Electrochemical Corrosion Behavior of Dental/Implant Alloys in Artificial Saliva

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The corrosion behavior and passive film characteristics of various dental alloys such as Co-Cr, Ni-Cr, Cu-Ni-Al, and commercially pure Ti (c.p. Ti) were evaluated in artificial saliva medium by utilizing electrochemical impedance spectroscopy (EIS),
  Electrochemical Corrosion Behavior of Dental/ImplantAlloys in Artificial Saliva Mohit Sharma, A.V. Ramesh Kumar, Nirbhay Singh, Nidhi Adya, and Bobin Saluja  (Submitted January 24, 2007; in revised form November 7, 2007) The corrosion behavior and passive film characteristics of various dental alloys such as Co-Cr, Ni-Cr, Cu-Ni-Al, and commercially pure Ti (c.p. Ti) were evaluated in artificial saliva medium by utilizing electro-chemical impedance spectroscopy (EIS), Tafel polarization, and cyclic polarization studies. EIS studies werecarried out for various durations viz. 1 h, 1 day, and 7 days to evaluate the stability of passive film andchange in corrosion characteristics with respect to time. Electrochemical parameters such as E corr , i corr ,corrosion rate, passive film characteristics with respect to time were obtained from various studies men-tioned above. The corrosion resistance decreased in the order Cu-Ni-Al>cp Ti>Co-Cr (Commercial)>Ni-Cr >Co-Cr (DRDO developed) in artificial saliva solution. Keywords  artificial saliva, corrosion, dental alloys, EIS study, passive film 1. Introduction  Non-precious metals and alloys are rapidly coming in voguereplacing precious or noble alloys in dentistry. They are beingused in full-cast and metal-ceramic restorations besides remov-able partial dentures. Approximately 90% of all removable partial dentures are now cast from non-precious alloys con-taining Co, Cr, Ni (Ref  1). These alloys possess advantageousmechanical properties, and therefore can be easily cast intodesired thinner shapes viz. crowns, bridges, fixed or removable partial dentures without compromising the rigidity (Ref  2-4). In oral cavity, the salinity of saliva approaches that of seawater and tends to be highly corrosive to most non-noble metals (Ref 1). The pH of saliva may vary between 2 and 11 while thetemperature in the oral cavity may be between 0   C and 70   C,all these variations mostly depend on the food intake (Ref  5).Thus corrosion resistance, besides other considerations suchas affordability and biocompatibility of alloys (Ref  6-8) play an important role. Although base alloys possess inferior corrosionresistance as compared to noble alloys (Ref  9), they outweighlatter in terms of mechanical properties. In designing non- precious alloys, chromium is added in the range of 15-30% toobtain an optimum value of corrosion resistance and mechan-ical strength (Ref  1). Ni addition is also performed to increasescorrosion resistance and mechanical properties. It has beenobserved that in nickel alloys chromium content of more than20% and a molybdenum content of greater than 4% can ensureadequate corrosion resistance (Ref  10, 11). A review of literature reveals various studies on dentalalloys. Ameer et al. (Ref  12) have studied potentiodynamic andelectrochemical impedance behavior of Co-Cr and Ni-Cr non- precious alloys and concluded that Co-Cr-Mo alloys are morecorrosion resistant as compared to Ni-Cr-Mo alloys. Huang(Ref  13) has investigated Ni-Cr alloy in artificial saliva usingelectrochemical impedance spectroscopy (EIS) study whereinthe film formation characteristics were studied after 2 h of filmstabilization. Manaranche and Hornberger (Ref  14) have carriedout corrosion and biocompatibility studies for Ni-Cr and Co-Cr dental alloys using inductively coupled plasma techniqueamong other methods. Takemoto et al. (Ref  15) have carriedout electrochemical corrosion behavior and dissolution studiesof cobalt-chromium alloy (Co-Cr) and commercially puretitanium (cp Ti) alloys in various gargle solutions. Viennot et al. (Ref  16) Eschler et al. (Ref  17) and Reclaru et al. (Ref  18, 19) Dong et al. (Ref  20) Lucas et al. (Ref  21) Elagli et al. (Ref  22) have carried out comparative electrochemical studies of cobalt-chromium dental alloys with those doped