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Potentiodynamic polarization study of the corrosion behavior of palladium-silver dental alloys
RESEARCH AND EDUCATION
Potentiodynamic polarization study of the corrosion behavior of palladium-silver dental alloys Desheng Sun, MAS, PhD,a William A. Brantley, PhD,b Gerald S. Frankel, ScD,c Reza H. Heshmati, DDS, MPH, MS,d and William M. Johnston, PhDe Because of high gold prices, palladium-silver (Pd-Ag) alloys have become popular for metalceramic restorations. The more expensive high-noble goldbased and gold-palladium alloys have been widely used for metal-ceramic restorations for several decades because of their excellent physical and mechanical properties, good porcelain adherence, high corrosion resistance, and superb biocompatibility.1 The Pd-Ag alloys, which were introduced in the 1970s,2 have values for elastic modulus and yield strength similar to those for the Au-Pd-Ag and Au-Pd alloys but with much lower densities.1 The Pd-Ag alloys also have high distortion resistance during porcelain firing,3 excellent metal-ceramic bond strength,4 and satisfactory tarnish and corrosion resistance.5,6 Moreover, earlier concerns that
ABSTRACT Statement of problem. Although palladium-silver alloys have been marketed for over 3 decades for metal-ceramic restorations, understanding of the corrosion behavior of current alloys is incomplete; this understanding is critical for evaluating biocompatibility and clinical performance. Purpose. The purpose of this in vitro study was to characterize the corrosion behavior of 3 representative Pd-Ag alloys in simulated body fluid and oral environments and to compare them with a high-noble Au-Pd alloy. The study obtained values of important electrochemical corrosion parameters, with clinical relevance, for the rational selection of casting alloys. Material and methods. The room temperature in vitro corrosion characteristics of the 3 Pd-Ag alloys and the high-noble Au-Pd alloy were evaluated in 0.9% NaCl, 0.09% NaCl, and Fusayama solutions. After simulated porcelain firing heat treatment, 5 specimens of each alloy were immersed in the electrolytes for 24 hours. For each specimen, the open-circuit potential (OCP) was first recorded, and linear polarization was then performed from e20 mV to +20 mV (versus OCP) at a rate of 0.125 mV/s. Cyclic polarization was subsequently performed on 3 specimens of each alloy from e300 mV to +1000 mV and back to e300 mV (versus OCP) at a scanning rate of 1 mV/s. The differences in OCP and corrosion resistance parameters (zero-current potential and polarization resistance) among alloys and electrolyte combinations were compared with the 2-factor ANOVA (maximum-likelihood method) with post hoc Tukey adjustments (a=.05). Results. The 24-hour OCPs and polarization resistance values of the 3 Pd-Ag alloys and the Au-Pd alloy were not significantly different (P=.233 and P=.211, respectively) for the same electrolyte, but significant differences were found for corrosion test results in different electrolytes (P<.001 and P=.032, respectively). No significant interaction was found between the factors of alloy and electrolyte (P=.249 and P=.713, respectively). The 3 Pd-Ag silver alloys appeared to be resistant to chloride ion corrosion, and passivation and de-alloying were identified for these alloys. Conclusions. The Pd-Ag alloys test results showed excellent in vitro corrosion resistance and were equivalent to those of the high-noble Au-Pd alloy in simulated body fluid and oral environments. Passivation, de-alloying, and formation of a AgCl layer were identified as possible corrosion mechanisms for Pd-Ag alloys. (J Prosthet Dent 2017;-:---)
Supported by National Institute of Dental and Craniofacial Research (grant DE10147). This study was based on a portion of a dissertation submitted by D.S., in partial fulfillment of the PhD degree, Graduate School of The Ohio State University. a Senior Materials and Corrosion Engineer, BP America, Naperville, Ill. b Professor, Division of Restorative Science and Prosthodontics, College of Dentistry, The Ohio State University, Columbus, Ohio. c Professor, Department of Materials Science and Engineering and Director, Fontana Corrosion Center, The Ohio State University, Columbus, Ohio. d Associate Professor, Clinical, Division of General Practice and Materials Science, College of Dentistry, The Ohio State University, Columbus, Ohio. e Professor Emeritus, Division of General Practice and Materials Science, College of Dentistry, The Ohio State University, Columbus, Ohio.
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Table 1. Nominal composition (wt%) of 4 alloys*
Clinical Implications The palladium-silver alloys evaluated showed excellent corrosion resistance and were resistant to chloride corrosion. This suggests that palladium-silver alloys will demonstrate good biocompatibility and satisfactory clinical performance with respect to corrosion resistance.
Pd-Ag alloys cause porcelain discoloration during firing to the metal substructure have been alleviated by the development of new porcelain compositions.1,7 The biocompatibility of Pd-Ag alloys could be better than that of high-palladium alloys, as they reduce the occurrence of “palladium allergy” that has been reported for high-palladium alloys.8-10 The role of the Ag/AgCl reaction11 for the corrosion process in Pd-Ag alloys has been proposed as a mechanism for blocking palladium allergy. The in vitro corrosion behavior of dental alloys has historically been investigated by using the technique of cyclic potentiodynamic polarization. Originally, Mezger et al5 characterized the in vitro corrosion of 5 commercial Pd-Ag alloys (all containing less than 40 wt% Ag) in 0.9% saline solution and artificial saliva. They found that the open-circuit potential (OCP) and corrosion current were similar for those Pd-Ag alloys and approximately the same magnitude as for the high-Pd alloys. They also reported that Pd-Ag alloys had lower OCP but similar corrosion current densities than high-Pd alloys in phosphate-buffered saline solution and that surface discoloration occurred on Pd-Ag alloys but not on highPd alloys after cyclic polarization in 0.9% saline solution.5 The elements in Pd-Ag dental alloys (Pd, Ag, Ga, In, and Sn) can be passivated in aqueous solution,12 and the corrosion mechanisms of pure Pd, pure Ag, and Pd-Ag binary alloys in a model saliva have been postulated based on thermodynamic calculations.13 Chloride and thiocyanate were identified as 2 critical ions in determining the corrosion behavior of Pd-Ag alloys. Preferential corrosion of Ag from the surface of Pd-Ag binary alloys has also been proposed.13 Passivation and dealloying are expected to occur for commercial Pd-Ag dental alloys in the oral environment. However, the corrosion mechanisms of these commercial alloys may be more complicated than for the binary alloys since the commercial alloys contain more elements and mucin in artificial saliva can behave as a cathodic inhibitor.14 Because information about the corrosion behavior and underlying mechanisms of currently marketed PdAg dental alloys is incomplete,15-17 the objective of this study was to investigate the corrosion behavior of 3
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Alloy (Manufacturer)
Pd
Au
Ag
Ga
In
Sn
Other
Rx 91 (Jeneric Pentron)
53.5
-
37.5
-
<1.0
8.5
Ru
Super Star (Jelenko)
59.8
-
28.1
<1.0
6.0
5.0
Ru, Re
W-1 (Ivoclar Vivadent AG)
53.3
-
37.7
-
<1.0
8.5
Olympia (Jelenko)
38.5
51.5
-
1.5
8.5
-
Ru, Li Ru
*Composition information provided by manufacturers.
