Biomaterials 26 (2005) 4747–4756 Electrochemical corrosion and metal ion release from Co-Cr-Mo prosthesis with titanium plasma spray coating Lucien Reclaru a, , Pierre-Yves Eschler a , Reto Lerf b , Andreas Blatter a a PX Holding S.A., Bldv des Eplature 42, 2304 La Chaux-de-Fonds, Switzerland b PI Precision Implants AG, Schachenallee 29, 5001 Aarau, Switzerland Received 8 July 2004; accepted 4 January 2005 Abstract The corrosion behavior of CoCrMo implants with rough titanium coatings, applied by different suppliers by either sintering or vacuum plasma spraying, has been evaluated and compared with uncoated material. The open-circuit potential, corrosion current and polarization resistance were determined by electrochemical techniques. The Co, Cr and Ti ions released from the samples into the electrolyte during a potentiostatic extraction technique were analyzed using ICP-MS. The Ti coatings from the different suppliers showed a different porous morphology, and the implants exhibited a distinct corrosion activity, underlining the importance of the coating process parameters. Among the titanium coated samples, the one with the sintered overcoat turned out to be the most resistant. Yet, on an absolute scale, they all showed a corrosion resistance inferior to that of uncoated CoCrMo or wrought titanium. r 2005 Elsevier Ltd. All rights reserved. Keywords: CoCrMo implant; Titanium plasma spray coating; Electrochemical test; Cations extraction; Corrosion 1. Introduction Co-based alloys are widely used for orthopedic implants because of their high strength, good biocom- patibility and wear resistance. Several ISO and ASTM standards specify such Co-based alloys for implant application. Cast CoCrMo alloys as specified in the ISO 5832-4 and the ASTM F-75 standards are very popular for components of total knee replacement, because precision casting can easily produce these complex geometrical forms. However, surfaces of the F-75 alloy are not optimal for osteointegration due to their relatively high release of Co and Cr ions [1,2]. By contrast, Ti surfaces—especially rough plasma sprayed Ti coatings (Ti VPS)—are known to offer excellent conditions for osteointegration [3,4]. Other surface treatments have also been investigated [5–12]. As a result of variable coating quality, some conflicting animal and clinical observations have been reported [13–16]. The idea to combine the advantages of cast CoCrMo alloy for knee components with an osteophilic Ti VPS coating is evident—high strength and durable osteointe- gration can be integrated into an optimal design of the implant. However, the issue of having two different materials implanted in direct contact with the human body has been reviewed critically on several occasions (e.g. [17–19]). This was the motivation to investigate the in vitro electrochemical behavior of different designs of prosthetic CoCrMo knee components with a rough or porous Ti coating. 2. Materials and methods For this study, four components for total knee replacement were chosen from four different suppliers, ARTICLE IN PRESS www.elsevier.com/locate/biomaterials 0142-9612/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.biomaterials.2005.01.004 Corresponding author. Tel.: +41 32 924 02 90; fax: +41 32 924 02 10. E-mail address: [email protected] (L. Reclaru).
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doi:10.1016/j.bi
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Biomaterials 26 (2005) 4747–4756
www.elsevier.com/locate/biomaterials
Electrochemical corrosion and metal ion release from Co-Cr-Moprosthesis with titanium plasma spray coating
Lucien Reclarua,�, Pierre-Yves Eschlera, Reto Lerfb, Andreas Blattera
aPX Holding S.A., Bldv des Eplature 42, 2304 La Chaux-de-Fonds, SwitzerlandbPI Precision Implants AG, Schachenallee 29, 5001 Aarau, Switzerland
Received 8 July 2004; accepted 4 January 2005
Abstract
The corrosion behavior of CoCrMo implants with rough titanium coatings, applied by different suppliers by either sintering or
vacuum plasma spraying, has been evaluated and compared with uncoated material. The open-circuit potential, corrosion current
and polarization resistance were determined by electrochemical techniques. The Co, Cr and Ti ions released from the samples into
the electrolyte during a potentiostatic extraction technique were analyzed using ICP-MS.
The Ti coatings from the different suppliers showed a different porous morphology, and the implants exhibited a distinct
corrosion activity, underlining the importance of the coating process parameters. Among the titanium coated samples, the one with
the sintered overcoat turned out to be the most resistant. Yet, on an absolute scale, they all showed a corrosion resistance inferior to
Co-based alloys are widely used for orthopedicimplants because of their high strength, good biocom-patibility and wear resistance. Several ISO and ASTMstandards specify such Co-based alloys for implantapplication. Cast CoCrMo alloys as specified in the ISO5832-4 and the ASTM F-75 standards are very popularfor components of total knee replacement, becauseprecision casting can easily produce these complexgeometrical forms. However, surfaces of the F-75 alloyare not optimal for osteointegration due to theirrelatively high release of Co and Cr ions [1,2].
