Electrochemical Corrosion and Surface Analyses of a Ni-Cr Alloy in Bleaching Agents

Electrochemical Corrosion and Surface Analyses of a Ni-CrAlloy in Bleaching AgentsEvsen Tamam, PhD, DDS,1 A. Kevser Aydın, PhD, DDS,2 & Semra Bilgic, PhD, MS3

1Research Associate, Department of Prosthodontics, Gazi University, Faculty of Dentistry, Ankara, Turkey2Emeritus Professor, Department of Prosthodontics, Ankara University, Faculty of Dentistry, Ankara, Turkey3Professor, Department of Physical Chemistry, Ankara University, Faculty of Science, Ankara, Turkey

Keywords

Ni- Cr alloys; bleaching agents; corrosion;oxide layer; XPS.

Correspondence

Evsen Tamam, Department ofProsthodontics, Gazi University, Faculty ofDentistry, 06510 Emek, Ankara, Turkey.E-mail: [email protected]

Presented at the 86th General Session &Exhibition of the International Association forDental Research (IADR), Toronto, Canada, 2-5, July 2008.

This research work was supported by AnkaraUniversity Research Foundation Grant No:2005.08.02.065.

The authors deny any conflicts of interest.

Accepted October 4, 2013

doi: 10.1111/jopr.12149

AbstractPurpose: The aim of this study was to evaluate the corrosion behavior of a Ni-Crdental casting alloy subjected to 10% hydrogen peroxide (HP) and 10% carbamideperoxide (CP) bleaching solutions and to determine the composition of the surfaceoxide layer formed on the alloy specimens.Materials and Methods: Ten cylindrical specimens (4 mm in diameter × 25 mmin height) were cast from a Ni-Cr alloy (Wiron 99) and divided into two groups(n = 5). A potentiodynamic polarization test was used to compare the corrosion ratesof specimens in HP and CP (pH = 6.5). Before cyclic polarization tests, all alloyspecimens were allowed to reach a steady open circuit potential (Ecorr) for a period of1 hour. Then tests were initiated at 100 mV versus standard calomel electrode (SCE)below Ecorr and scanned at a rate of 1 mV/s in the anodic direction until reaching1000 mV over the Ecorr value. The scan then was reversed back to the Ecorr of thespecimens. The open circuit potentials (Ecorr) and the current densities (Icorr) weredetermined using the anodic Tafel regions extrapolating from the curves. Differencesin Ecorr and Icorr were determined using one-way ANOVA (α = 0.05). In addition,corrosion rates were calculated from these curves. Before and after polarization tests, ascanning electron microscope (SEM) examination accompanied by energy dispersiveX-ray spectroscopy (EDS) was used to analyze the surface morphology. The surfacecharacterization of the passive film formed on alloy specimens was also performedby using X-ray photoelectron spectroscopy (XPS).Results: In this study, bleaching agents had an effect on the anodic process for twogroups. Although no statistical difference was identified between the groups for bothcorrosion parameters, results indicated that the effect of CP on the corrosion behaviorwas less than that of HP. These results agreed with the SEM observations. XPSdata showed that oxide layers formed on all groups contained mainly Cr2O3, NiO,and MoO3, and the amounts of oxides formed on CP-treated specimens were higherthan HP treated ones. Also, molybdenum rates were increased with CP applicationcompared to HP.Conclusions: The comparison of the effects of the two bleaching agents at 10%showed that the alloy suffered less corrosion with CP than HP. This result was alsoconfirmed by the SEM and XPS data. The presence of Mo on the oxide layer affectedthe oxide layer, leading to lower corrosion rates

Various metals and their alloys have been used to fabricatedental prostheses such as crowns, fixed partial dentures, cast-ing inlays, and denture bases.1 Although gold was tradition-ally the primary element in dental alloys, lately base metal al-loys for use in dentistry have been developed, decreasing costsremarkably.2,3

Dental alloys are expected to have certain properties, such asease of manipulation, reliability of treatment, sufficient rigidity

and durability for dental applications, biocompatibility, andgood esthetics.2-4 These materials are subjected to variableoral conditions, such as temperature varying between 5°C and55°C.4 Depending on food intake, the pH of saliva may varyfrom 2 to 11.5 Since saliva in the oral cavity is prone to bequite corrosive, dental metallic structures have to withstandthe adverse chemical atmosphere.5 Because dental alloys servefor long periods in the mouth, where they need to resist these

