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Influence of fluoride ions on corrosion performance of 316Lstainless steel as bipolar plate material in simulated PEMFCanode environments

Ying Yang a, Liejin Guo a,*, Hongtan Liu a,b,**a State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, PR ChinabDepartment of Mechanical and Aerospace Engineering, University of Miami, Coral Gables, FL 33124, USA

a r t i c l e i n f o

Article history:

Received 6 April 2011

Received in revised form

25 May 2011

Accepted 16 June 2011

Available online 27 July 2011

Keywords:

Bipolar plate

Corrosion

Stainless steel

Proton exchange membrane fuel cell

(PEMFC)

Passive film

* Corresponding author. Tel.: þ86 29 8266 38** Corresponding author. Department of Mech305 284 2019; fax: þ1 305 284 2580.

E-mail addresses: youngying@gmail.com0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.06.088

a b s t r a c t

Corrosion performance of 316L stainless steel as a bipolar plate material in proton

exchange membrane fuel cell (PEMFC) is studied under different simulated PEMFC anode

conditions. Solutions of 1 � 10�5 M H2SO4 with a wide range of different F� concentrations

at 70 �C bubbled with hydrogen gas are used to simulate the PEMFC anode environments.

Electrochemical methods, both potentiodynamic and potentiostatic, are employed to study

the corrosion behavior. Scanning electron microscope (SEM) and atomic force microscope

(AFM) are used to examine the surface morphology of the specimen after it is potentiostatic

polarized in simulated PEMFC anode environments. X-ray photoelectron spectroscopy

(XPS) analysis is used to identify the compositions and the depth profile of the passive film

formed on the 316L stainless steel surface after it is polarized in simulated PEMFC anode

environments. MotteSchottky measurements are used to characterize the semiconductor

passive films. The results of potentiostatic analyses show that corrosion currents increase

with F� concentrations. SEM examinations show that no localized corrosion occurs on the

surface of 316L stainless steel and AFM measurement results indicate that the surface

topography of 316L stainless steel becomes slightly rougher after polarized in solutions

with higher concentration of F�. From the results of XPS analysis and MotteSchottky

measurements, it is determined that the passive film formed on 316L stainless steel is

a single layer n-type semiconductor.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction a perfluorinated vinyl ether monomer containing a sulfonyl

The proton exchange membrane (PEM) widely used in

commercial proton exchange membrane fuel cell (PEMFC) is

Nafion developed by DuPont. Nafion is a kind of PEM with

perfluorinated sulfonic acid (PFSA) structure, which is derived

from the copolymerization of tetrafluoroethylene with

95; fax: þ86 29 8266 9033.anical and Aerospace Eng

(Y. Yang), lj-guo@mail.xj2011, Hydrogen Energy P

fluoride group in the side chain, and the sulfonyl fluoride

groups are chemically converted to the sulfonic acid groups

subsequently [1]. Under PEMFC working conditions, the PEM

will decompose due to the attacking of peroxy or hydroxy

radical on polymer endgroups with residual H-containing

terminal bonds [1,2]. During the process of PEM degradation,

ineering, University of Miami, Coral Gables, FL 33124, USA. Tel.: þ1

tu.edu.cn (L. Guo), hliu@miami.edu (H. Liu).ublications, LLC. Published by Elsevier Ltd. All rights reserved.

Table 1 e Main chemical composition of 316L stainlesssteel (wt. %).

C Cr Ni Mo Si Cu Fe

0.03 16.9 10.5 2.23 0.516 0.506 Balance

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 7 5e1 8 8 31876

the production of fluoride ions (F�) or hydrogen fluoride (HF)

will be released [1e5]. F� concentration in PEMFC environ-

ment is important due to two main reasons [6]: the amount of

F� in solution indicates degradation rates of the membrane

and is an indication of the expected life span of the

membrane, and F� help to induce corrosion of bipolar plate

materials as do other halide ions such as Cl� and Br�.It is obvious that different fuel cells are often operated

under different conditions, thus the degradation rates of the

membrane must be different, which leads to different

concentrations of F� in PEMFC environments. Curtin et al. [1]

reported that the degradation of PFSA polymer is the most

serious in the presence of peroxide radicals at low relative

humidity conditions and temperatures exceeding 90 �C.Besides, the flow fields and structures of bipolar plates are