representative Pd-Ag alloys by using the potentiodynamic polarization method and to compare their performance with that of a high-noble Au-Pd alloy with an excellent clinical history. The null hypotheses were that no significant differences would be found in the in vitro corrosion behavior of the 3 Pd-Ag alloys and that the AuPd alloy in each electrolyte and no significant differences would be found in the in vitro corrosion behavior of each of these alloys in all 3 electrolytes. MATERIAL AND METHODS Three representative Pd-Ag alloys (Rx 91; Jeneric Pentron, Super Star; Jelenko, and W-1; Ivoclar Vivadent AG) were selected. A well-known high-noble Au-Pd alloy (Olympia; Jelenko) was used as a control for comparison with the Pd-Ag alloys. The compositions of these alloys are listed in Table 1. (The Rx 91 alloy is no longer available; the Argelite 54B alloy manufactured by Argen has a very similar composition. The Jelenko alloy compositions are also now manufactured by Argen; the Argelite 60P alloy is very similar to Super Star, and the Argedent 52SF alloy is very similar to Olympia). The alloys were cast into disk-shaped specimens (12.5×1.3 mm) following standard dental laboratory procedures18 and the recommendations of each manufacturer. After the casting investment was removed, the specimens were oxidized following the instructions from each manufacturer. A simulated porcelain-firing heat treatment (IPS Classic; Ivoclar Vivadent AG) was then performed for all 4 alloys. All specimens were ground on both sides with 180-, 400-, and 600-grit silicon carbide abrasive papers (Buehler) and then polished with 5-, 1-, 0.3-, and 0.05-mm alumina slurries (Buehler). After they were polished, each specimen was ultrasonically cleaned for 10 minutes in both distilled water and ethanol. After being used in 1 experiment, each specimen was repolished and used again for another test. Corrosion test procedures were similar to those reported previously for high-Pd alloys.19 Following the protocol in that previous study, 5 specimens of each alloy were tested in 3 different electrolytes: 0.9% aqueous NaCl solution (using a chloride ion concentration similar to that in human saliva); 0.09% NaCl solution (similar to human body fluid); and an artificial saliva, Fusayama solution.20 Corrosion testing was performed at room
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Table 2. Mean ±SD potentiodynamic polarization results for 4 alloys in 3 electrolytes ZCP (mV Versus SCE)
(×10 U,cm ) 6
RP
2
0.09% NaCl solution Rx 91
224.6 ±97.9
71.9 ±23.0
0.4612 ±0.3059
Super Star
248.0 ±61.3
73.8 ±20.2
0.6919 ±0.3339
W-1
289.5 ±46.6
74.9 ±6.9
0.3616 ±0.1750
Olympia
373.3 ±105.1
193.0 ±104.4
1.3137 ±0.2752
Rx 91
249.9 ±36.8
92.9 ±42.9
1.2909 ±0.2948
Super Star
271.7 ±40.0
68.8 ±19.7
1.5954 ±0.6499
W-1
272.1 ±26.1
169.0 ±62.1
0.8609 ±0.4654
Olympia
245.7 ±127.4
NA
1.5995 ±0.9889
Rx 91
-397.7 ±23.0
-577.3 ±104.2
0.9347 ±0.7049
Super Star
-216.2 ±229.6
-364.0 ±152.1
1.0328 ±1.3095
W-1
-400.2 ±24.4
-513.9 ±56.5
0.8844 ±0.6809
Olympia
-312.5 ±268.5
-387.9 ±503.4
0.8397 ±1.4024
0.9% NaCl solution
Potential (V versus SCE)
Alloy
24-Hour OCP (mV Versus SCE)
1.60 1.40 1.20
Rx 91 0.09% NaCl
1.00 0.80 0.60 0.40 0.20 0.00 –0.20 –10
–9
Fusayama solution
Potential (V versus SCE)
temperature under aerated conditions for the 3 electrolytes. After a test specimen had been immersed in the electrolyte for 24 hours, the OCP (indicative of the tendency for corrosion in the absence of current flow) was recorded, and a linear polarization test was then carried out to determine the value of polarization resistance (used as a measure of corrosion resistance).19 For this linear polarization test, each of the 5 test specimens for each alloy was polarized in each electrolyte from e20 mV to +20 mV around the OCP to determine the value of polarization resistance (RP), by using a scan rate of 0.125 mV/s. Cyclic polarization was subsequently performed on 3 specimens of each heat-treated alloy in each electrolyte from e300 mV to +1000 mV and back to e300 mV (versus OCP), by using a scan rate of 1 mV/s. All potential values were referenced to the standard calomel electrode (SCE). The mean values of OCP and RP (slope of the anodic polarization curve near the zero-current potential [ZCP, defined as the potential at which the current changes polarity during the forward part of the cyclic polarization scan] for the forward scan: the Tafel approximation) for all 4 alloys in each electrolyte were compared using 2-factor ANOVA with the maximum likelihood method21 and SAS mixed procedure (SAS 9.4, SAS Institute, Cary NC), along with post hoc Tukey adjustments (a=.05).
–7
–6
–5
–4
–3
–2
A
1.60 1.40 1.20
Rx 91 0.9% NaCl
1.00 0.80 0.60 0.40 0.20 0.00 –0.20 –10
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
B
0.80
Potential (V versus SCE)
NA, not available.
–8
Log Current Density (A/cm2)
0.60 0.40
Rx 91 Fusayama solution
0.20 0.00 –0.20 –0.40 –0.60 –0.80 –10
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
C
Forward (anodic) direction Reverse (cathodic) direction
RESULTS
Figure 1. Cyclic polarization curves for Rx 91. A, In 0.09% NaCl solution. B, In 0.9% NaCl solution. C, In Fusayama solution. SCE, standard calomel electrode.
The corrosion parameters determined for each alloy from the potentiodynamic polarization experiments (24-hour OCP, ZCP, RP) are listed in Table 2. Representative cyclic polarization curves for the Pd-Ag alloys in the 3 electrolytes are shown in Figures 1 to 3.
Results of the statistical analyses for the corrosion behavior of the 4 alloys in the 3 electrolytes are summarized in Tables 3 and 4 for values of RP, and in Tables 5 and 6 for values of OCP.
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1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 –0.20 –10
–9
–8
–7
–6
–5
–4
–3
Log Current Density (A/cm2)
0.60 0.40 0.20 0.00 –9
–8
–7
–6
–5
–4
–3
Log Current Density (A/cm2)
0.20 0.00 –9
–8
–7
–6
–5
–4
–3
–2
A
1.20
W-1 0.9% NaCI
1.00 0.80 0.60 0.40 0.20 0.00 –9
–8
–7
–6
–5
–4
–3
Log Current Density (A/cm2)
–2
B
0.90 Super Star Fusayama solution
Potential (V versus SCE)
Potential (V versus SCE)
0.40
B
0.30 0.10 –0.10 –0.30 –0.50 –0.70 –0.90 –10
0.60
–0.20 –10
–2
0.90 0.50
0.80
1.40 Super Star 0.9% NaCI
0.80
0.70
1.00
A
1.00
–0.20 –10
-
W-1 0.09% NaCI
Log Current Density (A/cm2)
Potential (V versus SCE)
Potential (V versus SCE)
1.20
1.20
–0.20 –10
–2
1.60 1.40
Issue
1.40 Super Star 0.09% NaCI
Potential (V versus SCE)
Potential (V versus SCE)
1.60
-
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
C
Forward (anodic) direction Reverse (cathodic) direction
0.70 0.50
W-1 Fusayama solution
0.30 0.10 –0.10 –0.30 –0.50 –0.70 –0.90 –10
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
C
Forward (anodic) direction Reverse (cathodic) direction
Figure 2. Cyclic polarization curves for Super Star. A, In 0.09% NaCl solution. B, In 0.9% NaCl solution. C, In Fusayama solution. SCE, standard calomel electrode.
Figure 3. Cyclic polarization curves for W-1. A, In 0.09% NaCl solution. B, In 0.9% NaCl solution. C, In Fusayama solution. SCE, standard calomel electrode.
