Influence of Microstructural Evolution Processed by ECAP on

Corrosion Behavior of Pure Magnesium in RPMI-1640 Medium

Taito Hosaka

, Iman Amanina

, Naohiro Saruwatari

, Shoichiro Yoshihara

and Bryan J. MacDonald

Department of mechanicalEngineering, University of Yamanashi, 4-3-11, Takeda, Kofu-city, Yamanashi, Japan

School of Manufacturing and Mechanical Engineering, Dublin City University, Dublin 9, Ireland

Keywords: Microstructural Evolution, ECAP Process, Corrosion Behaviour, Pure Magnesium, RPMI 1640 Medium,

Biomaterial.

Abstract: Influence of microstructure changes caused by Equal-channel angular pressing (ECAP) process on

corrosion behavior of pure magnesium in RPMI-1640 medium was investigated. The grain size of ECAPed

samples (30µm) were greatly reduced compared with the grain size of the annealed sample (200µm). Then,

the immersion test has been carried out for a certain period of time. It was revealed that mass loss of the

ECAPed sample is larger than the as-received sample and the annealed sample. Thus, it could be considered

that many crystal defects yielded by ECAP process reduced the corrosion resistance. However, the

corrosion resistance has been improved to a certain extent according to reduction of crystal defects through

the heat treatment at the recrystallization temperature or lower. In addition, the amount of gas generation of

the ECAP sample after immersion test is larger compared with the as-received sample. Therefore,

correlation between the amount of gas generated and the mass loss was confirmed. Based on qualitative

identification of the elements by Energy Dispersive X-ray Spectrometry (EDS), the corrosion products of

the sample surface after the immersion test has been estimated to be a kind of calcium phosphate. These

above results have indicated the potential for fabrication of magnesium as bioabsorbable materials.

1 INTRODUCTION

Magnesium (Mg) is present in high concentrations in

sea water and the eighth most abundant element on

the earth. It has also excellent specific strength and

low density, only two-thirds that of Aluminum. Then,

Mg and its alloys could be adopted in many

applications including computer parts, mobile

phones, aerospace components and handheld tools

(Wu, 2007).

Mg alloys are also potentially useful for bone

implants and stent applications due to their low

density, inherent biocompatibility, adequate

mechanical properties and fracture toughness higher

than that of ceramics (Mani, 2007). Additionally, the

elastic modulus of Mg alloys (40–45GPa) is closer

to that of human bones (10–40GPa) than other

commonly used implant materials such as Titanium

(106GPa). Eventually, the stress-shielding

phenomena caused by current metallic implants

made of stainless steel or Titanium alloy could be

minimized (Xu, 2007).

Another advantage of Mg in relation to other

metallic implants is the degradability of Mg alloys

which offers the possibility of repair and

reconstruction of vascular compliance with

minimum inflammatory response (Loos, 2007).

In recent years, the application of bioabsorbable

materials for medical implants has drastically

increased (Busch, 2014). Since magnesium is an

essential element in our body, it can be absorbed

into the body without leaving any harm. Therefore,

many studies have been focused on using

magnesium and its alloys for medical devices such

as bioabsorbable stents (Hiromoto, 2012).

Nowadays, the traditional permanent metallic

materials for the stents are modernly manufactured

from stainless steel (316L), nitinol and cobalt-

chromium alloys which have high corrosion

resistance and remain as a permanent implant in the

body. Thus, many limitations such as risk of chronic

irritation caused by released toxic substances which

might appear after long-term placement of the stents.

118

Hosaka T., Amanina I., Saruwatari N., Yoshihara S. and J. MacDonald B.

Inﬂuence of Microstructural Evolution Processed by ECAP on Corrosion Behavior of Pure Magnesium in RPMI-1640 Medium.

DOI: 10.5220/0006143201180125

In Proceedings of the 10th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2017), pages 118-125

ISBN: 978-989-758-216-5

Magnesium is able to overcome many of these

limitations since it could be absorbed into the body.

