Anodized titanium and stainless steel in contact with CFRP: An electrochemical approach considering...

Anodized titanium and stainless steel in contactwith CFRP: An electrochemical approach consideringgalvanic corrosion

Yves Mueller,1 Roger Tognini,2 Joerg Mayer,3 Sannakaisa Virtanen4

1Laboratory for Corrosion and Materials Integrity, Empa Swiss Federal Laboratories for Materials Testing and Research,Duebendorf, Switzerland2Icotec AG, Altstaetten, Switzerland3TECIM Technologies for Implants and Materials, Niederlenz, Switzerland4Chair for Surface Science and Corrosion, Department of Materials Science and Engineering,Friedrich-Alexander University, Erlangen, Germany

Received 7 September 2006; revised 17 November 2006; accepted 29 November 2006Published online 2 March 2007 in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jbm.a.31198

Abstract: The combination of different materials in animplant gives the opportunity to better fulfill the require-ments that are needed to improve the healing process. How-ever, using different materials increases the risk of galvaniccoupling corrosion. In this study, coupling effects of gold-anodized titanium, stainless steel for biomedical applica-tions, carbon fiber reinforced polyetheretherketone (CFRP),and CFRP containing tantalum fibers are investigated elec-trochemically and by long-term immersion experiments insimulated body fluid (SBF). Potentiodynamic polarizationexperiments (i/E curves) and electrochemical impedancespectroscopy (EIS) of the separated materials showed a pas-sive behavior of the metallic samples. Anodized titaniumshowed no corrosion attacks, whereas stainless steel ishighly susceptibility for localized corrosion. On the otherside, an active dissolution behavior of both of the CFRPs inthe given environment could be determined, leading to

delaminating of the carbon fibers from the matrix. Long-term immersion experiments were carried out using a set-up especially developed to simulate coupling conditions ofa point contact fixator system (PC-Fix) in a biological envi-ronment. Electrochemical data were acquired in situ duringthe whole immersion time. The results of the immersionexperiments correlate with the findings of the electrochemi-cal investigation. Localized corrosion attacks were found onstainless steel, whereas anodized titanium showed no corro-sion attacks. No significant differences between the twoCFRP types could be found. Galvanic coupling corrosion incombination with crevice conditions and possible corrosionmechanisms are discussed. � 2007 Wiley Periodicals, Inc.J Biomed Mater Res 82A: 936–946, 2007

Key words: galvanic coupling; corrosion; electrochemicalimpedance; potentiodynamic polarization; implant materials

INTRODUCTION

Biomedical implants have to fulfill many require-ments in order to be functional. Besides biocompati-bility in the sense of biochemical acceptance of animplant, compatibility to the biological structure isneeded, as well.1 To achieve these requirements, of-ten at least two different types of materials have tobe combined. However, joining two materials withdifferent electrochemical potentials in a biologicalenvironment can lead to coupling and/or frettingcorrosion and, in the worst case, subsequently to a

defensive reaction of the biological system. This ispossible even in cases, when the two materials them-selves are biocompatible.2

Nowadays, because of their biocompatibility3,4

and increased structure compatibility,5–7 carbon fiberreinforced polyetheretherketone (CFRP) has beenconsidered as a promising candidate for bone platesto stabilize a fracture8 or even bone replacements.9

These bone plates have often been fixed with bonescrews of stainless steel because screws of carbonfiber composites are feared not to meet the mechani-cal requirements. However, after some time ofimmersion, corrosion attack has been observed at thescrew head on the contact area of plate and stainlesssteel screw.10 In this system, galvanic coupling corro-sion as well as crevice corrosion are responsible forthe damage.11 But also a mechanical aspect has to beconsidered. Because of micromotions between the

Correspondence to: Y. Mueller; e-mail: [email protected]; [email protected] grant sponsors: Swiss Innovation Promotion

Agency (CTI) and Icotec AG, Switzerland

' 2007 Wiley Periodicals, Inc.

coupled partners, the corrosion attack is increased.Even if repassivation takes place, the continuousactivation of the surface due to the friction leads to acontinuous attack of the surface and therefore to thedamage of the surface by fretting corrosion.

