degradation characteristics of poly-tetrafluoroethylene ... madalina 4 16.pdf · degradation...

http://www.revistadechimie.ro REV.CHIM.(Bucharest)♦ 67♦ No. 4 ♦ 2016800

Degradation Characteristics of Poly-tetrafluoroethyleneCoatings on Stainless Steel Orthodontic Wires Immersed

in Tuna Fish Derived Products

MADALINA NICOLETA MATEI1, KAMEL EARAR1*, LUCIA CARMEN TRINCA2*, DANIEL MARECI3, LENUTA FOTEA2,CATALINA ANISOARA PEPTU3, CRISTINA BICA4

1 Dunarea de Jos University of Galati, Medicine and Pharmacy Faculty, Department of Dentistry, 47 Domneasca Str., 800008, Galati,Romania2 Ion Ionescu de la Brad University of Agricultural Sciences and Veterinary Medicine, Faculty of Horticulture, 3 Mihail SadoveanuAlley, 700490, Iasi, Romania3 Gheorghe Asachi Technical University of Iasi, Faculty of Chemical Engineering and Environmental Protection, 73 D. MangeronBlvd., 700050, Iasi, Romania4 University of Medicine and Pharmacy of Tirgu Mures, Faculty of Dentistry, 38 Gheorghe Marinescu Str., Tirgu Mures, Romania

This paper objective was to evaluate the degradation characteristics of polytetrafluoroethylene (PTFE)coatings on stainless steel orthodontic wires with memory shape while immersed in tuna fish derivedproducts. EIS was the major investigative technique, since it has the potential to discover new informationon the processes occurring, while not interfering significantly with the mechanism operating. The spectra ofuncoated and coated orthodontics stainless steel wires were recorded, and the data were analyzed in orderto evaluate the equivalent circuit (EC) parameters and subsequently the degradation characteristics of eachmaterial. The corresponding corrosion morphologies after polarization test was also observed using scanningelectron microscopy (SEM). Comparison of the electrochemical behaviour, in the condition of exposure toaggressive media, for stainless steel orthodontic wires with memory shape, coated with a PTFE porouslayer versus the uncoated alloy sample was done. The coatings stainless steel orthodontic wires samplesexhibited more positive zero current potential, higher breakdown potential, and lower anodic current densitiesthan the uncoated ones. Impedance spectra were interpreted in terms of a duplex film, with corrosionresistance arising mainly due to the thin inner layer as the outer layer being more porous was less sealed.Pseudo-capacitive behaviour and high corrosion resistance were registered for the coated stainless steelalloy, in the medium to low frequency ranges. This paper results are valuable for estimation of coated anduncoated stainless steel orthodontic wires corrosion resistance and for anticipation of their effect uponbiochemical events in oral cavity.

Keywords: stainless steel dental alloy, memory shape, polytetrafluoroethylene coatings, tuna fish products, EIS, SEM

* email: [email protected]; [email protected]; Tel.: 0040745320134

Metal alloys are the main materials of orthodontics wiresand arch [1]. Formerly, the alloys used in orthodontics weregold-based alloys. Their high costs and low mechanicalproperties determined their replacement with moreeconomical materials like NiTi alloys and stainless steel.In the actual socio-economical conditions, the dentalphysician should select with discernment from the richoffer of alternative orthodontics materials existing currentlyon the market.

Currently, orthodontic materials with the inner ability oftransforming between the two phases (austenite/martensite) begin to be of interest for clinical testing, themore so as, the use of metal alloys with shape memoryproperty may reduce the time of treatment in daily practice.