For the patients with the coronary artery disease, the

magnesium stents could temporarily function as a

scaffold for supporting the vascular stenosis. The

disappearance of the magnesium alloys stent is

necessary after 6-12 (Werkhoven, 2011) months

period. Therefore, the structural integrity of the

implanted stent has to be preserve until the surgical

region has completely heal, hence the corrosion

behaviour of Mg and its alloys have received

attention in recent years. However, study on the

degradation behaviour and mechanisms of

magnesium and its alloys in physiological

environment such as inside the human body have not

been elucidated.

The main factors affecting the corrosion

performance of metallic materials as bioabsorbable

stent include internal factors which are all about the

material itself such as microstructure, distortion,

surface condition and external factors such as

influence of blood flow, body temperature and pH

value. The effect of grain refinement by various

severe plastic deformation methods on the corrosion

behaviour of materials has caught much attention in

recent years. During the last decade, equal-channel

angular pressing (ECAP) has considered as the most

appropriate procedure for the fabrication of

ultrafine-grained (UFG) metals and alloys for

industrial application. It has been widely-known that

ECAP process significantly affects the mechanical

properties of magnesium and its alloys. Although

many studies have been done on the materials

processed by ECAP, their corrosion resistance has

been rarely reported (Song, 2010). The present work

focused on the influence of microstructure change

on corrosion resistance of pure Mg processed by

ECAP.

Therefore, in this study, we attempted to

investigate the immersion test on ECAPed pure Mg

in RPMI 1640 medium. The results would be helpful

for better understanding the corrosion behaviours of

ECAPed pure Mg and its alloys, and explore their

possibility for engineering applications.

2 EXPERIMENTAL

2.1 ECAPed Specimen Preparation

The specimen used for ECAP process was cut from

the extruded pure Mg round bar (99.96%). The

schematic illustration of the ECAP process is shown

in Fig.1. The billets of the specimen for ECAP with

the size of diameter Φ6 mm × length 50 mm were

annealed for 24 hours at 723 K after cutting process.

Then, the billets were repeatedly pressed for 4 times

with a plunger speed of 4 mm/s at 573 K.

Molybdenum disulfide as lubricant was used to

reduce the friction coefficient between the billet and

the die inner wall. During the each pressing process,

the billet was inverted, and then rotated by 90° to the

circumferential direction.

Figure 1: Schematic of ECAP process.

2.2 Experimental Conditions

Table 1 shows the kind of the samples on several

experimental conditions of the different material

types and the subsequent annealing times. Type (a)

shows the ECAPed pure magnesium sample, type

(b) the extruded (as-received) pure Mg sample and

type (c) the annealed at 723 K for 24 hours pure Mg

sample have been studied in comparison with the

several conditions. Furthermore, the subsequent

annealing process below the re-crystallization

temperature was applied to ECAPed samples in

order to verify the effect of crystalline defects on

weakening pure Mg’s corrosion resistance without

increasing grain sizes. It has been processed at 423K

for type (d) 2hours, type (e) 4hours and type (f)

6hours respectively to confirm the influence of the

difference conditions on processing time. Besides,

Park has reported that the subsequent annealing

process could release the strain energy and weaken

deformation texture of ECAPed samples (Park,

2008).

2.3 Microstructure Observation

The samples, used for optical microstructure

observation were cut on perpendicular to the

pressing direction from the ECAPed billet, and

polished with distilled water and etched with acetic

picral (100 ml 6 wt.% picric acid, 28.8 ml 99.7 wt.%

acetic acid and 10 ml distilled water). Optical

microscopy (NICON ECLIPSE MA200, Japan) and

Inﬂuence of Microstructural Evolution Processed by ECAP on Corrosion Behavior of Pure Magnesium in RPMI-1640 Medium

119

Table 1: Samples on difference conditions of Mg.

scanning electron microscope (JEOL JSM-6500F,

Japan) equipped with the Electron Back Scatter

Diffraction (EBSD) (EDAX) camera were adopted

to observe the microstructures and more details of

the samples.