Crevices between bone plate and bone screw cannever be prevented. Therefore, two main problemsremain: potential of fretting corrosion and the elec-trochemistry of the two coupled materials. To de-crease the risk for fretting corrosion, an internal fixa-tion system with interlocked angular-stable screws(PC-Fix) has been developed.12 The fixation betweenthe screw head and the plate hole is made by a con-ing thread and in this way the relative motion of thetwo coupled partners is considerably reduced whencompared with the standard design being a sphericalscrew head in an ovaloide plate hole, e.g. in the lim-ited contact dynamic compression plate (LC-DCP)developed by the AO foundation.

To further reduce the risk for corrosive attack ofone of the components, careful selection of material’scombinations is needed. One possibility is the use ofanodized titanium screws. Apart from the fact thattitanium is more corrosion resistant in a chloridecontaining environment than stainless steel,13 ano-dized titanium shows very good biocompatibility.Furthermore, it has been shown that a combinationof the titanium alloy (Ti6Al4V) with glassy carbonand with carbon fiber reinforced carbon is more cor-rosion resistant than a combination of the carbonmaterials with stainless steel.14

In this investigation, standard electrochemicalmeasurement methods were used to determine theelectrochemical behavior of various material, i.e.stainless steel, cp-titanium, CFRP, and CFRP with asmall amount of tantalum fibers added to enhanceX-ray contrast, in different plate/screw materialcombinations. A simulated biological environment atdifferent pH-values is examined. To follow the long-term behavior of the materials in contact, a measure-ment set-up was developed, which allowed an elec-trochemical data acquisition in situ during the entiretime of immersion in the simulated biological envi-ronment. The differences between stainless steel,anodized titanium and CFRP as screw materials willbe discussed.

EXPERIMENTAL

Electrochemical investigation

Four different materials were used: commercially goldanodized titanium (grade 4, cold aged), stainless steel(316L), carbon fiber reinforced polyetheretherketone(CFRP), and CFRP containing 0.5 vol % tantalum fibers foran improved X-ray contrast. The tantalum fibers with a di-ameter of 50 lm have been added during the pultrusionprocess. Both CFRP materials had a carbon fiber content of62 vol % and the carbon fibers have been continuous anduniaxially aligned to the rod axis. Processing and proper-ties of these materials have been described elsewhere.15

The received metallic samples were circular plates of athickness of 3 mm and a diameter of 12 mm. The carbonfiber samples were experimentally pultruded rods of5 mm in diameter (CFRP) or rods of 3 mm (CFRP with Ta)embedded in epoxy resin.

For stainless steel and CFRP, no specific surface treat-ment is applied. The surface state is therefore not a criticalissue. For anodized titanium, the quality of surface treat-ment might play a significant role. For this reason, differ-ent surface layer qualities and thicknesses around goldanodizing conditions have been investigated to take intoaccount the possible scattering present in production pro-cesses. The result of different conditions can be seen opti-cal by changes in the coloration intensity and are mainlydue to small changes of the anodizing voltage and time.For better traceability, the titanium samples as well as theirsides were marked and photographed for the identificationafter the measurements (see Fig. 1).

In a first step, the electrochemical behavior of each ma-terial was investigated by electrochemical impedance spec-troscopy (EIS) and potentiodynamic polarization experi-ments (i/E curves). The electrolytes used were simulatedbody fluid (SBF)16 adjusted to pH 2.0 (no buffer), 5.0 (buf-fered with sodiumacetate-3-hydrate) and 7.4 (bufferedwith tris-(hydroxymethyl)-aminomethane). SBF at pH 5.0represents an intermediate solution according to its pHvalue. The electrochemical investigation with that electro-lyte showed also an expected intermediate behavior with-out specifically interesting phenomena. Therefore, only thetwo solutions representing the pH limits are furtherdescribed in detail.

