surface chemistry of bulk nanocrystalline pure iron and...

Surface chemistry of bulk nanocrystalline pure iron andelectrochemistry study in gas-flow physiological saline

F. L. Nie,1,2 Y. F. Zheng1,2,3

1State Key Laboratory for Turbulence and Complex System, College of Engineering, Peking University, Beijing 100871, China2Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China3Center for Biomedical Materials and Tissue Engineering, Academy for Advanced Interdisciplinary Studies,

Peking University, Beijing 100871, China

Received 30 November 2011; accepted 18 March 2012

Published online 7 May 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.b.32713

Abstract: Conventional microcrystalline pure iron (MC-Fe)

becomes a new candidate as biodegradable metals, which

has the insufficient physical feature and inferior biodegrada-

tion behavior. Novel bulk nanocrystalline pure iron (NC-Fe)

was fabricated via equal channel angular pressing technique

in the present work to overcome these problems. The contact

angle test with water and glycerol droplets shows a smaller

angle (though >90�) of NC-Fe than that of MC-Fe, which

implies a lower surface energy of NC-Fe. The surface rough-

ness of NC-Fe increased greatly than that of MC-Fe. A further

comparative study of corrosion and electrochemistry per-

formance between NC-Fe and its original MC-Fe was investi-

gated in physiological saline with different dissolved oxygen

concentration, aiming to in vitro simulate the corrosion pro-

cess of coronary stent occurred in physiological environment.

The electrochemical impedance spectra analysis and anodic

polarization measurements indicated that the NC-Fe exhibited

higher corrosion resistance than that of the MC-Fe; mean-

while obvious enhanced corrosion resistance with the decre-

ment of dissolved oxygen concentration was observed.

Related equivalent circuit model and surface reconstruction

process were further discussed, and the degradation mecha-

nism of the MC-Fe and NC-Fe were finally established. VC 2012

Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater

100B: 1404–1410, 2012.

Key Words: NC-Fe, surface characterization, biodegradation,

oxygen-dependent solution

How to cite this article: Nie FL, Zheng YF. 2012. Surface chemistry of bulk nanocrystalline pure iron and electrochemistry study ingas-flow physiological saline. J Biomed Mater Res Part B 2012:100B:1404–1410.

INTRODUCTION

Biodegradable metallic materials1–3 have been concentratedand investigated throughout the world, which are supposedto be the next generation biomaterials on prosthetics inplace of permanent implants in vivo. Free from long-termrisks of the postoperation inflammation and complicationderived from the everlasting materials as an ‘‘invasive’’ ex-otic with the host in the recent report,4 biodegradable mate-rials would completely disappear within the human bodyonce fulfilling their functionality and necessary task in theadorably appropriate duration.

Pure iron was lately developed as one of the revolution-ary biodegradable stent materials for its strong radialstrength, high elastic moduli, and relatively long degradationlife,4–6 in order to replace the traditional permanentimplants and bio-inert stent materials, such as 316L stain-less steel and L605 CoCr alloys within blood vessel. Prelimi-

nary tests from literature on microcrystalline pure iron(MC-Fe) stents indicated no obvious adverse effects hap-pened in rabbits or porcine models.4,7,8 The only negativereport4,9 was about an early failure owing to insufficientstrength caused by hydrogen embrittlement or aging. Toavoid the failure caused by strength lack, bulk nanocrystal-line pure iron (NC-Fe) by equal channel angular pressing(ECAP) was utilized in the present study. A combination ofelevated strength and adorable ductility was achieved dueto nanocrystallization.10 Meanwhile NC-Fe brings additionalbenefit for the stent design, that is, high strength willreduce the thickness of the stent with the same stiffness incomparison to MC-Fe, which could effectively enlarge thelumen of blood vessel and might prevent the restenosisfrom the ideally initial.