Significant differences were found overall in polarization resistance for the same alloy tested in the 3 electrolytes (P=.032), but no significant differences were found overall (P=.211) among the 4 alloys tested in each electrolyte (Table 3). Also, no significant interactions
(P=.713) were found between the factors of electrolyte and alloy for RP. Considering pairwise comparisons (Table 4), significant differences (P=.027) were found for RP with 0.09% NaCl solution compared with Rp in 0.9% NaCl solution but not for 0.09% NaCl solution compared
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Table 3. Summary of fixed effects for RP using Type 3 tests with SAS mixed procedure and maximum likelihood method for 2-way ANOVA21
Table 4. Pairwise comparisons of differences of least-squares means of polarization resistance values for alloys in 3 electrolytes using SAS mixed procedure and maximum likelihood method for 2-way ANOVA21
Numerator DF
Denominator DF
F
P
Electrolyte
2
48
3.69
.032
Electrolyte
Electrolyte
Standard Error
DF
t Value
Prob>|t|
Adj P*
Alloy
3
48
1.56
.211
0.09% NaCl
0.9% NaCl
0.2355
48
-0.267
.010
.027
Electrolyte×Alloy
6
48
0.62
.713
0.09% NaCl
Fusayama
0.2355
48
-0.92
.364
.633
0.9% NaCl
Fusayama
0.2355
48
1.76
.085
.195
Effect
RP, polarization resistance.
*Tukey adjustment.
Table 5. Summary of fixed effects for OCP using Type 3 tests with SAS mixed procedure and maximum likelihood method for 2-way ANOVA21 Numerator DF
Denominator DF
F
P
Electrolyte
2
48
169.25
<.001
Alloy
3
48
1.47
.233
Electrolyte×Alloy
6
48
1.36
.249
Effect
OCP, open-circuit potential.
Table 6. Pairwise comparisons of differences of least-squares means of OCP values for alloys in 3 electrolytes using SAS mixed procedure and maximum likelihood method for 2-way ANOVA21 Electrolyte
Electrolyte
Standard Error
DF
t Value
Prob>|t|
0.09% NaCl
0.9% NaCl
37.8989
48
0.63
.530
Adj P* .803
0.09% NaCl
Fusayama
37.8989
48
16.24
<.001
<.001
0.9% NaCl
Fusayama
37.8989
48
15.61
<.001
<.001
OCP, open-circuit potential. *Tukey adjustment.
with Fusayama solution (P=.633) or for 0.9% NaCl solution compared with Fusayama solution (P=.195). Significant differences (P<.001) were found overall in OCP for the same alloy in each of the 3 electrolytes (Table 5) but no significant difference overall in OCP for the 4 alloys in each electrolyte (P=.233). No significant interactions (P=.249) were found between the factors of electrolyte and alloy for OCP. Considering pairwise comparisons (Table 6), no significant differences were found in OCP for the alloys in 0.09% NaCl solution compared with that in 0.9% NaCl solution (P=.803), but significant differences (P<.001) in OCP were found for 0.09% NaCl solution compared with Fusayama solution and for 0.9% NaCl solution compared with Fusayama solution. The cyclic polarization curves in Figures 1 to 3 show passivation (decrease in current density with increasing voltage for the anodic scan) for all Pd-Ag alloys in the simulated body fluid and oral environments. In 0.9% NaCl solution, all 3 Pd-Ag alloys showed 3 ZCP values for the reverse scan (Figs. 1B, 2B, and 3B, prominent horizontal peaks). This phenomenon was observed less frequently in 0.09% NaCl solution and Fusayama solution. These steady-state potentials, named E1, E2 and E3, as well as the ZCP values, are listed in Table 7 and are almost identical for all 3 Pd-Ag alloys. DISCUSSION The maximum likelihood statistical analysis with 2-way ANOVA21 was used because of the substantial variances observed with the test sample groups used for the corrosion tests and to avoid concerns about the normality of the experimental data. From the results presented in Table 3, the first null hypothesis was accepted, whereas the second null hypothesis was rejected. No significant differences were found in corrosion resistance (measured as Rp) for the 4 alloys in each electrolyte. Significant Sun et al
Table 7. Mean ±SD ZCP and 3 zero-current potential values during reverse scan for 3 Pd-Ag alloys in 0.9% NaCl solution (n=3) Alloy
ZCP (mV vs SCE)
E1 (mV vs SCE)
E2 (mV vs SCE)
E3 (mV vs SCE)
Rx 91
92.9 ±42.9
1141.7 ±13.3
711.4 ±10.5
419.5 ±9.4
Super Star
68.8 ±19.7
1138.7 ±12.1
714.4 ±31.7
446.6 ±5.2
W-1
169.0 ±62.1
1162.0 ±6.2
683.9 ±12.9
428.4 ±4.1
ZCP, zero-current potential.
differences in corrosion resistance were observed when the electrolyte effects were compared for the same alloy. Results in Table 5 show no significant difference in corrosion tendency (measured as OCP) for the 4 alloys in the same electrolyte, whereas significant differences in OCP were observed when the 3 electrolytes were compared for the same alloy. The in vitro corrosion tendency and corrosion resistance of the 3 Pd-Ag alloys were equivalent to those of the high-noble Au-Pd alloy in the same electrolyte. This is consistent with the previously reported result that other commercial Pd-Ag alloys had in vitro corrosion resistance equivalent to the high-Pd alloys,5 which can be attributed to their chemical compositions. The amount of the noble element palladium in these Pd-Ag alloys is slightly more than 50%, substantially lower than that of the nobility 90% (palladium and gold) for Olympia. However, the nobility of the Pd-Ag alloys is still sufficiently high that their corrosion properties are not decreased. The similarity in corrosion properties among the 3 Pd-Ag alloys may be due to their similar compositions. For the Pd-Ag alloys, other base metal elements (such as Sn, In, and Zn) are present in amounts less than 10 wt%.1 The passivation of silver, as well as the base metal elements, on the surface evidently compensates for the reduced noble metal content and further contributes to the high corrosion resistance of the Pd-Ag alloys. THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 4 shows that a significant difference in polarization resistance was found for the alloys in 0.09% NaCl solution compared with 0.9% NaCl solution, but no significant differences were found when each saline solution was compared with Fusayama solution. Table 6 shows that no significant differences in OCP (corrosion tendency) were found when the alloys were immersed in the 2 saline solutions, but significant differences occurred when the OCP was compared for each saline solution with Fusayama solution. The chloride-ion corrosion resistance of the Pd-Ag alloys is attributed to both the nobility of these alloys and the passivation of the less noble elements in the alloy compositions. The much lower corrosion tendency (values of OCP) for the alloys in Fusayama solution (Table 2) indicate that Fusayama solution provides a less corrosive in vitro environment than the 2 saline solutions. This agrees with previous findings for high-palladium alloys.19 Mucin in Fusayama solution may act as a cathodic inhibitor,14 thus decreasing the corrosiveness of Fusayama solution. However, different results can be obtained from in vitro corrosion testing of dental alloys using potentiodynamic polarization and different electrolytes.22 Passivation was evident for all 3 Pd-Ag alloys in the 3 corrosion-testing media (Figs. 1-3), where a region of decreasing current density with increasing voltage occurred in the anodic (forward) portion of the polarization scan. It has already been shown that the elements Pd, Ag, Ga, In, and Sn can be passivated in aqueous solutions,12 resulting in passivation being a major mechanism for the high corrosion resistance of the PdAg alloys. Joska et al13 suggested that the passivation observed for the Pd-Ag alloys in artificial saliva could be due to the formation of palladium oxide and/or thiocyanate on the surface. The ennobling effect of palladium and its passivation behavior has been observed in a series of Pd-Ag binary model alloys in Ringer solution.10 The Ag/AgCl reaction AgCl + e− % Ag+ + Cl− has been proposed to increase the anodic oxidation process in these alloys and to serve as the mechanism for blocking the palladium allergy.8,9 Tarnish was observed on Pd-20Ag and Pd-40Ag (wt%) model alloys and on pure silver, and the attack was identified as initiating from grain boundaries and propagating into grains.10 AgCl may form at the OCP of Pd-Ag alloys, as these alloys have OCP values between 225 and 290 mVSCE in 0.09% and 0.9% NaCl solutions (Table 2), which are much higher than the equilibrium potential value for AgCl deposition/decomposition (E0=222 mVSHE= −19 mVSCE)11 and may favor the deposition of AgCl on the alloy surface. Thermodynamic calculation confirms that AgCl can be one of the corrosion products of Pd-Ag alloys in artificial saliva.13 Protection from AgCl deposition THE JOURNAL OF PROSTHETIC DENTISTRY