By contrast, Ti surfaces—especially rough plasmasprayed Ti coatings (Ti VPS)—are known to offerexcellent conditions for osteointegration [3,4]. Othersurface treatments have also been investigated [5–12]. As
e front matter r 2005 Elsevier Ltd. All rights reserved.
a result of variable coating quality, some conflictinganimal and clinical observations have been reported[13–16].
The idea to combine the advantages of cast CoCrMoalloy for knee components with an osteophilic Ti VPScoating is evident—high strength and durable osteointe-gration can be integrated into an optimal design of theimplant. However, the issue of having two differentmaterials implanted in direct contact with the humanbody has been reviewed critically on several occasions(e.g. [17–19]). This was the motivation to investigate thein vitro electrochemical behavior of different designs ofprosthetic CoCrMo knee components with a rough orporous Ti coating.
2. Materials and methods
For this study, four components for total kneereplacement were chosen from four different suppliers,
ARTICLE IN PRESSL. Reclaru et al. / Biomaterials 26 (2005) 4747–47564748
namely three tibial and one femoral part. As anexample, two of the implants investigated here areshown in Figs. 1 and 2. The surfaces adjacent to thebone of all these implants were covered by a Ti coating.Three of the coatings were applied by vacuum plasmaspraying (Ti VPS), and the fourth one was obtained bysintering a Ti powder onto the CoCrMo substrate. Forcomparison, reference samples in Ti implant (grade 2)
Fig. 1. Prosthesis #B.
Fig. 2. Prosthesis #C.
Table 1
Overview of the investigated components for total knee replacement
Type of implant Material (
Prosthesis #A Tibial component CoCrMo(
Prosthesis #B Tibial component CoCr Mo
Prosthesis #C Tibial component CoCr Mo
Prosthesis #D Femoral component CoCr Mo
and uncoated CoCrMo (ASTM F-75, Co-28Cr-6Mo)were included in this study. Table 1 provides anoverview of the studied components and their respectivecoatings.
2.1. Sample preparation
For the tests, sample discs of diameter 11mm were cutout from the implants by electro-erosion in water.Electro-erosion was chosen in order to preserve theintegrity of the crumbly, plasma sprayed coating.
The test samples were cleaned in a steam generator,rinsed with ethanol p.a. (Merck) and finally washed in asolution of ultra-pure water and ethanol p.a. (Merck).The sample holders, specially adapted to our tests toprevent possible crevice corrosion, were machined out ofPTFE (Fig. 3).
2.2. Electrochemical measurements
All the measurements were carried out on a PARModel 273 A potentiostat, using a rotating electrode at aspeed of 300 rpm. The counter-electrodes were made ofplatinum and the reference electrode was saturatedcalomel electrode (SCE). The test milieu was an artificialbone fluid according to Burks and Peck (Table 2), andthe testing temperature was 37 1C. A Faraday cageprotected the whole measurement set-up.
The samples were subjected to the following measure-ment cycle:
�
AS
F-7
(F-
(F-
(F-
Immersion in the electrolyte, de-aerated with N2.After 32 h, the open circuit potential Eoc was thenrecorded during the following 16 h.
�
Tracing of the linear polarization curves (720mV vs.Eoc) to calculate the polarization resistance Rp [20].The scanning rate was 0.1mV s�1.
�
Tracing of the polarization curves (7150 mV vs. Eoc)to calculate Tafel’s slope from which the corrosioncurrent density (icorr) was derived (ASTM G59-97,with PARCalc routine EG&G PARC ApplicationModel 352 SoftCorrTM II). The scanning rate was0.1mV s�1.
�
Tracing of the global polarization curve from�1000mV up to +1200mV (SCE) at a scanning rateof 0.25mV s�1.
TM) Ti coating Coating thickness
5) Ti VPS E700mm75) Ti VPS Nominally 250mm75) Ti VPS Nominally 400 mm75) Ti sintered Nominally 1000mm
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Fig. 3. Test sample and sample holder.