The samples for EBSD were also cut perpendicular

to the pressing direction, polished on the emery

papers and buffing with the abrasive diamond

solutions of particle size 1μm and 0.25μm.

Afterward, they were etched with the solution of

10ml HNO

, 30ml acetic acid, 40ml H

O and 120ml

ethanol for around 10s.

2.4 Microhardness Tests

The Vickers hardness test was carried out to

investigate the mechanical properties of the samples.

Vickers hardness measurements were performed

using micro-hardness testing device (AKASHI

MVK-G3500AT, Japan). The samples were cut from

billets perpendicular to the pressing direction and

polished. Measurements were carried out at least

five times for each sample. The load of Vickers

indenter was 25g.

2.5 Corrosion Tests

The corrosion behaviour of the ECAPed pure Mg

was investigated by the immersion test for certain

period. ECAPed samples used were cut from the

core of the ECAPed billets, cleaned and polished to

avoid the surface contamination of lubricant and so

forth. The surface of all specimens was polished

with #2000 grits emery paper to ensure the same

surface roughness, and then cleaned in acetone by

ultrasonic washing machine. The Size of the test

sample was diameter Φ6mm × thickness 3mm,

respectively. RPMI 1640 medium (Sigma-Aldrich

R8758, Japan) as a corrosion solution was used for

immersion test in all experiments to simulate the

internal environment. The temperature of the

corrosion solution was 310K constant．In this study,

Fetal Bovine Serum (FBS) was not added to the

medium. The initial samples were polished with

emery papers up to #2000 grits and cleaned. The

measuring procedure of the polished samples was

weighed at first, immersed in RPMI1640 medium

(pH=7.4) for various intervals in corrosion test,

cleaned with a corrosion product removal solution

(20g CrO

+ 2g Ba (NO

)

+ 1g AgNO

+ 100ml

Distilled water) after corrosion test, washed with

acetone and weighed without corrosion product. The

above steps were repeated and the mass loss in every

interval was measured to evaluate the corrosion rate

of ECAPed samples [g/m

]. Mass loss was obtained

by the following equation.

ML = ( W

- W ) / S (1)

where, W

[g] is mass of the sample before corrosion.

W [g] is mass of after corrosion. S [m

] is Initial

surface area of the sample.

Moreover, Changes in the pH value in the

immersion test was recorded for 7 days.

In addition, the surface of the ECAPed sample

after the immersion test for 24 hours was observed to

identify the corrosion products. After the immersion

test, the sample was rinsed with distilled water

quickly, and dried to observe their surface by an

optical microscope. The surface of the sample was

analyzed by the

energy dispersive X-ray

spectroscopy (EDS) with the scanning electron

microscope (Japan electron JSM-7100F)

Furthermore, the immersion test for 24 hours of

ECAPed sample and As-received sample was

conducted to measure the amount of generated gas

with the corrosion test of the pure magnesium in

RPMI 1640 medium. Fig.2 shows the method of

immersion test.

BIODEVICES 2017 - 10th International Conference on Biomedical Electronics and Devices

120

Figure 2: Photograph of immersion experiment.

3 RESULTS AND DISCUSSION

3.1 Microstructures of ECAPed Pure

Mg Samples

Fig. 3(a)-(f) indicates the optical microstructures of

the as-received, the annealed, the ECAPed, the

subsequent aging for 2, 4 and 6 hours of the pure

magnesium samples. The average grain size

distribution before ECAP is around 200μm. Thus,

after the ECAP process, the average grain size

distribution of 30μm was reduced. It is noteworthy

that some larger grains are surrounded by some

much smaller grains in the deformed

microstructures. This phenomenon might be resulted

from dynamic recrystallization since the temperature

(at 573K) of ECAP process is higher than the

recrystallization temperature of Mg.

Fig.4(a)–(f) shows the inverse pole figure map of

these samples also. In comparison with the as-

received, the crystal orientation of ECAPed sample

has significantly changed. Thus the effect of the

shear stress applied into the material could be

confirmed.