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would depend on the characteristics of the deposited AgCl layer and the bonding between this layer and the metal surface beneath. Ag is corroded to Ag+ ions from the alloy surface, and the Ag+ and Cl− ions then combine to form a AgCl deposit on the alloy surface. If the AgCl layer rapidly covers the alloy surface before the fresh alloy surface can corrode further and is tightly bonded to the alloy surface, then corrosion protection from AgCl deposition would be significant. Future studies are needed to obtain more information about the kinetics of this process and to characterize the deposition layer. Three zero-current potentials in the reverse scans, arising from separate electrochemical processes, were observed for all 3 Pd-Ag alloys in the 0.9% NaCl solution (Figs. 1B, 2B, and 3B), and these potentials were almost identical for all 3 alloys, despite their different compositions (Table 1). The key factor in this behavior may be the Cl− ions in the corrosion media. During the cyclic polarization experiment for each alloy specimen, the magnitude of polarization was greater than that during the initial linear polarization experiment. Thus, the protective oxide layer on the surface would be destroyed when the polarization potential was sufficiently high during the forward scan polarization. Alloying elements would also be oxidized to their ionic states, diffusing from the surface into the electrolyte at different potentials during the forward polarization; this process is facilitated by the presence of the Cl− ions. During reverse polarization, as the potential is decreasing, these cations could be plated out at high potentials, resulting in a large cathodic current. Depletion of the dissolved ion concentration by plating results in a decrease in the cathodic current as the potential is lowered further. As a result, a large nose-shaped region is often observed in which the anodic current density increases and then decreases with decreasing potential. This behavior is associated with active dissolution below the passivation potential, and the oxygen reduction reaction (the primary cathodic reaction during the forward scan) becomes dominant. This change in cathodic reaction and the emergence of the anodic nose-shaped region results in the 3 zero-current potentials. The appearance of 3 zero-current potentials in the reverse scans also indicates that de-alloying (separation of microstructural phases into the component elements) occurs at high potentials. This phenomenon has been previously observed in high-palladium alloys.19 Because the 3 Pd-Ag alloys have similar compositions, these 3 ZCPs were almost identical for each alloy. A recent in vitro study15 has also reported high corrosion resistance for 2 other Pd-Ag alloys in Fusayama artificial saliva. These studies used an alternative presentation of the cyclic polarization plots often found in materials science journals, in which the applied potential is shown on the horizontal axis, and the resulting current Sun et al
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density on the vertical axis. Another recent study16 reported the ion release, determined from inductively coupled mass spectroscopy, from a Pd-Cu-Ga alloy and another Pd-Ag alloy as a function of casting shape and surface preparation, noting the generally higher release of Pd ions from the Pd-Cu-Ga alloy. In a complementary in vitro study,17 the cytotoxicity of ions in concentrations released from Pd-based casting alloys was investigated using metal salts, mouse fibroblasts, and the MTT assay. The observation of some cytotoxic effects indicates that further studies of the biocompatibility of the palladiumbased alloys are warranted. Although it is accepted that useful screening results can be obtained for the corrosion properties of dental alloys, the cyclic polarization technique used in the present study is limited by the in vitro conditions, which differ from the complex oral environment, and in the electrochemical information that can be acquired. Uncertainty in locating the tangent line to the initial portion of the anodic polarization curve for the Tafel approximation can vary between plots for nominally identical replicate specimens. Inevitable variation in surface condition and surface microstructural phases also leads to differing electrochemical results for replicate specimens from the same sample group. Note also that while aerated electrolytes used in the present study simulate the general oral environment, deaerated electrolytes would better simulate the anaerobic conditions under bacterial plaque in vivo.19 Additional insight into fundamental corrosion processes can also be obtained by using the more sophisticated technique of electrochemical impedance spectroscopy. The results from a complementary electrochemical impedance spectroscopy study of the present 4 alloys will be presented in a future report. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. The in vitro corrosion characteristics of Pd-Ag alloys are equivalent to the those in high-noble Au-Pd control alloy in simulated body fluid and oral environments. 2. Pd-Ag alloys appear to be resistant to chloride ion corrosion and exhibit passivation. 3. De-alloying appears to occur during the corrosion of these alloys.
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REFERENCES 1. Brantley WA, Laub LW. Metal selection. In: Rosenstiel SF, Land MF, Fujimoto J, editors. Contemporary fixed prosthodontics. 5th ed. St. Louis: Mosby/Elsevier; 2016. p. 529-41. 2. Goodacre CJ. Palladium-silver alloys: a review of the literature. J Prosthet Dent 1989;62:34-7. 3. Papazoglou E, Brantley WA, Johnston WM. Evaluation of high-temperature distortion of high-palladium metal-ceramic crowns. J Prosthet Dent 2001;85: 133-40. 4. Papazoglou E, Brantley WA. Porcelain adherence vs force to failure for palladium-gallium alloys: a critique of metaleceramic bond testing. Dent Mater 1998;14:112-9. 5. Mezger PR, Vrijhoef MMA, Greener EH. The corrosion behavior of palladium-silver-ceramic alloys. Dent Mater 1989;5:97-100. 6. Canay S, Oktemer M. In vitro corrosion behavior of 13 prosthodontic alloys. Quintessence Int 1992;23:279-87. 7. O’Brien WJ, Boenke KM, Linger JB, Groh CL. Cerium oxide as a silver decolorizer in dental porcelains. Dent Mater 1998;14:365-9. 8. Berzins DW, Kawashima I, Graves R, Sarkar NK. Electrochemical characteristics of high-Pd alloys in relation to Pd-allergy. Dent Mater 2000;16: 266-73. 9. Sarkar NK, Berzins DW, Prasad A. Dealloying and electroformation in highPd dental alloys. Dent Mater 2000;16:374-9. 10. Vaidyanathan TK, Prasad A. In vitro corrosion and tarnish analysis of the AgPd binary system. J Dent Res 1981;60:707-15. 11. Bates RG, Macaskill JB. Standard potential of the silver-silver chloride electrode. Pure and Appl Chem 1978;50:701-6. 12. Pourbaix MJN. Atlas of electrochemical equilibria in aqueous solution. Oxford, UK: Pergamon Press; 1966. p. 258-363. 393-405, 428-35, 436-42, 475-84. 13. Joska L, Marek M, Leitner J. The mechanism of corrosion of palladium-silver binary alloys in artificial saliva. Biomaterials 2005;26: 1605-11. 14. Mezger PR, van’t Hof MAV, Vrijhoef MMA, Gravenmade EJS, Greener EH. Effect of mucin on the corrosion behaviour of dental casting alloys. J Oral Rehabil 1989;16:589-96. 15. Viennot S, Dalard F, Lissac M, Grosgogeat B. Corrosion resistance of cobaltchromium and palladium-silver alloys used in fixed prosthetic restorations. Eur J Oral Sci 2005;113:90-5. 16. Milheiro A, Muris J, Kleverlaan CJ, Feilzer AJ. Influence of shape and finishing on the corrosion of palladium-based dental alloys. J Adv Prosthodont 2015;7:56-61. 17. Milheiro A, Nozaki K, Kleverlaan CJ, Muris J, Miura H, Feilzer AJ. In vitro cytotoxicity of metallic ions released from dental alloys. Odontology 2016;104:136-42. 18. Carr AB, Brantley WA. New high-palladium casting alloys: Part 1. Overview and initial studies. Int J Prosthodont 1991;4:265-75. 19. Sun D, Monaghan P, Brantley WA, Johnston WM. Potentiodynamic polarization study of the in vitro corrosion behavior of high-palladium alloys in five media. J Prosthet Dent 2002;87:86-93. 20. Fusayama T, Katayori T, Nomoto S. Corrosion of gold and amalgam placed in contact with each other. J Dent Res 1963;42:1183-97. 21. Hartley HO, Rao JNK. Maximum-likelihood estimation for the mixed analysis of variance model. Biometrika 1967;54:93-108. 22. Holland RI. Corrosion testing by potentiodynamic polarization in various electrolytes. Dent Mater 1992;8:241-5.