The used electrochemical cell offers the possibility toarrange a three electrode configuration with the sample asworking electrode (WE), a platinum wire as counter elec-trode (CE) and a saturated calomel electrode (SCE) as ref-

Figure 1. Different intensity of the coloration due to different oxide thicknesses.

ANODIZED TITANIUM AND STAINLESS STEEL IN CONTACT WITH CFRP 937

Journal of Biomedical Materials Research Part A DOI 10.1002/jbm.a

erence electrode (RE). The measured area on the WE was acircle of 1 cm2. In addition, a hot tube heated with anexternal water bath as well as a fritted glass tube for nitro-gen flow was used (see Fig. 2). Nitrogen was used to keepthe oxygen concentration at a low and constant level andto stir the solution. In this way, the anodic dissolutionreactions can be analyzed under well defined conditions.However, the cathodic oxygen reduction reaction isdecreased compared to real human body conditions andwill not be discussed together with the potentiodynamicpolarization experiments.

The EIS-spectra were performed at the equilibriumpotential, which has been reached after the electrolyte washeated up to (37 6 2)8C. The AC potential amplitudeapplied for EIS measurement was 10 mV and the fre-quency was varied from 10 mHz up to 100 kHz. Immedi-ately after the EIS measurements, potentiodynamic polar-ization experiments were performed. The potential sweepwas started at the equilibrium potential and headed eitherto the anodic or cathodic direction with a sweep rate of1 mV/s. The potential limits were set to þ2000 and �1000mV (SCE). On each metallic sample one side was meas-ured in anodic, the other in cathodic direction.

A copper plate was used to contact the samples. Beforefixing the samples to the electrochemical cell, the metalplates were cleaned with ethanol. The CFRP samples werepolished on one side down to a 4000 emery paper. Theother side was glued to the copper plate by contact glue(ELCOLIT1 350) for an optimal electrical contact.

Long-term immersion experiment

To analyze the corrosion behavior of two coupled part-ners during the immersion in an electrolyte, an in situ re-cording of the electrochemical data is important. Further, it

is necessary that the measurement method does notstrongly influence the electrochemical system. Therefore, asimple contact, with a copper wire for example, is not suita-ble. Since different electrochemical potentials of contactedmaterials would cause galvanic coupling corrosion in theworst case, not only the contact between the examined part-ners but also the contact between the partners and theirconnection to the measuring device would be recorded, ifthe whole system is immersed in an electrolyte.

To realize near practical conditions, the specimens usedwere medical bone plates and screws. The innovative aspectof this system is the fixation between the screw head andplate hole, where a coning thread prevents relative motionsbetween the coupled partners of the fixation system, alsoknown as PC-Fix.12 As bone plate materials, only the twoCFRP types were used, which leads to eight possible materialcombinations (Table I). Both, CFRP bone plates and CFRPbone screws have been manufactured by composite flowmolding, a proprietary processing of Icotec AG Switzerland.

The plates were cut into pieces, so that every part had ascrew hole in the middle. On one cut plane, an isolatedwire was fixed with contact glue since on the cut planesmore carbon fibers, which are responsible for the electricalconductivity, can be contacted. Then both cut planes weresealed with silicon (DOW CORNING1 732) to preventpossible reactions caused by the electrolyte. To contact thescrews, a hole was drilled from the bottom of the screwalong its rotation axis. In this hole an isolated wire wasfixed by contact glue and the whole part was sealed by sil-icon. To increase the amount of contacted carbon fibersthat ends in the CFRP screw heads, these screws were cutat halve of their length before drilling. Screws and plateswere fixed using a torque screwdriver to a final torque of1.5 Nm. The resistance measured between the two coupledpartners after fixation were beyond 10 O independent ofthe combination of materials. This shows that the continu-ous carbon fibers in the composite offer a good conductiv-ity through the whole structure. After that, the sampleswere fixed in a plastic tube and mounted in the measuringset-up presented in Fig. 3. Five samples of the same mate-rial combination were mounted in each container. The con-tainers themselves were put in a water bath and thermo-statically hold at (37 6 0.2)8C. The electrolyte (SBF at pH7.4) in the containers was stirred by magnetic stirrers. Toreduce the risk for bacterial growth, the SBF electrolytewas cleaned by sterile filtration. Furthermore, the electro-lyte was monthly removed and fresh electrolyte was filledin the containers.