Fig. 5(a)–(f) shows the image quality map by

EBSD of these samples. The polishing method of the

sample in EBSD was all the same approach. Hence,

it was considered that there is no difference in strain

applied to the surface of each sample by the

polishing. The crystalline of the ECAPed sample

was considered low compared to the untreated

sample and the annealed sample.

On EBSD, The factor affecting the quality of

diffraction patterns in a materials science standpoint

is the perfection of the crystal lattice in the sample.

Thus, any distortions to the crystal lattice of the

sample would be produced to the lower quality

(more diffuse) diffraction patterns. It enables the

Image quality (IQ) parameter to be used to give a

qualitative description of the strain distribution.

As seen in figure (a), (b) and (c), the ECAP

sample is confirmed to be less crystalline in

comparison with the as-received sample and the

annealed sample. It could be considered that ECAP

process increases the crystal defects of the material

by introducing a shear strain to the material at the

bent portion of the die and refining the grains.

In addition，from figure (a), (d), (e) and (f), the

crystalline of the subsequent aged sample was

slightly increased. Hence, it could be concluded that

crystal defects such as dislocation and strain of

ECAPed sample was recovered by subsequent

annealing from ECAP.

3.2 Microhardness Test

Fig. 6(a)–(f) shows the Vickers hardness results.

Compared to the as-received samples and the

annealed samples, the ECAP processed samples

Figure 3: Microscopic images on surface of Mg.

Inﬂuence of Microstructural Evolution Processed by ECAP on Corrosion Behavior of Pure Magnesium in RPMI-1640 Medium

121

Figure 4: Inverse pole figure map of surface of Mg.

have higher value of Vickers hardness. From figure

(a), (d), (e h) and (f), Vickers ardness of the

subsequent aged samples was slightly decreased as

the aging time increases the Vickers hardness

decreases. Based on grain observation in optical

microscope and EBSD, there is no change in grain

size of the ECAP sample and the subsequent aged

sample. Accordingly, It was considered that the

Vickers hardness of the subsequent aged samples

were reduced by the recovery of crystal defects in

the heat treatment below the recrystallization

temperature.

3.3 Corrosion Behaviour in Constant

Immersion Test

Constant immersion test is a more direct and

illustrative approach to detect corrosion behaviour of

a material. Fig.7 shows the mass loss of the

ECAPed, the as-received, the annealed and the

subsequent aging for 2, 4 and 6 hours pure Mg

samples after 168 hours (7 days) immersed in RPMI

1640 medium. From Fig.7, it is obvious result that

the ECAP sample has the largest mass loss in these

conditions. Additionally, it shows the annealed

sample has the most favourable corrosion resistance

among the samples. From the both results of the

grain size observation and the mass loss, during the

corrosion test of the pure Mg in RPMI 1640 medium,

there is no significant effect in grain size, however,

it could be suggested that the other corrosion factor

has remarkable effect. Furthermore, the corrosion

resistance of the subsequent samples was improved

as the heat treatment time increased.

Some researchers have reported that the

corrosion resistance of materials have been

improved by grain refinement which has been

decreasing general corrosion rate and alleviating

corrosion localization (Janecek, 2005). Also, the

better corrosion resistance of ECAPed Cupper (Cu),

Titanium (Ti) and industrial pure Aluminium (Al)

with deformed microstructures than the coarse

grains one have been indicated (Blyanov, 2004).

However, unexpectedly, the ECAP process might

weaken the corrosion resistance of the pure Mg. It

could be considered that many crystal defects

yielded by ECAP process reduced the corrosion

resistance.It was considered that this negative

phenomenon was related to the microstructure

characterization and the corrosion mechanism of

ECAPed pure Mg. It has been verified that the

corrosion has been caused primarily by surface

defects such as grain boundaries and dislocations

(Aust, 1994).

The above results have demonstrated that the

strain-induced crystalline defects are unfavourable

for the corrosion resistance of pure Mg. However the

corrosion behaviour of ECAPed pure Mg could be

improved by subsequent annealing process.