different, so F� concentration must be different at different

locations. Furthermore, some previous studies [2,6,7] show

that the fluoride ion concentrations in realistic PEMFC envi-

ronments are complicated. Borup et al. [6] suggested that the

PEMFC operating conditions as pH 3.60 with F� concentration

of 1.8 ppm at anode and pH 4.02 with F� concentration of

1.1 ppm at cathode. Healy et al. [2] provided that the results of

F� concentration ranging from 5 � 10�6 M to 2 � 10�4 M in

product water from PEMFC in their experiments. Agneaux

et al. [7] analyzed working solutions from different PEMFC

anode and cathode after 500 h operation. They pointed out the

anode solutions were more lightly loaded with ions than the

cathode solutions, considering the overall contents of all

elements. The F� concentration is highest and ranges from

5.1 � 10�6 M to 5.7 � 10�4 M with pH 5.7e6.3 at anode side and

from 9.2 � 10�6 M to 4.1 � 10�4 M with pH 4.9e6.3 at cathode

side. Fukutsuka et al. [8] pointed out that it is very low of F�

concentration in PEMFC environments so that its effect on

corrosion resistance of bipolar plate materials can be

neglected.

Considering the above, it is very important to study the

corrosion performance of bipolar plate materials in PEMFC

environments containing different F� concentrations. Little

attention [9,10] has been focused on the effect of fluoride ions,

a kind of important aggressive ions in PEMFC environments,

on corrosion performance of bipolar plate materials. Li et al.

[10] studied corrosion performance of 316 stainless steel in F�

containing dilute hydrochloric acid and acetic acid solutions

aerated with oxygen gas at room temperature employing

electrochemical impedance spectroscopy method, and they

reported that passive impedance of 316 stainless steel

decreased slightly with increasing F� concentration. Yang

et al. [9] studied the effect of F� on the corrosion behavior of

316L stainless steel bipolar plates in simulated PEMFC cathode

environments employing Auger electron spectroscopy and

photo-electrochemical method, and they concluded the

passive film formed on 316L stainless steel was a bi-layer

semiconductor structure and the corrosion resistance of

316L stainless steel decreased with the increase of F�

concentration in the solution. However, scarcely any study

has paid attention to the effect of F� on the corrosion perfor-

mance of bipolar plate materials in PEMFC anode environ-

ments. Thus, this paper is focused on the anode

environments. In this study, a solution of 1 � 10�5 M H2SO4

was chosen as the basic solution [9,11] and different

concentrations of F� (NaF as the source of the F�) were used to

simulate different PEMFC environments. Since 316L stainless

steel is probably the most studied for metallic bipolar plates it

is chosen in this work. Corrosion performance of 316L stain-

less steel is evaluated with electrochemical methods. Scan-

ning electron microscope (SEM), atomic force microscope

(AFM) and X-ray photoelectron spectroscopy (XPS) are

employed to character the surface topography and composi-

tion of the passive film after 316L stainless steel is polarized in

different simulated PEMFC anode environments.

2. Experimental systems and methodologies

2.1. Material and simulated solutions

The main chemical composition of 316L stainless steel is

shown in Table 1.

316L stainless steel specimens were machined into cylin-

ders with a diameter of 10 mm and a length of 7 mm. One end

and the side of the specimenswere sealed and connectedwith

a copper wire, while another end was exposed as working

surface and polished with 800-grit silicon carbide abrasive

paper. The surface of specimens was rinsed with acetone and

de-ionized water before measurements.

To examine the effect of F� concentration on corrosion

performance of 316L stainless steel under PEMFC anode

condition, a solution of 1 � 10�5 M H2SO4 with different

concentrations of F� (0, 3 � 10�4, 6 � 10�4, 1 � 10�3 and

5 � 10�3 M) was used as corrosion environment in this study.

Pre-bubbling hydrogen gas was carried out 1 h before each

measurement and the solution was bubbled with hydrogen

gas during measurement.

2.2. Electrochemistry analyses

The electrochemical experiments were carried out in a corro-

sion cell consisting of a three-electrode arrangement with one

gas inlet tube and one gas outlet tube. The specimen served as

the working electrode and a platinum sheet as the counter

electrode. A saturated calomel electrode (SCE) connected to

a salt-bridge probe with a Vycor frit tip served as the reference

electrode. All the potentials are referenced to the SCE except

stated otherwise.