Table 2
Chemical composition of artificial bone fluid according to Burks and
Peck Peck (NF S 90-701and ISO 10993-9)
Conc (mgL�1) Molarity (M)
NaCl 680 117.24� 10�3
CaCl2 200 1.8 0� 10�3
KCl 400 5.41� 10�3
MgSO4 100 0.83� 10�3
NaHCO3 2200 26.20� 10�3
Na2HPO4 126 0.89� 10�3
NaH2PO4 26 0.22� 10�3
L. Reclaru et al. / Biomaterials 26 (2005) 4747–4756 4749
It is important to note that the measured corrosioncurrents will normally be expressed in units of currentdensities (A cm2) throughout the paper, normalized tothe geometrical test surfaces rather than to the electro-chemically active surface. Since the geometrical samplesurfaces are close to 1 cm2, currents and current densitiesreturn the same values.
0 5 10 15
Time (hours)
–500
–400
–300
–200
–100
0
Op
en c
ircu
it p
ote
nti
al (
mV
SC
E)
#A
#B
#C
#D
Fig. 4. Potential curves as recorded under open circuit conditions.
2.3. Extraction measurements
For the measurement of a possible release of Co, Crand Ti ions, a potentiostatic technique of controlledpotential coulometry, adapted from ASTM F746-1998,was employed. The technique consists in carrying out anexcitation at 800mV SCE during 26 s and then to set afixed potential during 36min. Potentiostatic measure-ments were carried out at five different potentials, fixedat 500, 550, 600, 650 and 700mV SCE, respectively. Ateach level, 10 measurement cycles were run, resulting ina total test duration of 30 h.
With this technique, the total quantity of the electricalcharge consumed during the experiment is easilyobtained. This total electrical charge is a useful
parameter, as it can be related directly to the concentra-tion of electro-active species in the cell.
The extraction milieu was a solution of 9 g/L NaCl(ASTM F746-1998) in ultra-pure water (electricalresistance 18MO cm). The volume of the electrolytewas 50mL for a sample surface of 0.95 cm2.
The quantity of Cr, Co and Ti ions released intothe electrolyte during the extraction was analyzedat the end of the experiment by inductively coupledplasma mass spectroscopy (ICP-MS thermo-OptekPQ2+).
3. Results and discussion
3.1. Open-circuit potential (Eoc)
Fig. 4 shows the evolution of the potential recordedunder open-circuit conditions for the titanium coatedsamples #A, #B, #C and #D. The potential was tracedduring the last 16 h of the total immersion of 48 h.
This electrochemical variable is not specific toreversible phenomena and therefore Nernst’ s equili-brium equation is no longer valid. The nature ofthe metal–solution interface varies with time andconsequently the open-circuit potential is no longera characteristic of the metal. It also depends onthe experimental conditions, particularly on the electro-lyte composition, the temperature and the oxygencontent of the electrolyte, and on the surface state ofthe metal [21].
The open-circuit potential after 24 h immersion allowsto draw a relative comparison of the nobility of thealloys in the milieu under consideration, and toconstruct a galvanic series.
It is seen in Fig. 4 that the sinter-coated sample #Dattains a positive open circuit potential of about 20mVwhile the Eoc values of the Ti VPS coated samples #Athrough #C are significantly lower and remain negative.
In terms of open-circuit potential, the measuredvalues suggest the following ranking #D–#B–#C–#A.
L. Reclaru et al. / Biomaterials 26 (2005) 4747–47564750
3.2. Polarization resistance (Rp) and corrosion current
density icorr
In a next step, the polarization curves I ¼ f ðEÞ wererecorded by varying the potential in the vicinity of theopen circuit potential (Eoc720mV) while measuring theresulting current I. The polarization resistances Rp ofthe samples were then determined from the slopes ofthese curves at I ¼ 0: Polarization resistances Rp and thecorrosion current densities icorr; derived from the Tafelslopes calculated from the polarization curves, aresummarized in Table 3 together with Eoc:
Rp is representative for the degree of protection of thepassivation layer at the metal surface. The higher thevalue of Rp; the lower the corrosion current density icorr;and hence the better the metal resists to corrosion. Inthis regard, the polarization resistances in Table 3deserve some comments:
The polarization resistances suggest the same rankingamong the coated samples as the open-circuit potentials.The sinter-coated sample #D again shows the bestbehavior while sample #A is worst.
With values of the order of mAcm�2, the corrosioncurrent densities are excessively high for the Ti VPScoated samples #A–#C. For the sinter coated sample#D, icorr is still of the order of several hundred nA cm�2.
