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PergamonCorrosion Science, Vol. 37, No. 9, pp. 1333-1340, 1995

Copyright 0 1995 Elsevier Science LtdPrinted in Great Britain. All rights reserved

ool(r93sx/95 %9.50+0.00

0010-938X(95)00033-X

A COMPARISON BETWEEN THE CORROSION BEHAVIOUROF SINTERED AND UNSINTERED POROUS TITANIUM

K.H.W. SEAH, R. THAMPURAN, X. CHEN* and S.H. TEOH

Department of Mechanical Engineering, National University of Singapore, 10 Kent Ridge Crescent,Singapore 0511, Singapore

Abstract-Corrosion tests were performed on sintered and unsintered titanium porous compacts of variousporosities, as well as on solid titanium which acted as a control material. The tests were conducted in 0.9%aqueous NaCl solution maintained at 37C to simulate the physiological environment encountered bysurgical implants in the human body. The results confirm that solid titanium and sintered porous titaniumboth possess a distinct passivation range. Unsintered porous titanium does not seem to passivate at all andsuffers greater corrosion than solid titanium. Unsintered specimens compacted at higher pressuresexperienced more corrosion than those compacted at lower pressures, although sintered specimensbehaved in the exact opposite fashion. An explanation is given in the paper for these phenomena.

INTRODUCTION

For centuries now, metallic devices have been used to repair and replace parts of thehuman body. It became apparent, from post-surgical infections, that the success ofimplant surgeries greatly depended on the properties of the implant material. Bannonand Mild listed the characteristics that make a good implant material. These are: (i)no tissue reaction to the implant, (ii) no corrosion of the implant, (iii) design andfunctionality of the implant, (iv) mechanical properties of the implant, and (v) surgicalimplications. In addition to stainless steels and Cr-Co alloys, titanium has been foundto satisfy the above criteria.* By the mid-1960s, titanium successfully replaced stainlesssteel and Cr-Co alloys in nearly all such implants, with no reported failures3

Hall and Hackerman were the first to observe pitting corrosion of commerciallypure (CP) at 10 V after some samples were anodically polarised in 0.5 M NaClsolution. Later, Tomashov et al. concluded that different conditions of oxidationproduced different oxide forms for titanium. Hoar and Mears6 made a comprehensivestudy of corrosion of surgical implant materials in Hanks physiological solution andvarious NaCl solutions at 37C, and found that titanium and its alloys formed a stablepassivating film on the surface and possessed very high breakdown potentials. Solar eta1.7 confirmed these findings and showed that the corrosion results for titanium inhuman blood, Hanks solution and chloride solutions were similar. This led to theconclusion that inorganic solutions are satisfactory substitutes for blood, and

probably other extracellular fluid, for studying the corrosion behaviour of titanium.

* Metals and Advanced Materials Centre, Singapore Institute of Standards and Industrial Research, 1Science Park Drive, Singapore 051 I, Singapore.

Manuscript received 14 June 1994; in amended form 19 September 1994.

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1334 K.H.W. Seah et al

As early as 1951, Leventhals reported that his studies on rabbits and rats showedthat titanium is an inert metal and appears to be ideal for fixation of fractures ~~ aconclusion confirmed by other research workers. Methods of fixation include

impaction, mechanical internal fixation, as well as in-situ methyl methacrylatepolymerisation. Failures of these fixation techniques stimulated interest in the fieldof biological growth into inert porous materials. Improved distribution of stress, abiological repair system, a biological defense against infection, and a more permanentimplant resulting in greater patient comfort are some advantages of adopting such afixation technique, through which there is bone in-growth into the pores of poroustitanium. Parametric studies of the effect of pore size on bone in-growth wereperformed by Nelson et al. and Ducheyne et ~1.~ The biomedical properties ofporous implant materials are important as lower elastic modulus materials withmoduli closer to that of bone are able to transfer forces to surrounding bone morereadily, thereby reducing stress-shielding effects and enhancing bone in-growth.14

Lucas et a1.15 studied the corrosion behaviour of passivated and steam-sterilisedporous titanium in 0.9% aerated NaCl solution at 37C. They found that the porouswire mesh portion of the specimen (having an inherently larger surface area ascompared to the solid substrate) experienced a higher current density at each level ofpotential. Buchanan et ~1.~ conducted anodic polarization tests for porous CPtitanium surface layers on solid Ti-6A14V substrates in 10% calf serum and 90%isotonic saline at 37C and found that the anodic current densities for the poroustitanium were significantly larger than those for the solid substrate. They concludedthat the larger current density for the porous portion was due to the larger true surfacearea of the porous portion.