1Journal of Prosthodontics 00 (2014) 1–10 C© 2014 by the American College of Prosthodontists

Corrosion and Surface Analyses of a Ni-Cr Alloy Tamam et al

corrosive environments, it is important to know their biocom-patibility and resistance to corrosion,6 which can be determinedby the study of the electrochemical behavior of the material.4

The interface between biomaterial and tissue can be definedas an electrochemical system, supporting the implementationof electrochemical methods of evaluation of metal biomaterialsurfaces.4

One of the determining factors of corrosion in terms ofbiocompatibility is defining which elements are released andmeasuring their amounts. Since adverse biologic effects suchas toxicity, allergy, or mutagenicity are directly affected byreleased elements, corrosion behavior of an alloy should beevaluated from the standpoint of biocompatibility. Release ofmetallic ions into the oral cavity originates from the metallo-lattice in base metal alloys. Thermostable substances, like chlo-rides, sulfides, and oxides, result from the corrosion processand are called corrosion products.7 The biologic reaction be-tween tissue and the released elements depends on the type ofthe element and the duration of exposure.2 Elements releasedfrom dental alloys in the oral environment in relation to lev-els, kinetics, and chemical state are crucial in the evaluationof possible biologic response of patients wearing such an al-loy. Even a low dose of allergen can be sufficient to initiateallergic reaction in an individual prone to allergy. Most dentalallergies, as with nickel-based alloys, are composed of type IVhypersensitivity reactions, cell-mediated by T-lymphocytes.8

Therefore, corrosion generates adverse biological effects ofdental alloys, but it is not the only factor. According to Mulderset al,9 the handling of nonprecious alloys is restricted by theirtendency to corrode, because corrosion products can be bothtoxic and allergy-stimulating in the oral cavity. Toxicity is thedegree of dissolution and release of chemical ingredients fromthe material, that can stimulate systemic or allergic responses,as well as carcinogenic and mutagenic effects.10 Schmalz andGarhammer8 have classified the biological interactions betweenalloys and target tissues as follows: bacterial adhesion, toxicity,subtoxic effects, and allergies.

Chromium and nickel contribute to the corrosion resistanceand mechanical strength of base metal alloys. In Ni alloys a Crcontent over 20%, and Mo content of greater than 4% can pro-vide sufficient corrosion resistance.5,11 Similarly, the presenceof Mo in the Ni-Cr alloy improves the resistance to localizedcorrosion in the chloride containing media.12 McGinley et al13

studied the biocompatibility of base metal alloys with differentlevels of constitutive elements. In their study, Ni-based alloyshad a high Ni content (75.4%) and a Cr content of 12.6%,lower than the 16% to 20% Cr claimed in the literature to pro-vide corrosion protection. An alloy with a 58.9% Ni and 25%Cr content is within the range generally advocated. The ICP-MS results of the study showed that metal ion release couldnot be directly compared to the alloy elemental composition.Conversely, alloys with a Cr content below the lowest proposedlevel released significantly less Ni, Cr, Mo, Fe, and Cu com-pared with the other Ni-based alloy. McGinley et al stated thatjustifying a lowest Cr content of 16 to 20 mass% in the dentalliterature seems deceptive.13 After a metal restoration is placedin the oral cavity, it will be subjected to low pH saliva, then apassive film will form on the alloy surface. Corrosion resistanceof this passive film is important in terms of biological reactions.

Table 1 Bulk composition of Wiron 99 R© as supplied by the manufac-turer, from EDS and XRF analyses (mass, wt%)

EDS (#) EDS ($) XRF Manufacturer

Elements (at%) (at%) (mass%) (mass%)

Ni 63.50 64.19 61.7 65.0Cr 24.85 24.70 22.1 22.5Mo 8.93 9.89 8.6 9.5Si 2.72 1.21 2 1.0Nb ND∗ ND∗ 0.9 1.0Fe ND∗ ND∗ 0.2 0.5Ce ND∗ ND∗ ND∗ 0.5

*Not detected.#Data obtained from the alloy surface.

$Data obtained from the sectioned surfaces.

