The Effect of Porcelain Firing on Electrochemical Behavior of a Dental Alloy in Hydrogen Peroxide Evs¸en Tamam, DDS, PhD,1 A. Kevser Aydın, DDS, PhD,2 & Semra Bilgic¸, MS, PhD3 1
Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey Department of Prosthodontics, Faculty of Dentistry, Ankara University, Ankara, Turkey 3 Department of Physical Chemistry, Faculty of Dentistry, Ankara University, Ankara, Turkey 2
Keywords Ni-Cr alloy; corrosion; porcelain firing; hydrogen peroxide; SEM. Correspondence Evs¸en Tamam, Department of Prosthodontics, Faculty of Dentistry, Gazi University, 06510 Emek, Ankara, Turkey. E-mail: [email protected] Presented at the 30th Annual Conference of the European Prosthodontic Association, London, England, 2 to 4 November 2006. This investigation was supported by Ankara University Research Foundation Grant No: 2005.08.02.065. The authors deny any conflicts of interest. Accepted April 22, 2014
Abstract Purpose: The aim of this study was to evaluate changes in electrochemical corrosion properties of porcelain firing simulated nickel-chromium dental casting alloy exposed to a 10% hydrogen peroxide bleaching agent. Materials and Methods: The electrochemical corrosion behavior of a Ni-Cr alloy was evaluated by cyclic polarization test in the aerated electrolyte (pH = 6.5). Test groups were produced in as-cast (group 1, control group) and simulated porcelain firing (group 2: heat-treated/mean value; group 3: heat-treated/cycle) conditions. Scanning electron microscopy (SEM) was also used to examine the alloy surfaces before and after the corrosion test. Results: The ranking of the groups with respect to Ecorr and Icorr was as follows: 1, 2, 3 and 3, 1, 2, respectively. Group 3 exhibited the greatest and group 2 displayed the least corrosion tendencies. An increase in corrosion rates was observed after heat treatment/cycle state. Post-corrosion SEM photographs were also consistent with the test results. Conclusion: Within the parameters of this study, a single heat treatment is insufficient to cause upheaval in corrosion behavior of a Ni-Cr alloy subjected to 10% hydrogen peroxide.
doi: 10.1111/jopr.12215
Ni-Cr dental alloys are used both for denture frameworks and reasonable alternatives to noble alloys for fixed prosthodontics because of their advantageous porcelain-fused-to-metal (PFM) systems.1,2 Regarding biocompatibility, corrosion behavior of a dental alloy is an important factor. Corrosion occurs on the surface of the alloy in the form of electrochemical reactions. It is a continuous process that causes ion loss.3-5 Dental alloys must be resistant to some corrosive situations normally present in the mouth (i.e., humidity, temperature changes, and pH changes).6-9 The effect of the PFM procedure on corrosion behavior and surface properties of Ni-based alloys has been pointed out in a few studies. The firing temperatures during the PFM process may lead to changes in the surface of Ni-Cr alloys with different elemental content.1 Such changes in the surface would affect the corrosion behavior of the alloy and thus release of the elements. Roach et al2 underlined that porcelain firing had a debilitative effect on corrosion properties of Ni-Cr alloys, with 14% to 22% Cr and 9% to 17% Mo. Although Wylie et al10 reported that small changes in microstructure occurred during the porcelain firing process and these did not influence the corrosion behavior
for the alloys investigated (containing higher [25%] and lower [12.6%] levels of Cr), this study demonstrated the importance of the level of Cr in Ni-based alloys. Tooth bleaching is a popular and conservative option for discolored teeth. Home bleaching is probably the most widely used bleaching technique because of its relative ease of use, low cost, safety, and high success rate.11 Because all other treatments must be completed before bleaching application, PFM restorations that may exist in the patient’s mouth would be exposed to the bleaching agent. A study that included mercury release from dental amalgam exposed to bleaching agent showed that this procedure caused mercury release in solution.11 Canay et al12 studied the effect of 10% carbamide peroxide on electrochemical corrosion of dental casting alloys used for fixed partial dentures and amalgam and revealed that unpolished amalgam and Ni-Cr alloy specimens had the highest and noble alloys had the lowest corrosion rates. Ameer et al13 examined electrochemical behavior of a nonprecious dental alloy in bleaching agents. They concluded that all tested agents affected the alloy’s corrosion behavior. Our previous study14 showed
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that a bleaching agent caused elemental release from NiCr alloy. The aim of this study was to evaluate the effect of various heat treatments simulating porcelain firing on the corrosion behavior and alterations on the surface of a base metal alloy exposed to 10% hydrogen peroxide bleaching agent. The null hypothesis is that porcelain firing procedures may alter the corrosion behavior of Ni-Cr alloy treated with 10% hydrogen peroxide.
Table 1 Summary of the cp results for the test groups Groups
Ecorr (mV)
Icorr (μA/cm2 )
Corrosion Rate (μm/year)
Group 1 Group 2 Group 3
256.2 (35.975) 254.2 (32.2) 237.6 (27.26)∗
0.1626 (0.012) 0.0638 (0.0017) 0.3708 (0.044)∗
0.07106 0.04361 0.10043
SD in parentheses. *Indicates sample group significantly different than other sample groups at a 95% confidence interval.