Figure 2. Set-up of the i/E and EIS measurements.

TABLE ICoupled Partners for the Long-Term

Immersion Experiment

Combination No. Screw Material Plate Material

1 CFRP CFRP2 CFRP CFRP w. Ta3 CFRP w. Ta CFRP4 CFRP w. Ta CFRP w. Ta5 Stainless steel CFRP6 Stainless steel CFRP w. Ta7 Anodized titanium CFRP8 Anodized titanium CFRP w. Ta

938 MUELLER ET AL.


The potential of each contacted component was mea-sured periodically using a SCE reference electrode. Aslong as the coupled partners are in electrical contact, nodifference of the potential between screw and plate isexpected. However, if corrosion of the metal or degrada-tion of the carbon fibers occurs along the contact area, theelectrical contact can be reduced or even interrupted. As aresult, the electrical resistance between the coupled part-ners will increase and the potential of the screw and theplate may differ in the case of complete interrupt. There-fore, the resistance between the coupled partners wasdetermined using a LCR-meter. The described set-up allowto measure relevant corrosion data without influencing thesystem too much, since no current (potential measurement)or only a small AC-current (resistance) is applied.

RESULTS

i/E- and EIS measurements

Stainless steel

Typical i/E- and EIS-curves of stainless steel inSBF at pH 7.4 and 378C are shown in Fig. 4. Threecurves in anodic and cathodic direction are pre-sented. As it can be seen, the measurements are wellreproducible. The open circuit potential (OCP) ismeasured in the range between �50 and �100 mV(SCE). Pitting corrosion occurs at potentials aboveþ650 mV (SCE). The curves show a scattering of200 mV for the pitting potential. In the region beforepitting, current transients are observable indicatinglocal activation/passivation processes on the metalsurface. In the cathodic direction, a plateau exists ataround �500 mV (SCE) representing the diffusioncontrolled cathodic oxygen reduction reaction. The

further increase of the current to lower potentialscan be assigned to the Hþ reduction.

The EIS measurements show a phase shift ofaround �80 deg at 10 Hz. This value is nearly con-stant over the whole frequency range down to thelowest frequencies. These curves are typical for apassive surface and indicate good corrosion resist-ance. The highly reproducible measurements indi-cate a uniform surface.

Using SBF at pH 2.0 (Fig. 5), the corrosion poten-tial shifts to more positive potentials. According tothe Nernst’s law, the OCP changes with �59 mV/pH, hence an OCP shift of þ320 mV is expected.This is in good agreement with the experimentalfinding of a shift of þ350 mV. Furthermore, the pit-ting potential is slightly shifted in the negative direc-

Figure 3. Set-up of the long-term immersion experiment.

Figure 4. (a) i/E and (b) EIS curves of stainless steel inSBF pH 7.4 at 378C.



tion, and the scatter in the value of the pitting poten-tial is reduced in the acidified solution.

In Figure 5a, one cathodic polarization curve showsa much lower corrosion potential corresponding to anactive dissolution process. The same sample showslower impedance values in the EIS spectrum indicat-ing a lower polarization resistance (Fig. 5b). Further-more, the phase shift decreases significantly at lowerfrequencies. This behavior is mainly caused by diffu-sion phenomena due to dissolution reactions. Exam-ining the surface after the measurement with scan-ning electron microscope (SEM), damages of the sur-face such as scratches and small deformations couldbe detected (Fig. 6). Therefore, a former mechanicaldamage is a probable origin of the less noble natureof the surface of this sample. Such an effect cannot beneglected on real implant systems.

On the other hand, the SEM examination of thesurface of the stainless steel samples polarizedanodically showed small pits with corrosion prod-ucts (Fig. 7). Therefore, the electrochemical result ofpitting can be confirmed.