To our knowledge, there were plenty of reports on thecorrosion behavior of MC-Fe in quasi-industrial environment

Correspondence to: Y. F. Zheng; e-mail: [email protected]

Contract grant sponsor: National Basic Research Program of China (973 Program); contract grant number: 2012CB619102

Contract grant sponsor: Research Fund for the Doctoral Program of Higher Education; contract grant number: 20100001110011

Contract grant sponsor: National High Technology Research and Development Program of China (863 Program); contract grant numbers:

2011AA030101, 2011AA030103

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 31170909

1404 VC 2012 WILEY PERIODICALS, INC.

or more aggressive acid/alkaline solutions,11–13 yet the cor-rosion of MC-Fe in body-like fluids is rarely studied, to saynothing of the NC-Fe. The air-related corrosive nature ofFe-based materials14 intrigues taking the oxygen concentra-tion, one dominant influence factor of the biocorrosion behav-ior of the metal stent used within blood vessel,15 intoaccount. Thus, gas-flow physiological saline (PS) with variousoxygen contents was adopted as electrolyte here at body tem-perature. The purpose of the present study is to investigatethe electrochemical property and surface characterizations ofNC-Fe and make clear of its biodegradation mechanism ingas-flow PS, in comparison with the MC-Fe as the control.

MATERIALS AND METHODS

Materials preparationAs-received commercial MC-Fe (>99.8 wt %, with averagegrain size of 50 lm) and resulting NC-Fe (with grain sizeranging from 80 to 200 nm10) were employed in this study.NC-Fe was fabricated after eight passes and following heattreatment by ECAP technique via Bc way and experimentaldetails can be found in our previous work.16 All the sampleswith the size of U 8 mm � 1 mm were mechanicallypolished up to 2000# grit, ultrasonically cleaned in acetone,absolute ethanol and distilled water, and then dried.

Surface characterizationThe topography and roughness of naked surface from NC-Feand MC-Fe samples were recorded by AFM (SPA-400,SIINT). The dimensions of the explored zone were 10 � 10lm2 and triplicated measurements were done for eachgroup of samples.

Two different liquids were used to carry out the contactangle (CA) measurements: distilled water and glycerol drop-lets. The values of the polar and dispersive components oftheir surface tension are shown in Table I. The testing ofCAs were conducted by the sessile-drop method with a CAsystem at the ambient temperature. From the CA resultsobtained with the two liquids studied, the surface freeenergy (SFE) of each surface was calculated by Owens–Wendt–Kaelble theory.17

The surface morphology and microstructure of thesamples before and after corrosion were characterized byenvironmental scanning electron microscopy (ESEM,AMRAY-1910FE). The change of surface chemical composi-tion of experimental samples was analyzed by XPS (AXISUltra, Kratos).

Electrochemical measurementA three-electrode cell was used for electrochemical meas-urements. The counter electrode was made of platinum andthe reference electrode was saturated calomel electrode.The exposed area of the working electrode to the solutionwas 0.1256 cm2. Electrolytes with different oxygen contentwere used by consistent flowing nitrogen or oxygen afterpreaerating for 15 min, including oxygen-free PS (withoxygen concentration of 1.62 lg/L), oxygen-rich PS (withoxygen concentration of 19.24 lg/L), and naturally aeratedPS (normal PS, with oxygen concentration of 9.17 lg/L).Anodic polarization measurements (with scan rate of0.00033 V/s) and electrochemical impedance spectra (EIS)tests were all carried out on an electrochemical workstation(CHI660C, China) at the temperature of 37�C. The worksta-tion is sealed and the oxygen concentration was controlledby continuous air flowing at a steady velocity. EIS data wereobtained with signal amplitude of 10 mV around opencircuit potential values in the frequency range of 105–10�2

Hz after 7200-s immersion in the given solution.The corrosion rate was calculated according to ASTM-

G59-9718 and ASTM-102-8919 using the following formula:vcorr ¼ K1 Icorr EW/q, MR ¼ K2 Icorr EW, where vcorr or CRis the corrosion rate in terms of penetration rate, MR is thecorrosion rate in terms of mass loss rate, EW is the equiva-lent weight, K1 ¼3.27 � 10�3mm g/(lA cm year) and K2 ¼8.954 � 10�3g cm2/(lA m day). CR is given in mm/year,MR in g/m2/day, and Icorr in lA/cm2.

The inductively coupled plasma atomic emission spec-trometry (Leeman, Profile ICP-AES) was employed to mea-sure the concentration of Fe ion dissolved from the samplesinto the solution.

Statistical analysisThe measurement of the CAs and AFM tests were per-formed at least three times, data averaged and expressed asmean 6 standard deviation. Statistical analysis was per-formed using one-way analysis of variance and significancewas considered at p = 0.05.