Materials and methods The materials used in this study were 10% H2 O2 (HP) soluR tion (pH = 6.5) and a Ni-Cr dental casting alloy (Wiron 99 ; BEGO, Bremen, Germany). Cylinder-shaped (4 mm diameter, 25 mm high) specimens (15) were cast following the recommendations of the manufacturer, and then randomly divided into three groups (n = 5). After that, heat treatment procedures and polishing were carried out as described below. Group 1 (control group): casting and polishing (without any heat treatment/cast state). Group 2: casting, firing at 950°C for 5 minutes, and polishing (fired/mean value state). Group 3: casting, firing with full cycles of PFM systems (without applying any porcelain), and polishing (fired/cycle state). Polishing procedure was performed until obtaining clinically acceptable surfaces in accordance with the recommendations of the manufacturer, as explained in detail in our previous work.14 Briefly, after devesting, the alloys were blasted with 250 μm Al2 O3 and finished with diamond milling tools. Then, the surfaces were blasted with 50 μm Al2 O3 , polished with R (BEGO), polished with rubber polishers, finished Perlablast with a polishing paste, and steam cleaned for all groups. For the second and third groups, specimens’ surfaces were blasted again with 250 μm Al2 O3 , and steam cleaned immediately after the finishing stage. The specimens of the second group then were heated at 950°C for 5 minutes (chosen as a mean value of the porcelain firing procedure).15 For the third group, to simulate a general porcelain firing cycle, alloys were heated in five phases (950°C for 1 minute, 930°C for 1 minute, 930°C for 1 minute, 930°C for 1 minute, and 900°C for 1 minute for the stages of first opaque, second opaque, first dentin, second dentin, and glaze stages, respectively) following Vita VMK R 95 (Vident, Brea, CA) dental porcelain’s prospectus without porcelain application before the polishing stage. As detailed in previous studies,15-17 each specimen was cleaned by scrubbing with a soft toothbrush and 2% glutaraldehyde solution, rinsing with distilled water, and ultrasonic treatment in 70% isopropyl alcohol. After cleaning, specimens were disinfected in 70% isopropyl alcohol for 20 minutes, followed by two rinses with sterile double-distilled water to remove the alcohol. Electrochemical measurements were carried out for the ascast and fired specimens using a potentiostat (Gamry PCl4/ 750TM Potentiostat; Gamry Instruments Inc., Warminster, PA). Polarization experiments were carried out in a conventional three-electrode glass cell with a platinum counter electrode and saturated calomel electrode (SCE) as a reference, with a Lug402
gin capillary bridge. The working electrode was constructed by embedding a cylindrical alloy specimen in methyl methacrylate resin. The testing area of the specimen was 0.1256 cm2 . Before each experiment, the electrode was polished with 1200grit emery paper, washed thoroughly with bidistilled water, and then transferred to the cell. The water-jacketed cell was connected to a constant temperature circulator operating at 37 ± 1°C to simulate mouth conditions.17 The 10% HP solution was prepared by diluting the 30% HP solution in 0.1 M phosphate buffer solution (pH = 6.5) as electrolyte. The volume of the electrolyte in the corrosion cell was 50 mL. The working electrode was placed in the holder/electrode assembly when the electrolyte reached the test temperature. During each experiment, solutions were mixed with a magnetic stirrer.17 Before cyclic polarization (cp) testing, all test specimens were allowed to reach a steady-state potential (Ei = 0) for a period of 1 hour. Then tests were initiated at 100 mV versus SCE below this potential and scanned at a rate of 1 mV/s in the anodic direction until 1000 mV over the value of steady-state potential. The scan then was reversed back to the Ei = 0 of the specimens. Each measurement was repeated five times for each specimen (5 specimens × 5 measurements), and mean values for corrosion parameters were calculated (means were calculated from 25 measurements; n = 25). Data (the corrosion potentials, Ecorr , and the current densities, Icorr ) from these curves were acquired using the anodic Tafel regions extrapolating through computer software (Gamry ECHEM analyst DC105TM Corrosion Techniques Software; Gamry Instruments Inc.). Hysteresis in the polarization curves was used to characterize alloy susceptibility to pitting and crevice corrosion.2 Scanning electron microscopy (SEM, Jeol JSM- 6400, Tokyo, Japan) was used to examine the alloy surfaces (two specimens of each group) before and after cp testing. One-way ANOVA was applied to compare the corrosion potential and current density among the three groups. Tukey’s test (α = 0.05) was chosen as the multiple comparison technique when necessary.
Results The corrosion parameters, as determined through cp testing, are listed in Table 1. The representative cp curves for groups 1, 2, and 3 are provided in Figure 1 to show the range of corrosion behavior of the as-cast and fired conditions. Repeating heat treatment (firing procedure/cycle condition) resulted in an increase in Ecorr values of the alloy when compared to group 2 (firing procedure/mean value condition). In
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Corrosion of Porcelain-Fired Ni-Cr Alloy in H2 O2
Figure 1 Representative cyclic polarization curves for testing groups: (A) Group 1, (B) Group 2, and (C) Group 3.
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addition, group 1 exhibited a wider passive region in the polarization curve, while firing/cycle condition led to more narrow passive region than firing/mean value condition. According to the Ecorr values (mV), the groups could be ranked in decreasing order as follows: group 1, group 2, and group 3. The ranking of the Icorr values (μA/cm2 ) among test groups was as follows: 3 > 1 > 2. The material with higher Icorr at a given potential is more prone to corrode. Group 2 had the most passive and group 3 had the most active Icorr value. The lowest Ecorr value for group 3 agreed with the highest Icorr value obtained from the same group. This finding was statistically significant (α = 0.05). Although a remarkable diversity was found for Icorr value between the first and the second groups, this difference was not statistically significant. On the other hand, the differences between groups 1/3 and 2/3 were statistically significant for the values for Ecorr and Icorr (α = 0.05). The pre-corrosion surface textures of the W99 specimens were almost similar for all groups (Fig 2). Post-corrosion examination with SEM revealed that the highest surface erosion occurred in group 3 and the lowest for group 2 (Fig 3).
