corrosion behavior of biodegradable mg alloys in...

21. – 23. 9. 2011, Brno, Czech Republic, EU

CORROSION BEHAVIOR OF BIODEGRADABLE Mg ALLOYS IN EMEM MEDIUM

František HNILICAa, Luděk JOSKAb, Jaroslav MALEKa, Vítězslav BREZINAc, Bohumil SMOLAd,

Ivana STULIKOVAd

a Czech Technical University in Prague, Faculty of Mechanical Engineering, Technická 4, 166 07 Prague 6,

Czech Republic, [email protected]

b Institute of Chemical Technology Prague, Faculty of Chemical Technology, Technická 5, 166 28 Praha 6

c University of South Bohemia, Institute of Physical Biology, Zámek 136, 373 33 Nové Hrady,

Czech Republic, [email protected]

d Charles University Prague, Faculty of Mathematics and Physics, Ke Karlovu 5, 121 16 Prague 2,

Czech Republic

Abstract

Corrosion behavior and cytotoxicity testing of magnesium alloys with nominal composition Mg5Y4Nd (WE54)

and Mg4Y2Nn1Sc1Mn (WES) and different way of processing were investigated. The WE54 and WES alloys

contain several types of nano-sized precipitates of transient and stable phases that influence corrosion

degradation in the EMEM medium. Corrosion behavior is more influenced by larger structural components

(oxide particles and eutectic on grain boundaries) than by the nanoparticles. Spreading of MG63 cells in all

extracts is relatively good. Spreading index values reach or exceed 0.7. Viability index diminishes with time

slightly for the WE54 extracts, but the values after 24 hours exposure do not drop below 0.5. The extract of

the WES alloys in both conditions (as received and heat treatment T5) is cytotoxic.

Keywords: corrosion, cytocompatibility, Mg alloys, nano-sized precipitates

1. INTRODUCTION

Degradable implants invoke an increased interest as repeat surgery increases costs of the health care and

further patient morbidity [1,2]. Magnesium-based implants have the potential to serve as biocompatible,

osteoconductive, degradable implants for load-bearing applications. The weakness of pure Mg is its low

strength and poor corrosion resistance especially in human body environment compared to the bone

properties. An appropriate alloy composition can improve corrosion resistance, mechanical properties and

manufacturing of magnesium-based materials [3, 4]. Rare earth elements in combination with a small

amount of zirconium alloyed to Mg are recently investigated as prospective materials for temporary implants

[5, 6]. Precipitation hardening of Mg-rare earth alloys leading to microstructure with nano-scaled precipitates

oriented in the Mg matrix changes mechanical properties and corrosion rate essentially [7]. The alloy

production technology influences the final product properties, too. The aim of the work was the corrosion

behavior investigation of magnesium alloys with various rare earth combination and way of processing and

cytotoxicity testing.

2. EXPERIMENTAL PROCEDURE

2.1 Materials

Chemical composition of studied alloys, their processing and heat treatment are given in Table 1.

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Table 1 Composition of studied alloys (in wt.%) and their processing way

Alloy notation

Nominal composition

Y Nd Sc Mn RE Mg Way of processing

WE54T4 Mg5Y4Nd 4.75-5.5 1.5-2.0 - - 1-2 rest T4 (525 oC/8 h )

WE54T6 Mg5Y4Nd 4.75-5.5 1.5-2.0 - - 1-2 rest T6(T4+200 oC/24 h)

WE54PM400 Mg5Y4Nd 4.75-5.5 1.5-2.0 - - 1-2 rest Powder metallurgy, extruded at 400

oC

WESAC Mg4Y2Nn1Sc1Mn 3.71 2.12 1.28 1.1 rest As received

WEST5 Mg4Y2Nn1Sc1Mn 3.71 2.12 1.28 1.1 rest T5 (200 oC/5 h)

The WE54 alloy investigated was supplied by Magnesium Elektron Ltd. The nominal composition is

guarantied by the supplier as follows (in wt. %): 4.75 - 5.5 Y, 1.5 – 2.0 Nd, 1.0 – 2.0 heavy RE (Yb, Er, Dy,

Gd) and minimally 0.4 Zr [8]. Two heat treated WE54 alloys conventionally cast and one prepared by powder

metallurgy (PM) method were investigated. The cast alloy was T4 treated (solution treatment at 525 °C for 8

h) or T6 treated (T4 + aging at 200 °C for 24 h), notation WE54T4 and WE54T6 respectively. The PM

material was prepared from the supplied material by gas-atomization using Ar + 1 % O2, sieved powder (~30

µm) was consolidated at 400 °C by hot extrusion to bars of ~15 mm diameter and slowly cooled to room

temperature. The alloy marked as WES was studied in as-received state (WES AC notation) as well as after

the T5 treatment (aging at 200 oC for 45 hours) (WES T5 notation).

2.2 Experimental methods

Microstructure of Mg alloys analysis

Microstructure of all alloys was investigated by means of optical metallography (LM), scanning electron

microscopy (SEM), transmission electron microscopy (TEM) and electron diffraction (ED).