Stainless steels are substantially used in many countriesas main materials of orthodontic wires and arches. Amongthem, AISI 316L stainless steels presented, in vitro, thesame corrosion behaviour after immersion inelectrochemical media that simulates the inner humanbody physiological state versus pathological one [2].However, the performance of an orthodontic wire isconditioned by its corrosion behavior in the oralenvironment, an aggressive and complex electrolyticmedium [3]. Ion leaching and corrosion processes trigger

the releasing of the material’s different componentparticles, depending on solubility, pH, wear induced bybiomechanical forces, electrochemical processes at theinterface and outer factors. The investigations focusingcorrosion and using different in vitro models may contributeto the selection of an alloy with enhanced properties [4-15], or a special coated layer as coating is one of the mostwidely used method for reducing biomaterials corrosion.The electrochemical techniques for evaluation of corrosionresistance are rapid, reproducible, reliable, and also provideinformation about corrosion mechanism involved [22-24].

The increased consumption of refined products derivedof fruits, vegetables, milk, meat and fish [16-20], raisedthe prevalence of orthodontic wires corrosion in the mouth,especially in the last years. However, the paucity of reportedstudies on the corrosion behaviors of orthodontic wires inrefined products, explains the limited information regardingthe corrosion behavior of stainless steel orthodontic wiresin fish derived products [21].

This work presents the electrochemical study ofpolytetrafluoroethylene (PTFE) coatings on stainless steelsorthodontic wires in contact with derived fish products.The electrochemical behaviour of each stainless steelorthodontic wire was evaluated by potentiodynamic

REV.CHIM.(Bucharest)♦ 67♦ No. 4 ♦ 2016 http://www.revistadechimie.ro 801

polarization and electrochemical impedance spectroscopy(EIS) after different immersion time in derived fishproducts.

EIS was the major investigative technique, since it hasthe potential to discover new information on the processesoccurring, while not interfering significantly with themechanism operating. The spectra of uncoated and coatedorthodontics stainless steel wires were recorded, and thedata were analyzed in order to evaluate the equivalentcircuit (EC) parameters and subsequently the degradationcharacteristics of each material. The correspondingcorrosion morphologies after polarization test was alsoobserved using scanning electron microscopy (SEM).

The obtained data are valuable for estimation of coatedand uncoated stainless steel orthodontic wires corrosionresistance and for anticipation of their effect uponbiochemical events in oral cavity.

Experimental partMaterials and methodsMaterials

Tow different uncoated and coated with PTFEorthodontics stainless steel, both manufactured byDentsply Gac International Inc. (Bohemia NY, USA), wereconsidered in this investigation. The chemical compositionshas been determined by EDX analysis using a scanningelectron microscope (SEM) Quanta 3D Model AL99/D8229(FEI, Hillsboro, OR, USA) operating with beam energy 30kV. The chemical compositions of uncoated and coatedwith PTFE orthodontics stainless steel are presented infigures 1-2.

Differential scanning calorimetry (DSC)The stainless steel orthodontic wires were examined

with DSC technique in order to determine theaccompanying transformations with temperature.

The sample was placed in an aluminum crucible, andsealed. An empty aluminum crucible served as a referenceduring the DSC measurements with DSC Model 1MettlerToledo. Nitrogen gas (120 mL/min) was used in order toprevent condensation of water vapors and oxidation of

stainless steel orthodontic wires. The temperature of thecrucible was scanned from -40 to + 50 oC at 15 oC/min.

Electrochemical mediaElectrochemical tests were performed in two different

media: the first medium used was the fish paste obtainedby mixing (with a blender) the content of a tuna filet insunflower oil can, that consisted in sliced tuna filet insunflower oil, having 185g net weight, 140 drained weight,and containing Skipjack tuna (Katsuwonus Pelamis),sunflower oil and salt as ingredients. The pH of the pasteobtained from tuna filet in sunflower oil was 5.79.

The second medium, was the paste resulted by mixing(with a blender) the content of tuna filet in brine can,consisting in sliced Skipjack tuna steak (KatsuwonusPelamis) in brine, having 195g net weight, 150 drainedweight, and containing tuna steak, water and salt asingredients. The pH of the paste obtained from tuna filet inbrine was 5.83.

