Studies on the corrosion behavior of the dental alloys.

Tatarciuc, Monica ; Vitalariu, Anca

1. INTRODUCTION

Corrosion behaviour of dental alloys is one factor used to determine the material degradation and biocompatibility. It can be assumed that saliva, food components and beverages can degrade and age dental restoratives under in vivo conditions. Many researches have studied the corrosion behaviour of dental alloys in physiological solutions (Aeimbhu et al, 2005). To measure the corrosion potential is a relatively simple concept used to evaluate the behaviour of dental alloys into the oral cavity. Measuring the corrosion potential we can appreciate the active or passive behaviour of the dental alloys (Kobayashi et al, 2005). Using the Evans diagram we can evaluate the corrosion from the thermodynamic point of view.

2. MATERIAL AND METHOD

Materials used in this study were five commercially available alloys: Gold, Palliag, Gaudent, Verasoft and Amalgam with different contents described in Table 1.

The potential of corrosion [E.sub.cor] is a measure that expresses the tendency of corrosion of a metal or alloy introduced into an electrolytic fluid. The potential of corrosion can be directly measured in report to a reference electrode that is characterised by a potential half-cell very stable. A reference electrode is introduced into a corrosive medium with the studied alloy and using a milivoltmeter with high impedance we can directly measure the potential. The potential of corrosion can be indirectly evaluated using the curves of linear polarisation, the diagram Evans. In the diagrams it is represented the logarithm of the density of the electric power depending on the potential of the electrode corresponding to a limit of over potential equal to [+ or -]50.... 60Mv. The crossing of the linear portion of the anodic and catholic lines of the polarization curve indicate on the potential axe the value of the corrosion potential [E.sub.ctor].

The electrodes were realised from dental alloys used in oral prosthesis removed from some of our treated patients. The surface of the electrodes was planed and ground on 320, 500, 800, 2400 silicon carbide abrasive papers followed by 1[micro]m alpha alumina polishing and 0.3 [micro]m finishing. After that, the electrodes were sheathed with epoxy resin. The electrode surface was planted with a grinding paper, cleaned and ultrasound treated in a solution of ethylic acid, distilled water and acetone.

Three types of electrolytes fluid were studied, represented by three artificial saliva: Aynor, Fusayama Meyer and Rondelli. The composition and the pH are described in Table 2.

3. EQUIPMENT AND DEVICES

To evaluate the corrosion potential and to register the potentiodynamic polarisation we used the potentiostat PGP201, from the Economic Electrochemical Lab: VoltaLab 21 (Radelkis Copenhagen).

VoltaLab 21 is a compact potentiostat/galvanostat with built-in signal generator that can be used as a stand-alone instrument when it is programmed through its front panel. In this manual mode, the potentiostat scan rate can be selected up to 2.5 V/s. With VoltaMaster 4, the maximum scan rate is 10 mV/s. Ideal for corrosion studies, VoltaLab 21 records the polarization resistance and the corrosion potential over very long periods. The resolution of the electric power was 100Pa. The electrochemical cell also contained a saturated calomel electrode as reference. Then the alloys were anodically polarized. We registered the polarisation curve for a opened potential circuit (OPC) between--300 Mv and +300 Mv with a scanning rate of the potential equal to 0,5 mV/s (Kim et al, 2005). The potential into an opened circuit was evaluated into the Evans diagram. The examination of the surfaces of the electrodes was performed with an optical microscope.

4. RESULTS

The crossing point of the anodic and cathodic curves indicates the corrosion potential. All the values registered for the OPC and for the corrosion potential are showed in Table 3.

With only one exception (Palliag into the Afnor solution), the corrosion potential appreciated by the Evans diagram was smaller in comparison to the potential into the open circuit. That can be explained because the potential into the open circuit polarisation is measured into statically conditions and in the Evans method into dynamic conditions (the potential applied on the electrode being different to the equilibrium potential). The Evans diagram reflecting the potential in opened circuit was registered for each alloy. In figures 1-3 are shown the Evans diagrams registered for gold, palliag and amalgam.

The highest corrosion probability was registered for the amalgam and the lowest for Palliag. The corrosion of dental amalgam is thus a complex process, which involves contributions from each of the phase present as well as intergranular corrosion (Brett et al, 2002).

The artificial medium Fusayama Meyer saliva was the lowest corrosive medium and the Rondelli saliva had the highest corrosive potential.

The choice of a dental alloy became a real challenge because of wide variety of products offered by the dental market. The results of our study are useful for the practitioners (the dentist and dental technician) representing a guide in the selection of the appropriate dental alloy for each patient.