By contrast, the uncoated CoCrMo and Ti implant(grade 2) samples, measured under the same conditions,both show a substantially higher Rp and a correspond-ingly lower icorr: There values lie in the range aspreviously reported for titanium and CoCrMo used indental applications [22].
2
4
6
Cu
rren
t I (
mA
/cm
2 )
#A
#B
3.3. Potentiodynamic polarization
Potentiodynamic polarization curves were system-atically traced for all samples after the immersion in the
Table 3
Electochemical parameters of the test samples: open circuit potential
Eoc; polarization resistance Rp as calculated from the linear polariza-
tion curves, and corrosion current densities icorr as derived from the
Tafel slopes
Sample Open circuit
potential Eoc
(mV)
Polarization
resistance Rp
(KO)
Corrosion
current
density icorr(nA cm2)
#D 20 111 315
#B �153 34.6 2245
#C �209 24 3367
#A �331 9.7 7990
CoCr Mo
(uncoated) 17 788 24
Ti implant
(grade 2)
�38 1586 68
artificial bone fluid during 48 h. Fig. 5 displays the datain the semi-logarithmic Anglo-Saxon format and Fig. 6in linear axes to make appear the breakdown potentialsmore clearly.
The potentiodynamic polarization curves in Figs. 5and 6 also reveal a relative poor corrosion resistance ofthe Ti-coated CoCrMo samples: there corrosion rates(i.e. corrosion currents) are about 100 times higher thanthose of the uncoated titanium implant (grade 2) andCoCrMo alloy. The global polarization curve is theresultant of anodic and cathodic polarization curves.The value of the potential EðI ¼ 0Þ indicates the passage
-1000 -500 0 500 1000
Potential E (mV SCE)
-2
0
-500 -400 -300 -200 -100 0 100 200 300 400 500
Potential E (mV SCE)
-0.2
-0.1
0
0.1
0.2
Cu
rren
t I (
mA
/cm
2 )
#A
#B #C#D
CoCr
(a)
(b)
Fig. 6. (a) Potentiodynamic polarization curves; linear presentation.
(b) Zoom in the breakdown potential range of Fig. 6a.
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Table 4
Coulometric zone analysis
Sample Eði ¼ 0Þ (mV) Zone I Eði ¼
0Þ to 300mV
(mC)
Zone II 300 to
600mV (mC)
CoCrMo
substrate
�377 0.4 1.5
Ti implant
(grade 2)
�115 2.0 16.8
#D �368 95.4 398.5
#C �217 170.4 430.6
#B �165 866.4 1318.8
#A �346 1937.2 2984.2
10
15
20
Am
per
s/cm
2 )
L. Reclaru et al. / Biomaterials 26 (2005) 4747–4756 4751
of the cathodic zone to the anodic zone (Fig. 5). Inmost cases, the values of the corrosion potentialsEði ¼ 0Þ are smaller than those corresponding to Eoc
(Tables 3 and 4). The variation is probably due to adepassivation phenomena on the surface during catho-dic scanning.
Fig. 6 invites for a classification according to thebreakdown potential Eb: This is the potential at whichthe anodic current (and hence the corrosion rate) startsto increase significantly. According to ASTM G15-97a,the breakdown potential is the least noble potentialwhere pitting and/or crevice corrosion will initiate andpropagate. Even though the determination of Eb is notalways obvious (Fig. 6b), the test samples can accord-ingly be classified as follows:
0
5
t (m
icro
�
–5
Cu
rren
Sample #A shows a poor corrosion resistance, sincean important anodic current starts to develop at anEb as low as about �160mV.
–10
� 0 6 12 18 24 30 36
Time (min)
Sample #B is not much better, with an Eb in the samerange (about �180mV) and an anodic current onlyslightly below that of #A.
Fig. 7. Sample #C: potentiostatic cycles recorded during 36min each
�
0 6 12 18 24 30 36
Time (min)
–10
0
10
20
30
40
50
Cu
rren
t (m
icro
Am
per
s/cm
2 )
Fig. 8. Sample #D: potentiostatic cycles recorded during 36min each
at 650mV SCE.
at 650mV SCE.
Sample #C is the Ti VPS coated sample with thehighest and slightly positive Eb:
The sinter coated sample #D resists much better tocorrosion, since its Eb appears at significantly highervalues (400mV).
