00216-005-3093-8

14]; nevertheless, there is still a fundamental need for asimple analytical characterisation of dental alloys toallow an assessment of their electrochemical behaviour.

Two techniques are described for corrosion analysisin the ISO standard 10271 [15], the static immersion testconnected to an ICP analysis of the corrosion solutionand the electrochemical measurement with help of aclassical corrosion measurement cell. Two new methodsbased on these concepts, voltammetry of immobilisedmicroparticles (ViMP) onto lead and cyclic voltammetrymeasurements with help of the mini-cell system (MCS),were used to analyse different metallic materials.

ViMP, formerly known as abrasive stripping vol-tammetry, was presented in the mid-1980s by Scholz forthe identification of materials using voltammetry. Par-ticles are extracted from the materials throughmechanical abrasion. These are adhered at graphite leadand then analysed by differential pulse voltammetry(DPV). Several alloys were tested using this method andreproducible results were obtained, showing that ispossible to detect differences in composition and corro-sion stability of metallic materials [16–22].

MCS is based on a transportable mini-cell whichperforms non-destructive electrochemical measurementson solid samples, without additional preparation of thespecimen. MCS can analyse any metallic biomaterialincluding those applied in patients. The reference elec-trode is inside a tube, which contains a saturated cal-omel electrode and the counter electrode (a platinumwire); the sample is the working electrode. The wholesystem is connected to a potentiostat and a computer.[23, 24]

From the graphic results obtained and by usingCorrview for Windows, the following electrochemicalparameters are calculated: E0 is the corrosion potential,the potential at which the total oxidation rate is equal tothe total reduction rate, and by definition the current atthis point is zero; I0 is the exchange current density, avalue which is determined by indirect methods such asby the Tafel equation. The corrosion rate (Crate) can alsobe calculated through Faraday’s law and represents theloss of mass of material as a function of time and perunit area. The polarization resistance (Rp), that is thetotal resistance in the corrosion circuit, is calculated byusing Ohm’s law.

The purpose of this work was to show the possibilitiesand limitations of ViMP at graphite lead and of MCSfor the assessment of dental alloys.

Experimental

Voltammetry of immobilized microparticles (ViMP)

Microparticles were extracted by scratching the surfaceof the materials listed in Table 1 with a diamond-cov-ered instrument.

The particles were then attached onto a freshlyground surface of a graphite lead (graphite type H4;Fig. 1).

The electrolyte was 1% NaCl, purged for 8 min withnitrogen. The electrochemical measurements were per-formed using DPV connected to a potentiostat (EG&G,Versastat). The system was plugged into a computer anddriven by the software Corrware for Windows. ViMPwas performed at a scan rate of 10 mV s�1 with a po-tential range between �1.16 and +1.54 V (versus nor-mal hydrogen electrode, NHE); the pulse amplitude was25 mV and the pulse duration of 50 ms. Each materialwas tested three times.

Voltammetry with mini-cell system (MCS)

The pure metals were polished with sandpaper (400 SiC)and cleaned with acetone; the alloys were cleaned withacetone only. The materials analysed were the same asthose used for ViMP (Table 1). The electrolyte was anon-purged solution of 1% NaCl. The electrochemicalmeasurements were performed using MCS connected toa potentiostat (EI 1286, Schlumberger). The system wasplugged into a computer and driven by the softwareCorrware for Windows. Single sweep voltammogramswere performed in a potential range between �1 and+1.45 V (versus NHE) at a scan rate of 10 mV s�1.The measurements were repeated three times for eachmaterial.

Results and discussion

Voltammetry of immobilized microparticles (ViMP)

The results obtained with ViMP for the metals and al-loys are presented in the Figs. 2 and 3, respectively.Notice that the obtained voltammograms are charac-teristic for each element and alloy.

Table 1 Description andcomposition of the materialsanalysed

Metals (p.a. grade) Alloys (% mass composition)

Ag Silver (Bego) 1.AuroLloyd KF 55 Au; 29.2 Ag; 10 Pd; 3.5 In;Au Galvan 110 gold 1.2 Zn; 1 SnCu Copper (Aldrich) 2.BegoPal 300 75.4 Pd; 6.2 Ag; 6 Au; 6.3 In; 6 GaPd Palladium (Bego) 3.Alloy 1 43 Ag; 40 Cu; 17 SnSn Tin (Aldrich) 4.Alloy 2 65 Ag; 17 Cu; 23 SnZn Zinc (Apolda) 5.SiPal 65 Ag; 25 Pd; 8.3 Cu; 1.5 Zn; 0.2 Au

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Comparison of the element curves with those of al-loys can give interesting information as to which elementa certain curve shape is associated with in the case ofalloys, and therefore the element curves can be used as areference.

Significant differences were observed with twoexperimental alloys of silver–copper–tin (alloys 3 and 4).If one considers that 12% Cu can be dissolved in Ag, orvice versa that 8% Ag can be dissolved homogeneouslyin Cu, then some differences should appear in the elec-trochemical behaviour of a heterogeneous mixture of Agand Cu.

In the case of alloy 3, it was expected that the a-phaseof the Ag–Cu alloy should mix with a Cu–Sn compo-nent. This seems to be the situation occurring because

the Sn peak does not appear in the alloy curve, andinstead the DPV peak is very near that of pure copper.In contrast, the DPV peak of alloy 4 shows a shapeidentical to pure Ag. Therefore, it seems that Cu and Snparts are alloyed with Ag and disappear as an electro-chemically active element, because the alloy has its ownidentity.