Discussion Temperatures reached during firing may alter the composition of the surface oxides on the alloys, which may, in turn, alter their corrosion behavior and host tissue reactions.2 In this study, the corrosion behavior of the W99 alloy was due to the effect of heat treatment in a bleaching solution. As can be seen in Figure 1, all groups exhibited different behavior in 10% HP solution. Khamis and Seddik18 found that the Ni-Cr cast alloys show a high susceptibility to pitting corrosion; however, in this study, all groups showed little or no hysteresis between the forward and reverse scans (Fig 1), indicating that W99 alloy for all test conditions was resistant to pitting corrosion. These findings agree with the other studies2,19 that used the same alloy. It also has been reported that the Ni-Cr alloy presents high resistance to corrosion both in vitro and in vivo.20 Although tested alloys were cobalt-based alloys, Nascimento et al21 reported that pitting corrosion capability seemed to be directly related to lower chromium content and to the complexity of the composition of the alloy. For Fe-Ni-Cr-Mo stainless steel, the relative effectiveness of Cr and Mo content on pitting or crevice corrosion usually can be assessed qualitatively by pitting resistance equivalent (PRE), which is represented by the empirical equation: PRE = %Cr + 3.3%Mo.19 According to Huang, “A PRE above 38 is supposed to provide good resistance to pitting corrosion in a Cl−1 -containing environment. For Ni-Cr-Mo alloy, a PRE of at least 43 is required to avoid pitting corrosion in the clinically relevant pH range of 4 to 7.”19,21 According to the PRE equation mentioned above, the PRE value for W99 in this study was 49.03. The firing temperatures chosen in this study were a mean value and a full cycle of a PFM procedure without applying any porcelain. The mean heating value selected in this study was 950°C for 5 minutes in accordance with Wataha et al,15 and also consistent with the “degassing” (oxide firing) value suggested 404
for many brands of PFM systems. The full cycle was conveR nient with the Vita VMK 95 porcelain system recommended by the alloy manufacturer. Wylie et al10 performed electrochemical tests on two Ni-based alloys fired with a full cycle of a porcelain veneer application. They reported that porcelain firing simulation had no effect on the anodic reactivity of tested Ni-based alloys and that the current was found to be identical to those of the as-cast state. Although small changes in the alloy microstructures were identified following simulated porcelain firing, no differences in corrosion behavior were detected. In contrast, Roach et al found a decrease in corrosion resistance following simulated porcelain firing.2 However, according to Wylie et al, “the differences were dependent upon the alloy tested, and thus other alloying combinations may show more significant alterations. Therefore, simulated porcelain firing remains an important consideration when evaluating the corrosion behavior of Ni-based alloys.”10 In the W99 instruction book, the manufacturer emphasizes that oxide firing is not necessary. Since no statistical differences were identified between groups 1 and 2 for corrosion parameters (Table 1), this information was confirmed; however, the Icorr values determined for groups 1 and 2 showed that degassing procedure reduced the current density on the alloy surface. The corrosion parameters measured in this study were corrosion potential, corrosion current density, and the corrosion rate. All the corrosion testing and data interpretation were performed in accordance with the international standards for these types of tests.22-23 Ecorr is the potential at which the metal oxidation begins in a given solution. It means that same metal will exhibit different potential values in different solutions. The other parameters indicate whether the electrical current (Icorr ) is flowing, or whether the material loss as a function of time (corrosion rate) exists. If the Icorr has been measured, it could be used to calculate corrosion rate of the metal.23-25 Thus, from the clinical point of view, Icorr and corrosion rate values are the most remarkable data because they represent how much material is lost per year. In this study, we found group 3 (fired/cycle state) was the most prone to corrosion. This could be explained by the negative effect of repeated heat treatments on the thin oxide film, which may alter the corrosion behavior of the W99 alloy. The polishing traces observed on the specimen surfaces before the cp testing (Fig 2) almost disappeared after the cp testing (Fig 3). These changes in the surface topography were the most remarkable for group 3 (Fig 3C) and the least remarkable for group 2 (Fig 3B). These findings may be correlated with the Icorr values obtained for testing groups (Table 1). In addition, the corrosion rates of the groups (Table 1) confirmed that group 3 with 0.10043 μm/year value was to be exposed to the most detrimental effect of 10% HP solution. Various factors, including alloy type, creep at high temperatures, release of induced stresses from casting and cold working, and different coefficients of thermal expansion of alloy and porcelain, have been suggested as the reason for metal distortion during porcelain firing.21 The clinical porcelain firing process requires four stages of high-temperature cycles from 950 to 1010°C, inevitably altering the microstructure of cast alloys in a variety of ways, including homogenization, phase transformation, and oxidation. These changes in microstructure may affect the development of protective surface oxides that would
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Figure 2 SEM micrographs before cp testing for W99 alloy comparing as-cast (A) and after simulated porcelain firing states (B: fired/mean value and C: fired/cycle condition), respectively.
Figure 3 SEM micrographs after cp testing for W99 alloy comparing as-cast (A) and after simulated porcelain firing states (B: fired/mean value and C: fired/cycle condition), respectively.