The corrosion cell was immersed in a temperature

controlled water bath. The corrosion solution was kept at

70 �C [12] andwas bubbled thoroughly with hydrogen gas prior

to and during the electrochemical measurements. The tests

were conducted using a PAR 273A potentiostat (EG&G) coupled

with a 5210 lock-in Amplifier (Signal recovery). The Power-

Suite software was used for electrochemical data acquisition

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 7 5e1 8 8 3 1877

and processing. Each experiment was repeated by using three

specimens to ensure reproducibility of the results.

2.2.1. Potentiodynamic and potentiostatic measurementsAt the beginning of each potentiodynamic experiment, the

specimen was polarized cathodically for 10 min to remove

oxides on the specimen surface and then stabilized at open

circuit potential (OCP) for 1 h. Then the potentiodynamic test

was carried out. In potentiodynamic tests, specimens were

polarized at a scanning rate of 1 mV s�1 in a potential range

from �0.2 V vs. OCP to 1.2 V vs. SCE. In potentiostatic tests,

specimens were held at a potential of �0.1 V to simulate the

PEMFC anode condition. After polarized cathodically for

10 min, the specimen was then potentiostatic polarized for

5 h.

2.2.2. MotteSchottky measurementIn this measurement, potential sweeps were carried out from

anodic potential of 0.4 V to cathodic potential of �0.3 V at

a scan rate of 25 mV per step. The data acquisition frequency

of 188 Hz is chosen in the flat region of capacitance vs.

frequency plots, so that the influence of acquisition frequency

to capacitance measurement can be minimized.

2.3. Surface morphology

Scanning electron microscope (SEM) and atomic force micro-

scope (AFM) was used to characterize the surface morphology

of specimens potentiostatic polarized at �0.1 V for 5 h in

1 � 10�5 M H2SO4 solutions with different F� concentrations

(0, 3 � 10�4, 6 � 10�4, 1 � 10�3 and 5 � 10�3 M). The SEM

measurements were carried out by SSX-550 (SHIMADZU) and

the AFM measurements were carried out by Solver Next type

AFM (NT-MDT). In the AFM measurements, semi-contact

mode was operated, scan rate was 2 Hz and scan area was

5 mm � 5 mm. Surface morphology and roughness were

analyzed by “Image Analysis” software (NT-MDT, version

3.5.0.1016).

0 500 1000 1500 2000 2500 3000 3500-0.70

-0.68

-0.66

-0.64

-0.62

-0.60

-0.58

-0.56

-0.54

-0.52

-0.50

e

dcb

1x10-5 M H2SO4 (a)

1x10-5 M H2SO4 + 3x10-4 M F- (b)

1x10-5 M H2SO4 + 6x10-4 M F- (c)

1x10-5 M H2SO4 + 1x10-3 M F- (d)

1x10-5 M H2SO4 + 5x10-3 M F- (e)

E corr /

VSC

E

Elapsed time / s

a

Fig. 1 e Open circuit potential (OCP) curves of 316L stainless

steel in at 70 �C in 1 3 10L5 M H2SO4 solution with different

concentrations of FL (0, 3 3 10L4, 6 3 10L4, 1 3 10L3,

5 3 10L3 M) bubbled with hydrogen gas.

2.4. Composition of passive films

To identify the composition and the depth profile of passive

film on the specimen surface, X-ray photoelectron spectros-

copy (XPS) was taken after specimen was potentiostatic

polarized at �0.1 V for 5 h in 1 � 10�5 M H2SO4 solutions with

different F� concentrations. The composition profiles of the

elements Fe 2p, Ni 2p, Cr 2p and O 1s in the passive film were

analyzed, and was carried out with an AXIS UltraDLD X-ray

photoelectron spectroscope (Kratos Analytical) using a mono-

chromatic Al Ka radiation X-ray source (1486.6 eV). Binding

energies were referenced to the C1s peak at 285.1 eV. The

chamber base pressure was 1.7 � 10�9 Torr (2.266 � 10�7 Pa).

Sputtering was performed at a pressure of about

1.7 � 10�6 Torr (2.266 � 10�4 Pa) with 3.0 keV argon ions beam,

while the sputtered area was 2 mm � 2 mm. The sputtering

rate was determined to be around 3.3 nm min�1. The data

processing of the different peaks were calculated by “Vision

Processing” software (Kratos Analytical).

3. Results and discussion

3.1. Electrochemistry

It is well known that PEMFC anode environment is de-aerated.