Seah and Chen17 tested the corrosion characteristics of solid 316 stainless steel,solid titanium and porous titanium of different porosities in 0.9% NaCl solution at37C. Unfortunately, they made a detailed study of the effect of porosity for onlyporous titanium specimens which were compacted and left unsintered. Unsinteredspecimens are invariably brittle and are not suitable for surgical implants. It isconjectured that sintered and unsintered specimens will exhibit different corrosionbehaviour.

The aim of this present research is to compare the corrosion characteristics of

sintered and unsintered porous titanium compacts, produced by the technique ofpowder metallurgy, of varying degrees of porosity in 0.9% NaCl solution at 37C. Asolid titanium specimen was used as a control for the tests. These parameters werechosen to simulate the environment which a typical titanium surgical implant wouldencounter in the human body.

EXPERIMENTAL METHOD

The main equipment used for this series of experiments was an automatic polarisation unit comprising

the following components: arbitrary function generator (Hokuto Denko HB-105), GPIB potentiostat:galvanostat (Hokuto Denko HB-SOIG); NEC Ih-bit CPU (PC-901 RX); NEC monitor (PC-KD854n); NECprinter (PCPRIOIGS); and a Techne water bath (Tempette TE-RA). Polarisation was controlled by thepotentiostat and the input signal was provided by the function generator. The potentiostat included a highimpedance (> 10 a), low rise time (below 0.3 ps at no-load condition) electrometer for measuring thepotential between the anode and the reference electrode, as well as an electronic milli-ammeter with amaximum output of i_ I A. The instrument was able to test up to a potential of + IOV.

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A comparison between the corrosion behaviour of sintered and unsintered porous titanium 1335

Heater

Constant -temperaturebath

Fig. 1. A schematic diagram of the polarisation test system

I II Referenceelectrode

I I Beaker(test cell)

WorkingU

electrode

c- >(specimen)

---_ Counterelectrode

An electrometer of high impedance was used for electrochemical potentiometry to avoid polarising thereference electrode. In order to generate anodic polarisation curves by the data acquisition of the computersystem, current and potential measurements had to be converted to the digital form by a two-channelanalog-digital (AD) converter. AD converters are typically fairly high in impedance and, most importantly,

they are connected to the system of electrodes only for the duration of AD conversion (approx. 1 ps), thuseliminating the problem of reference electrode polarisation. The potentiostat had a three-electrode system,namely, a reference electrode, an auxiliary (counter) electrode and a working electrode which was thespecimen of interest. A saturated calomel electrode (SCE) served as a reference electrode. A platinisedtitanium wire was made the counter electrode to complete the circuit. The wire was properly insulated,leaving only an exposed area of 1 cm. A schematic diagram of the arrangement is shown in Fig. 1.

Test speci mens

Commercial 99% pure grade 4 titanium powder was compacted to make the test specimens. Scanningelectron microscope (SEM) studies revealed the mean particle size to be 150 pm. The various particle shapesof the powder are observed to be ligamental, aggregate, rounded, flake and irregular, which conform to thequalitative description of the shapes prescribed by German. Each porous titanium compact was made by

compacting about 10 g of powder in a punch-die set of diameter 19.2 mm using a hydraulic-drivencompaction machine. To obtain compacts of four different porosities for sintering, the powders wererespectively compacted to four different pressure levels (5, 10, 14 and 18 t). To obtain compacts of threedifferent porosities for making the unsintered specimens, the powders were, respectively, compacted to threedifferent pressure levels (5, 12 and 18 t). Compaction was done gradually (in steps of2,5,8, 10, 12, 14, 16 and18 t) to allow the pressurised powder particles to adjust and settle. Each compact produced was at least 10mm thick.

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1336 K.H.W. Seah a al.

Sintering

A vacuum oven capable of attaining a vacuum of up to 10~~ mbar was used for sintering so that thecompacts would not oxidise in the process. The compacts to be sintered were put into small ceramic tubes 10cm long whose ends were covered using stainless steel foil, in order to prevent contamination of the vacuumoven by titanium powder. These tubes were then inserted into the vacuum oven and the heating cycle for thesintering process (as shown in Fig. 2) was carried out. Figure 3 is a graph showing the relationship betweenporosity (measured in terms of ~01%) and compaction pressure for sintered and unsintered titaniumspecimens.