Evaluating the properties of the passive film is a prerequisitefor its biocompatibility. The damaged passive film (usually inthe oxide form) is the main source of the release of corrosionproducts to adjacent and systemic tissues.11

Most mouthguard bleaching procedures are thought compar-atively safe with respect to systemic effects. Clinically, Ni-Crdental casting alloys are still in use for premolar and molar teeth,and individuals who are considering bleaching treatment mayhave crown-restored teeth. Although as a common procedurebleaching is primarily applied to the anterior teeth, excessiveamounts of bleaching material may come in contact with metalcrowns and alter the corrosion behavior of these restorations.14

Therefore, determining the corrosion behaviors of these alloysin commonly used bleaching agents will be useful in choosingthe alloy to be used for certain dental treatments.

In a previous study,15 we investigated the amounts of releasedelements by immersion testing and how the alloy surfaces wereaffected by the bleaching agents. The purpose of this study wasto assess the electrochemical corrosion behavior of a Ni-Crdental casting alloy treated with 10% hydrogen peroxide (HP)and 10% carbamide peroxide (CP) solutions and to identifythe composition of the surface oxide layer formed on the alloyspecimens.

Materials and methods

The materials used in this study were 10% hydrogen peroxide(HP) and 10% carbamide peroxide (CP) solutions at pH = 6.5and a Ni-Cr dental casting alloy (Wiron 99 R©; BEGO, Bremen,Germany). The bulk composition for the alloy is listed in Ta-ble 1. Specimen preparation was conducted in the same wayas a previous report15 as follows: Ten cylindrical specimens (4mm diameter, 25 mm high) were cast following the recommen-dations of the manufacturer and then divided into two groups(n = 5) randomly.

A polishing procedure was performed until clinically ac-ceptable surfaces were obtained in accordance with the rec-ommendations of the manufacturer. Briefly, after devesting,the alloys were sandblasted with 250 μm Al2O3 and finishedwith diamond milling tools. The surfaces were sandblastedwith 50 μm Al2O3, then polished with Perlablast R© (BEGO),

2 Journal of Prosthodontics 00 (2014) 1–10 C© 2014 by the American College of Prosthodontists

Tamam et al Corrosion and Surface Analyses of a Ni-Cr Alloy

Figure 1 Corrosion cell, original and schematic view.

rubber-polished with rubber polishers, finish-polished with apolishing paste, and steam cleaned.

Finally, each specimen was cleaned by scrubbing with a softtoothbrush and 2% glutaraldehyde solution, rinsing with dis-tilled water, and ultrasonic treatment in 70% isopropyl alcohol.After cleaning, specimens were disinfected in 70% isopropylalcohol for 20 minutes, followed by two rinses with steriledouble-distilled water to remove the alcohol.16,17

Electrochemical measurements were implemented using apotentiostat (Gamry PCl4/ 750TM Potentiostat; Gamry Instru-ments Inc., Warminster, PA). Polarization experiments werecarried out in a conventional three-electrode glass cell with

a platinum counter electrode and saturated calomel electrode(SCE) as a reference, with a Luggin capillary bridge (Fig 1).The working electrode was constructed by embedding a cylin-drical alloy specimen in methyl methacrylate resin. The testingarea of the specimen was 0.1256 cm2. Before each experiment,the electrode was polished with 1200-grit emery paper, washedthoroughly with double-distilled water, and then transferredto the cell. The water-jacketed cell was connected to a con-stant temperature circulator operating at 37 ± 1°C to simulatemouth conditions. To use the test solutions as electrolyte, theywere prepared by diluting the 30% HP solution in 0.1 M phos-phate buffer solution. The 10% CP solution was prepared by

Journal of Prosthodontics 00 (2014) 1–10 C© 2014 by the American College of Prosthodontists 3


dissolving carbamide peroxide powder (Sigma, St Louis, MO)in 0.1 M PBS, adjusting the pH (6.5) accordingly. The volumeof the electrolyte in the corrosion cell was 50 ml. Electrolyteswere renewed for every measurement. The working electrodewas placed in the holder/electrode assembly when the elec-trolyte reached the test temperature. During each experiment,solutions were mixed with a magnetic stirrer.

Before cyclic polarization (cyp) testing, all test specimenswere allowed to reach a steady state potential (Ei = 0) for a pe-riod of 1 hour. Then tests were initiated at 100 mV versus SCEbelow this potential and scanned at a rate of 1 mV/s in the anodicdirection 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 corro-sion parameters were calculated (means were calculated from25 measurements; n = 25). Data acquisition (the corrosion po-tentials, Ecorr, and the current densities, Icorr) from these curveswas done through a software program (Gamry ECHEM analystDC105TM Corrosion Techniques Software Version 5; GamryInstruments Inc.). The corrosion rates were also calculated fromthese curves through Faraday’s law using the same computersoftware and represented the loss of material mass as a functionof time and per unit area. The differences between the meansof the corrosion potential and current density values measuredwere analyzed with the Kolmogorov-Smirnov test (for normal-ity) followed by one-way ANOVA at a significance level of0.05.