Immersion test

Disc-shaped specimens (8 mm in diameter, 3 mm thick) for immersion test were machined from all studied

alloys. All samples were ground with SiC emery papers of up to 2000 grid and then cleaned in alcohol for 5

min, dried in air and sterilized by UV radiation. Four discs from each alloy ( 0.28 g) were immersed in

EMEM solution (28 ml, pH = 7.2, 37oC) without BFS (bovine fetal serum) and shaken (60 rpm) for 5 days.

EMEM solution consist of 200 mg/l CaCl2, 400 mg/l KCl, 97.7 mg/l MgSO4 (anhydrous), 6 800 mg/l NaCl, 140

mg/l NaH2PO4, 2 200 mg/l NaHCO3 + amino acids and vitamins. Specimens were taken out after 3 or 5 days

and rinsed in alcohol.

Corrosion surface layers characterization

Both surface layer top-views and cross-sections of the corrosion layers were examined by LM and SEM.

Energy dispersive X-ray analysis (EDX) was used for the chemical analysis of the corrosion layers.

Corrosion layers cross-sections perpendicularly to the corroded surface were prepared. The embedded

samples were grinded and mechanically polished with diamond suspension in a water free lubricant. The

specimens were then etched by 2% Nital etchant.

Cytotolerance test

BFS was added into the resulted extracts and spreading and viability of MG63 cells (supplied by Sigma,

collection ECACC) were determined by time lapse micro cinematography using Olympus IX51 microscope in

air atmosphere with 5% CO2 at 37oC during 24 hours. Spreading index value after the 1. hour in the extracts

and time dependence of viability index were determined. Cell spreading index is defined as the ratio of the


spread cells number to the total cells number and viability index as the ratio the vital cells number to the total

cells number.

3. EXPERIMENTAL RESULTS AND DISCUSSION

3.1 Structure of alloys

The structure of both WE54T4 and WE54T6 consist of equiaxed grains with the size of (130 ± 10) µm and

(102 ± 8) µm respectively (Fig. 1). Smaller grain size in the WE54T6 is most probably caused by a recovery

of sub-grains that were introduced by quenching of the specimen after the T4 treatment into water at RT.

Fig. 1 Microstructure of WE54T6 alloy (LM) Fig. 2 D019 and Cbco phase prismatic plates in WE54T6 specimen. TEM image, zone [0001]α-Mg

Fine prismatic plates of the metastable D019 (hexagonal structure 63/mcc, a 2aMg, c cMg) and of the C-

base centered orthorhombic (Cbco, a 2aMg, b Mghkl 0110d8 , c cMg) phases precipitated during aging at

200 o

C in a dense triangular arrangement (Fig. 2). SEM images of PM prepared WE54 after extrusion at

400oC are in Fig. 3 and 4, respectively. Alloy structure consists of long fibers elongated in the direction of

extrusion, with the size of fibers in the transverse direction of (5.6 ± 0.9) μm. Oxide lines are located along

fibers with an oxide size of tenth of μm. The fibers are composed of cells surrounded by a phase containing

more of heavy elements than the α-Mg matrix solid solution in cell interiors.

Fig. 3 SEM images of WE54 alloy prepared PM

after extrusion (longitudinal orientation)

Fig. 4 SEM images of WE54 alloy prepared PM

after extrusion (transverse orientation)

This is confirmed by TEM image (Fig. 5) and ED [9]. Analysis of the ED pattern from this area can be

consistently indexed on the base of the stable Mg5Gd-type phase (fcc, a Mghkl 0110d8 ) that is observed in

WE alloys [10, 11]. Some particles of another stable phase of the Mg41Nd5- type (bct, a = 1.47 nm, c = 1.03

nm) were identified in cell interiors. The obtained ED patterns do not exclude existence of some other

metastable phases known to precipitate in WE43 and WE54 alloys, namely D019, Cbco and β1 phase (fcc, a

= 0.74 nm), as they have plane spacing values very near to those of the stable β phase. Structure of the


WESAC alloy is shown in Fig 6. Grain boundaries are decorated by eutectics of the α-Mg and fcc phase with

structure isomorphous with the Mg5Gd (fcc phase) containing a mixture of Y and Nd instead of Gd. A dense

Fig. 5 TEM image of WE54PM400 Fig. 6 Grain boundary eutectic in WES alloy

dispersion of metastable Cbco phase (prismatic platelets of 40 nm diameter, 5 nm thickness) in a triangular

arrangement precipitates in the matrix along the boundary eutectics during the T5 heat treatment (Fig.7).

Very thin basal plates containing Y and Mn (thickness about 1 nm) and Mn2Sc basal discs (diameter of 20

nm, thickness of 5 nm) were observed in the matrix, too.

Fig. 7 TEM image of the WEST5 alloy Fig. 8 Corrosion morphologies

(view perpendicular to surface)

3.2 Surface corrosion layers characterization

The morphology of the corroded surface is shown in the SEM images - Figs. 8, 9. Two morphologically

distinguishable regions are characteristic for almost all samples. The first is an area with more or less

smooth surface (Fig. 8) and the second an area with ragged surface covered by small globular particles (Fig.