Both types of tuna filet cans (Nixe®, Ecuador) wereobtained from supermarket.

Biochemical freshness and proximate compositionparameters were determined in order to evaluate themain variations of both electrochemical mediumscontaining the tuna fish paste (in sunflower oil / in brine),during 8 h storage at room temperature.

The pH was determined on homogeneous solution ofsample and distilled water (1:10, w:v), using a InoLab 730pH metre. Also, moisture, protein, colagen, lipid and saltscontents, as the main proximate composition parameterswere determined by NIRS Near Infrared SpectrophotometerFood Check Bruins.

Electrochemical measurementsA three-electrode corrosion flow cell kit (C145/170,

Radiometer, France) with platinum as counter electrodeand saturated calomel reference electrode (SCE) asreference electrode was employed for both electro-chemical measurements. The electrochemical cellcontained a freely adjustable Luggin capillary that housedthe reference electrode. The sample surface of the

Fig. 2. EDX spectrameasured at the

surface of the uncoatedorthodontics stainless

steel

Fig. 1. EDX spectrameasured at the

surface of the coatedorthodontics stainless

steel


uncoated and coated stainless steel orthodontic wiresexposed to the electrolyte testing was 0.05 cm2 . Allpotentials in this paper are referred to the SCE. Allelectrochemical measurements were carried out on aPrinceton Applied Research potentiostat Model PARSTAT4000 (Princeton Applied Research, Princeton, NJ, USA). Theinstrument was controlled by a personal computer andspecific software (Versa Studio, PAR, Princeton, NJ, USA).

Electrochemical impedance spectroscopy (EIS)measurements were also performed using the sameinstrumentation. The perturbation amplitude was 10 mVand the frequency ranged down from100 kHz to 10 mHz.Five points were recorded for each frequency decade. TheEIS spectra were obtained at different times: 1 hour, 2 h ,and 8 h after electrode immersion in tuna filet paste insunflower oil, and in brine. The EIS experimental data wereanalyzed in terms of equivalent circuits (EC) usingZSimpWin 3.22 software [25]. Measurement ofpotentiodynamic polarization curves (PPC) was initiatedafter 8 hours exposure to the test.

Scanning electron microscopy (SEM) of corroded surfacesIn order to observe the surface morphology the

polarization tested samples have been analyzed byscanning electron microscopy (SEM) using a HITACHI-SU1510 microscope.

Results and discussionsDSC analysis of the uncoated and coated stainless steelorthodontic wires

The DSC heating and cooling curves of the uncoatedand coated stainless steel orthodontic wires are shown infigures 3 and 4, respectively. The endothermic peaksappear to be constant at 11-12 oC. Such type of endothermicpeaks are attributed to the transformation of martensite toaustenite due to presence of alloy intermetallic phases.The endothermic peak from the heating curve of theuncoated and coated stainless steel orthodontic wires isthe austenite corresponding transformation temperaturewhile the exothermic peak from the cooling curves isconsidered as the corresponding martensite transformationtemperature.

Electrochemical analysisWhen an alloy is placed in the oral cavity environment,

it takes place an electrochemical interaction (i.e., corrosionprocess).

The experimental impedance data measured at theopen-circuit potential of both the uncoated and coatedstainless steel orthodontic wires immersed for differentperiods of time in both type of derived tuna fish products,at 37 oC are presented as Nyquist diagrams in figures 5-6.

Nyquist plots showed that the impedance of thematerials decreased with the elapse of time, more

Fig. 4. The DSC thermogram of the coated stainless steelorthodontic wires

Fig. 3. The DSC thermogram of the uncoated stainless steelorthodontic wires

Fig. 5. Nyquist plots of the impedance spectra measured for:(a) coated and (b) uncoated stainless steel orthodontic wires as

a function of immersion time in the paste of tuna filet insunflower oil


pronounced for the uncoated material. All the spectraexhibit a capacitive loop (open arc). The diameter of theopen arc provided an estimation of the polarizationresistance. The decrease in diameter indicated a reductionof the corrosion resistance.