Mounling ofcorrosion specimen

Each of the specimens was milled to a cubical shape of side 1 cm and to an accuracy of + I % as specifiedby ASTM.2 A wire was connected to one side of the specimen by means of a steel screw as shown in Fig. 4.The exposed surface of the specimen was limited to 1 cm2 by cold mounting it using a thermosetting resinwhich acted as an insulator for the other five sides. The hole for the screw was drilled and tapped. This severewear and deformation produced a pore-free tapped hole. Therefore, no electrolyte would have been able tocome in contact with the screw.

The exposed surface was polished with close adherence to ASTM G5-82. There were two stages m thepolishing process, namely mechanical grinding and electropolishing. Mechanical grinding was done to

, lhr , Zhrs 2hrs-----c------- 2hrs I

Tme

Heating cycle for sintering process

Fig. 2. Heating cycle for sintering process

x Umintered+ Sintered

P1, 30-

VIe

= 20-

10 12 11 16 18

Compaction Pressure t tonnes)

Fig. 3. Graphs of porosity vs compaction pressure.

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A comparison between the corrosion behaviour of sintered and unsintered porous titanium 1337

Screw/

Polymerresin

Wire

Id

\Specimen

Wire lug

Fig. 4. A schematic diagram of the mounted test sample

remove any resin that may have overflowed on to the test surface. This was done in four stages using SICpaper of 320, 600, 800 and 1000 grit in that order. The samples were then degreased by swabbing withacetone, followed by rinsing in distilled water.

Subsequent electropolishing was necessary to re-open pores closed by the mechanical grinding. Also,electropolishing was used to remove wear debris and relieve surface stresses introduced by grinding. Theelectropolishing was done using the Electromet polishing machine III (Model no: 70-1730) and a voltagesetting of 60 V for I min. The electrolyte used for electropolishing was a mixture of 940 ml of acetic (100%glacial) acid and 60 ml of 60% perchloric acid.** Care was taken to add the perchloric acid to the acetic acidand not the other way round. Also, the used electrolyte was not poured away, but stored in a waste bottle for

proper disposal. After electropolishing, the test surface was again meticulously washed with acetone anddistilled water before drying.

Test procedureThe experimental set-up is shown schematically in Fig. I. The electrolyte used was 0.9% aqueous NaCl

solution as specified by ASTM. Prior to immersion of the electrodes, the beaker of electrolyte was heatedto a constant temperature of 37C to simulate the physiological environment in a constant temperaturewater bath. The test surface of the sample (working electrode) was positioned as close as possible to thereference electrode (SCE) and care was taken to ensure that the sample was at a depth of more than 2 cmfrom the liquid/air interface. This was necessary to ensure that oxygen supply was identical for all samples.*

Following immersion of the electrodes, the corrosion potential of the test surface was continuouslymonitored for I h. Anodic polarisation measurements started only after the I h period. A scan speed of

25 mV/min was decided on after referring to several reports of past trials6. Data collection was done atI s intervals and was halted only when the anodic polarisation range of 2500 mV was exceeded. The durationof each test was about 160 min. Every corrosion test was repeated several times to ensure repeatability.

EXPERIMENTAL RESULTS AND DISCUSSION

Unsinteredporous titaniumFigure 5 shows the plots of the potential-current curves for unsintered porous

titanium. The curve for solid titanium is included in the figure for comparison. All theresults were found to be repeatable. The large passivation region seen in the solid

titanium is expected,4.7 and so is the fact that porous titanium is more susceptible tocorrosion than solid titanium.Sm17 Porous titanium does not seem to exhibit anypassivation region at all. What is really surprising, however, is that with increase incompaction pressure (which corresponds to a lowering of porosity), current densityincreases. Previous research workers 15- have shown that a porous specimen will havea higher corrosion current density than a solid one since its true surface area is larger

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1338 K.H.W. Seah et al .

0 I , , _,I .;

0.01 0.1

-500 1Log Current Density (IJ.A/cm2)

Anodc poarisation curves for unsintered porous titamum

Fg 5. Anodic polarisation curves for unsintered porous titanium.

than the nominal surface area. By the same reasoning, a more porous specimen should

also suffer more corrosion than a less porous one. The corrosion results obtained forunsintered porous titanium are therefore contrary to this commonly held view.