Anodized titanium

Figure 8 shows i/E- and EIS-curves of three ano-dized titanium samples in SBF at pH 7.4 and 378C.Clearly, the results significantly differ from sample tosample. A comparison of the sample coloration (Fig.1) and the experimental data shows a logical correla-tion: the darker the sample, indicating the presenceof a thicker oxide layer on the surface, the lower thepassive current density determined from potentiody-

Figure 5. (a) i/E and (b) EIS curves of stainless steel inSBF pH 2.0 at 378C.

Figure 6. SEM micrograph of surface damages on stain-less steel showing active behavior.

Figure 7. SEM micrograph showing pitting corrosion onstainless steel after potentiodynamic polarization in anodicdirection.

940 MUELLER ET AL.


namic polarization experiments as well as the higherthe polarization resistance determined from EIS-spec-tra. Nevertheless, even the highest passive currentdensities (&14 nA/cm2) are orders of magnitudelower than the comparable current densities of theinvestigated stainless steel (&6 lA/cm2).

Regarding the EIS spectra in Fig. 8b, the edge ofthe phase shift curve of sample Ti02, which has a vis-ually thicker anodic oxide layer, is moved to higherfrequencies indicating a higher dielectric constant ofthe anodic oxide layer. Another finding is that thesame sample show two time constants determined bytwo phase shift maxima representing a more complexand also more stable structure of the oxide layer. Fur-thermore, the values are higher over the whole fre-quency range. The oxide layer behaves more as a con-denser, which means a strongly suppressed chargetransfer through the layer. Corrosion processes

are therefore extremely inhibited. On the other hand,the two samples with thinner oxide layers have onlyone time constant and the values are lower. Togetherwith the lower values of the impedance curves,charge transfer through the oxide layer is less hin-dered. At low frequencies, the phase shift increasesagain indicating diffusion controlled processes.

The average OCP is found to be at þ50 mV (SCE)but the scattering domain is 200 mV. At a lower pHof the electrolyte, the scattering interval of the corro-sion potential even further increases. No pitting wasdetected in the investigated potential region inde-pendent of the electrolyte used, which is in agree-ment with findings by many others reporting thatpitting of titanium in chloride containing solutionsonly takes place at much higher voltages.17 Oxida-tion peaks were measured at potentials above 1500mV (SCE), which may be caused by oxidation ofspecies in the electrolyte (e.g., Cl�).

Carbon fiber reinforced polyetheretherketone (CFRP)

The i/E as well as the EIS measurements with bothtypes of CFRPs are well reproducible (Fig. 9). In theanodic direction, which represents corrosion pro-cesses, the curves follow the Tafel slope until reachingthe current density limit of the experiment. This indi-cates active dissolution processes. Also in the cathodicdirection the current densities follow the Tafel slope,but at lower potentials a current density plateau can beobserved indicating the diffusion controlled oxygenreduction in the electrolyte. The differences betweensamples with and without tantalum fibers are a noisiersignal as well as a smaller mean current density in theplateau region on samples with tantalum fibers.

The EIS spectra show small differences betweenthe single measurements but are comparable to thereproducibility of the measurements with stainlesssteel. Additional to possible heterogeneities of thematerial, the variations can also be the result of dif-ferent contact quality between the sample and thecopper plate by the silver glue.

The phase shift curve of both CFRP types show sig-nificant differences compared to the ones of the metal-lic samples. The values are always lower compared tothe metallic samples. Therefore, charge transfer is lesshindered. Furthermore, the phase shift value isdecreasing from 100 Hz downwards to lower frequen-cies. This behavior can be explained using a transmis-sive boundary model where only the bottom of a holeis conducting whereas its wall is isolating. Regarding adissolving carbon fiber in a polyetheretherketone ma-trix, this model represents exactly a real phenomenon.

Furthermore, SEM investigations after the electro-chemical measurements show corrosion attacks onthe carbon fibers supporting the mentioned model

Figure 8. (a) i/E and (b) EIS curves of anodized titaniumin SBF pH 7.4 at 378C.