Corresponding author: Dr William A. Brantley The Ohio State University College of Dentistry Division of Restorative Science and Prosthodontics 305 West 12th Ave Columbus, OH 43210 Email: [email protected] Copyright © 2017 by the Editorial Council for The Journal of Prosthetic Dentistry.
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Potentiodynamic polarization study of the corrosion behavior of palladium-silver dental alloys Desheng Sun, MAS, PhD,a William A. Brantley, PhD,b Gerald S. Frankel, ScD,c Reza H. Heshmati, DDS, MPH, MS,d and William M. Johnston, PhDe Because of high gold prices, palladium-silver (Pd-Ag) alloys have become popular for metalceramic restorations. The more expensive high-noble goldbased and gold-palladium alloys have been widely used for metal-ceramic restorations for several decades because of their excellent physical and mechanical properties, good porcelain adherence, high corrosion resistance, and superb biocompatibility.1 The Pd-Ag alloys, which were introduced in the 1970s,2 have values for elastic modulus and yield strength similar to those for the Au-Pd-Ag and Au-Pd alloys but with much lower densities.1 The Pd-Ag alloys also have high distortion resistance during porcelain firing,3 excellent metal-ceramic bond strength,4 and satisfactory tarnish and corrosion resistance.5,6 Moreover, earlier concerns that
ABSTRACT Statement of problem. Although palladium-silver alloys have been marketed for over 3 decades for metal-ceramic restorations, understanding of the corrosion behavior of current alloys is incomplete; this understanding is critical for evaluating biocompatibility and clinical performance. Purpose. The purpose of this in vitro study was to characterize the corrosion behavior of 3 representative Pd-Ag alloys in simulated body fluid and oral environments and to compare them with a high-noble Au-Pd alloy. The study obtained values of important electrochemical corrosion parameters, with clinical relevance, for the rational selection of casting alloys. Material and methods. The room temperature in vitro corrosion characteristics of the 3 Pd-Ag alloys and the high-noble Au-Pd alloy were evaluated in 0.9% NaCl, 0.09% NaCl, and Fusayama solutions. After simulated porcelain firing heat treatment, 5 specimens of each alloy were immersed in the electrolytes for 24 hours. For each specimen, the open-circuit potential (OCP) was first recorded, and linear polarization was then performed from e20 mV to +20 mV (versus OCP) at a rate of 0.125 mV/s. Cyclic polarization was subsequently performed on 3 specimens of each alloy from e300 mV to +1000 mV and back to e300 mV (versus OCP) at a scanning rate of 1 mV/s. The differences in OCP and corrosion resistance parameters (zero-current potential and polarization resistance) among alloys and electrolyte combinations were compared with the 2-factor ANOVA (maximum-likelihood method) with post hoc Tukey adjustments (a=.05). Results. The 24-hour OCPs and polarization resistance values of the 3 Pd-Ag alloys and the Au-Pd alloy were not significantly different (P=.233 and P=.211, respectively) for the same electrolyte, but significant differences were found for corrosion test results in different electrolytes (P<.001 and P=.032, respectively). No significant interaction was found between the factors of alloy and electrolyte (P=.249 and P=.713, respectively). The 3 Pd-Ag silver alloys appeared to be resistant to chloride ion corrosion, and passivation and de-alloying were identified for these alloys. Conclusions. The Pd-Ag alloys test results showed excellent in vitro corrosion resistance and were equivalent to those of the high-noble Au-Pd alloy in simulated body fluid and oral environments. Passivation, de-alloying, and formation of a AgCl layer were identified as possible corrosion mechanisms for Pd-Ag alloys. (J Prosthet Dent 2017;-:---)
Supported by National Institute of Dental and Craniofacial Research (grant DE10147). This study was based on a portion of a dissertation submitted by D.S., in partial fulfillment of the PhD degree, Graduate School of The Ohio State University. a Senior Materials and Corrosion Engineer, BP America, Naperville, Ill. b Professor, Division of Restorative Science and Prosthodontics, College of Dentistry, The Ohio State University, Columbus, Ohio. c Professor, Department of Materials Science and Engineering and Director, Fontana Corrosion Center, The Ohio State University, Columbus, Ohio. d Associate Professor, Clinical, Division of General Practice and Materials Science, College of Dentistry, The Ohio State University, Columbus, Ohio. e Professor Emeritus, Division of General Practice and Materials Science, College of Dentistry, The Ohio State University, Columbus, Ohio.
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Table 1. Nominal composition (wt%) of 4 alloys*
Clinical Implications The palladium-silver alloys evaluated showed excellent corrosion resistance and were resistant to chloride corrosion. This suggests that palladium-silver alloys will demonstrate good biocompatibility and satisfactory clinical performance with respect to corrosion resistance.