The comparison of the alloys with different content ofsilver and other alloying elements can be seen forexample in the combination of silver with palladium inalloy 5, which is a homogeneous alloy. The activity ofthe alloy in this case is shifted in the direction of purepalladium.

In the case of alloying palladium with a small amountof gold, as in the case of alloy 2, no sign of Pd appears in

Fig. 2 ViMP voltammogram of the analysed pure metals (potentialversus NHE)

Fig. 1 Scanning electronmicroscopy picture of thegraphite lead used for ViMP,containing gold particles on itssurface

Fig. 3 ViMP voltammogram of the analysed alloys (potentialversus NHE)

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the curve; instead, the peak is identical to alloy 1, withhigh gold content. This can be explained by the solidsolution mixture of both elements.

Voltammetry with mini-cell system (MCS)

The first task with this setup was to check the repro-ducibility of the MCS. The scan rate was 10 mV s�1 andis improved by a factor of 60 as recommended for thestandard measurements of corrosion test withv=0.1667 mV s�1.

Because of the reduction of the measurement area to0.008 cm2 (cf. 1 cm2), which corresponds to a ratio of125, a high charging process can be excluded.

Another feature of the MCS is the low volume (lessthan 8·10�4 cm3) which is involved in the electrochem-ical interactions; based on this, thin-layer conditions canbe assumed. The position of the cathodic threshold po-tential or even the holding time at this potential shouldonly ensure that enough hydrogen will be developed toreduce the solved oxygen in the vicinity of the metalsurface. Simultaneously oxidised forms of the elementsfixed in the lattice, as well as solved ions in the doublelayer, should be reduced and integrated in the surface.

A good example to demonstrate such effects in thereproducibility of the results is provided by observingthe current density (I) versus potential (E) curves for

pure zinc (Fig. 4). With help of Origin 6.0 a mean curveof all single measurements can be created for each metal(Fig. 5).

These curves show all characteristic features; forexample, a symmetric shape around the zero currentpotential, which is correlated with the corrosion poten-tial, can be estimated with a very high precision togetherwith other electrochemical parameters (Table 2).

Based on the Nernst equation, the concentration ofoxidised-state ions in the double layer can be calculated.For zinc this means that under the measurement con-ditions in the vicinity of the surface, a small layer ofZn2+ ions was created with a concentration of 1 Mbecause the equilibrium potential could be measurednear �0.76 V versus NHE, the standard potential forzinc. The shift in the cathodic direction is related to pHconditions (pH near 6) and with the presence of chlorideions [25]. The curves for the alloys have a characteristicshape of I versus E curve, alike was observed in theViMP results, as shown in Fig. 6.

In making an assessment of the corrosion behaviourof dental alloys, a comparison with the element curvesand cyclic measurements is necessary. Such investiga-tions very rapidly give information about the pittingcorrosion capability, in particular for non-noble dentalalloys and amalgams [26].

Another interesting feature about the application ofthe MCS seems to be the characterisation of modified

Fig. 4 Linear sweepvoltammogram for zinc withcurrent density presented inlogarithmic and linear scales(potential versus NHE)

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implant surfaces. In combination with impedancetechniques, cyclic voltammetric measurements andsolid-state analysis, a characterisation of the interfaceproperties of metallic biomaterials in situ is possible.

Conclusions

The ViMP results give a better idea about the compo-sition of the alloys owing to the reduced quantity ofparticles involved in the analysis. However, the infor-mation is still only qualitative. This technique might beuseful for analysis of samples taken directly from thepatient, and helps to avoid the application of dentalmaterials with galvanic corrosion possibilities.

The MCS gives the possibility of direct measurementat the specimen surfaces without special preparation,and therefore the extracted data reflects the real elec-trochemical behaviour of the materials as applied. It was

shown that the results of electrochemical measurementshave a high reproducibility and can give information foruse in the description and assessment of the electro-chemical behaviour of metals and dental alloys. Thissetup might help us to understand the electrochemicalbehaviour of the materials, starting in simple electrolytesituations up to more complex systems as in the body.

Both investigation techniques have shown that dentalalloys have characteristic polarization curves, which canbe used as fingerprints for their characterisation andassessment. The results are comparable, but the infor-mation is of different kind, since the ViMP results in-volve a limited particle number, whereas MCS analysesthe complete surface. This means that their informationis more complementary than equivalent.

The MCS has the additional advantage of being veryeasy to handle for the reliable data it gives; it is thereforeespecially applicable to investigations involving smallspecimens. The application of this setup to different

Fig. 5 Linear sweepvoltammogram for the puremetals with current densitypresented in logarithmic andlinear scales (potential versusNHE)

Table 2 Electrochemicalparameters obtained for zinc in1% NaCl

Corrosion rate (mm PY) Rp (W cm2) I0(A cm2) E0 (V)

6.15 64 4.10·10�4 �0.7816.71 58 4.47·10�4 �0.7731.19 330 7.91·10�5 �0.8802.01 195 1.34·10�4 �0.8277.10 55 4.73·10�4 �0.7541.22 322 8.11·10�5 �0.8281.85 211 1.24·10�5 �0.7880.57 676 3.86·10�5 �0.8380.81 480 5.44·10�5 �0.829

Mean±SEM 3.07±0.91 266±70 1.92·10�4±6.40·10�5 �0.811±0.013

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types of investigations (including training courses forstudents) is also a great advantage of this method.

Acknowledgments We are grateful to the Socrates/Erasmus Pro-gram for support, as well as to Humboldt University and Univer-sity of Madeira.

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Macmillan, New York, p 18426. Articles in preparation

Fig. 6 Linear sweepvoltammogram for the alloyswith current density presentedin logarithmic and linear scales(potential versus NHE)

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