In such an acid environment, the metal corrosion process is

an electrochemical processwhich involves both oxidation and

reduction reactions [13]. The oxidation reactions can be rep-

resented as:

M/Mnþ þ ne� (1)

where M represents surface metal elements, Mnþ the corre-

sponding metal ions in the solution and e� free electrons.

The reduction reaction can be represented as:

2Hþ þ 2e�/H2 (2)

where Hþ represents proton adsorbed on the metal surface

and H2 produced hydrogen.

However, with regard to stainless steel, which may be

passivated in many corrosion environments and thus a layer

of passive film are formed on the surface of stainless steel.

Although the passive films are very thin, normally less than

5 nm thick, they separate the stainless steels and the envi-

ronments thus hinder the stainless steels react spontaneously

and violently in corrosion environments [14]. In acidic envi-

ronment, chromium oxide in passive film is stable chemically

and electrochemically but iron and nickel oxides are unstable

and easy to be dissolved, so that this kind of chromium oxide

rich film will protect the stainless steel substrate. However,

the metallurgical structure of the stainless steel bulk was not

uniform and solid, defects such as inclusions, scratches and

micro-cracks may lie in the surface, which may cause local-

ized corrosion [15].

3.1.1. Open circuit potentialFig. 1 shows the open circuit potential (OCP) curves of 316L

stainless steel in solutions containing different concentration

1E-7 1E-6 1E-5 1E-4

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

a

d

b

e

1x10-5 M H2SO4 (a)

1x10-5 M H2SO4 + 3x10-4 M F- (b)

1x10-5 M H2SO4 + 6x10-4 M F- (c)

1x10-5 M H2SO4 + 1x10-3 M F- (d)

1x10-5 M H2SO4 + 5x10-3 M F- (e)

Pote

ntia

l / V

SCE

Current density / A cm-2

c

PEMFC anode potential

a

ED

(Pitting potential)

Epp

(Passivation potential)icorr

(Corrosion current density)

Pote

ntia

l

Log (Current density)

Ecorr

(Corrosion potential)

b

Fig. 2 e (a) Polarization curves for 316L stainless steel at

70 �C in 1 3 10L5 M H2SO4 solution with different

concentrations of FL (0, 3 3 10L4, 6 3 10L4, 1 3 10L3,

5 3 10L3 M) bubbled with hydrogen gas. (b) Schematic of

polarization curves for 316L stainless steel in simulated

PEMFC anode environment.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 7 5e1 8 8 31878

of F� at 70 �Cbubbledwithhydrogengas. It canbeobserved that

the OCP increases rapidly after the surface of electrodes

reducing pre-treatment cathodically, then the OCP stabilizes

gradually after about 170e600 s in solutions, which shows 316L

stainless steel can passivated naturally in simulated PEMFC

anode environments. It is found that the time needed for OCP

stabilization becomes longer with increasing F� concentration

in solutions, which means the increase of F� concentration

Table 2e Polarization parameters of 316L stainless steel at 70 �Cof FL (0, 3 3 10L4, 6 3 10L4, 1 3 10L3, 5 3 10L3 M) bubbled hy

Concentration of F�

MEcorr

V vs. SCEicorr

mA cm�2

Without F� �0.520 43.24

3 � 10�4 �0.549 15.59

6 � 10�4 �0.565 10.39

1 � 10�3 �0.569 6.29

5 � 10�3 �0.617 3.67

impedes the growth of passive film on the surface of 316L

stainless steel under free corrosion condition.

3.1.2. Potentiodynamic studyFig. 2 displays potentiodynamic polarization curves for 316L

stainless steel at 70 �C in 1 � 10�5 M H2SO4 solution with

different F� concentrations bubbled with hydrogen gas. It can

be seen from Fig. 2(a) that 316L stainless steel display typical

passive behavior in all simulated PEMFC anode environments,

the potentiodynamic polarization curves pass through active

region, activeepassive transition region, passive region and

transpassive region with increasing potential from free

corrosion potential. The passive region is broad (around 0.8 V)

which indicates the passive ability of 316L stainless steel is

good in PEMFC anode environments. From around 0.5 V to

around 0.8 V, there are peaks which could be attributed to

transpassivation due to the formation of high valence chro-

mium [16].

Table 2 shows corresponding polarization parameters and

Fig. 2(b) illustrates them in polarization curve, which were

obtained from the polarization curves shown in Fig. 2(a) by

using the PowerSuite software. In Table 2 and Fig. 2(b), Ecorrand icorr represent the corrosion potential and the corrosion

current density, respectively. Epp and ED represent the

passivation potential and the pitting potential, respectively.