RESULTS

Surface characterizationsThe wettability was investigated by the static CA measure-ment. Figure 1 shows the results of CA on the flat surfaceof MC-Fe and NC-Fe samples under water and glycerol drop-lets, respectively. Hydrophobic surface can be seen withnaked substrate by water droplet, with the CA over 90� for

TABLE I. Parameters of Surface Physics of NC-Fe and MC-Fe

SFE (mJ/m2) ct cd cp Contact Angle (�) Ra (nm)

MC-Fe 33.42 6 14.99 26.77 6 16.92 6.65 6 1.95 93.90 6 4.55 76.17 6 5.23NC-Fe 30.65 6 13.09 25.49 6 15.57 5.16 6 2.52 91.33 6 1.84 152.80 6 13.75Surface tension (mN/m)a t Dispersive component Polar component – –Water 72.8 21.8 51 – –Glycerol 63.4 37.4 26 – –

a Shown here stands for the proper value of the separate liquids in order to calculate the SFE.

ORIGINAL RESEARCH REPORT

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH B: APPLIED BIOMATERIALS | JUL 2012 VOL 100B, ISSUE 5 1405

both MC-Fe and NC-Fe. Taken from the plot, it can be seenthat the value of CA for NC-Fe is smaller than that of MC-Fe,although there is no significant difference between them.Talking about surface energy of the experimental samplesreflected by glycerol drip, values of total SFE, dispersive andpolar components for NC-Fe are smaller than those for MC-Fe. As for the measurement of morphology and roughnessunder AFM, as shown in Figure 2, it can be clearly seenthat the surface of NC-Fe is rougher than that of MC-Fe(152.80 nm vs. 76.17 nm).

Electrochemical corrosion and ion releaseFigure 3 shows the polarization curves of NC-Fe and MC-Fetested in oxygen-free PS, normal PS, and oxygen-rich PS.The shape and alignment of all the plots behave active-alike,without neither the presence of passivation nor pitting cor-rosion. Various electrochemical property data (icorr andEcorr, which were calculated from the intersection of theTafel-domain slopes) were deduced from Figure 3 and listedin Table II, from which MC-Fe exhibits an accelerated corro-sion progress 1–5 times faster than that of NC-Fe. Figure 4displays Nyquist plot and Bode diagram of experimental

samples, from which it can be clearly seen that NC-Fe showsmuch broader contour of the semicircle than MC-Fe at thesame oxygen level.

An equivalent circuit model Rs (QRt) for both NC-Fe andMC-Fe is proposed and inset in Figure 4. As listed in TableII, Rt is higher (about 1.1–1.3 times) in the case of NC-Fethan that of MC-Fe, which keeps reciprocally proportional tothe corrosion rate. The curve and peak under oxygen-freePS shifted toward the low frequency when the grain sizedecreases from MC-Fe to NC-Fe (from 100.54 to 10�0.21 Hz),which downgrades the Rt value and corresponds to the low-frequency minimum magnitude of the impedance. dmax (themaximum phase angle) and xd

max (the frequency corre-sponding to the maximum phase angle) of NC-Fe in oxygen-containing PS are approaching identical. The high-frequencyregion ending with its phase angle about �40� implies athinner and loosen layer. For both NC-Fe and MC-Fe in oxy-gen-free PS, elevated values of peak and Zim stand upwardthan those in oxygen-containing PS, all of which give anexplicit clue of increased corrosion resistance.

Figure 5 displays the concentration of Fe ions collectedin the given medium after the electrochemical test. Theamount of dissolved Fe slowly climbed up from 3.5 to 5.7lg/mL for MC-Fe when the oxygen concentration in theelectrolyte changes from low to normal value. As more oxy-gen is bubbled into the solution till its saturation, there is amuch sharp ascent of Fe ion, �13.4 lg/mL for MC-Fe. Whilefor NC-Fe, a much gentle increase slope of Fe ion can beobserved during the course of different solutions.

Figure 6 reveals the XPS characterization of surfacechemistry change of MC-Fe and NC-Fe after corrosion fromthe normal PS, oxygen-free PS, and oxygen-rich PS, respec-tively. Representative peaks of O 1s (530.14 eV) and Fe 2p(710.84 eV) position can be distinctly recognized amongthe spectral lines of all the samples, as well as seldom dis-tributed Cl 2p (198.20 eV) and Na 1s (1071.50 eV). It indi-cates that a mixture of different stoichiometry compositionof iron and oxygen/hydroxy (FexOy or FeOOH) and sodiumsalt were formed on the surface.