then result in changes to the alloys’ corrosion resistance. Lai et al26 concluded that new surface oxides were formed on the alloy surfaces after the PFM firing. Therefore, to minimize the release of alloy-corrosion products from the exposed metal surfaces of metal-ceramic restorations into the patient’s tissues, it is important to evaluate the effects of the porcelain firing process on the corrosion behavior of base-metal dental alloys.26 Ardlin et al27 studied the effect of high temperature on CoCr alloys. They found that firing at 980°C reduced the release of ions from two Co-Cr alloys into standard saline lactic acid solution. In the current study, oxide firing (firing with mean value), which may have a slight effect on the surface oxide layer and/or dendritic microstructure of alloy, also supports the idea that degassing procedure reduces corrosion. It was shown that dendritic microstructures and grain boundaries of base metal alloys are affected after porcelain firing simulation.27 Namely, in the as-cast state, Ni-Cr alloys revealed a dendritic structure. After porcelain firing, the alloys’ microstructures displayed homogenization in the dendritic structure, which exhibited further grain boundary homogeneity. Li et al29 reported that the as-cast microstructures of Pd-Ag alloys were inhomogeneous. After simulated porcelain firing heat treatment, the microstructures of the specimens were homogenized and contained numerous precipitates, along with the palladium solid solution matrix. In our study, full-cycle firing simulation may have caused more critical outcomes, including homogenization, phase transformation, and/or oxidation, in addition to corrosion behavior. Porcelain-bonding alloys were distinct from full-cast alloys largely because the former required melting ranges that could
survive the application of porcelain (typical firing temperatures of 870 to 1370°C).30 Also, the application of porcelain clinically requires high-temperature firing cycles, inevitably resulting in changes in the microstructure during the porcelain firing process.2,10 According to Wataha, “The thickest oxide layers occur in nickel- and cobalt-based alloys because these alloys contain elements that form oxides easily during the initial oxidation step prior to firing of the opaque porcelain.”30 Wylie et al10 also indicated that the presence of crevices combined with variations of elemental distribution in the microstructure can lead to accelerated corrosion of Ni-based alloys with lower Cr contents. This evidence, along with the data obtained in this study, provide support for the hypothesis that PFM firing causes changes in alloy microstructures that result in altered surface oxides and corrosion behavior for tested Ni-Cr alloy. Hydrogen peroxide and carbamide peroxide are the most favored agents for bleaching applications, and the active substance is HP at different proportions for both.31 In this study, HP was preferred due to the higher concentration of active material content. If the first study had been conducted with lower concentration (carbamide peroxide), corrosion would not have been observed. Carbamide peroxide solution and HP with different concentrations will be the subject of our future studies. Although the stability of Ni-based alloys has been shown to be significantly reduced when placed in more acidic environments, thus enhancing metal ion release,16 the pH was 6.5 in the current study because popular commercial bleaching agents have an average pH of 5 to 6.5. Al-Hity et al3 reported that although many factors may contribute to the results, heat treatment decreased the corrosion resistance
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for gold alloy, especially in an acidic environment in their test conditions. As Wylie et al noted, “It is important to assess the corrosion behavior of dental alloys in acidic solutions due to the low pH encountered in crevices in addition to the variable pH conditions routinely encountered in the oral environment through the variety of foods and liquids ingested.”10 In the current study, electrochemical measurements were performed under the aerated conditions in order to provide a medium closely resembling the oral cavity. Deaerated conditions may affect the results. Mouthguard bleaching usually is performed for 8 hours a day, and when the patients are asleep, where the flow of saliva decreases. If patients have full-cast and/or PFM restorations, bleaching treatment must be applied very carefully because excessive material may contact these restorations and alter the corrosion properties and/or tissue responses. Further investigations are also needed with different alloys, different bleaching agents, and different pH values.
Conclusions Within the parameters of this study, a single heat treatment is insufficient to cause upheaval in corrosion behavior of a NiCr alloy subjected to 10% hydrogen peroxide. Although the null hypothesis was rejected, if full-cast metal restorations are planned, degassing application will be better for this type of alloy in terms of improving the corrosion properties; however, multiple heat treatments, as in alloys veneered with porcelain, cause significantly more corrosion potential. For this reason, the clinician should be cautious in the use of commercial bleaching material for patients with existing base metal alloy restorations.
Acknowledgments The authors are grateful to Dils¸en Tamam for statistical analysis.
References 1. Huang H-H: Surface characterization of passive film on NiCr-based dental casting alloys. Biomaterials 2003;24:1575-1582 2. Roach MD, Wolan JT, Parsell DE, et al: Use of x-ray photoelectron spectroscopy and cyclic polarization to evaluate the corrosion behavior of six nickel-chromium alloys before and after porcelain-fused-to-metal firing. J Prosthet Dent 2000;84:623-634 3. Al-Hity RR, Kappert HF, Viennot S, et al: Corrosion resistance measurements of dental alloys, are they correlated? Dent Mater 2006;23:679-687 4. Venugopalan R, Messer RLW: The effect of a microcarrier suspension cell culture system on polarization measurements from Ni-Cr dental casting alloys. Dent Mater 2005;21:993-998 5. Gil FJ, S´anchez LA, Esp´ıas A, et al: In vitro corrosion behaviour and metallic ion release of different prosthodontic alloys. Int Dent J 1999;49:361-367 6. Kedici SP, Aks¨ut AA, Kılıc¸arslan MA, et al: Corrosion behaviour of dental metals and alloys in different media. J Oral Rehabil 1998;25:800-808 7. Bayramo˘glu G, Alemdaro˘glu T, Kedici S, et al: The effect of pH on the corrosion of dental metal alloys. J Oral Rehabil 2000;27:563-575 406