[Fig. 10(a)]. Experiments with real composite flowmolded CFRP bone plates show however an additionalcorrosion phenomenon due to the different alignmentof the carbon fibers. The result of this corrosion attackis the loosening of the bending to the polyetherether-ketone matrix that leads to a rise of fibers from thesubstrate under mechanical tension [Fig. 10(b)].

Long-term immersion experiment

The measurements of the electrochemical potentialreferred to the SCE electrode as well as the electricalresistance between the two coupled partners showedthree main types of behavior:

I. The potentials of both coupled partners remainconstant and identical at around 40 mV,whereas the resistance continuously increasesduring the whole immersion time [Fig. 11(a)].This behavior indicates active corrosion of oneor both coupled partners.

II. The resistance remains stable and the poten-tials increase in the beginning. After a certaintime, the potentials suddenly drop and remainconstant in a potential domain similar to typeI whereas the resistance starts to increase [Fig.11(b)]. The potential increase at the beginningindicates a passive behavior. The potentialdrop can be caused by a sudden activation.Due to small differences of the OCP betweenthe screw and the plate materials, slightchanges of the local chemistry in crevice canresult in exchange of the location of the anodicand cathodic sites. Therefore, the potentialdrop can be assigned to active dissolution ofone or both contact partners. Identification ofthe actual corrosion sites is however difficult.

III. The resistance remains unchanged duringimmersion and the potentials increase in thebeginning and level off after a certain time ataround 140 mV [Fig. 11(c)]. This behavior istypical for a stable passive system.

Figure 9. (a) i/E and (b) EIS curves of CFRP and (c) i/E and (d) EIS curves of CFRP with tantalum fibers in SBF pH 7.4at 378C.

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Two samples, which included both stainless steelscrews, showed a different behavior not fitting thethree main types. The potential jumps several timesto low potential value similar to type I after an ini-tial potential increase. The resistance stays constantas long as the potential is high and increases whenthe potential jumps to lower values (Fig. 12). Thesepotential jumps are typical for ongoing activationand repassivation processes.

All the curves of the different coupling systemshave been assigned to the three types. The two spe-cial samples were assigned to the second typebecause of the initial passive behavior. Table IIshows the distribution. It can be seen that the cou-pling systems with metallic screws prefer the behav-ior of type II, whereas the CFRP combinations seemto prefer type I. Type III only seldom occurred.

After the immersion experiment the screws weredisassembled and the screw threads were examinedwith SEM. The two coupling systems containingstainless steel screws, which showed a behavior not

fitting to the three main types, were attacked by localcorrosion in the turns of the threads (Fig. 13). Theother coupling systems exhibit no similar corrosionattacks. However, the surfaces of the screw headsare abraded during the assembly and disassembly.Therefore, small signs of corrosion attacks can becovered by surface features related to friction. Fig. 14shows this effect for titanium where the differentsurface morphologies of anodized titanium screwthreads before the assembly and after the immersionexperiment and disassembly are clearly visible.

Figure 10. (a) Corrosion attack on a carbon fiber and (b)delaminating fibers due to corrosion attack at the interfacefiber/matrix after electrochemical experiment in SBF pH2.0 at 378C.

Figure 11. Three different types of the long-term behav-ior. The vertical arrows mark the time of the electrolyterefreshments.



Due to friction during assembly and disassembly,it was impossible to clearly characterize corrosionattacks at the threads of the CFRP screws and plates.However, it can be assumed that the morphology ofthe corrosion attacks on CFRP look similar to theones observed after the electrochemical measure-ments (Fig. 10).

DISCUSSION

i/E and EIS

The wide scattering of the passive current densitydetermined by potentiodynamic polarization experi-ments (i/E) and the scattering of the electrochemicalimpedance spectra (EIS) of the anodized titaniumsamples confirm the earlier mentioned differences inthe coloration and the oxide thickness (Fig. 1). Therelevant values for the corrosion behavior are thepassive current density, the polarization resistance,and the pitting potential. Considering these aspects,anodized titanium is a better option regarding corro-sion protection in spite of the good reproducibilityof the measurements with stainless steel indicating amore reliable life-time prediction.