Fig.8(a)–(f) shows the pH value. The pH value in all

conditions was increased for approximately over 0-

12 hours and decreased over 12-24 hours. After 24

hours, the pH value was slightly decreased. This

tendency was particularly remarkable in the pure Mg

ECAPed sample and the subsequent aging for 2, 4

and 6 hours samples. In this result, FBS was not

added to the RPMI 1640 medium. There is a need to

investigate the pH level in the case of enhancing the

buffer function of the medium from now on. The

rapid increase in the pH of the early stage of

immersion test has not been sufficiently examined in

the present study.

Fig.9 shows the amount of gas generated the

ECAPed and the as-received sample after immersion

for 24 hours. The amount of gas generated of the

ECAPed sample was 1.72 ml/cm

, the as-received

BIODEVICES 2017 - 10th International Conference on Biomedical Electronics and Devices

122

Figure 5: Image quality map on surface of Mg.

sample was 1.28 ml/cm

. The ratio on gas generation

volume of the as-received sample and the ECAPed

sample were similar to the ratio on the mass loss of

the as-received sample and the ECAPed sample.

Therefore, it was suggested that there is a correlation

between the amount of mass loss and the amount of

gas generation.

Fig.10 shows the corroded surface of the

ECAPed sample after immersion test for 24 hours by

SEM. The Localized corrosion products could be

observed on the sample surface. Fig.11 shows the

result of the chemical element analysis in the same

microscopic field by EDS. From the result, the main

component elements of the corrosion products

present in the sample surface were oxygen O,

phosphorus P and calcium Ca. It could be assumed

that the chemical compound produced on sample

surface during the corrosion of pure magnesium in

RPMI 1640 medium was a type of calcium

phosphate. The peak of hydrogen of the light metal

could not be detected by EDS.

Figure 6: Vikers hardness test results.

Figure 7: Relationship between mass loss and immersion

time.

Figure 8: Relationship between pH and Immersion time.

Inﬂuence of Microstructural Evolution Processed by ECAP on Corrosion Behavior of Pure Magnesium in RPMI-1640 Medium

123

Figure 9: Volume of gas generation on ECAPed and As-

received Mg.

Figure 10: SEM image of ECAPed pure Mg sample after

immersional test.

4 CONCLUSIONS

Fine grained pure Mg could be manufactured

through the ECAP process at 573K. Also, the

subsequent annealing process below the re-

crystallization temperature was applied by using the

ECAPed samples in order to verify the effect of

crystalline defects on weakening corrosion

resistance of pure Mg without increasing grain sizes.

The effect of microstructure on the corrosion

behavior of the ECAPed pure Mg was investigated

by optical microscopy and SEM observation,

microhardness test and certain period immersion

tests in RPMI 1640 medium.

(1) The difference in the microstructure by ECAP

processing was observed by optical microscope and

SEM with EBSD. The average grain size of ECAPed

samples (30µm) were greatly reduced compared

with the annealed sample (200µm). From the result

of EBSD, the crystal orientation of ECAPed sample

in comparison with the as-received and the annealed

sample has significantly changed. The ECAPed

sample was found to be less crystalline alongside of

the as-received and the annealed sample. It could be

considered that the crystal defects of the material by

introducing a shear strain to the material have been

increased by ECAP process. In addition, it has been

confirmed that crystal defects such as dislocation

and strain of ECAPed sample was recovered by

subsequent aging.

(2) The corrosion behaviour of the ECAPed pure

Mg was investigated by the immersion test for

certain period. From the results of the immersion test,

it has been revealed that the corrosion resistance of

pure Mg in RPMI 1640 medium has been reduced

ECAP process. The strain-induced crystalline

defects were unfavourable for the corrosion

resistance of pure Mg. However, the corrosion

resistance of ECAPed pure Mg could be improved

by subsequent aging process. On the other hand, the

ratio on gas generation volume of the as-received

sample and the ECAPed sample were similar to the

ratio on the mass loss of the as-received sample and

the ECAPed sample. Therefore, it was suggested

Figure 11: EDS images of ECAPed pure Mg sample after immersion test.