Pd-Ag alloys cause porcelain discoloration during firing to the metal substructure have been alleviated by the development of new porcelain compositions.1,7 The biocompatibility of Pd-Ag alloys could be better than that of high-palladium alloys, as they reduce the occurrence of “palladium allergy” that has been reported for high-palladium alloys.8-10 The role of the Ag/AgCl reaction11 for the corrosion process in Pd-Ag alloys has been proposed as a mechanism for blocking palladium allergy. The in vitro corrosion behavior of dental alloys has historically been investigated by using the technique of cyclic potentiodynamic polarization. Originally, Mezger et al5 characterized the in vitro corrosion of 5 commercial Pd-Ag alloys (all containing less than 40 wt% Ag) in 0.9% saline solution and artificial saliva. They found that the open-circuit potential (OCP) and corrosion current were similar for those Pd-Ag alloys and approximately the same magnitude as for the high-Pd alloys. They also reported that Pd-Ag alloys had lower OCP but similar corrosion current densities than high-Pd alloys in phosphate-buffered saline solution and that surface discoloration occurred on Pd-Ag alloys but not on highPd alloys after cyclic polarization in 0.9% saline solution.5 The elements in Pd-Ag dental alloys (Pd, Ag, Ga, In, and Sn) can be passivated in aqueous solution,12 and the corrosion mechanisms of pure Pd, pure Ag, and Pd-Ag binary alloys in a model saliva have been postulated based on thermodynamic calculations.13 Chloride and thiocyanate were identified as 2 critical ions in determining the corrosion behavior of Pd-Ag alloys. Preferential corrosion of Ag from the surface of Pd-Ag binary alloys has also been proposed.13 Passivation and dealloying are expected to occur for commercial Pd-Ag dental alloys in the oral environment. However, the corrosion mechanisms of these commercial alloys may be more complicated than for the binary alloys since the commercial alloys contain more elements and mucin in artificial saliva can behave as a cathodic inhibitor.14 Because information about the corrosion behavior and underlying mechanisms of currently marketed PdAg dental alloys is incomplete,15-17 the objective of this study was to investigate the corrosion behavior of 3
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Alloy (Manufacturer)
Pd
Au
Ag
Ga
In
Sn
Other
Rx 91 (Jeneric Pentron)
53.5
-
37.5
-
<1.0
8.5
Ru
Super Star (Jelenko)
59.8
-
28.1
<1.0
6.0
5.0
Ru, Re
W-1 (Ivoclar Vivadent AG)
53.3
-
37.7
-
<1.0
8.5
Olympia (Jelenko)
38.5
51.5
-
1.5
8.5
-
Ru, Li Ru
*Composition information provided by manufacturers.
representative Pd-Ag alloys by using the potentiodynamic polarization method and to compare their performance with that of a high-noble Au-Pd alloy with an excellent clinical history. The null hypotheses were that no significant differences would be found in the in vitro corrosion behavior of the 3 Pd-Ag alloys and that the AuPd alloy in each electrolyte and no significant differences would be found in the in vitro corrosion behavior of each of these alloys in all 3 electrolytes. MATERIAL AND METHODS Three representative Pd-Ag alloys (Rx 91; Jeneric Pentron, Super Star; Jelenko, and W-1; Ivoclar Vivadent AG) were selected. A well-known high-noble Au-Pd alloy (Olympia; Jelenko) was used as a control for comparison with the Pd-Ag alloys. The compositions of these alloys are listed in Table 1. (The Rx 91 alloy is no longer available; the Argelite 54B alloy manufactured by Argen has a very similar composition. The Jelenko alloy compositions are also now manufactured by Argen; the Argelite 60P alloy is very similar to Super Star, and the Argedent 52SF alloy is very similar to Olympia). The alloys were cast into disk-shaped specimens (12.5×1.3 mm) following standard dental laboratory procedures18 and the recommendations of each manufacturer. After the casting investment was removed, the specimens were oxidized following the instructions from each manufacturer. A simulated porcelain-firing heat treatment (IPS Classic; Ivoclar Vivadent AG) was then performed for all 4 alloys. All specimens were ground on both sides with 180-, 400-, and 600-grit silicon carbide abrasive papers (Buehler) and then polished with 5-, 1-, 0.3-, and 0.05-mm alumina slurries (Buehler). After they were polished, each specimen was ultrasonically cleaned for 10 minutes in both distilled water and ethanol. After being used in 1 experiment, each specimen was repolished and used again for another test. Corrosion test procedures were similar to those reported previously for high-Pd alloys.19 Following the protocol in that previous study, 5 specimens of each alloy were tested in 3 different electrolytes: 0.9% aqueous NaCl solution (using a chloride ion concentration similar to that in human saliva); 0.09% NaCl solution (similar to human body fluid); and an artificial saliva, Fusayama solution.20 Corrosion testing was performed at room
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Table 2. Mean ±SD potentiodynamic polarization results for 4 alloys in 3 electrolytes ZCP (mV Versus SCE)
(×10 U,cm ) 6
RP
2
0.09% NaCl solution Rx 91
224.6 ±97.9
71.9 ±23.0
0.4612 ±0.3059
Super Star
248.0 ±61.3
73.8 ±20.2
0.6919 ±0.3339
W-1
289.5 ±46.6
74.9 ±6.9
0.3616 ±0.1750
Olympia
373.3 ±105.1
193.0 ±104.4
1.3137 ±0.2752
Rx 91
249.9 ±36.8
92.9 ±42.9
1.2909 ±0.2948
Super Star
271.7 ±40.0
68.8 ±19.7
1.5954 ±0.6499
W-1
272.1 ±26.1
169.0 ±62.1
0.8609 ±0.4654
Olympia
245.7 ±127.4
NA
1.5995 ±0.9889
Rx 91
-397.7 ±23.0
-577.3 ±104.2
0.9347 ±0.7049
Super Star
-216.2 ±229.6
-364.0 ±152.1
1.0328 ±1.3095
W-1
-400.2 ±24.4
-513.9 ±56.5
0.8844 ±0.6809
Olympia
-312.5 ±268.5
-387.9 ±503.4
0.8397 ±1.4024
0.9% NaCl solution
Potential (V versus SCE)
Alloy
24-Hour OCP (mV Versus SCE)
1.60 1.40 1.20
Rx 91 0.09% NaCl
1.00 0.80 0.60 0.40 0.20 0.00 –0.20 –10
–9
Fusayama solution
Potential (V versus SCE)
temperature under aerated conditions for the 3 electrolytes. After a test specimen had been immersed in the electrolyte for 24 hours, the OCP (indicative of the tendency for corrosion in the absence of current flow) was recorded, and a linear polarization test was then carried out to determine the value of polarization resistance (used as a measure of corrosion resistance).19 For this linear polarization test, each of the 5 test specimens for each alloy was polarized in each electrolyte from e20 mV to +20 mV around the OCP to determine the value of polarization resistance (RP), by using a scan rate of 0.125 mV/s. Cyclic polarization was subsequently performed on 3 specimens of each heat-treated alloy in each electrolyte from e300 mV to +1000 mV and back to e300 mV (versus OCP), by using a scan rate of 1 mV/s. All potential values were referenced to the standard calomel electrode (SCE). The mean values of OCP and RP (slope of the anodic polarization curve near the zero-current potential [ZCP, defined as the potential at which the current changes polarity during the forward part of the cyclic polarization scan] for the forward scan: the Tafel approximation) for all 4 alloys in each electrolyte were compared using 2-factor ANOVA with the maximum likelihood method21 and SAS mixed procedure (SAS 9.4, SAS Institute, Cary NC), along with post hoc Tukey adjustments (a=.05).
–7
–6
–5
–4
–3
–2
A
1.60 1.40 1.20
Rx 91 0.9% NaCl
1.00 0.80 0.60 0.40 0.20 0.00 –0.20 –10
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
B
0.80
Potential (V versus SCE)
NA, not available.
–8
Log Current Density (A/cm2)
0.60 0.40
Rx 91 Fusayama solution
0.20 0.00 –0.20 –0.40 –0.60 –0.80 –10
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
C
Forward (anodic) direction Reverse (cathodic) direction
RESULTS
Figure 1. Cyclic polarization curves for Rx 91. A, In 0.09% NaCl solution. B, In 0.9% NaCl solution. C, In Fusayama solution. SCE, standard calomel electrode.
The corrosion parameters determined for each alloy from the potentiodynamic polarization experiments (24-hour OCP, ZCP, RP) are listed in Table 2. Representative cyclic polarization curves for the Pd-Ag alloys in the 3 electrolytes are shown in Figures 1 to 3.
Results of the statistical analyses for the corrosion behavior of the 4 alloys in the 3 electrolytes are summarized in Tables 3 and 4 for values of RP, and in Tables 5 and 6 for values of OCP.