FIGURE 1. Hydrophilicity and surface energy of NC-Fe and MC-Fe.

FIGURE 2. Surface roughness and contour profile of NC-Fe and MC-

Fe. [Color figure can be viewed in the online issue, which is available

at wileyonlinelibrary.com.]FIGURE 3. Representative anodic polarization curves of NC-Fe and

MC-Fe in saline.

1406 NIE AND ZHENG SURFACE CHEMISTRY OF BULK NANOCRYSTALLINE PURE IRON

DISCUSSION

Some properties such as mechanical strength and corrosionresistance were greatly improved in the cases of bulk metalswhen their grain size downgrades to nanoscale in litera-ture,10,20 even the cellular responses or other biological per-formances.21 Taking the surface topography in Figure 2 forexample, a much rough surface can be illustrated in thecase of NC-Fe, which might be good for the protein adsorp-tion,22 cell spread, and attachment.23 Wettability, to someextent, as a reflection of surface chemistry or structure, is

another bioavailability-dependent feature of surface. It isreported that a-Fe2O3 as hematite covered on the surfaceproved to be intrinsically hydrophilic.24 A hydrophobic sur-face (CA > 65� over or SFE < 50 mJ/m2) was demonstratedon both MC-Fe and NC-Fe in the present study, which canbe well explained by the identical coverage of chemical com-position and phase constitute for MC-Fe and NC-Fe from theBragg pattern of XRD in our previous study.10 Moreover,similar tendency of a much rough surface with a smaller CAand lower free energy had been reported on the SPD-induced NiTi samples.22,25

It is easily seen in Figure 3 that the body curve of NC-Fein oxygen-free PS stays significantly top-left above that ofMC-Fe, representing a much lower current density andpretty higher corrosion potential of NC-Fe. However, slightdifferences were shown between NC-Fe and MC-Fe in oxy-gen-containing PS (representing both normal PS and oxy-gen-rich PS), due to the oxygen-dependent corrosion natureof pure iron. The data of corrosion potential and the rele-vant electrochemical parameters from Figure 3 and Table IIdemonstrate the influence of grain refinement effect, anddissolved oxygen concentration on the degradation behaviorof pure iron can be more clearly concluded. The currentdensity of both NC-Fe and MC-Fe, which is a mainly and

TABLE II. Specific Data of Electrochemical Parameters and Corrosion Rates Derived From That of NC-Fe and MC-Fe in the

Given Solution

OCP(V)

Tafel

CRcorr

(mm/year)

MRcorr

(g/m2/day)

EIS

icorr(A/cm2)

Ecorr

(V)

Rs

(Ohmcm2) CPE-T CPE-P

Rt

(Ohmcm2)

xdmax

(Hz)dmax

(�)

MC-Fe in oxygen-rich PS �0.637 1.094 � 10�5 �0.496 0.127 2.743 192.9 1.6082 � 10�4 0.67693 2406 100.64 36.6NC-Fe in oxygen-rich PS �0.633 8.59 � 10�6 �0.571 0.100 2.154 212.5 2.3194 � 10�4 0.7871 3213 100.11 47.2MC-Fe in normal PS �0.643 1.01 � 10�5 �0.532 0.117 2.532 207.8 3.911 � 10�4 0.6438 2789 100.02 36.1NC-Fe in normal PS �0.693 8.189 � 10�6 �0.583 0.095 2.053 219.6 2.3918 � 10�4 0.79945 3181 100.1 47.8MC-Fe in oxygen-free PS �0.645 3.413 � 10�6 �0.8 0.039 0.856 169.3 9.8028 � 10�5 0.79149 5503 100.54 55.2NC-Fe in oxygen-free PS �0.674 7.775 � 10�7 �0.739 0.009 0.195 98.1 6.9077 � 10�4 0.68363 8858 10�0.21 55.8

CR denotes corrosion rate or penetration rate, MR denotes mass loss rate.

FIGURE 4. (a) Bode plots and (b) Nyquist plots of NC-Fe and MC-Fe

in saline. The fitted or simulated curves were obtained using the

model.