with preciousmetals in artificial saliva viz. Fusayama solution/corrosivemedium. Duffo and Castillo (Ref  23) evaluated corrosion behavior of Cu-Ni-Al alloy in natural as well as in variousartificial saliva solutions using electrochemical techniques.Johanson and co-workers (Ref  24) have studied corrosion behavior of copper, nickel alloys in artificial saliva and salinesolutions. Taira et al. (Ref  25) have conducted study on Tialloys and found that the alloys studied showed a stable passivity in the corrosion test.For laboratory studies generally an artificial solution is usedsince natural saliva composition cannot be generalized. Liter-ature survey reveals various studies which have proposeddifferent compositions for artificial saliva solution. A compar-ative study of such solutions has been performed by Duffo andCastillo (Ref  23).The presence of Cr improves the corrosion resistance of alloys in a corrosive environment due to the formation of aCr-rich, passive oxide film which is highly resistant to acidattack. Similarly, presence of molybdenum in the Ni-Cr based Mohit Sharma, A.V. Ramesh Kumar,  and  Nirbhay Singh , DefenceMaterials and Stores Research and Development Establishment (DMSRDE), Kanpur 208013, India; and  Nidhi Adya , and  BobinSaluja , Institute of Nuclear Medicine and Allied Research (INMAS),Timarpur, Delhi, India. Contact e-mail: kumarmohitsharma@rediffmail.com. JMEPEG (2008) 17:695–701   ASM InternationalDOI: 10.1007/s11665-008-9198-4 1059-9495/$19.00Journal of Materials Engineering and Performance Volume 17(5) October 2008—695  alloy increases the resistance to localized corrosion in thechloride containing environment (Ref  26). Therefore, for Ni-Cr-based dental alloys, the addition of 12% Cr (minimumvalue) and 2-5% Mo to the alloy bulk is well recommendedfrom the corrosion resistance point of view (Ref  3). cp Ti has been widely used as dental implant material (Ref  27).Since dental alloy implants are always in touch with media(saliva), it is felt that Electrochemical Impedance Spectroscopystudies should be carried out for long duration, which can provide wealth of mechanistic information. However, nostudies were reported on time-dependent corrosion character-istics of dental implants. In this study, the corrosion behavior and passive film characteristics have been studied on variousalloys for different duration, which can throw light on the passive film stability for these alloys. These results werecorroborated with cyclic polarization studies.Present investigation is aimed at electrochemical corrosionstudy of various alloys viz. Co-Cr (developed by DefenceResearch and Development Organization, India) and Co-Cr (Commercially available), Ni-Cr, Cu-Ni-Al and cp Ti by usingTafel polarization, cyclic polarization and EIS methods. Allstudies were carried out in artificial saliva Solution. 2. Experimental 2.1 Materials and Methods  Four different types of dental base alloys for implant Supraconstructions along with cp Ti were used for the study.The composition of Ni-Cr alloy was 75.8% Ni, 15% Cr, 2.8%Mo, 0.7% Nb, 2.9% Al, and 1.5% Mn. The commerciallyavailable Be free Co-Cr alloy was of the composition 65% Co,28% Cr, 4.5% Mo, and 1.6% Si. The composition of Cobalt-Chromium-Molybdenum-Silicon alloy (Indigenously devel-oped by DRDO, India) was 61% Co, 31% Cr, 3.6% Mo, and1.3% Si. The composition of Copper-Nickel-Aluminum Alloy(Cunial Alloy) is proprietary. Besides these alloys commer-cially pure Ti was also used in this investigation.For all the experiments AR Grade chemicals/reagents wereused and solutions were prepared in double distilled water.Experiments were carried out in a three cell assembly withPlatinum as a counter electrode and standard calomel electrode(SCE) as a reference electrode. Potentiostat/Galvanostat model283 coupled with frequency response detector model 1025 were both supplied by EG&G Instruments, USA was used in thestudies. Data acquisition was done through a computer softwareM398, Version 1.30, EG&G PAR, USA. The experiments werecarried out at ambient temperature. Samples were prepared bycutting the alloys into 1 cm 2  pieces and then mounting theminto epoxy base, thus leaving only the test specimen area intothe contact of the test electrolyte. Samples were polished withsuccessively finer grade of emery papers (up to 800 grit) andthen degreased with toluene. Modified Fusayama solution (Ref 28) was used as artificial saliva whose composition was NaCl(0.4 g/L), KCl (0.4 g/L), CaCl 2 Æ 2H 2 O (0.795 g/L), NaH 2 PO 4 Æ- H 2 O (0.690 g/L), KSCN (0.3 g/L), and urea (1.0 g/L).The following electrochemical studies were carried out:(i)  Open Circuit Potential (OCP):  Open circuit potentials weremeasured in the electrolytes before carrying out the experi-ments. The OCP was measured for duration of 3000 s.