Sinteredporous t i tanium

Figure 6 shows the plots of the potential-current curves for sintered poroustitanium. The curve for solid titanium is included in the figure for comparison. All theresults were found to be repeatable. Once again, the large passivation region seen in

Compadin Ressure _- 18 onnes

Log Current Oensity ( M/cm2 I

Anodc poarisotion curves for sintered porous titamum

Fg 6. Anodic polarisation curves for sintered porous titanium.

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A comparison between the corrosion behaviour of sintered and unsintered porous titanium 1339

the solid titanium is expected,4,7 and so is the fact that porous titanium is moresusceptible to corrosion than solid titanium.5-7 However, there is only a smallpassivation region in the former case. In this figure, it can be observed that with

increase in compaction pressure (which corresponds to a lower porosity), currentdensity decreases. This effect is contrary to what is seen in Fig. 5.

Explanation for the discrepancyIn order to explain why a difference is observed between the corrosion behaviour

of sintered and unsintered porous titanium, it is necessary to study the poremorphology of porous structures. In the case of green (unsintered) compacts, thepores/crevices are sharper, smaller and closer together when the compaction pressureis high. In order to function as corrosion sites, the crevices must be wide enough toallow liquid entry and, at the same time, sufficiently narrow to maintain stagnancy ofthe electrolyte. Highly dense green compacts appear to have a pore morphology thatpromotes crevice corrosion. In highly porous green compacts, however, poremorphology is predominantly one with wide, interconnected channels that allow thefree flow of liquid and, thus, fewer sites are available for promoting crevice corrosion.These differences in pore morphology seem to account for the greater corrosionresistance observed in the highly porous green compacts.

In sintered compacts, however, particle-to-particle diffusion and grain boundarymovement (caused by the sintering process) change the pore morphology. In highlyporous compacts (produced under low compaction pressure), the wide open channelsare narrow and isolated, thereby providing a large number of sites capable of trappingliquid that eventually promote crevice corrosion. In the low porosity compacts(produced under high compaction pressure), the high densification promotes poreclosure during sintering, and the resultant number and size of crevices are smaller,causing these compacts to be more resistant to crevice corrosion attack.

Polarisation curves for the 5 t specimensIn both Figs 5 and 6, it can be seen that the polarisation curves for the 5 t specimens

are quite far away and different from the curves for the samples compacted to highertonnages. This can be explained by considering the relationship between the porosity

of a specimen and its compaction pressure, as shown in Fig. 3. In this figure, it can beobserved that as compaction pressure increases, porosity decreases at a decreasingrate. The drop in porosity is less and less severe as compaction pressure increases. Thismeans that the 5 t specimen has a porosity that is significantly higher than thosecompacted to higher pressures. Therefore, it can be deduced that the 5 t specimensshould have a pore morphology that is significantly different from that of the otherspecimens, the latter ones having greater similarity in corrosion behaviour with oneanother since their porosities are closer to one another.

CONCLUSIONS

The experiments conducted confirm that solid titanium and sintered poroustitanium both possess a distinct passivation range. However, unsintered titanium doesnot seem to passivate at all and suffers greater corrosion than solid titanium.Moreover, unsintered specimens which were compacted at higher pressuresexperienced more corrosion than those compacted at lower pressures, although

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1340 K.H.W. Seah et al.

sintered specimens behaved in the exact opposite fashion. Pore morphology of thecompacts is a possible explanation for this observed discrepancy.

Acknowledgements- -The authors would like to express their appreciation to Anuj Agrawal for his assistancein the collection of data.

I.2.3.4.5.6.7.8.9.

IO.Il.12.13.14.

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15. L.C. Lucas, J.E. Lemons, J. Lee and P. Dale, ASTM STP 953, 124 (1987).16. R.A. Buchanan, E.D. Kigney Jr and CD. Griffin, ASTM STP 953, 105 (1987).17. K.H.W. Seah and X. Chen, Corros. Sci. 34, 1841 (1993).

IX. D.C.Tipton, ASTM STP 866, 24 (1985).19. R.R. German, Powder Metallurgy Science. Metal Powder Industries Federation, Princeton, NJ (1984).20. Annual Book q f ASTM Standards, p. 102 (1991).21. Annual Book of ASTM Standards, p. 73 (1991).22. R.R. Ogden, Titanium in Rare Met&s Handbook. Prentice-Hall, NJ (1965).23. Annual Book qf ASTM Standards, p.29 (1991).

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