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1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 –0.20 –10
–9
–8
–7
–6
–5
–4
–3
Log Current Density (A/cm2)
0.60 0.40 0.20 0.00 –9
–8
–7
–6
–5
–4
–3
Log Current Density (A/cm2)
0.20 0.00 –9
–8
–7
–6
–5
–4
–3
–2
A
1.20
W-1 0.9% NaCI
1.00 0.80 0.60 0.40 0.20 0.00 –9
–8
–7
–6
–5
–4
–3
Log Current Density (A/cm2)
–2
B
0.90 Super Star Fusayama solution
Potential (V versus SCE)
Potential (V versus SCE)
0.40
B
0.30 0.10 –0.10 –0.30 –0.50 –0.70 –0.90 –10
0.60
–0.20 –10
–2
0.90 0.50
0.80
1.40 Super Star 0.9% NaCI
0.80
0.70
1.00
A
1.00
–0.20 –10
-
W-1 0.09% NaCI
Log Current Density (A/cm2)
Potential (V versus SCE)
Potential (V versus SCE)
1.20
1.20
–0.20 –10
–2
1.60 1.40
Issue
1.40 Super Star 0.09% NaCI
Potential (V versus SCE)
Potential (V versus SCE)
1.60
-
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
C
Forward (anodic) direction Reverse (cathodic) direction
0.70 0.50
W-1 Fusayama solution
0.30 0.10 –0.10 –0.30 –0.50 –0.70 –0.90 –10
–9
–8
–7
–6
–5
–4
Log Current Density (A/cm2)
–3
–2
C
Forward (anodic) direction Reverse (cathodic) direction
Figure 2. Cyclic polarization curves for Super Star. A, In 0.09% NaCl solution. B, In 0.9% NaCl solution. C, In Fusayama solution. SCE, standard calomel electrode.
Figure 3. Cyclic polarization curves for W-1. A, In 0.09% NaCl solution. B, In 0.9% NaCl solution. C, In Fusayama solution. SCE, standard calomel electrode.
Significant differences were found overall in polarization resistance for the same alloy tested in the 3 electrolytes (P=.032), but no significant differences were found overall (P=.211) among the 4 alloys tested in each electrolyte (Table 3). Also, no significant interactions
(P=.713) were found between the factors of electrolyte and alloy for RP. Considering pairwise comparisons (Table 4), significant differences (P=.027) were found for RP with 0.09% NaCl solution compared with Rp in 0.9% NaCl solution but not for 0.09% NaCl solution compared
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Table 3. Summary of fixed effects for RP using Type 3 tests with SAS mixed procedure and maximum likelihood method for 2-way ANOVA21
Table 4. Pairwise comparisons of differences of least-squares means of polarization resistance values for alloys in 3 electrolytes using SAS mixed procedure and maximum likelihood method for 2-way ANOVA21
Numerator DF
Denominator DF
F
P
Electrolyte
2
48
3.69
.032
Electrolyte
Electrolyte
Standard Error
DF
t Value
Prob>|t|
Adj P*
Alloy
3
48
1.56
.211
0.09% NaCl
0.9% NaCl
0.2355
48
-0.267
.010
.027
Electrolyte×Alloy
6
48
0.62
.713
0.09% NaCl
Fusayama
0.2355
48
-0.92
.364
.633
0.9% NaCl
Fusayama
0.2355
48
1.76
.085
.195
Effect
RP, polarization resistance.
*Tukey adjustment.
Table 5. Summary of fixed effects for OCP using Type 3 tests with SAS mixed procedure and maximum likelihood method for 2-way ANOVA21 Numerator DF
Denominator DF
F
P
Electrolyte
2
48
169.25
<.001
Alloy
3
48
1.47
.233
Electrolyte×Alloy
6
48
1.36
.249
Effect
OCP, open-circuit potential.
Table 6. Pairwise comparisons of differences of least-squares means of OCP values for alloys in 3 electrolytes using SAS mixed procedure and maximum likelihood method for 2-way ANOVA21 Electrolyte
Electrolyte
Standard Error
DF
t Value
Prob>|t|
0.09% NaCl
0.9% NaCl
37.8989
48
0.63
.530
Adj P* .803
0.09% NaCl
Fusayama
37.8989
48
16.24
<.001
<.001
0.9% NaCl
Fusayama
37.8989
48
15.61
<.001
<.001
OCP, open-circuit potential. *Tukey adjustment.
with Fusayama solution (P=.633) or for 0.9% NaCl solution compared with Fusayama solution (P=.195). Significant differences (P<.001) were found overall in OCP for the same alloy in each of the 3 electrolytes (Table 5) but no significant difference overall in OCP for the 4 alloys in each electrolyte (P=.233). No significant interactions (P=.249) were found between the factors of electrolyte and alloy for OCP. Considering pairwise comparisons (Table 6), no significant differences were found in OCP for the alloys in 0.09% NaCl solution compared with that in 0.9% NaCl solution (P=.803), but significant differences (P<.001) in OCP were found for 0.09% NaCl solution compared with Fusayama solution and for 0.9% NaCl solution compared with Fusayama solution. The cyclic polarization curves in Figures 1 to 3 show passivation (decrease in current density with increasing voltage for the anodic scan) for all Pd-Ag alloys in the simulated body fluid and oral environments. In 0.9% NaCl solution, all 3 Pd-Ag alloys showed 3 ZCP values for the reverse scan (Figs. 1B, 2B, and 3B, prominent horizontal peaks). This phenomenon was observed less frequently in 0.09% NaCl solution and Fusayama solution. These steady-state potentials, named E1, E2 and E3, as well as the ZCP values, are listed in Table 7 and are almost identical for all 3 Pd-Ag alloys. DISCUSSION The maximum likelihood statistical analysis with 2-way ANOVA21 was used because of the substantial variances observed with the test sample groups used for the corrosion tests and to avoid concerns about the normality of the experimental data. From the results presented in Table 3, the first null hypothesis was accepted, whereas the second null hypothesis was rejected. No significant differences were found in corrosion resistance (measured as Rp) for the 4 alloys in each electrolyte. Significant Sun et al
Table 7. Mean ±SD ZCP and 3 zero-current potential values during reverse scan for 3 Pd-Ag alloys in 0.9% NaCl solution (n=3) Alloy
ZCP (mV vs SCE)
E1 (mV vs SCE)
E2 (mV vs SCE)
E3 (mV vs SCE)
Rx 91
92.9 ±42.9
1141.7 ±13.3
711.4 ±10.5
419.5 ±9.4
Super Star
68.8 ±19.7
1138.7 ±12.1
714.4 ±31.7
446.6 ±5.2
W-1
169.0 ±62.1
1162.0 ±6.2
683.9 ±12.9
428.4 ±4.1
ZCP, zero-current potential.