Surface analysis

Scanning electron microscopy (SEM, Jeol JSM-6400; JEOLLtd., Tokyo, Japan), with an energy dispersive spectroscopy(EDS, NORAN System Six-NSS300; Thermo Scientific, Mid-dleton, VA) facility, was used to examine the alloy surfaces (2specimens of each group) before and after cyp testing. Prior tocasting, both the outermost and the inner regions of the ingotwere examined with EDS. An ingot was also examined us-ing X-ray fluorescence spectroscopy (XRF, RIX 3000; RigakuIndustrial Co., Yamaguchi City, Japan) to determine the com-position of the alloy. The EDS analysis of the inner regions wasperformed as follows: the ingot was cut into two pieces hor-izontally using a water-cooled precision low-speed diamondsaw (Microcut 175, Metkon, Bursa, Turkey), and the sectionedsurfaces of these specimens were examined.

In addition, identification of the surface passive film formedon the alloy surfaces was performed via X-ray photoelectronspectroscopy (XPS) both before and after bleaching applica-tions. XPS analysis was performed on a SPECS system (SPECSSurface Nano Analysis GmbH; Berlin, Germany) using un-monochromatized MgKα radiation (1253.6 eV) as an X-rayanode operated at 100 W. One cylinder from each group18 wasanalyzed by XPS. Spectra were acquired from a 4 × 7 mm2

sample region. The pressure in the analysis chamber was main-tained < 10−7 mbar during measurements. All experimentswere done with a 90° angle between specimen and electronanalyzer. Survey and high-resolution spectra were collected us-ing 144 and 48 eV pass energy, respectively. The backgroundsubtraction, peak integration, and atomic composition were es-

Table 2 Summary of the cyp results for the test groups (SD in paren-theses)

HP CP

Ecorr (mV) 256.2 (35.975) 204 (62.37)Icorr (μA/ cm2) 0.1626 (0.012) 0.05662 (0.0101)Corrosion rates (mpy

× 10−4)27.98 4.68

timated using the SpecsLab software (Version1.0). For C 1s,O 1s, and Ti 2p, the Shirley approximation was used for thebackground subtraction; for all other elements, the backgroundwas subtracted linearly. To convert peak areas to surface con-centration, sensitivity factors published by Evans et al19 wereused. The binding energy scale was referenced by setting the Cto H peak maximum of surface in the C1s spectrum to 285.0 eV.To obtain composition as a function of depth (depth profiling),the alloy surface was etched using argon sputtering with an ionbeam for times between 0 and 3480 seconds with 120-secondintervals.

According to Roach et al,20 “XPS uses X-rays to cause theejection of electrons, known as photoelectrons, from atoms. Thephotoelectrons are separated according to their energies in anelectron analyzer, detected, and recorded. Because the energyof each ejected photoelectron is characteristic of the atom fromwhich it came, the resulting electron energy spectrum may beused to identify the elements present.”

Results

Table 1 shows the EDS and XRF ingot compositional analysesof the Wiron 99 R© (W99) alloy compared to the manufacturer’sinformation. The results are very similar for the major elements,with the only differences being the minor elements, identifiedby EDS and XRF, but which the manufacturers do not alwaysquote. The corrosion parameters, as determined through cyptesting, are listed in Table 2. The Kolmogorov-Smirnov testshowed that the data distribution was normal. The cyp curvesare provided in Figure 2 to show the range of corrosion behaviorof alloy in HP and CP electrolytes.

HP treatment resulted in an increase in Ecorr values (256.2mV) of the alloy when compared to the CP treated group (204mV). In addition, the HP group exhibited a wider passive regionin the polarization curve than the CP group did.

The ranking of the Icorr values (μA/cm2) between the testgroups was as follows: HP > CP (0.1626 μA/cm2, 0.0566μA/cm2, respectively). Corrosion rates (mpy) Icorr valuesranked as 27.98 mpy for HP and 4.68 mpy for CP. The ma-terial with higher Icorr at a given potential is more prone tocorrosion. The HP group had a more active Icorr value than theCP group did. Although a remarkable difference was found forboth Ecorr and Icorr values between the groups, this differencewas not statistically significant (α = 0.05).