FIGURE 5. Ions released into the solution for NC-Fe and MC-Fe after

electrochemical tests. [Color figure can be viewed in the online issue,

which is available at wileyonlinelibrary.com.]



directly corresponding parameter to dynamically character-ize the corrosion rate, was found to exhibit an oxygen-de-pendent tendency and increase with the increment of oxy-gen content. Moreover, EIS in Figure 4 gives a map that alarger diameter corresponding to the extended arc indicatesa bigger R value and stronger corrosion resistance. Thesemicircles exhibiting equivalent arc pattern with centersbelow the real axis thus indicate the charge-transfer processcontrol of the dissolution in pure iron. The overlapped Bodeof NC-Fe in normal PS and oxygen-rich PS showed little dis-parity due to the ineffective regulation out of the dissolvedoxygen threshold, similar to MC-Fe. Both the typical sharppeak from the Nyquist diagram and unique capacitive looprepresent the charge transfer resistance (Rt) in parallel onthe double layer capacity.

Corrosion rate either in terms of penetration rate ormass loss in the present study shows that an evidentlyslower degradation process is claimed in the case of NC-Feunder the normal PS. Although under the same environ-ment, MC-Fe undertakes a faster corrosion process, in anequivalent agreement with their corrosion rate data in ourprevious work on as-cast and as-rolled Fe.26 Together with

the present results and our anterior investigations, thisimplies that the corrosion rate of pure iron lies in the multi-ple factors such as the oxygen concentration (rich or free),the microstructure (nanocrystalline or microcrystalline), andthe size of the samples (film or bulk) in the given solutionwith preliminary pH and temperatures. In normal PS, thecorrosion rate of MC-Fe is 0.117 mm/year or equivalent2.532 g/m2/day and that for NC-Fe is 0.095 mm/year or2.053 g/m2/day. Although the oxygen was aerated till thesaturation or depletion in the normal PS (which is denotedas oxygen-rich PS or oxygen-free PS), the corrosion rate ofMC-Fe comes to faster or slower compared to that in thenormal PS, so does it with NC-Fe. The reason why nanocrys-talline microstructure affects a lot on the slow corrosionmechanism can be attributed to the balance of loweramount of impurities around the boundaries with slow-down penetration rate due to activation energy in theatomic-scale model.27 Whenever the dependent oxygen con-centration decreases or increases, both of the samples post-pone or accelerate its corrosion procedure correspondinglydue to the oxygen-involving corrosion nature. However, thegap of the corrosion rate between NC-Fe and MC-Feenlarges and this means that lagging response of NC-Fe cor-relates with its more stable status derived from nanocrystal-line texture and lower free energy when interacted with ox-ygen. Talking about the influence of peri-shape or size ofthe samples, the order of corrosion rate should be alignedas bulk < sheet < film accordingly,10 amid which evenslower degradation process was observed when the ironstent was implanted into the in vivo circulation system thanthat in vitro simulation.

Based on the SEM surface observation of the experimen-tal samples after electrochemical testing, Figure 7 compre-hensively illustrates the interface change and mass/electrontransfer for both MC-Fe and NC-Fe in oxygen-free PS andoxygen-containing PS, which reflects the dynamical responseat the surface/interface for different grain-sized pure ironunder different oxygen concentration. For MC-Fe, previouswork indicated that grooves and other defects would domi-nate the grain boundary,28 along which oxygen or hydrogenwas prone to get accumulated. The corrosive surface mor-phologies in Figure 7 also confirmed the typical grainboundary corrosion in the case of MC-Fe. In oxygen-free PS,hydrogen is inevitably involved at the first stage29 and obvi-ously predominate the dissolution. Hydrogen ions migrateand get adsorbed onto the sublayer of the samples [Figure7(a,b)], and an electrochemical desorption Volmer–Heyrov-sky model on hydrogen evolution is applied here owing tothe charge transfer in the intermediate hydrogen adsorp-tion-absorption (Hads and Habs) pattern.30 In comparison toMC-Fe [Figure 7(a)], NC-Fe [Figure 7(b)], lack of defects andgroove-like structures, would postpone the first stage ofhydrogen adsorption and prevent the further diffusion ofhydrogen into the bulk metal body due to its isotropicmicrostructure,31 thus slows down the corrosion rate. Underoxygen-containing condition [Figure 7(c,d)], the oxygen-con-suming corrosion nature of pure iron would accelerate thecorrosion of MC-Fe and NC-Fe under dissolved oxygen

FIGURE 6. XPS spectra of (a) NC-Fe and (b) MC-Fe in oxygen-depend-

ent saline. [Color figure can be viewed in the online issue, which is

available at wileyonlinelibrary.com.]


medium, in spite of its surface oxide coverage. The intergra-nular corrosion occurred with grains peeling off from sur-face for MC-Fe [Figure 7(c)], leaving the grain boundariesobservable in the etched holes. Slight cracks and thicker oxi-dation film can be found for NC-Fe [Figure 7(d)] due to itsnondeficient surface that can protect internal bulk metaleffectively.