Yet, the corrosion resistance of all the Ti-coatedsamples is not comparable to that of the uncoatedCoCrMo alloy, as this latter does not develop asignificant anodic current up to Eb ¼ 750mV:
The Ti-coated samples all show an anodic currenteven at negative potentials, though it may be low in therange of newampheres. Therefore, they will undergo aweak corrosion process already well below the break-down potential. An ‘‘immunity zone’’, in which corro-sion is weak or insignificant, may be defined as thepotential range between the corrosion potential Eði ¼ 0Þand the breakdown potential (Eb). The more extended
this zone, the lower the risk that a polarization or agalvanic coupling may induce a corrosion of the alloy.The extent of the immunity zone of the Ti-coatedsamples may be insufficient for a conscientious applica-tion. Generally speaking, the breakdown potentialshould lie at 900mV at least to speak of a no-risksituation.
3.4. Coulometric zone analysis
The coulometric zone analysis provides a quantitativeestimate of the degradation process of a specific alloyover the relevant potential range. This analysis consistsin separating the anodic polarization curves into two (ormore) distinct ‘‘risk’’ zones: zone I from Eði ¼ 0Þ up to+300mV, and zone II from +300mV to +600mV.Although the importance of the electrochemical degra-dation phenomenon may vary from one person toanother, a real risk may be expected in zone I, since thisis close to real clinical conditions, and exceptionally alsoin zone II.
The surfaces under the polarization curves areintegrated for each zone using the Monte-Carlo method.
ARTICLE IN PRESSL. Reclaru et al. / Biomaterials 26 (2005) 4747–47564752
The results are expressed in units of an electrical charge(mC). This charge corresponds to the (ionic) currenttransport associated with the electrochemical degrada-tion of the alloy during the anodic sweeping through therespective potential zone. Table 4 presents the values ofthe electrical charges obtained for zones I and II.
Table 4 reveals the importance of the surfacetreatment for the corrosion related risks that can beattributed to these implants. The Ti-coated samplesshow very high charges even in the high-risk zone I,namely hundred to thousands of mC, as compared tothe uncoated samples with charges of the order of 1mC.
To get a better feeling on the importance of the chargevalues obtained for the Ti-coated samples, reference ismade to an earlier study on approximately 100dental alloys [23]. In this study, it was found that theelectrical charges associated with zone I of dental alloysgenerally do not exceed 1mC and often are substantiallylower.
The high charges measured with the coated samplessuggest that the Ti-coatings favor the liberation of metal
Table 5
Sample #C: electrical charges (mC) transported during the extraction tests at
Cycle no. Level (500mV) Level (550mV) Level (60
1 131.3 88.4 90.2
2 33.4 31.6 28.6
3 4.3 5.4 9.8
4 2.0 4.1 8.5
5 1.7 3.8 8.3
6 1.5 3.7 8.1
7 1.6 3.6 8.0
8 1.3 3.6 8.00
9 1.2 3.4 7.8
10 1.2 5.4 17.0
Total 179.5 153.0 194.3
Table 6
Sample #D: electrical charges (mC) transported during the extraction tests a
Cycle no. Level (500mV) Level (550mV) Level (600m
1 170.0 82.8 75.0
2 21.2 27.6 32.0
3 8.9 11.8 19.0
4 7.0 8.7 15.4
5 3.8 6.9 13.7
6 3.2 6.4 12.5
7 2.2 5.8 11.9
8 2.0 5.4 11.4
9 1.7 6.0 10.9
10 1.5 5.7 10.8
Total 221.5 167.1 212.6
ions (charge carriers) that generate the corrosioncurrent. Extraction tests as described in the next sectionwere therefore carried out in order to quantify anypossible Ti, Co or Cr liberation.
3.5. Extraction tests
The extraction method is described in the experi-mental Section 2.3. As an example of the resultsobtained by this method, Figs. 7 and 8 present the 10cycles of the potentiostatic measurement at 650mV SCEfor the samples #C and #D. The measured currentdensities are of the order of mAcm�2.
The electrical charges associated with the extraction,obtained by integration of the potentiostatic curves, aresummarized in Tables 5 and 6. In both cases theelectrical charge decreases progressively with each cycle,unveiling a tendency for passivation. Nonetheless, theobserved electrical charges are quite important, throughnot significantly different.
different potential levels.