The SEM micrographs showed almost the same picturesof the untreated and differently treated Ni-Cr specimens (Fig3). The post-corrosion examination with SEM revealed thatboth HP and CP solutions caused surface erosion compared to



Figure 2 Cyclic polarization curves for HP- and CP-treated groups.

untreated specimens; however, the surface erosion that occurredwith HP solution was more significant with respect to CP so-lution. While the HP treated group (Fig 3B) exhibited a morehomogeneously corroded surface, the CP treated group (Fig3C) shows a more disordered and patchy surface.

XPS was employed to follow and characterize the surfacebefore and after the bleaching treatment and for further supportof the corrosion results. A representative XPS survey spectrumis shown in Figure 4A; XPS depth profile analysis of the Ni-Cralloy is shown in Figure 5. The XPS survey spectra of untreatedand tested specimens in Figures 4 and 5 confirmed the presenceof Ni, Cr, Mo, and O. Surfaces are composed of Cr, Mo, andNi oxides for all groups. The C 1s signal indicates the presenceof carbonaceous contamination due to C-containing moleculesremaining after the cleaning procedure or adsorbed later on theair-exposed surfaces. Results of XPS analysis of the surfaceoxide layer formed on the Ni-based alloy specimens beforeand after bleaching applications are listed in Tables 3 and 4,showing the oxidation state and percent atomic composition,respectively.

For the control group (untreated specimen), oxide forms ofNi, Cr, and Mo were detected. Ni 2p peak was present onthe surface with BE that correlated with elemental Ni (Ni0) andNiO. A complex asymmetric Cr 2p peak indicating the presenceof Cr2O3 as well as elemental Cr (Cr0) was identified. A surfaceexhibited a complex Mo 3d peak composed of elemental Mo(Mo0) and MoO3 (Tables 3, 4). Depth profile analysis resultsshowed that oxide forms of Ni, Cr, and Mo were detected foronly argon sputtering times of 1200 to 1440 seconds. After thistime, there was no evidence of their oxide forms. O1s accounted

for 26.4 atomic percent of the surface. The minor alloy elements(Si, Nb, Ce, Fe) were not detected.

For the CP-treated group, Ni was present only in the oxideform in whole argon sputtering stages, but no evidence wasfound of its elemental state. This suggested that Ni is relativelymore prone to oxidation than other elements present on thesurface. Mo was detected mainly in the elemental state, butoxide forms of Mo were present as well. The peaks of Cr(Cr 2p1/2 at 575.7 eV, Cr 2p3/2 at 577 eV) demonstrated thepresence of Cr2O3. In addition, CrOOH and Cr(OH)3 were alsodetermined by depth profiling analysis only for argon sputteringtimes between 0 to 840 seconds. The complex Nb 3d peaks werepresent on the surface with BE correlating with Nb2O5 in thewhole depth profiling. The NbO2 form was found only in thefirst 10 sputtering stages, whereas the metallic Nb (Nb0) form isprominent throughout only the last 20 sputtering stages. Si andCe were present in both oxide and elemental states. O accountedfor 62.3 atomic percent of the oxide layer. A trace amount ofFe was also detected. Compared to the untreated group, bothHP and CP surface atomic percent values increased for Mo3d, and Si 2p peaks, whereas Ni 2p and Cr 2p peak intensitiesdecreased (Table 4). This result indicates that the oxide layerfor both specimen groups consists of oxide forms of Mo andSi.

For the HP-treated group, Ni was present in both oxide andelemental states. Compared to the other two groups (controland CP), a significant amount of Ce (at 2.227%) was observed.High-resolution analysis of the binding energies indicated thatCr, Mo, Si, Ce, and Fe were only in the forms of oxides. O1saccounted for 23.05 atomic percent of the oxide layer. Mo and



Figure 3 SEM micrographs and corresponding EDS element profiles for W99 alloy comparing before (A) and after corrosion measurements [(B)HP-treated specimens, and (C) CP-treated specimens].

O levels significantly decreased, while all the other elementswere increased compared to the CP-treated group (Table 4).

Depth profile analysis showed that the oxides formed bothafter CP and HP treatment were thicker than those of the un-treated group. Since the oxide compounds were present duringthe complete argon sputtering procedures (0 to 3480 seconds),the thickness variation between the CP- and HP-treated groupscould not be determined.