differences in corrosion resistance were observed when the electrolyte effects were compared for the same alloy. Results in Table 5 show no significant difference in corrosion tendency (measured as OCP) for the 4 alloys in the same electrolyte, whereas significant differences in OCP were observed when the 3 electrolytes were compared for the same alloy. The in vitro corrosion tendency and corrosion resistance of the 3 Pd-Ag alloys were equivalent to those of the high-noble Au-Pd alloy in the same electrolyte. This is consistent with the previously reported result that other commercial Pd-Ag alloys had in vitro corrosion resistance equivalent to the high-Pd alloys,5 which can be attributed to their chemical compositions. The amount of the noble element palladium in these Pd-Ag alloys is slightly more than 50%, substantially lower than that of the nobility 90% (palladium and gold) for Olympia. However, the nobility of the Pd-Ag alloys is still sufficiently high that their corrosion properties are not decreased. The similarity in corrosion properties among the 3 Pd-Ag alloys may be due to their similar compositions. For the Pd-Ag alloys, other base metal elements (such as Sn, In, and Zn) are present in amounts less than 10 wt%.1 The passivation of silver, as well as the base metal elements, on the surface evidently compensates for the reduced noble metal content and further contributes to the high corrosion resistance of the Pd-Ag alloys. THE JOURNAL OF PROSTHETIC DENTISTRY
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Table 4 shows that a significant difference in polarization resistance was found for the alloys in 0.09% NaCl solution compared with 0.9% NaCl solution, but no significant differences were found when each saline solution was compared with Fusayama solution. Table 6 shows that no significant differences in OCP (corrosion tendency) were found when the alloys were immersed in the 2 saline solutions, but significant differences occurred when the OCP was compared for each saline solution with Fusayama solution. The chloride-ion corrosion resistance of the Pd-Ag alloys is attributed to both the nobility of these alloys and the passivation of the less noble elements in the alloy compositions. The much lower corrosion tendency (values of OCP) for the alloys in Fusayama solution (Table 2) indicate that Fusayama solution provides a less corrosive in vitro environment than the 2 saline solutions. This agrees with previous findings for high-palladium alloys.19 Mucin in Fusayama solution may act as a cathodic inhibitor,14 thus decreasing the corrosiveness of Fusayama solution. However, different results can be obtained from in vitro corrosion testing of dental alloys using potentiodynamic polarization and different electrolytes.22 Passivation was evident for all 3 Pd-Ag alloys in the 3 corrosion-testing media (Figs. 1-3), where a region of decreasing current density with increasing voltage occurred in the anodic (forward) portion of the polarization scan. It has already been shown that the elements Pd, Ag, Ga, In, and Sn can be passivated in aqueous solutions,12 resulting in passivation being a major mechanism for the high corrosion resistance of the PdAg alloys. Joska et al13 suggested that the passivation observed for the Pd-Ag alloys in artificial saliva could be due to the formation of palladium oxide and/or thiocyanate on the surface. The ennobling effect of palladium and its passivation behavior has been observed in a series of Pd-Ag binary model alloys in Ringer solution.10 The Ag/AgCl reaction AgCl + e− % Ag+ + Cl− has been proposed to increase the anodic oxidation process in these alloys and to serve as the mechanism for blocking the palladium allergy.8,9 Tarnish was observed on Pd-20Ag and Pd-40Ag (wt%) model alloys and on pure silver, and the attack was identified as initiating from grain boundaries and propagating into grains.10 AgCl may form at the OCP of Pd-Ag alloys, as these alloys have OCP values between 225 and 290 mVSCE in 0.09% and 0.9% NaCl solutions (Table 2), which are much higher than the equilibrium potential value for AgCl deposition/decomposition (E0=222 mVSHE= −19 mVSCE)11 and may favor the deposition of AgCl on the alloy surface. Thermodynamic calculation confirms that AgCl can be one of the corrosion products of Pd-Ag alloys in artificial saliva.13 Protection from AgCl deposition THE JOURNAL OF PROSTHETIC DENTISTRY
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would depend on the characteristics of the deposited AgCl layer and the bonding between this layer and the metal surface beneath. Ag is corroded to Ag+ ions from the alloy surface, and the Ag+ and Cl− ions then combine to form a AgCl deposit on the alloy surface. If the AgCl layer rapidly covers the alloy surface before the fresh alloy surface can corrode further and is tightly bonded to the alloy surface, then corrosion protection from AgCl deposition would be significant. Future studies are needed to obtain more information about the kinetics of this process and to characterize the deposition layer. Three zero-current potentials in the reverse scans, arising from separate electrochemical processes, were observed for all 3 Pd-Ag alloys in the 0.9% NaCl solution (Figs. 1B, 2B, and 3B), and these potentials were almost identical for all 3 alloys, despite their different compositions (Table 1). The key factor in this behavior may be the Cl− ions in the corrosion media. During the cyclic polarization experiment for each alloy specimen, the magnitude of polarization was greater than that during the initial linear polarization experiment. Thus, the protective oxide layer on the surface would be destroyed when the polarization potential was sufficiently high during the forward scan polarization. Alloying elements would also be oxidized to their ionic states, diffusing from the surface into the electrolyte at different potentials during the forward polarization; this process is facilitated by the presence of the Cl− ions. During reverse polarization, as the potential is decreasing, these cations could be plated out at high potentials, resulting in a large cathodic current. Depletion of the dissolved ion concentration by plating results in a decrease in the cathodic current as the potential is lowered further. As a result, a large nose-shaped region is often observed in which the anodic current density increases and then decreases with decreasing potential. This behavior is associated with active dissolution below the passivation potential, and the oxygen reduction reaction (the primary cathodic reaction during the forward scan) becomes dominant. This change in cathodic reaction and the emergence of the anodic nose-shaped region results in the 3 zero-current potentials. The appearance of 3 zero-current potentials in the reverse scans also indicates that de-alloying (separation of microstructural phases into the component elements) occurs at high potentials. This phenomenon has been previously observed in high-palladium alloys.19 Because the 3 Pd-Ag alloys have similar compositions, these 3 ZCPs were almost identical for each alloy. A recent in vitro study15 has also reported high corrosion resistance for 2 other Pd-Ag alloys in Fusayama artificial saliva. These studies used an alternative presentation of the cyclic polarization plots often found in materials science journals, in which the applied potential is shown on the horizontal axis, and the resulting current Sun et al
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density on the vertical axis. Another recent study16 reported the ion release, determined from inductively coupled mass spectroscopy, from a Pd-Cu-Ga alloy and another Pd-Ag alloy as a function of casting shape and surface preparation, noting the generally higher release of Pd ions from the Pd-Cu-Ga alloy. In a complementary in vitro study,17 the cytotoxicity of ions in concentrations released from Pd-based casting alloys was investigated using metal salts, mouse fibroblasts, and the MTT assay. The observation of some cytotoxic effects indicates that further studies of the biocompatibility of the palladiumbased alloys are warranted. Although it is accepted that useful screening results can be obtained for the corrosion properties of dental alloys, the cyclic polarization technique used in the present study is limited by the in vitro conditions, which differ from the complex oral environment, and in the electrochemical information that can be acquired. Uncertainty in locating the tangent line to the initial portion of the anodic polarization curve for the Tafel approximation can vary between plots for nominally identical replicate specimens. Inevitable variation in surface condition and surface microstructural phases also leads to differing electrochemical results for replicate specimens from the same sample group. Note also that while aerated electrolytes used in the present study simulate the general oral environment, deaerated electrolytes would better simulate the anaerobic conditions under bacterial plaque in vivo.19 Additional insight into fundamental corrosion processes can also be obtained by using the more sophisticated technique of electrochemical impedance spectroscopy. The results from a complementary electrochemical impedance spectroscopy study of the present 4 alloys will be presented in a future report. CONCLUSIONS Based on the findings of this in vitro study, the following conclusions were drawn: 1. The in vitro corrosion characteristics of Pd-Ag alloys are equivalent to the those in high-noble Au-Pd control alloy in simulated body fluid and oral environments. 2. Pd-Ag alloys appear to be resistant to chloride ion corrosion and exhibit passivation. 3. De-alloying appears to occur during the corrosion of these alloys.
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Corresponding author: Dr William A. Brantley The Ohio State University College of Dentistry Division of Restorative Science and Prosthodontics 305 West 12th Ave Columbus, OH 43210 Email: [email protected] Copyright © 2017 by the Editorial Council for The Journal of Prosthetic Dentistry.
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