In the case of all anodized titanium samples, boththe passive current density and the impedance value

at low frequencies (&polarization resistance) indi-cate orders of magnitude lower corrosion rates thanwith the other materials examined in the present

Figure 12. Potential jumps between CFRP plate and stain-less steel screw during long-term immersion indicatingmultiple activation/repassivation events.

TABLE IILong-Term Experiments Distributed to the Different

Types of Behavior Depending on the Coupling System

Combination No. Type I Type II Type III

1 2 2 12 1 2 23 3 2 04 3 1 15 1 4 06 2 2 17 1 3 18 2 3 0

Figure 13. SEM micrograph showing local corrosionattacks at the bottom of the thread groove of a stainlesssteel screws after the long-term immersion experiment.

Figure 14. SEM micrographs showing anodized titaniumsurfaces in the threads of the screws, (a) before and (b)after assembly, long-term immersion experiment, anddisassembly.

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investigation [compare Fig. 4(a) and 8(a)]. Further-more, pitting on anodized titanium in chloride solu-tions only takes place at very high potentials (>10 VSCE) that are not relevant for the biomedical applica-tion. Therefore, in spite of the large scattering in thebehavior of the Ti samples, the danger of failure canbe considered significantly smaller than with theother materials studied.

One parameter, which influences the corrosionbehavior significantly, is the acidity of the electrolyte.A decrease of the pH value of the electrolyte decreasesthe pitting potential of stainless steel and increases theopen-circuit potential [Figs. 4(a) and 5(a)]. As a result,the potential range of stable passivity is remarkablyshortened. Considering a lower pH in the region of aninflammation in the body, pitting corrosion of stainlesssteel could take place. On the contrary, again no corro-sion signs could be found on the titanium surfaces.

Considering the danger of galvanic coupling corro-sion, an important factor is the difference of theopen-circuit potentials of the two coupled partners.The open-circuit potentials of stainless steel and car-bon fiber reinforced polyetheretherketone (CFRP) dif-fer very slightly and therefore indicate a small riskfor galvanic coupling corrosion. Because of the scat-tering in the oxide thickness and the wide passiveregion, the differences of the equilibrium potentialsbetween anodized titanium and CFRP are slightlyhigher and the risk of galvanic coupling corrosioncould increase. Nevertheless, as Ti shows very stablepassivity under these conditions, galvanic coupling isnot expected to lead to a serious corrosion problem.

Long-term immersion

The understanding of the electrochemical behaviorof the different materials is important to estimate thecorrosion resistance in general or the risk of occur-rence of different corrosion modes (uniform vs. local-ized corrosion). However, exact determination of theelectrochemical behavior of the coupled partners onreal galvanic coupling corrosion is difficult, since notonly the materials but also geometric considerations,such as crevices that may be present between thecoupled partners, can influence the corrosion behav-ior. Therefore a set-up close to real condition isrequired for testing. In this study, such geometricconditions could be generated with the set-up used todetermine the long-term behavior of the differentcoupling systems (Fig. 3). Furthermore, electrochemi-cal data such as the open circuit potential (OCP) ofthe single parts and resistances between the coupledpartners could be determined in situ. The used LCR-method to measure the resistance between thecoupled partners, including a component from theelectrolyte, applies an AC-potential on the systemand determines the current answer. In this way, a

smaller disturbance of the electrochemical equilib-rium occurs when compared with a method applyinga DC-current on the system. However, the maximumvalue of resistance that can be measured depends onthe conductivity of the electrolyte.

The electrochemical investigations showed thatcarbon fibers dissolve actively in the given environ-ment. An exchange current density of 100 nA/cm2

can be determined from the i/E diagrams (Fig. 9) bythe Tafel slope method. This means that even inopen circuit condition, degradation of the carbonfibers occurs and increases in contact with a morenoble material such as anodized titanium. Stainlesssteel on the other side has a lower OCP than CFRP.In this case, CFRP will be cathodically protected andstainless steel will be attacked. But since the differ-ence of the OCPs of CFRP and stainless steel is onlyabout 50 mV, not a full protection of CFRP isexpected and stainless steel can behave as an anodedepending on the local chemistry.