Discussion

In this study, the corrosion behavior of the W99 alloy was in-vestigated under the effect of bleaching solutions. The resultsshowed that the bleaching treatment affected the corrosion be-havior of Ni-based alloy. In addition, bleaching agents causedalterations to the surface chemistry and the surface morphologyof this alloy.

Compared to information available from the manufacturer,the bulk compositions determined by EDS and XRF were foundto be similar in general with slight deviations in detail (Table 1).This could be interpreted as a lack of precise information fromthe manufacturer and/or artifact due to specimen preparation,or evaporation of certain elements from the surface may havetaken place.21 In the case of a Ni-based alloy with a lower Crcontent (12.6 wt%), corrosion resistance decreased compared tothe higher Cr content (25 wt%) in the bulk alloy.22 Huang23 alsodetermined that corrosion resistance diminished for alloys with12.4 wt% to 13.6 wt% Cr, contrasted with alloys containing

more than 21 wt% Cr, which exhibited high levels of corrosionresistance. In the current study the Cr content of the Wiron99 R© was 22.5 wt% in the bulk, exceeding the referred level forcorrosion resistance.

Ni-based alloys are generally used for prosthetic purposesfor prolonged periods. Therefore, choosing which kind of alloyis proper in the oral cavity, which combinations are reliable, orhow the biocompatibility is affected after dental application isimportant. If an alloy exhibits a low corrosion resistance evenat 1% NaCl solution,4 it will not be able to withstand the muchmore corrosive environment of the human body and saliva.Moreover, dental treatments like bleaching may also contributeto the corrosion process, because bleaching agents can act likean electrolyte.

The results of electrochemical measurements have high re-peatability and are informative for defining and interpreting theelectrochemical behavior of dental alloys.4 This explains why,in the current study, cyp testing was preferred to investigate thecorrosion behavior of the tested alloy. For potentiodynamic po-larization tests, the three-electrode setup is usually used, witha working electrode prepared from the dental alloy, a counterelectrode (usually platinum) to flow charge, and a referenceelectrode (usually a saturated calomel electrode, SCE, or a sat-urated Ag/AgCl electrode).24 One electrochemical techniquethat has gained widespread acceptance as a general tool forassessing the possibility of an alloy suffering corrosion is thecyclic potentiodynamic polarization technique. This techniqueis used to generate the polarization scan. This technique has



Figure 4 Representative XPS survey scan. A) An XPS widescan of CP-treated group; B) An XPS narrow scan of untreated group showing Ni peakspread over 852.6- to 853.8 eV and covering Ni0 and NiO and showing that elemental oxidation can be in various forms.

been especially useful to assess localized corrosion for passi-vating types of alloys such as 316ss, nickel-based alloys con-taining chromium, and refractory-type alloys such as titaniumand zirconium, all in an array of environments.25-27 Accordingto Silverman,26 “The voltage applied to an electrode made fromthe alloy under study is ramped at a continuous, often slow, raterelative to a reference electrode using a potentiostat. The volt-age is first increased in the anodic or noble direction (forwardscan). The voltage scan direction is reversed at some chosencurrent or voltage and progresses in the cathodic or active di-rection (backward or reverse scan). The scan is terminated atanother chosen voltage, usually either the corrosion potentialor some potential active with respect to the corrosion potential.The potential at which the scan is started is usually the corrosionpotential.” The specimen is often immersed in the environmentlong enough for the corrosion potential to reach steady state.The corrosion behavior is predicted from the structure of thepolarization scan.

Ameer et al28 investigated the electrochemical behavior of anonprecious dental alloy in bleaching agents. They reported that

W99 alloy exhibited Ecorr = 241.7 mV, Icorr = 0.682 μA/cm2,and corrosion rate = 8.45×10−4 mpy values in 10% HP solu-tion; Ecorr = 241.2 mV, Icorr = 0.264 μA/cm2, and corrosionrate = 2.73×10−4 mpy values in 14.85% CP solution. Theseobservations are different from those made in the present study,and this might be related to the different experimental condi-tions used. In fact, they obtained the electrolytes using differenttechniques and concentrations but never mentioned the pHvalues. In this study, the HP group exhibited 256.2 mV, 0.162μA/cm2, and 27.98 × 10−4 mpy; whereas the CP group ex-hibited 204 mV, 0.056 μA/cm2, and 4.68 × 10−4 mpy Ecorr,Icorr and corrosion rate values, respectively. Canay et al29 foundthat Ni-Cr alloy in 10% carbamide peroxide solution displayed300 mV and 5.2 μA/cm2 Ecorr and Icorr values, respectively.Although they used the same pH and the same solution prepa-ration technique as in the present study, the compositions of thetested alloys were different.