0mV) Level (650mV) Level (700mV) Total
94.5 101.7 506.1
30.4 45.6 169.6
19.8 42.9 82.2
19.6 43.6 77.8
19.4 43.8 77.0
19.3 43.3 75.9
19.0 42.7 74.9
18.8 42.0 73.7
18.5 42.1 73.0
35.9 73.8 133.3
295.2 521.5 1343.5
t different potential levels
V) Level (650mV) Level (700mV) Total
78.01 107.4 513.2
47.8 80.9 209.5
35.2 71.5 146.4
30.6 65.6 127.3
29.0 61.6 115.0
25.8 56.8 104.7
24.5 56.0 100.4
23.9 51.9 94.6
22.5 49.8 90.9
21.7 48.2 87.9
339.0 649.7 1589.9
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After the extraction tests, the electrolyte solutionswere analyzed by ICP-MS in order to determine thechemical nature and concentrations of the chargecarriers. The results are listed in Table 7 in units ofmicrograms per liter.
The analytical results in Table 7 reveal high quantitiesof cobalt and chromium in the electrolyte. Theconcentration of titanium, in contrast, was found to besignificantly lower, although it is this element that makesup the surface.
3.6. Ti-coated CoCrMo surfaces
The presence of cobalt and chromium in the electro-lyte implies that the Ti-coatings are permeable and letpass the metal ions extracted at the CoCrMosurface underneath the coating. Their high concentra-tions as compared to that of titanium mean that it ismainly the CoCrMo substrate that contributes to thecorrosion current by the release of its constituent ions.Now, bear in mind that the uncoated CoCrMo sampledeveloped substantially lower corrosion currents, i.e.
Table 7
Elementary ICP-MS analysis of the electrolytes after the extraction
tests of samples #C and #D
Cation Blank (mg l�1) #C (mg l�1) #D (mg l�1)
Cobalt 0.3 1700 2240
Chrome 0.1 555 470
Titanium 0.08 4.9 3.9
Fig. 9. Coating morphology of sample #A.. 5� ;
they released much less ions, than the Ti-coatedCoCrMo surfaces.
This suggests an accelerated and selective corrosionprocess at work at the Ti–CoCrMo interface. The originof this phenomenon is not clear. Galvanic coupling ofthe dissimilar materials with the titanium in the cathodicposition seems to be ruled out as an explanation sincethe tests were conducted under potentiostatic control, inwhich the different nobility of substrate and coatingcannot act. Another explanation may be the cationsbeing released into a confined environment at theinterface. Chromium ions released in cavities is knownto provoke local acidification of the electrolyte [1],conditions that will amplify localized corrosion.
The permeability of the Ti coatings is not in factsurprising when looking at the morphologies shown inthe micrographs in Figs. 9–13. It is intuitive that themore porous the morphology, the easier for theelectrolyte to penetrate through the coating onto theCoCrMo substrate. As seen in the micrographs, the Ti-VPS morphologies become apparently denser, i.e. lessporous and hence less permeable, in the series#A–#B–#C. This mirrors in fact the sequence of theobserved corrosion resistance found throughout thisstudy.
This sequence of denser morphologies and highercorrosion resistance goes along, with thinner coa-ting as can be seen in Table 1. Coating thicknessand morphology are related to the plasma sprayparameters. The coating quality, in terms of corrosionresistance, hence subtly depends on the proper choiceand control of spray temperature, time and impactvelocity.
50� Protected zone; 50� corroded zone.
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Fig. 10. Coating morphology of sample #B. 5� ; 50� protected zone; 50� corroded zone. (The black spot is for identification purposes).
Fig. 11. Coating morphology of sample #C. 5� ; 50� protected zone; 50� corroded zone. (The black spots are for identification purposes).
L. Reclaru et al. / Biomaterials 26 (2005) 4747–47564754
4. Conclusions
The titanium plasma spray technology used forcoating on CoCrMo plays an important role in thecorrosion resistance of the prosthesis. Interface defectssuch as flaws, adhesion, porosity, etc. generate corrosionprocesses accompanied by the release of Cr, Co and Tications.
The relatively low corrosion resistance of the Ti-coated samples can be explained by the coating’spermeability to the electrolyte, related to theirporous morphologies. The penetration of the electrolyteleads to a localized corrosion at the coating–substrate
interface with a predominant attack of the CoCrMosubstrate, as evidenced by the important release ofchromium and cobalt ions as compared to that oftitanium ions.
It is worth to note that cobalt and chromium arerenowned potential allergens, and that the migration oftitanium particles in the body tissue also had recentlybecome an issue of clinical studies [24–27].
In conclusion, the concept of a Ti-VPS coatedCoCrMo implant poses a dilemma: in view of optimalosteointegration, the coating should have an openmorphology, but for a good corrosion resistance, itshould be impermeably dense.
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Fig. 13. Cross-section through the coating of sample #C, showing the