In SBF media, the electrochemical investigation ofthe separated single materials shows a low risk of gal-vanic coupling corrosion for the combination of stain-less steel and CFRP because of the small differencesof the open circuit potentials measured in this case.However, the electrolyte can be more aggressive increvice condition due to reduced exchange of speciessuch as corrosion products or reactants needed forrepassivation of local corrosion events. Under theseconditions, the risk of local corrosion is stronglyincreased on passive surfaces. Once local attacksoccur, even small differences of the OCPs can increasethe dissolution kinetics of the corrosion processes.

On the surfaces of titanium and titanium alloys,crevice corrosion effects can be neglected, since tita-nium and titanium alloys also show high corrosionresistance in concentrated chloride solutions presentin crevices. However the more aggressive environ-ment in the crevice can lead to an increased dissolu-tion of the carbon fibers.

The results show that even when using an internalfixation system (such as PC-fix) to suppress frettingduring the immersion, the combination of stainlesssteel screws and CFRP bone plates causes localizedcorrosion on the contact area of the screws. Thisindicates that not the friction alone is responsible forcorrosion attacks during the time of immersion inthe human body,10 but galvanic coupling corrosionunder crevice condition plays an important role.

The assignment of the long-term immersion behav-ior show the tendency that all combination withCFRP and CFRP tantalum screws prefer active disso-lution (type I) and the combination with the metallicscrews prefer type II (Table II and Fig. 11). In agree-ment with the electrochemical investigation of theseparated materials, it shows that carbon fibers willalways dissolve actively whereas in combination



with metallic screws a passive behavior is observedfirst. After activation, local attacks on the metal(mainly on stainless steel) and degradation of the car-bon fibers lead to a loss of electrical contact betweensome of the fibers of the plate and the surface of thescrew head. As a result, the effective contact areatested with the LCR-meter is reduced inducing themeasurement of a higher contact resistance. In thisway, local damages can be indirectly monitored witha very efficient and simple technique.

Because stainless steel is susceptible to localizedcorrosion and because carbon fibers dissolveactively, the combination of anodized titanium withCFRP has to be preferred with respect to galvaniccoupling corrosion problems. Additionally, no signif-icant differences could be found between the be-havior of CFRP and the CFRP containing tantalumfibers. Therefore, plates made of CFRP containingtantalum fibers for an improved X-ray contrast willbe preferred from a surgery point of view.

CONCLUSION

The electrochemical study showed that the investi-gated stainless steel is susceptible to pitting corrosionwhereas anodized titanium stays always passive. Foranodized titanium, the passivity is stable over a largepotential range in all cases and this stability mightinduce scattering of the OCP due to undefined redoxreactions. This is in contrast to the stainless steel,where the pH strongly influences the pitting behavior.Considering the electrochemical behavior of CFRP, noremarkable differences between CFRP with and with-out tantalum fibers could be determined. However,both CFRP types show an active corrosion behavior.

The monitoring of the electrochemical parameters(OCP and resistance) of the coupled partners duringthe immersion experiment can be used as an indica-tion of the onset of localized corrosion attacks. Thisway, it was concluded that even without fretting, thestainless steel screws show localized corrosionattacks after the long-term immersion experiment.On the other hand, no localized corrosion attackscould be found on the anodized titanium screws.

The knowledge about the electrochemical behaviorof the single materials enables a first estimation ofthe corrosion behavior in a coupling system. How-ever, long-term experiments of a real coupling sys-tem are necessary to prove the assumptions madebased on the electrochemical behavior of the singlecoupled partners. The developed long-term measure-ment method represented in this paper offers thepossibility to analyze changes of the electrochemicalparameters in situ. Therefore not only the initial andfinal condition of the surfaces can be assessed, butelectrochemical changes as a function of time can be

investigated as well. A combination of both methods(electrochemical characterization of the coupled part-ners and the long-term immersion experiments) pro-vides information about the risk of time dependentgalvanic coupling corrosion and allows formulatingpossible mechanisms.

References

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