In our previous study,15 as opposed to the current work, weonly performed a 30-day immersion test. Then total mass andindividual elements (Ni, Cr, and Mo), which were released into



Figure 5 Representative XPS depth profile analysis of oxide film formed on the surface of HP- treated group.

solution (CP and HP), were measured by means of atomic ab-sorption spectroscopy (AAS). Immersion study determines themass per unit area. The results of our previous work15 demon-strated that the total amounts of released elements were 171.2μg/cm2 and 59.7 μg/cm2 exposed to HP and CP, respectively.Within this period, total mass released was only 4.9 μg/cm2 forthe control (nonbleached) group. For all tested solutions, theindividual element releasing at the highest level was Ni, and thelowest was Cr. Although tested alloy and bleaching solutionswere the same, in the present study, we performed disparatemeasurements. This article is a new collaborative study thatattempted to enhance and support our previous findings. Theresults of electrochemical measurements have indicated the cor-rosion potentials and the passage of current through the surface,which resulted in corrosion, and corrosion rate. Corrosion rate,specified in mpy, shows wear as a function of time (per year).Thereby, it allows estimation of the thickness of the annualabrasion as distinct from the previous work.15 In this study,the corrosion rate findings (27.98 × 10−4 mpy for HP and4.68 × 10−4 mpy for CP) also confirmed the destructive effectsof bleaching agents.

Bleaching products are in contact with intraoral structures forprolonged periods in a bleaching tray. Therefore, the productsshould possess a relatively neutral pH to reduce the probabledamage that could be derived from particularly acidic agentsas well as quite basic ones. Currently available products for themouthguard bleaching technique, with 10% and higher concen-trations of HP and CP, have a pH close to neutral.17 CP decom-poses intraorally to urea, ammonia, carbonic acid, and hydrogenperoxide at a low concentration. These products have been re-ported to be less acidic due to the presence of ammonia andcarbon dioxide, both byproducts of the urea breakdown cycle.

Table 3 Results of XPS chemical analysis of surface passive film be-fore and after elemental release measurements, showing the compoundstructure

NiO Cr2O3 MoO3 SiO2 Nb2O5 CeO2 Fe2O3

Untreated + + + − − − −HP + + + + + + +CP + + + + + + +

These more stable and alkaline pH conditions (approximately6.5) are compatible with oral tissues.29 Price et al30 measuredthe pH of 26 commercially available tooth-whitening productsand reported that the pH ranged from 3.67 (highly acidic) to11.13 (highly basic). They emphasized that dentist-supervisedhome bleaching products had a mean pH of 6.48 (range 5.66 to7.35), while the pH of the over-the-counter products could behighly acidic or highly basic. Therefore, a pH of 6.5 was cho-sen in the current study by ignoring over-the-counter products.It should be noted that different results may be achieved withdifferent pH solutions.

Corrosion behavior is complex and includes many param-eters like composition and the surface treatment of alloys,pH of the oral cavity, presence of proteins, masticatory func-tion, saliva, and its characteristics.2,3,8,10,31 For corrosion test-ing in vitro, synthetic environments are used more frequentlythan natural saliva (due to easy access to the artificial salivasolutions). These solutions may also possess a higher degreeof stability compared to natural saliva. In addition to the dif-ferent compositions between individuals, pH of saliva can alsovary. Many factors such as food, oral hygiene products, age,



Table 4 XPS depth profile analysis of the surface oxide layer beforeand after elemental release measurements, showing the compositioncontent in atomic percent (at%)

Ni2p Cr2p Mo3d Si2p O1S Nb3d Ce3d Fe2p

Untreated 33.7 7.2 1.9 0 26.4 − − −HP 6.4 2.8 2.2 7.5 23.05 0.4 2.2 0.02CP 0.2 1.4 5.8 6.2 62.3 0.04 0.1 0.2

periodontal health, oral habits like smoking, and systemic dis-eases could influence the pH and composition of saliva.32-34 Tosee the effects of the pure bleaching agents, in this study, HP andCP were used, whereas in clinical conditions, bleaching agentsmerge with saliva and are probably diluted by it. It would bebetter to perform corrosion analysis in natural and/or artificialsaliva to make comparisons between bleached and nonbleachedconditions. Corrosion behavior of Ni-Cr alloys in different so-lutions such as artificial saliva, NaCl, and saline solutions arewell documented. Therefore, a nonbleached group analysis interms of corrosion has not been implemented. The presenceof saliva could change the results. Corrosion behavior analysisin artificial saliva, natural saliva, HP, and CP solutions mixedwith artificial or natural saliva will be the subject of our futurestudies.