¨ 8. Canay S¸, Oktemer M: In vitro corrosion behavior of 13 prosthodontic alloys. Quintessence Int 1992;23:279-287 9. Wataha JC: Biocompatibility of dental casting alloys: a review. J Prosthet Dent 2000;83:223-234 10. Wylie CM, Shelton RM, Fleming GJP, et al: Corrosion of nickel-based dental casting alloys. Dent Mater 2006;23:714-723 11. Rotstein I, Avron Y, Shemesh H, et al: Factors affecting mercury release from dental amalgam exposed to carbamide peroxide bleaching agent. Am J Dent 2004;17:347-350 12. Canay S¸, C ¸ ehreli MC, Bilgic¸ S: In vitro evaluation of the effect of a current bleaching agent on the electrochemical corrosion of dental alloys. J Oral Rehabil 2002;29:1014-1019 13. Ameer MA, Khamis E, Al- Motlaq M: Electrochemical behavior of non-precious dental alloys in bleaching agents. Electrochim Acta 2004;50:141-148 14. Tamam E, Aydin AK: Surface characterization of passive film and elemental release analysis of a Ni- Cr alloy during bleaching. Part 1: effects of different bleaching agents. Clin Oral Investig 2011;15:375-382 15. Wataha JC, Lockwood PE, Mettenburg D, et al: Toothbrushing causes elemental release from dental casting alloys over extended intervals. J Biomed Mater Res Part B: Appl Biomater 2003;65B:180-185 16. Wataha JC, Lockwood PE, Khajotia SS, et al: Effect of pH on element release from dental casting alloys. J Prosthet Dent 1998;80:691-698 17. Khamis E, Seddik M: Corrosion evaluation of recasting nonprecious dental alloys. Int Dent J 1995;45:209-217 18. Huang H-H: Effect of chemical composition on the corrosion behavior of Ni-Cr- Mo dental casting alloys. J Biomed Mater Res 2002;60:458-465 19. Benatti OFM, Miranda Jr WG, Muench A: In vitro and in vivo corrosion evaluation of nickel- chromium- and copperaluminum- based alloys. J Prosthet Dent 2000;84:360-363 20. Nascimento ML, Mueller W-D, Carvalho AC, et al: Electrochemical characterization of cobalt-based alloys using the mini-cell system. Dent Mater 2006;23:369-373 21. Lakhani SA, Ercoli C, Moss ME, et al: Influence of cold working and thermal treatment on the fit of implant supported metal ceramic fixed partial dentures. J Prosthet Dent 2002;88:159-169 22. Babolan R: Corrosion Tests and Standards: Application and Interpretation (ed 2). West Conshohocken, PA, ASTM International Publications, 2005, Number: MNL20-2ND 23. ISO Standards (1995. ISO 11845:1995. Corrosion of metals and alloys- General principles for corrosion testing 24. ISO Standards (1999. ISO 8044:1999). Corrosion of metals and alloys- Basic terms and definitions 25. Lai HY, Bowers B, Wolan JT, et al: Metallurgical, surface, and corrosion analysis of Ni-Cr dental casting alloys before and after porcelain firing. Dent Mater 2008;24:378-385 26. Qui J, Yu W-Q, Zhang F-Q, et al: Corrosion behaviour and surface analysis of a Co-Cr and two Ni-Cr dental alloys before and after simulated porcelain firing. Eur J Oral Sci 2011;119:93-101 27. Ardlin BI, Dahl JE, Tibballs JE: Static immersion and irritation tests of dental metal–ceramic alloys. Eur J Oral Sci 2005;113:83-89 28. Li D, Baba N, Brantley WA, et al: Study of Pd-Ag alloys: examination of effect of casting porosity on fatigue behavior and microstructural analysis. J Mater Sci: Mater Med 2010;21:2723-2731 29. Wataha JC: Alloys for prosthodontic restorations. J Prosthet Dent 2002;87:351-363 30. El-Murr J, Ruel D, St-Georges AJ: Effects of external bleaching on restorative materials: a review. J Can Dent Assoc 2011;77:b59
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Department of Prosthodontics, Faculty of Dentistry, Gazi University, Ankara, Turkey Department of Prosthodontics, Faculty of Dentistry, Ankara University, Ankara, Turkey 3 Department of Physical Chemistry, Faculty of Dentistry, Ankara University, Ankara, Turkey 2
Keywords Ni-Cr alloy; corrosion; porcelain firing; hydrogen peroxide; SEM. Correspondence Evs¸en Tamam, Department of Prosthodontics, Faculty of Dentistry, Gazi University, 06510 Emek, Ankara, Turkey. E-mail: [email protected] Presented at the 30th Annual Conference of the European Prosthodontic Association, London, England, 2 to 4 November 2006. This investigation was supported by Ankara University Research Foundation Grant No: 2005.08.02.065. The authors deny any conflicts of interest. Accepted April 22, 2014
Abstract Purpose: The aim of this study was to evaluate changes in electrochemical corrosion properties of porcelain firing simulated nickel-chromium dental casting alloy exposed to a 10% hydrogen peroxide bleaching agent. Materials and Methods: The electrochemical corrosion behavior of a Ni-Cr alloy was evaluated by cyclic polarization test in the aerated electrolyte (pH = 6.5). Test groups were produced in as-cast (group 1, control group) and simulated porcelain firing (group 2: heat-treated/mean value; group 3: heat-treated/cycle) conditions. Scanning electron microscopy (SEM) was also used to examine the alloy surfaces before and after the corrosion test. Results: The ranking of the groups with respect to Ecorr and Icorr was as follows: 1, 2, 3 and 3, 1, 2, respectively. Group 3 exhibited the greatest and group 2 displayed the least corrosion tendencies. An increase in corrosion rates was observed after heat treatment/cycle state. Post-corrosion SEM photographs were also consistent with the test results. Conclusion: Within the parameters of this study, a single heat treatment is insufficient to cause upheaval in corrosion behavior of a Ni-Cr alloy subjected to 10% hydrogen peroxide.