(ii)  Tafel Studies:  Tafel plots of various alloys were ob-tained by exposing them into respective electrolytes and polarizing from Ecorr   - 250 mV to +250 mV vs. SCEwith scan rate of 0.166 mV/sec.(iii)  Cyclic Polarization Studies:  The specimens were polar-ized in a cyclic manner from  - 250 mV from OCP to avertex potential of 1.2 Volts and final potential of thecyclic scan was  - 250 mV vs. OCP. Scan rate duringthe experiment was 1 mV/Sec.(iv)  Electrochemical Impedance Spectroscopy:  Single sineAC Impedance studies were carried out in the fre-quency range 100 kHz-10 mHz. The AC signal im- posed during the experiment was of 5 mV rmsamplitude. Data were recorded at 5-frequencies/decade.In this article, bode plots are used for analysis of thecorrosion behavior of various alloys. Bode plots areLog frequency versus phase shift of AC sine wave. In bode plots, it is easy to understand how the impedance/  phase angle depends on frequency. At low and higher frequency the behavior of the cell (sample + electrolyte)is resistor like and phase angle is nearly zero. At inter-mediate frequency, the phase angle shifts toward 90  and for a perfect capacitor, the phase angle is 90  .Depending on the value of phase angle maxima, theinformation regarding equivalent circuit model can bededuced. For example, if there is only one phase max-ima peak, then the equivalent circuit model correspondsto a simple Randles circuit. A detailed account of  Fig. 1  Time potential curves of various dental materials Fig. 2  Current density-potential curves (Tafel plots) of various den-tal materials 696—Volume 17(5) October 2008 Journal of Materials Engineering and Performance  impedance plots may be obtained from the research pa- pers of Walter (Ref  29) and Kolman (Ref  30). The plots were analyzed to obtain equivalent circuits by Zsimp-win software provided by EG&G. 3. Results and Discussion Figure 1 shows the time potential plots of various dentalmaterials viz. (i) Ni-Cr, (ii) Co-Cr (Commercial), (iii) Cu-Ni-Al, (iv) Co-Cr (DRDO), and (v) cp Ti exposed to artificialsaliva solution. Upon exposure of these samples in artificialsaliva (modified fusayama solution), it was observed that allmaterials except Cu-Ni-Al showed a shift in the potentialtoward noble direction. The E corr   shift was maximum (251 mV)in case of Co-Cr (Commercial) while minimum E corr   shift wasobserved in case of Cu-Ni-Al (22 mV). This is possibly due to better passive film formation in Co-Cr (commercial) alloy.Finally, the order of E corr   after stabilization was Co-Cr (Commercial)>Ni-Cr >Co-Cr (DRDO)>Cu-Ni-Al>cp Ti.Figure 2 shows the current density-Potential characteristics(Tafel plots) of various dental materials exposed to artificial Fig. 3  Cyclic polarization curves of alloys. (a) Ni-Cr, (b) Co-Cr (Commercial), (c) Co-Cr (DRDO), (d) Cu-Ni-Al and, (e) cp Ti Table 1 Various DC electrochemical parameters calculated from Tafel plots for various dental materials S. No. Material  b a ,  ·  10 - 3 b c ,  ·  10 - 3 R  p , KOhms E Corr , mV i Corr ,  l A/cm 2 Corr. Rate,  ·  10 - 3 , mpy 1 Ni-Cr 544.2 257.0 387.4  - 172.8 0.198 88.432 Co-Cr (Commercial) 404.9 248.5 130.0  - 207.9 0.4793 214.13 Cu-Ni-Al 8968 405.8 14.63  - 283.8 11.200 50024 Co-Cr (DRDO) 273.3 182.3 909.8  - 221.0 0.05442 24.305 cp Ti 439.3 414.0 128.8  - 342.5 0.6980 311.7 Journal of Materials Engineering and Performance Volume 17(5) October 2008—697  saliva solution. It is evident form the figure that the Tafel behavior of all these alloys is highly distinct. Corrosion current was maximum in Cu-Ni-Al and it decreased in the order cpTi>Co-Cr (Commercial)>Ni-Cr >Co-Cr (DRDO). The elec-trochemical parameters are compiled in Table 1.It can be seen from the figure that corrosion rate of Co-Cr (DRDO) was minimum possibly because of better spontaneous passive film formation due to much higher chromium content.The electrochemical reactions occurring on these alloys appear to be more or less mixed anodic and cathodic control except Cu-Ni-Al alloy where very high  b C  was observed (Table 1).Cyclic polarization study of Ni-Cr alloy exposed to modifiedFusayama solution is shown in Fig. 3a. Ni-Cr alloy showedactive-passive behavior with a pitting potential of 818 mV. Onreversing the potential the reverse scan almost traces throughthe forward scan up to 900 mV and further reversal leads toshift of hysteresis loop toward lower current region indicatingno pitting tendency under these conditions. The Co-Cr (Commercial) and Co-Cr (DRDO) also showed a similar  behavior (Fig. 3 b, c). However, in Cu-Ni-Al alloy on reversingthe potential, the scan initially follows a lower current path andintersects the forward scan just above the pitting potential andthereafter traces a higher current path (Fig. 3d) possiblyindicating a poor repassivation by this alloy in artificial saliva.Therefore, this alloy has a tendency for localized corrosion. The passivation current density of Ni-Cr, Co-Cr (Commercial), Co-Cr (DRDO), and cp Ti were 1.65, 1.7, 1.989, and 7.122  l A/ cm 2 , respectively. The lower passivation current in the first three alloys are due to chromium factor present. However, in cpTi the pitting potential was attained up to a vertex potential of 1.4 V. Moreover on reversing, the scan traced through a lower current region with hysteresis loop area far higher than anymaterial mentioned above indicating excellent repassivationand no tendency for pitting corrosion.Most of the studies regarding dental alloys in saliva to the best of our knowledge, deal with potentiodynamic polarizationof these alloys where almost all dental alloys showed active- passive behavior (Ref  31-33). However, cyclic polarization  behavior was studied by Gil et al. (Ref  34) non-dental alloyswhere the passive behavior with respect to surface roughness inHanks solution was evaluated. A linear behavior was observedwhen current density was plotted against roughness. However,our studies more or less deal with overall corrosion behavior of these materials in artificial saliva. A summary of various parameters obtained from cyclic polarization experiments isgiven in Table 2.Figures 4-6 represent Bode phase plots of various dental alloys in artificial saliva (modified Fusayama Solution) for 1 h,1 day, and 7 days duration, respectively. After 1 h of filmstabilization, Ni-Cr alloy showed maximum phase angle 75.1  Table 2 Various parameters obtained from cyclic polarization data S. No. Material E pass , mV i pass ,  l A/cm 2 E pit , mV i pit ,  l A/cm 2 1 Ni-Cr 79.0 1.65 578 5.72 Co-Cr (Commercial) 102.0 1.7 638 7.7333 Cu-Ni-Al  - 190.0 31.11 274 42.114 Co-Cr (DRDO) 31.0 1.989 586 5.75 cp Ti  - 134.0 7.122  … … Fig. 4  Bode plots for various alloys after 1 h of film stabilization 698—Volume 17(5) October 2008 Journal of Materials Engineering and Performance  at 3.98 Hz. Co-Cr (Commercial) alloy showed maximum phaseangle 76.1   at 1.58 Hz. Cu-Ni-Al alloy showed maximum phase angle 64.5   at 6.31 Hz. Co-Cr alloy (DRDO) displayedmaximum phase angle 81.1   at 1.58 Hz. cp Ti showedmaximum phase angle 68.7   at 0.398 Hz. frequency. After 1 day exposure, Ni-Cr alloy showed maximum phase angle75.2   at 0.398 Hz. Co-Cr (Commercial) alloy exhibited max-imum phase angle 74.6   at 0.631 Hz. Cu-Ni-Al alloy showedmaximum phase angle 73   at 1580 Hz. Co-Cr (DRDO) showedmaximum phase angle 79.6   at 0.631 Hz. Cp Ti showedmaximum phase angle 66   at 0.1 Hz. After 7 days of exposure,as evident from Fig. 6, Ni-Cr alloy, Co-Cr (Commercial) andCo-Cr (DRDO) alloys showed maximum phase angles at 0.158 Hz. with the values being 75.4  , 76.1  , and 77.1  , Fig. 6  Bode plots of various alloys after 7-day period Fig. 5  Bode plots of various alloys after 1-day period Journal of Materials Engineering and Performance Volume 17(5) October 2008—699
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