The 10% CP solutions have a 3% to 3.5% equivalent concen-tration of HP, an effective yet safe level for tooth whitening thatis more diluted than the potent 10% HP concentrations.17,32 Itmeans that both HP and CP bleaching agents have the sameactive agent but different concentrations. For these reasons,CP bleaching has been suggested as a safer alternative to theharsher HP-based systems.12,21 This study shows a differencein the Ecorr and Icorr values between HP and CP agents, but withno significant difference at the 95% confidence level. Althoughno statistical difference was found, it was shown that increasingperoxide concentration led to an increase in Icorr and corrosionrate values (Table 2). In accordance with the proportion of ac-tive agents between the HP and CP solutions, the Icorr valuefor the HP group was approximately three times as much com-pared to the CP group. The specimens tested in the CP solutionyield the weakest corrosion current and the best corrosion re-sistance. These results also agreed with the SEM observations.This behavior could be attributed to the high electron density onnitrogen and oxygen active centers of CP that absorbed on themetallic surface, forming a protective layer for the corrosion.28

Interestingly, although 30 days exposure time was applied inour previous work,15 a distinctly slight surface erosion can beenseen in the SEM observations compared to current ones. Thiscould be attributed to the thickness of surface wear defined fora year following electrochemical testing.

XPS data showed that both HP and CP solutions resulted inan oxide film on the surface of specimens. The layers formed onall groups contained mainly Cr2O3, NiO, and MoO3 (Table 3).Bleaching applications caused a thicker oxide layer comparedto the untreated group. Since the O concentrations were ap-proximately the same, the amount of oxides formed on bothHP and untreated specimens were similar, but the composi-tion of the oxides was somehow different (Tables 3, 4). Higher

concentration of oxygen on the surface of the CP group maybe attributed to the formation of a higher amount of surfaceoxide films. For the HP group, Cr and Ni levels were higherthan the CP-treated specimen. In addition, a significant amountof Ce (at 2.227%), which can be easily connected to O, wasalso observed. Except for the trace elements, only the Mo levelwas increased with CP compared to HP treatments. Since thetested alloy and bleaching agents were identical to our previouswork,15 it is not surprising to obtain similar results in terms ofthe compound structures of the passive film. But the contentratios are a good bit different, showing that surface interactionsdiffered according to test and exposure time. XPS data alsoagreed with the cyp and SEM results. Although surface oxidefilm formed on all groups, the results of this study may indi-cate the instability of the passive film that formed after highlyconcentrated HP (10%) treatment. HP may have acceleratedthe degradation of the alloy surface by affecting the protectivesurface film. Finally, the results also showed that the presenceof especially Mo on the oxide layer as well as Cr improved thegrowth of the oxide layer, leading to a better corrosion behav-ior. From our XPS results, it appears that both CP and HP cancause changes in surface concentration levels of the elements.Such changes may be caused by enhanced oxidation, corrosion,and dissolution of the alloy surfaces that take place during theexposure to the HP and other decomposition byproducts of thebleaching agents. Further studies with higher concentrations ofbleaching agents, different pH values, different alloy types, dif-ferent commercial bleaching products, and different solutions,such as peroxides mixed with artificial saliva or natural salivaare necessary for better understanding of this matter.

Conclusions

The following conclusions can be drawn from our study:

1. Bleaching agents had an effect on the anodic process forthe tested alloy. The comparison of the effects of the twobleaching agents at 10% showed that the alloy sufferedless corrosion rates in the presence of CP than HP. Thisresult was also confirmed by the SEM and XPS data.

2. XPS revealed that an HP solution resulted in a strongercorrosion and modification of the composition of thealloy surface.

3. Mo positively contributes to formation of the surfaceoxide film as well as Cr.

Acknowledgments

X-ray Photoelectron (XPS) measurements were done by METUCentral Laboratory. The authors are grateful to Dilsen Tamamfor statistical analysis.

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Electrochemical Corrosion and Surface Analyses of a Ni-Cr Alloy in Bleaching Agents