doi: 10.1111/jopr.12215
Ni-Cr dental alloys are used both for denture frameworks and reasonable alternatives to noble alloys for fixed prosthodontics because of their advantageous porcelain-fused-to-metal (PFM) systems.1,2 Regarding biocompatibility, corrosion behavior of a dental alloy is an important factor. Corrosion occurs on the surface of the alloy in the form of electrochemical reactions. It is a continuous process that causes ion loss.3-5 Dental alloys must be resistant to some corrosive situations normally present in the mouth (i.e., humidity, temperature changes, and pH changes).6-9 The effect of the PFM procedure on corrosion behavior and surface properties of Ni-based alloys has been pointed out in a few studies. The firing temperatures during the PFM process may lead to changes in the surface of Ni-Cr alloys with different elemental content.1 Such changes in the surface would affect the corrosion behavior of the alloy and thus release of the elements. Roach et al2 underlined that porcelain firing had a debilitative effect on corrosion properties of Ni-Cr alloys, with 14% to 22% Cr and 9% to 17% Mo. Although Wylie et al10 reported that small changes in microstructure occurred during the porcelain firing process and these did not influence the corrosion behavior
for the alloys investigated (containing higher [25%] and lower [12.6%] levels of Cr), this study demonstrated the importance of the level of Cr in Ni-based alloys. Tooth bleaching is a popular and conservative option for discolored teeth. Home bleaching is probably the most widely used bleaching technique because of its relative ease of use, low cost, safety, and high success rate.11 Because all other treatments must be completed before bleaching application, PFM restorations that may exist in the patient’s mouth would be exposed to the bleaching agent. A study that included mercury release from dental amalgam exposed to bleaching agent showed that this procedure caused mercury release in solution.11 Canay et al12 studied the effect of 10% carbamide peroxide on electrochemical corrosion of dental casting alloys used for fixed partial dentures and amalgam and revealed that unpolished amalgam and Ni-Cr alloy specimens had the highest and noble alloys had the lowest corrosion rates. Ameer et al13 examined electrochemical behavior of a nonprecious dental alloy in bleaching agents. They concluded that all tested agents affected the alloy’s corrosion behavior. Our previous study14 showed
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that a bleaching agent caused elemental release from NiCr alloy. The aim of this study was to evaluate the effect of various heat treatments simulating porcelain firing on the corrosion behavior and alterations on the surface of a base metal alloy exposed to 10% hydrogen peroxide bleaching agent. The null hypothesis is that porcelain firing procedures may alter the corrosion behavior of Ni-Cr alloy treated with 10% hydrogen peroxide.
Table 1 Summary of the cp results for the test groups Groups
Ecorr (mV)
Icorr (μA/cm2 )
Corrosion Rate (μm/year)
Group 1 Group 2 Group 3
256.2 (35.975) 254.2 (32.2) 237.6 (27.26)∗
0.1626 (0.012) 0.0638 (0.0017) 0.3708 (0.044)∗
0.07106 0.04361 0.10043
SD in parentheses. *Indicates sample group significantly different than other sample groups at a 95% confidence interval.
Materials and methods The materials used in this study were 10% H2 O2 (HP) soluR tion (pH = 6.5) and a Ni-Cr dental casting alloy (Wiron 99 ; BEGO, Bremen, Germany). Cylinder-shaped (4 mm diameter, 25 mm high) specimens (15) were cast following the recommendations of the manufacturer, and then randomly divided into three groups (n = 5). After that, heat treatment procedures and polishing were carried out as described below. Group 1 (control group): casting and polishing (without any heat treatment/cast state). Group 2: casting, firing at 950°C for 5 minutes, and polishing (fired/mean value state). Group 3: casting, firing with full cycles of PFM systems (without applying any porcelain), and polishing (fired/cycle state). Polishing procedure was performed until obtaining clinically acceptable surfaces in accordance with the recommendations of the manufacturer, as explained in detail in our previous work.14 Briefly, after devesting, the alloys were blasted with 250 μm Al2 O3 and finished with diamond milling tools. Then, the surfaces were blasted with 50 μm Al2 O3 , polished with R (BEGO), polished with rubber polishers, finished Perlablast with a polishing paste, and steam cleaned for all groups. For the second and third groups, specimens’ surfaces were blasted again with 250 μm Al2 O3 , and steam cleaned immediately after the finishing stage. The specimens of the second group then were heated at 950°C for 5 minutes (chosen as a mean value of the porcelain firing procedure).15 For the third group, to simulate a general porcelain firing cycle, alloys were heated in five phases (950°C for 1 minute, 930°C for 1 minute, 930°C for 1 minute, 930°C for 1 minute, and 900°C for 1 minute for the stages of first opaque, second opaque, first dentin, second dentin, and glaze stages, respectively) following Vita VMK R 95 (Vident, Brea, CA) dental porcelain’s prospectus without porcelain application before the polishing stage. As detailed in previous studies,15-17 each specimen was cleaned by scrubbing with a soft toothbrush and 2% glutaraldehyde solution, rinsing with distilled water, and ultrasonic treatment in 70% isopropyl alcohol. After cleaning, specimens were disinfected in 70% isopropyl alcohol for 20 minutes, followed by two rinses with sterile double-distilled water to remove the alcohol. Electrochemical measurements were carried out for the ascast and fired specimens using a potentiostat (Gamry PCl4/ 750TM Potentiostat; Gamry Instruments Inc., Warminster, PA). Polarization experiments were carried out in a conventional three-electrode glass cell with a platinum counter electrode and saturated calomel electrode (SCE) as a reference, with a Lug402
gin capillary bridge. The working electrode was constructed by embedding a cylindrical alloy specimen in methyl methacrylate resin. The testing area of the specimen was 0.1256 cm2 . Before each experiment, the electrode was polished with 1200grit emery paper, washed thoroughly with bidistilled water, and then transferred to the cell. The water-jacketed cell was connected to a constant temperature circulator operating at 37 ± 1°C to simulate mouth conditions.17 The 10% HP solution was prepared by diluting the 30% HP solution in 0.1 M phosphate buffer solution (pH = 6.5) as electrolyte. The volume of the electrolyte in the corrosion cell was 50 mL. The working electrode was placed in the holder/electrode assembly when the electrolyte reached the test temperature. During each experiment, solutions were mixed with a magnetic stirrer.17 Before cyclic polarization (cp) testing, all test specimens were allowed to reach a steady-state potential (Ei = 0) for a period of 1 hour. Then tests were initiated at 100 mV versus SCE below this potential and scanned at a rate of 1 mV/s in the anodic direction until 1000 mV over the value of steady-state potential. The scan then was reversed back to the Ei = 0 of the specimens. Each measurement was repeated five times for each specimen (5 specimens × 5 measurements), and mean values for corrosion parameters were calculated (means were calculated from 25 measurements; n = 25). Data (the corrosion potentials, Ecorr , and the current densities, Icorr ) from these curves were acquired using the anodic Tafel regions extrapolating through computer software (Gamry ECHEM analyst DC105TM Corrosion Techniques Software; Gamry Instruments Inc.). Hysteresis in the polarization curves was used to characterize alloy susceptibility to pitting and crevice corrosion.2 Scanning electron microscopy (SEM, Jeol JSM- 6400, Tokyo, Japan) was used to examine the alloy surfaces (two specimens of each group) before and after cp testing. One-way ANOVA was applied to compare the corrosion potential and current density among the three groups. Tukey’s test (α = 0.05) was chosen as the multiple comparison technique when necessary.