Because of the increased aesthetic demands of the patients, the orthodontic fixed appliances became very frequent among the teenagers and young patients. This is the reason why our future researches will be focused on the evaluation of the corrosion behaviour of dental alloys used in orthodontic treatments.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

5. CONCLUSION

The influence of different artificial saliva on metallic corrosion was investigated by measurements of open circuit potential, polarisation curves, chosen owing to its high rate of corrosion. The corrosion of the dental alloys in different artificial saliva brought clearly into evidence the importance of the contacting solutions on the rate of corrosion (Brett & Muresan, 2002).

Our results are clinically significant because demonstrate that the individually oral conditions, the specific parameters and composition of the oral fluids are responsible for the corrosion behaviour and the biocompatibility of the precious and non-precious dental alloys. The selection of the dental alloy should be based on the salivary pH evaluation of each patient.

6. REFERENCES

Aeimbhu, A.; Castle, J. & Singjai, P. (2005). The influence of albumin on the electrochemical behaviour of commercial titanium. Key Engineering Materials, Vol.288-289, (2005), pp. 615-618

Brett, C.; Acciari, H.& Guastaldi, A.(2002). Corrosion of dental amalgams-studies of individual phases. Key Engineering Materials, Vol.230-232, (2002), pp. 463-466

Brett, C. & Muresan, I. (2002). The influence of artificial body fluids on metallic corrosion. Key Engineering Materials, Vol.230-232 (2002), pp. 459-462

Kim, Y.S.; Yoo, R.Y. & Sohn, C.G. (2005). Role of alloying elements on the cytotoxic behavior and corrosion of austenitic stainless steels. Materials Science Forum, Vol. 475-479, (2005), pp. 2295-2298

Kobayashi, S.; Ohgoe, Y. & Ozeki, K. (2005). Biocompatibility of diamond-like carbon coated Ni-Ti ortodontic wirw and acrylic resin teeth. Key Engineering Materials, Vol. 284-286, (2005), pp. 783-786

Tab. 1. The composition of the studied alloys

 Density
 (g/
Sample Alloy Composition [cm.sub.3])

1 GOLD Au-77%, Ag-13,5%, 16.5
 Pt-1,5%

2 PALLIAG Ag-58,5%, Pd-27,4%,
 Cu10,5%, Au2%, 11,1
 Zn1,5%, Ir-0,1%

3 GAUDENT Cu-82%, Al-9,97%,
 Ni-4,35, Fe-1,32%, 7,8
 Mn-2,04%

4 VERASOFT Ni-53,6%, Mn-19,5%,
 Cr-14,5%, Cu-9,5%, 7,7
 Al-1,6%, Si-1,5%

5 AMALGAM Alloy (Ag-69,7%,
 Sn17,7%, Cu-12%, 11,6
 Zn-0,9%)(50%)+Hg50%

Tab. 2. The pH and composition of the corrosion medium

Corrosion Composition pH
medium

Afnor Na Cl-0,7 g/L, KCl-1,2g/L,
saliva [Na.sub.2]HP[O.sub.4] 6,78
 [H.sub.2]O-0,26g/L,
 NaHC[O.sub.3]-1,5g/L,
 KSCN-0,33g/L, urea-1,35g/L

Fusayama Na Cl-0,40 g/L, KCl-0,40 g/L, 6,24
saliva [Na.sub.2]HP[O.sub.4]
 [H.sub.2]O-0,69 g/L,
 urea-1,00g/L, [Na.sub.2]S.9
 [H.sub.2]O-0,005 g/L,
 Ca[Cl.sub.2]-0,79 g/L

Rondelli KCl-1,47g/L, 7,52
saliva NaHC[O.sub.3]-1,25g/L,
 KSCN-0,520g/L,
 K[H.sub.2]P[O.sub.4]
 [H.sub.2]O-0,190 g/L

Tab. 3. The potential in open circuit and the
corrosion potential for the studied alloys

 [E.sub.cor]
ALLOY Artificial saliva OPC mV (ESC)
 (Evans)

Gold Afnor -137 -201
 Fusayama-Meyer -152 -181
 Rondelli -249 -287
Palliag Afnor -3 +2,3
 Fusayama-Meyer -22 -117
 Rondelli -65 -194
Gaudent Afnor -323 -353
 Fusayama-Meyer -170 -174
 Rondelli -268 -356
Verasoft Afnor -430 -599
 Fusayama-Meyer -143 -181
 Rondelli -237 -302
Amalgam Afnor -480 -668
 Fusayama-Meyer -322 -388
 Rondelli -449 -654