Bode plots of the two materials immersed for differentperiods of time in fish products are shown in figures 7-8.

The advantage of the Bode plot is that the data for allmeasured frequencies are shown and also a wide range ofimpedance values are displayed.

The frequency dependence of the phase angle indicatesthe number of time constants presented in the system andcan be used to determine the parameters values in theequivalent circuit. Two peaks were observed in the Bode-phase plots determined for the coated stainless steelorthodontic wires, whereas only one was observed for theuncoated stainless steel orthodontic wires. This indicatesthe involvement of two time constants in the open circuitpotential for the coated stainless steel orthodontic wiresimmersed in both type fish products. The rather highimpedance values (of the order of 104 Ω cm2), registeredin the medium to low frequencies domaine, revealed thepresence of the corrosion resistance of the coated stainlesssteel orthodontic wires in the fish products used aselectrochemical medium.

The impedance modulus (|Z|) of stainless steelorthodontic wires slowly decreased with the time of sampleimmersion. All the spectra showed that in the higherfrequency region, log |Z| tends to become constant. Thisis a typical response for the resistive behavior andcorresponds to the solution resistance, Rsol. In the mediumfrequency range, a linear relationship between log |Z| andlog f was observed in all cases, though with different slopes(always less than –1), whereas the maxima in the phase

Fig. 7. Bode plots of the impedance spectra measured for:(a) coated and (b) uncoated stainless steel orthodontic wires as a

function of immersion time in the paste of tuna filet in sunflower oil

Fig. 6. Nyquist plots of the impedance spectra measured for: (a)coated and (b) uncoated stainless steel orthodontic wires as afunction of immersion time in the paste of tuna filet in brine

Fig. 8. Bode plots of the impedance spectra measured for: (a)coated and (b) uncoated stainless steel orthodontic wires as afunction of immersion time in the paste of tuna filet in brine


angle plots are smaller than –90o, indicating that the bothstainless steel orthodontic wires were not completelycapacitive.

For the interpretation of the electrochemical behaviorof a system from EIS spectra, an appropriate physical modelof the electrochemical reactions occurring on theelectrodes is necessary. The electrochemical system maybe represented by an equivalent circuit (EC). In the case ofthe uncoated stainless steel orthodontic wires, the spectracould be satisfactorily simulated using the simplified ECcircuit shown in figure 9a. The occurrence of a secondoverlapping wave in the phase shift response of thestainless steel orthodontic wires indicated that the spectracannot be explained by the simple equivalent circuit basedon a single parallel combination of a resistance and aconstant phase element, as shown in figure 9a. Therefore,fitting of the impedance was done with the EC depicted infigure 9b using a series combination of the solutionresistance, Rsol (75±10 W cm2), with two RQ parallelcombinations: Rsol (R1Q1) (R2Q2). The proposed EC give theelectrical representation of two-layer surface filmsconsisting of a barrier-type inner layer and a relatively porouscoated outer layer [26 – 28].

Very good agreement between the simulated andexperimental data was obtained. The values of the

Fig. 9. Equivalent circuits used for fitting themeasured impedance spectra

Table 1ELECTROCHEMICAL PARAMETERS RESULTED FROM EIS SPECTRA BY USING

THE SELECTED EC FOR THE PTFE COATED STAINLESS STEEL ORTHODONTICWIRES AFTER DIFFERENT IMMERSION TIME IN THE PASTE OF TUNA FILET IN

SUNFLOWER OIL AT OPEN CIRCUIT POTENTIAL

Table 2ELECTROCHEMICAL

PARAMETERS RESULTEDFROM EIS SPECTRA USING

THE SELECTED EC FOR THEUNCOATED STAINLESSSTEEL ORTHODONTIC

WIRES AFTER DIFFERENTIMMERSION TIME

IN THE PASTE OF TUNAFILET IN BRINE AT OPEN

CIRCUIT POTENTIALimpedance parameters determined from the fits arepresented in tables 1 and 2 for the uncoated and coatedstainless steel orthodontic wires, respectively.