Results The corrosion parameters, as determined through cp testing, are listed in Table 1. The representative cp curves for groups 1, 2, and 3 are provided in Figure 1 to show the range of corrosion behavior of the as-cast and fired conditions. Repeating heat treatment (firing procedure/cycle condition) resulted in an increase in Ecorr values of the alloy when compared to group 2 (firing procedure/mean value condition). In
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Figure 1 Representative cyclic polarization curves for testing groups: (A) Group 1, (B) Group 2, and (C) Group 3.
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addition, group 1 exhibited a wider passive region in the polarization curve, while firing/cycle condition led to more narrow passive region than firing/mean value condition. According to the Ecorr values (mV), the groups could be ranked in decreasing order as follows: group 1, group 2, and group 3. The ranking of the Icorr values (μA/cm2 ) among test groups was as follows: 3 > 1 > 2. The material with higher Icorr at a given potential is more prone to corrode. Group 2 had the most passive and group 3 had the most active Icorr value. The lowest Ecorr value for group 3 agreed with the highest Icorr value obtained from the same group. This finding was statistically significant (α = 0.05). Although a remarkable diversity was found for Icorr value between the first and the second groups, this difference was not statistically significant. On the other hand, the differences between groups 1/3 and 2/3 were statistically significant for the values for Ecorr and Icorr (α = 0.05). The pre-corrosion surface textures of the W99 specimens were almost similar for all groups (Fig 2). Post-corrosion examination with SEM revealed that the highest surface erosion occurred in group 3 and the lowest for group 2 (Fig 3).
Discussion Temperatures reached during firing may alter the composition of the surface oxides on the alloys, which may, in turn, alter their corrosion behavior and host tissue reactions.2 In this study, the corrosion behavior of the W99 alloy was due to the effect of heat treatment in a bleaching solution. As can be seen in Figure 1, all groups exhibited different behavior in 10% HP solution. Khamis and Seddik18 found that the Ni-Cr cast alloys show a high susceptibility to pitting corrosion; however, in this study, all groups showed little or no hysteresis between the forward and reverse scans (Fig 1), indicating that W99 alloy for all test conditions was resistant to pitting corrosion. These findings agree with the other studies2,19 that used the same alloy. It also has been reported that the Ni-Cr alloy presents high resistance to corrosion both in vitro and in vivo.20 Although tested alloys were cobalt-based alloys, Nascimento et al21 reported that pitting corrosion capability seemed to be directly related to lower chromium content and to the complexity of the composition of the alloy. For Fe-Ni-Cr-Mo stainless steel, the relative effectiveness of Cr and Mo content on pitting or crevice corrosion usually can be assessed qualitatively by pitting resistance equivalent (PRE), which is represented by the empirical equation: PRE = %Cr + 3.3%Mo.19 According to Huang, “A PRE above 38 is supposed to provide good resistance to pitting corrosion in a Cl−1 -containing environment. For Ni-Cr-Mo alloy, a PRE of at least 43 is required to avoid pitting corrosion in the clinically relevant pH range of 4 to 7.”19,21 According to the PRE equation mentioned above, the PRE value for W99 in this study was 49.03. The firing temperatures chosen in this study were a mean value and a full cycle of a PFM procedure without applying any porcelain. The mean heating value selected in this study was 950°C for 5 minutes in accordance with Wataha et al,15 and also consistent with the “degassing” (oxide firing) value suggested 404
for many brands of PFM systems. The full cycle was conveR nient with the Vita VMK 95 porcelain system recommended by the alloy manufacturer. Wylie et al10 performed electrochemical tests on two Ni-based alloys fired with a full cycle of a porcelain veneer application. They reported that porcelain firing simulation had no effect on the anodic reactivity of tested Ni-based alloys and that the current was found to be identical to those of the as-cast state. Although small changes in the alloy microstructures were identified following simulated porcelain firing, no differences in corrosion behavior were detected. In contrast, Roach et al found a decrease in corrosion resistance following simulated porcelain firing.2 However, according to Wylie et al, “the differences were dependent upon the alloy tested, and thus other alloying combinations may show more significant alterations. Therefore, simulated porcelain firing remains an important consideration when evaluating the corrosion behavior of Ni-based alloys.”10 In the W99 instruction book, the manufacturer emphasizes that oxide firing is not necessary. Since no statistical differences were identified between groups 1 and 2 for corrosion parameters (Table 1), this information was confirmed; however, the Icorr values determined for groups 1 and 2 showed that degassing procedure reduced the current density on the alloy surface. The corrosion parameters measured in this study were corrosion potential, corrosion current density, and the corrosion rate. All the corrosion testing and data interpretation were performed in accordance with the international standards for these types of tests.22-23 Ecorr is the potential at which the metal oxidation begins in a given solution. It means that same metal will exhibit different potential values in different solutions. The other parameters indicate whether the electrical current (Icorr ) is flowing, or whether the material loss as a function of time (corrosion rate) exists. If the Icorr has been measured, it could be used to calculate corrosion rate of the metal.23-25 Thus, from the clinical point of view, Icorr and corrosion rate values are the most remarkable data because they represent how much material is lost per year. In this study, we found group 3 (fired/cycle state) was the most prone to corrosion. This could be explained by the negative effect of repeated heat treatments on the thin oxide film, which may alter the corrosion behavior of the W99 alloy. The polishing traces observed on the specimen surfaces before the cp testing (Fig 2) almost disappeared after the cp testing (Fig 3). These changes in the surface topography were the most remarkable for group 3 (Fig 3C) and the least remarkable for group 2 (Fig 3B). These findings may be correlated with the Icorr values obtained for testing groups (Table 1). In addition, the corrosion rates of the groups (Table 1) confirmed that group 3 with 0.10043 μm/year value was to be exposed to the most detrimental effect of 10% HP solution. Various factors, including alloy type, creep at high temperatures, release of induced stresses from casting and cold working, and different coefficients of thermal expansion of alloy and porcelain, have been suggested as the reason for metal distortion during porcelain firing.21 The clinical porcelain firing process requires four stages of high-temperature cycles from 950 to 1010°C, inevitably altering the microstructure of cast alloys in a variety of ways, including homogenization, phase transformation, and oxidation. These changes in microstructure may affect the development of protective surface oxides that would
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Figure 2 SEM micrographs before cp testing for W99 alloy comparing as-cast (A) and after simulated porcelain firing states (B: fired/mean value and C: fired/cycle condition), respectively.