CONCLUSIONS

A much rough surface with smaller CA and lower freeenergy is found for NC-Fe, compared with MC-Fe. Because

of its nanocrystalline nature, NC-Fe exhibits a lower currentdensity from polarization curves. Much broader semicircleand R values from EIS can be exactly obtained for the caseof NC-Fe in oxygen-free PS, normal PS, and oxygen-rich PS,which reflects a superior corrosion resistance performanceof NC-Fe in comparison with MC-Fe. NC-Fe is less sensitiveto the oxygen content than that of MC-Fe in 0.9% NaCl solu-tion; meanwhile a hydrogen adsorption mechanism controlthe corrosion procedure in oxygen-free solution, whereas aoxygen-consuming mechanism dominate the corrosion pro-cedure in oxygen-containing solutions.

FIGURE 7. Combined schematic diagrams showing corrosion mechanism and corresponding typical surface SEM images of (a) MC-Fe in oxy-

gen-free PS; (b) NC-Fe in oxygen-free PS; (c) MC-Fe in oxygen-containing PS; and (d) NC-Fe in oxygen-containing PS. [Color figure can be

viewed in the online issue, which is available at wileyonlinelibrary.com.]



REFERENCES1. Scharnweber D. Biodegradation of metals. In: Encyclopedia of

Materials: Science and Technology 2001; Elsevier, Oxford. pp

555–560.

2. Colombo A, Karvouni E. Biodegradable stents: ‘‘Fulfilling the mis-

sion and stepping away’’ Circulation 2000;102:371–373.

3. Heublein B, Rohde R, Kaese V, Niemeyer M, Hartung W, Haverich

A. Biocorrosion of magnesium alloys: A new principle in cardio-

vascular implant technology? Heart 2003;89:651–656.

4. Peuster M, Wohlsein P, Brugmann M, Ehlerding M, Seidler K,

Fink C, Brauer H, Fischer A, Hausdorf G. A novel approach to tem-

porary stenting: Degradable cardiovascular stents produced from

corrodible metal—Results 6-18 months after implantation into

New Zealand white rabbits. Heart 2001;86:563–569.

5. Hermawan H, Dube D, Mantovani D. Developments in metallic

biodegradable stents. Acta Biomaterialia 2010;6:1693–1697.

6. Mani G, Feldman MD, Patel D, Agrawal CM. Coronary stents: A

materials perspective. Biomaterials 2007;28:1689–1710.

7. Peuster M, Hesse C, Schloo T, Fink C, Beerbaum P, von Schna-

kenburg C. Long-term biocompatibility of a corrodible peripheral

iron stent in the porcine descending aorta. Biomaterials 2006;27:

4955–4962.

8. Waksman R, Pakala R, Baffour R, Seabron R, Hellinga D, Tio FO.

Short-term effects of biocorrodible iron stents in porcine coronary

arteries. J Interv Cardiol 2008;21:15–20.

9. Hermawan H, Moravej M, Dub�e D, Fiset M, Mantovani M. Degra-

dation behaviour of metallic biomaterials for degradable stents.

Adv Mater Res 2007;15–17:113–118.

10. Nie FL, Zheng YF, Wei SC, Hu C, Yang G. In vitro corrosion, cyto-

toxicity and hemocompatibility of bulk nanocrystalline pure iron.

Biomed Mater 2010;5:065015.

11. Shao HB, Wang JM, He WC, Zhang JQ, Cao CN. EIS analysis on

the anodic dissolution kinetics of pure iron in a highly alkaline so-

lution. Electrochem Commun 2005;7:1429–1433.

12. Tang YB, Liu L, Li Y, Wang FH. Evidence for the occurrence of

electrochemical reactions and their interaction with chemical reac-

tions during the corrosion of pure Fe with solid NaCl deposit in

water vapor at 600�C. Electrochem Commun 2010;12:191–193.