For the uncoated and coated stainless steel orthodonticwires, the parameters R1 and Q1 account for the reactionsat the stainless steel orthodontic wires layer/solutioninterface and determine the impedance behavior in thehigh frequency range of the spectrum. The constant phaseelement Q1 represents the double layer pseudo-capacitance of the stainless steel layer, as shown by thehigh value of the n1 exponent [29].

Corrosion may occur in pores as the result of the directlymetal expose to the aggressive attack of the electrolyte.Pores in the coating layer may act as paths for theelectrolyte attack to the substrate.

Therefore for coated stainless steel orthodontic wires,the parameters R2 and Q2 describe the properties of thePTFE layer.

As immersion time increases from 1 hour to 8 h , theresistance (R2) of the PTFE coated layer decreases slowly.However, the values of R2 are about 10 times bigger thanthose of R1 for all time exposures, which revealing thatPTFE coated layer provides most of the corrosion protectionto the stainless steel orthodontic wires. Therefore, theperformance of the PTFE coated stainless steel orthodontic

Table 3pH AND PROXIMATE

COMPOSITION VALUESOF THE PASTE FROM TUNAFISH MEAT CANS SAMPLES

DURING 8 HOURS STORAGE


wires in the both fish products was superior to that of theuncoated material.

Therefore, both type (coated and uncoated) of stainlesssteel orthodontic wires presented higher corrosionresistance values in the paste of tuna filet in sunflower oil.In the case of fish product containing tuna meat andsunflower oil, the formation (hypothetically) of a fullycovered oil film would prevent corrosion [30-31] of thestainless steel surface by separating the water phase, saltand the others main biochemical compounds of the tunafish meat (table 3) Practically, when the paste of tuna fishin oil was prepared by well blending the can’s content, thewater phase was embedded into the oil phase, and theresulted stable emulsion, that showed non significantchanges of the proximate composition during 8 hoursstorage (table 3) exerted an inhibitory effect on thecorrosion process [32-33].

In the case of the paste of tuna filet in brine, the smallercorrosion resistance values registered for both type ofstainless steel orthodontic wires can be explained by theenhanced corrosive effect of sodium chloride [34] and citricacid from the brine aqueous solution [30].

Semi-logarithmic plots between -1000 mV and 1000mV vs. SCE of the uncoated stainless steel wires in bothtunafish derived products are displayed in figures 10-11.

The zero current potential (ZCP) and corrosion current(jcorr) values were determined by Tafel analysis of both theanodic and cathodic branches of the polarization plots. TheZCP is defined as the potential at which the current reachesa minimum during the forward potentiodynamic

Fig. 10. Potentiodynamic polarization curves for coated anduncoated stainless steel orthodontic wires in the paste of tuna filet

in sunflower oil

Fig. 11. Potentiodynamic polarization curves for coated anduncoated stainless steel orthodontic wires in the paste of tuna filet

in brine

polarization scan. As a result of the coated samples, thezero-current potential of the uncoated stainless steelorthodontic wires was shifted towards a more noble valuein both paste of tuna filet products.

The corrosion current is representative for thedegradation degree of each sample. The average values ofZCP and jcorr determined from the polarization curves arepresented in tables 4 and 5.

Corrosion may occur in pores as the result of the metalbeing directly exposed to the aggressive attack of theelectrolyte. Data from figures 10-11 reveals an evidentdecrease of the anodic current for the coated stainlesssteel orthodontic wires as compared with the uncoatedones. This fact proves that the coated sample has a greatercorrosion resistance compared to uncoated sample, whichdetermine the correspondently smaller measured valuesof the corrosion current densities. Low corrosion currentdensity values were obtained from the potentiodynamicpolarization curves of investigated samples in the paste ofthe tuna filet in sunflower oil. These results are in agreementwith EIS data.