Figure 3 SEM micrographs after cp testing for W99 alloy comparing as-cast (A) and after simulated porcelain firing states (B: fired/mean value and C: fired/cycle condition), respectively.
then result in changes to the alloys’ corrosion resistance. Lai et al26 concluded that new surface oxides were formed on the alloy surfaces after the PFM firing. Therefore, to minimize the release of alloy-corrosion products from the exposed metal surfaces of metal-ceramic restorations into the patient’s tissues, it is important to evaluate the effects of the porcelain firing process on the corrosion behavior of base-metal dental alloys.26 Ardlin et al27 studied the effect of high temperature on CoCr alloys. They found that firing at 980°C reduced the release of ions from two Co-Cr alloys into standard saline lactic acid solution. In the current study, oxide firing (firing with mean value), which may have a slight effect on the surface oxide layer and/or dendritic microstructure of alloy, also supports the idea that degassing procedure reduces corrosion. It was shown that dendritic microstructures and grain boundaries of base metal alloys are affected after porcelain firing simulation.27 Namely, in the as-cast state, Ni-Cr alloys revealed a dendritic structure. After porcelain firing, the alloys’ microstructures displayed homogenization in the dendritic structure, which exhibited further grain boundary homogeneity. Li et al29 reported that the as-cast microstructures of Pd-Ag alloys were inhomogeneous. After simulated porcelain firing heat treatment, the microstructures of the specimens were homogenized and contained numerous precipitates, along with the palladium solid solution matrix. In our study, full-cycle firing simulation may have caused more critical outcomes, including homogenization, phase transformation, and/or oxidation, in addition to corrosion behavior. Porcelain-bonding alloys were distinct from full-cast alloys largely because the former required melting ranges that could
survive the application of porcelain (typical firing temperatures of 870 to 1370°C).30 Also, the application of porcelain clinically requires high-temperature firing cycles, inevitably resulting in changes in the microstructure during the porcelain firing process.2,10 According to Wataha, “The thickest oxide layers occur in nickel- and cobalt-based alloys because these alloys contain elements that form oxides easily during the initial oxidation step prior to firing of the opaque porcelain.”30 Wylie et al10 also indicated that the presence of crevices combined with variations of elemental distribution in the microstructure can lead to accelerated corrosion of Ni-based alloys with lower Cr contents. This evidence, along with the data obtained in this study, provide support for the hypothesis that PFM firing causes changes in alloy microstructures that result in altered surface oxides and corrosion behavior for tested Ni-Cr alloy. Hydrogen peroxide and carbamide peroxide are the most favored agents for bleaching applications, and the active substance is HP at different proportions for both.31 In this study, HP was preferred due to the higher concentration of active material content. If the first study had been conducted with lower concentration (carbamide peroxide), corrosion would not have been observed. Carbamide peroxide solution and HP with different concentrations will be the subject of our future studies. Although the stability of Ni-based alloys has been shown to be significantly reduced when placed in more acidic environments, thus enhancing metal ion release,16 the pH was 6.5 in the current study because popular commercial bleaching agents have an average pH of 5 to 6.5. Al-Hity et al3 reported that although many factors may contribute to the results, heat treatment decreased the corrosion resistance
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for gold alloy, especially in an acidic environment in their test conditions. As Wylie et al noted, “It is important to assess the corrosion behavior of dental alloys in acidic solutions due to the low pH encountered in crevices in addition to the variable pH conditions routinely encountered in the oral environment through the variety of foods and liquids ingested.”10 In the current study, electrochemical measurements were performed under the aerated conditions in order to provide a medium closely resembling the oral cavity. Deaerated conditions may affect the results. Mouthguard bleaching usually is performed for 8 hours a day, and when the patients are asleep, where the flow of saliva decreases. If patients have full-cast and/or PFM restorations, bleaching treatment must be applied very carefully because excessive material may contact these restorations and alter the corrosion properties and/or tissue responses. Further investigations are also needed with different alloys, different bleaching agents, and different pH values.
Conclusions Within the parameters of this study, a single heat treatment is insufficient to cause upheaval in corrosion behavior of a NiCr alloy subjected to 10% hydrogen peroxide. Although the null hypothesis was rejected, if full-cast metal restorations are planned, degassing application will be better for this type of alloy in terms of improving the corrosion properties; however, multiple heat treatments, as in alloys veneered with porcelain, cause significantly more corrosion potential. For this reason, the clinician should be cautious in the use of commercial bleaching material for patients with existing base metal alloy restorations.
Acknowledgments The authors are grateful to Dils¸en Tamam for statistical analysis.
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