BIODEVICES 2017 - 10th International Conference on Biomedical Electronics and Devices

124

that there is a correlation between the amount of

mass loss and the amount of gas generation. (3) The

surface of the ECAPed sample after the immersion

test for 24 hours was observed to identify the

corrosion products by EDS. The localized corrosion

products were observed on the sample surface. Then,

from the result, it could be assumed that the

chemical compound which has been produced on

sample surface during the corrosion of pure

magnesium in RPMI 1640 medium was a type of

calcium phosphate.

ACKNOWLEDGEMENTS

This work was supported by JSPS KAKENHI Grant

Numbers JP16K05974, JP25420010.

REFERENCES

K. T. Aust, U. Erb, G. Palumbo, 1994. Interface control

for resistance to intergranular cracking, Mater. Sci.

Eng. A, 176, pp. 329–334.

A. Blyanov, J. Kutnyakova, N.A. Amirkhanova, V.V.

Stolyarov, R.Z. Valiev, X.Z. Liao, Y.H. Zhao, Y.B.

Jiang, H.F. Xu, T.C. Lowe, Y.T. Zhu, 2004. Corrosion

resistance of ultra fine-grained Ti, Scr. Mater., 51, pp.

225–229.

Hiromoto, S., 2014. Corrosion behavior of magnesium

alloys for biomaterial use, Corrosion engineering of

Japan, 63, pp. 371–377.

Jae Yeol Park, Dong Nyung Lee, 2008. Deformation and

annealing textures of equal-channel angular pressed

1050 Al alloys strips, Mater. Sci. Eng. A, 497, pp.

395–407.

M. Janecek, B. Hadzima, R.J. Hellmig, Y. Estrin, 2005.

The influence of microstructure on the corrosion

properties of cu polycrystals prepared by ECAP,

Metall. Mater., 43, pp. 258–271.

A. Loos, R. Rohde, A. Haverich, S. Barlach, 2007. In vitro

and in vivo biocompatibility testing of absorbable

metal stents, Macromol. Symp., 253, pp. 103–108.

G. Mani, M.D. Feldman, D. Patel, C.M. Agrawal, 2007.

Coronary stents: a materials perspective,

Biomaterials, 28, pp. 1689–1710.

Raila, B., Anne, S., Stefanie R., Simon W., Mathias B.,

Svea P., Stephan F., Katrin S., 2014. New stent surface

materials: The impact of polymer-dependent

interactions of human endothelial cells, smooth muscle

cells, and platelets, Acta Biomaterialia, 10, pp. 688–

700.

Dan Song, AiBin Ma, , Jinghua Jiang, Pinghua Lin,

Donghui Yang, Junfeng Fan, Jung-Gu Kim, 2010.

Corrosion behavior of equal-channel-angular-pressed

pure magnesium in NaCl aqueous solution, Corrosion

Science, 52, pp. 481–490.

Werkhoven R. J.,Sillekens W. H., J. B. J. M. van Lieshout,

2011. Effect of surface roughness on the in vitro

degradation behavior of a biodegradable magnesium-

based alloy, Magnesium Technology, pp. 419–422.

C. S. Wu, Z. Zhang, F.H. Cao, L.J. Zhang, J.Q. Zhang,

C.N. Cao, 2007. Study on the anodizing of AZ31

magnesium alloys in alkaline borate solutions, Appl.

Surf. Sci., 253, pp. 3893–3898.

L. Xu, G. Yu, E. Zhang, F. Pan, K. Yang, 2007. In vivo

corrosion behavior of Mg–Mn–Zn alloy for bone

implant application, J. Biomed. Mater. Res. A., 83, pp.

703–711.

Inﬂuence of Microstructural Evolution Processed by ECAP on Corrosion Behavior of Pure Magnesium in RPMI-1640 Medium

125