13. Bastos AC, Ferreira MG, Simoes AM. Corrosion inhibition by

chromate and phosphate extracts for iron substrates studied by

EIS and SVET. Corr Sci 2006;48:1500–1512.

14. Morita M, Sasada T, Nomura I, Wei YQ, Tsukamoto Y. Influence

of low dissolved oxygen concentration in body fluid on corrosion

fatigue behaviors of implant metals. Ann Biomed Eng 1992;20:

505–516.

15. Levesque J, Hermawan H, Dube D, Mantovani D. Design of a

pseudo-physiological test bench specific to the development

of biodegradable metallic biomaterials. Acta Biomater 2008;4:

284–295.

16. Yang G, Huang CX, Wang C, Zhang LY, Hu C, Zhang ZF, Wu SD.

Enhancement of mechanical properties of heat-resistant marten-

sitic steel processed by equal channel angular pressing. Mater Sci

Eng A 2009;515:199–206.

17. Owens DK, Wendt RC. Estimation of the surface free energy of

polymers. J Appl Polym Sci 1969;13:1741–1747.

18. American Society for Testing and Materials. ASTM G59 practice for

conducting potentiodynamic polarization resistance. In: Annual

Book of ASTM Standards. Philadelphia (PA): ASTM; 1986;1–4.

19. American Society for Testing and Materials. ASTM-G102-89,

standard practice for calculation for corrosion rates and related

information from electrochemical measurements. In: Annual Book

of ASTM Standards. Philadelphia (PA): ASTM; 1999;1–7.

20. Valiev RZ, Korznikova GF, Mulyukov Kh Ya, Mishra RS, Mukherjee

AK. Bulk nanostructured materials from severe plastic deforma-

tion. Phil Mag B 1997;75:103–189.

21. Salda~na L, M�endez-Vilas A, Jiang L, Multigner M, Gonz�alez-Carra-

sco JL, P�erez-Prado MT, Gonz�alez-Martın ML, Munuera L, Vilaboa

N. In vitro biocompatibility of an ultrafine grained zirconium. Bio-

materials 2007;28:4343–4354.

22. Michiardi A, Aparicio C, Ratner BD, Planell JA, Gil J. The influ-

ence of surface energy on competitive protein adsorption on oxi-

dized NiTi surfaces. Biomaterials 2007;28:586–594.

23. Faghihi S, Zhilyaev AP, Szpunar JA, Azari F, Vali H, Tabrizian M.

Nanostructuring of a titanium material by high-pressure torsion

improve pre-osteoblast attachment. Adv Mater 2007;19:1069–1073.

24. Lu HB, Liao L, Li JC, Shuai M, Liu YL. Hematite nanochain net-

works: Simple synthesis, magnetic properties, and surface wett-

ability. Appl Phys Lett 2008;92:093102.

25. Nie FL, Zheng YF, Cheng Y, Wei SC, Valiev RZ. In vitro studies on

microcrystalline, nanocrystalline and amorphous Ni50.2Ti49.8

alloy fabricated by high pressure torsion. Mater Lett 2010;64:

983–986.

26. Liu B, Zheng YF. Effects of alloying elements (Mn, Co, Al, W, Sn,

B, C and S) on biodegradability and in vitro biocompatibility of

pure iron. Acta Biomater 2011;7:1407–1420.

27. Beaunier L, Froment M, Vignaud C. A kinetical model for the elec-

trochemical grooving of grain boundaries. Electrochim Acta 1980;

25:1239–1246.

28. Kim TN, Balakrishnan A, Lee BC, Kim WS, Smetana K, Park JK,

Panigrahi BB. In vitro biocompatibility of equal channel angular

processed (ECAP) titanium. Biomed Mater 2007;2:S117–S120.

29. Wilson RE. The mechanism of the corrosion of iron and steel in

natural waters and the calculation of specific rates of corrosion.

Ind Eng Chem 1923;15:127–133.

30. Amokrane N, Gabrielli C, Ostermann E, Perrot H. Investigation of

hydrogen adsorption–absorption on iron by EIS. Electrochim Acta

2007;53:700–709.

31. Tao NR, Wang ZB, Tong WP, Sui ML, Lu J, Lu K. An investigation

of surface nanocrystallization mechanism in Fe induced by sur-

face mechanical attrition treatment. Acta Mater 2002;50:

4603–4616.


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