9). Formations with holes in the middle resembling volcanoes occur with different frequencies on all surfaces

(Fig. 8 and 9). Corroded surfaces of the WES alloys are the most rough. Cross-sections of the corrosion

layers formed in 5 days are shown in Figs. 10, 11 and 12. The boundary between the corrosion layer and

the alloy are not comparably smooth, the corrosion process is more localized at some places. The WE54T4

and WE54T6 alloys corrode inside grains (Fig. 10). A preferential corrosion along grain boundaries was not

observed. A different corrosion mechanism for the T4 state and the T6 state is not visible qualitatively. In

contrast, a significant difference between the cast alloys (WE54T4, WE54T6) and the alloy prepared by

powder metallurgy (WE54PM400) was observed. The interface between the alloy prepared by powder

metallurgy and its corrosion layer consists of steps (Figs. 11), which indicates that the corrosion process is

influenced by the fibrous structure and by the presence of oxide particles arranged in rows. Fig. 12 shows a

cross-sectional view of WESAC after corrosion test. The Mg matrix is mainly attacked, while the dendritic

phase is relatively protected.


Fig. 9 Corrosion morphologies


Fig. 10 Corrosion morphologies of WESAC sample


Fig. 11 Cross-sectional view of WE54PM400 sample

after corrosion (backscatter electron)

Fig. 12 Cross-sectional view of WESAC sample after

corrosion (LM)

Corrosion layers are inhomogeneous in the PM material as clearly seen in Fig. 11. A different chemical

composition at different locations of the layers was confirmed by energy dispersive X-ray analysis. The

results indicate that the corrosion layers consist mainly of Mg, O, Y, C, Ca and a small amount of Cl, S and

Na. Some spectra show a very high fraction of Mg and O and a small fraction of other elements. A stronger

Y and Ca signal was detected in other measurements. It should be noted that it is not possible to detect

hydrogen with EDX and that the signal comes from a material volume of several micrometers in size.

Measurement results confirmed influence of microstructure on corrosion behavior. To determine influence of

alloying and heat treatment other experimental approaches are planned to be used in future.

3.3 Cytotolerance test

Spreading index of MG63 cells determined after the first exposition hour is shown in Fig. 13. MG63 cells

were placed only into the EMEM+BFS instead into the extract in the control test. Spreading of MG63 cells in

all extracts is relatively good; the spreading index values reach or exceed 0.7. However, they are lower than

the values in the control test. Spreading index for the WE54 alloy extracts gradually decreases in the

sequence of WE54T4, WE54T6 and WE54PM400 states. The values for both states of the WES alloy are

almost identical and comparable with the value for the WE54 in the T6 condition. The time dependence of

cells viability in the range of 0-24 hours is shown in Fig. 14. Viability index diminishes with time slightly for

the WE54 extracts. Viability index decreases with a rate increasing in the sequence of WE54T4, WE54T6

and WE54PM400, but the values after 24 hours exposure do not drop below 0.5. The extract of the WES

alloys in both conditions (AC and T5) is cytotoxic. The viability indexes lie below 0.5 even at the experiment

beginning and fall to 0.2 after 24 hours.


0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

WE54

T4

WE54

T6

WE54

PM400

WES T5 WES AC Control

sp

red

ing

in

dex a

fter

1 h

ou

r

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 500 1000 1500 2000

time [min]

via

bilit

y in

dex WE54 T4

WE54 T6

WE54 PM400

WES T5

WES AC

control

Fig. 13 Spreading index of MG63 cells in extracts of

studied alloys

Fig. 14 Time dependence of viability index for MG63

cells in extracts of studied alloys

CONCLUSIONS

The investigated WE54 and WES alloys contain several types of nano-sized precipitates of transient and

stable phases that influence corrosion degradation in the EMEM medium. Corrosion behavior is more

influenced by larger structural components (oxide particles and eutectic on grain boundaries) than by the

nanoparticles.

Spreading of MG63 cells in all extracts is relatively good. Spreading index values reach or exceed 0.7, however, they are lower than the values in the control test.

Viability index diminishes with time slightly for the WE54 extracts. Viability index values decrease with a rate

increasing in the sequence of WE54T4, WE54T6 and WE54PM400 but the values after 24 hours exposure

do not drop below 0.5. The extract of the WES alloys in both conditions (AC and T5) is cytotoxic.

ACKNOWLEDGEMENT

The financial support by the Czech Science Foundation (project No. GACR 106/09/0407) is gratefully acknowledged.

LITERATURE

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[7] Neubert V, Stulıkova I, Smola B, Mordike B.L, Vlach M, Bakkar A, Pelcova, J. Mat. Sci.Eng.A 462,2007, 329–333

[8] www.magnesium-elektron.com, datasheet 480, 18.7.2011

[9] Vostry P, Stulikova I, Smola B, Samatova M, Hort N, Knoop F.M. Annealing response of gas atomized and hot extruded

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