The susceptibility of an alloy to pitting corrosion in acertain medium [35] can be characterized in terms of thebreakdown potential (Ebd) relative to the zero-currentpotential value (ZCP). The potential range situatedbetween ZCP and Ebd represents the passivity zone in whichthe corrosion rate is low or even insignificant. As thedifference between Ebd and ZCP becomes smaller, the alloyis expected to become more susceptible to pitting

Table 4THE MEAN VALUES OFELECTROCHEMICAL

PARAMETERS MEASUREDAND CALCULATED

FOR THE PASTE OF TUNAFILET IN SUNFLOWER OIL

(37 OC)

Table 5THE MEAN VALUES OFELECTROCHEMICAL

PARAMETERS MEASUREDAND CALCULATED

FOR THE SAMPLES IN TUNAFILET IN BRINE (37 OC)


Fig. 12. Surface attack morphologyafter potentiodynamic polarization of

the uncoated stainless steelorthodontic wires in the paste of tuna

filet : (a) in brine, and (b) insunflower oil

Fig. 13. Surface attack morphologyafter potentiodynamic polarizationfor the coated PTFE stainless steelorthodontic wires in the paste oftuna filet: (a) in brine, and (b) in

sunflower oil

corrosion. Also, the area of hysteresis loop (Qhysteresis) reflectsthe susceptibility to localized corrosion.

The analysis of the potentiodynamic polarization curvesproved that the uncoated stainless steel alloy exhibited amore area of hysteresis loop and a more negativebreakdown potential value than coated stainless steelorthodontic wires. These results indicating that uncoatedstainless steel orthodontic wires exhibited a greatersusceptibility to localized corrosion, are consistent also withthe conclusions derived from Qhysteresis presented above.

A comparison with other published is not applicable inthis case, since the corrosion and anodic current densitiesof stainless steel orthodontic wires depend on potentialscanning rate, surface preparation or exposure time.However, our results sustain the recommendation ofcaution against consumption of products derived fromtuna filet in brine for patients with stainless steel orthodonticwires.

The surface morphologies of coated and uncoatedstainless steel orthodontic wires polarized at +1 V in thepaste of tuna filet in sunflower oil and in brine wereexamined by scanning electrochemical microscopy (SEM).

The analysis of figures 12-13 indicated the appearanceof localized corrosion at the surface of coated and uncoatedstainless steel orthodontic wires. The localized corrosionat surface of both coated and uncoated stainless steelorthodontic wires was evident, even the morphology ofthe electrochemically polarization tests was extremelyirregular.

ConclusionsLow corrosion current densities of the potentiodynamic

polarization curves for the coated stainless steelorthodontic wires showed more positive zero current (ZCP)and breakdown (Ebd) potentials than the uncoated stainlesssteel orthodontic wires. However, the PTFE protective layersfrom the surface of stainless steel orthodontic wires areprone to localized breakdown in both tuna filet products ascharacterized by the measurement of pitting corrosionpotential chloride-containing solutions. The EIS spectra ofthe coated stainless steel samples in the paste of tuna filetexhibited two time constants: the first accounting for thecharacteristic reactions at the stainless steel layer/solution

interfaces, and the second corresponding to the PTFEcoated layer . The experimental potentiodynamicpolarization curves, EIS diagrams, and the equivalent circuitparameters showed that the coated stainless steelorthodontic wires in both tuna filet products presented goodelectrochemical corrosion resistance .

Acknowledgements: This work was supported by the RomanianNational Authority for Scientific Research, CNCS-UEFISCDI (projectnumber PN-II-IDPCE-2011-3-0218).

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Manuscript received: 3.10.2015

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