I�0.1 V vs. SCE represents the corrosion current density of 316L

stainless steel in simulated PEMFC anode environment at

polarized potential of �0.1 V. Although the Ecorr of 316L

stainless steel in solutions decreases with the increase of F�

concentration, icorr does not increase, indicating that the

increase of F� concentration could provide more corrosion

inhibiting effect under free corrosion conditions (not an

operational PEMFC). The Epp decreases with the increase of F�

concentration in solutions, thus the higher the F� concentra-

tion in solutions is, the easier is passivation of 316L stainless

steel. The ED also decreases with the increase of F� concen-

tration in solutions, which means localized corrosion such as

pitting occurs more easily on 316L stainless steel with

increasing F� concentration. However, the potential of

a PEMFC anode under operation conditions ranges from

�0.19 V to 0.04 V [17,18], which lies in the passivation region of

polarization curves of 316L stainless steel in all simulated

PEMFC anode environments, thus it is suggested that pitting

corrosion of 316L stainless steel hardly takes place in PEMFC

anode environments.

The horizontal line marked in Fig. 2(a) indicates the typical

anode potential (�0.1 V) in PEMFC when the output voltage is

0.7 V, the corresponding corrosion current densities of 316L

in 13 10L5MH2SO4 solutionwith different concentrationsdrogen gas.

EppV vs. SCE

EDV vs. SCE

I�0.1 V vs. SCE

mA cm�2

�0.299 0.502 9.8

�0.364 0.457 8.9

�0.384 0.446 8.1

�0.415 0.435 8.0

�0.485 0.369 9.2

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 7 5e1 8 8 3 1879

stainless steel in different solutions are shown in Table 2. It

can be seen that the corrosion current of 316L stainless steel at

�0.1 V does not show regular changes with increasing F�

concentration and the value ranges 8e10 mA cm�2.

3.2. Potentiostatic studies

To study the long-term corrosion behavior of 316L stainless

steel in 1 � 10�5 M H2SO4 with different F� concentrations

bubbled with hydrogen gas, potentiostatic measurement was

performed for 5 h and the results are shown in Fig. 3.

From Fig. 3, it can be seen that there was a fast decay for the

current density of 316L stainless steel in every solution after it

was appliedwith a constant potential of�0.1 V, due to the fresh

surface pre-treatment cathodically of 316L stainless steel is

passivated rapidly. After 316L stainless steel specimens are

applied potential of �0.1 V for 1700e2400 s, the corrosion

current densities are lower than US DOE 2015 target, 1 mA cm�2

[19]. The currents of 316L stainless steel in all solutions are

stabilized after about 3000 s, indicating that a stable passive

statewas established. FormFig. 3, it can be seen that thehigher

the F� concentration is, the higher is the corrosion current

density, and the current density of 316L stainless steel after

polarized for 5 h is 0.16, 0.16, 0.17, 0.20 and 0.29 mA cm�2,

respectively. It is found that the trend of corrosion current

density is not similar to that found in potentiodynamic tests,

which suggests that only potentiodynamic characterization for

evaluation of bipolar plate materials is not enough and poten-

tiostatic test is necessary to reveal the long-term corrosion

behavior of bipolar plate materials. It can also be seen that

current density decreases continuously with continued poten-

tiostatic polarization,whichmeans that thepassivation of 316L

stainless steel is stable and passive film can protect the

substrate of 316L stainless steel in PEMFC anode environments.

3.3. X-ray electron spectroscopy analysis

The composition profiles of elements Fe 2p, Cr 2p, Ni 2p and O

1s in the passive films formed on 316L stainless steel which

Fig. 3 e Potentiostatic plots for 316L stainless steel at 70 �Cin 1 3 10L5 M H2SO4 solution with different concentrations

of FL (0, 3 3 10L4, 6 3 10L4, 1 3 10L3, 5 3 10L3 M) bubbled

with hydrogen gas.

polarized at�0.1 V for 5 h at 70 �C in 1� 10�5 M H2SO4 solution

with different concentrations of F� bubbled with hydrogen

gas, were obtained by X-ray electron spectroscopy (XPS) and

the results are showed in Fig. 4. The depth profiles of Cr/Fe

atomic ratio for the passive films are shown in Fig. 5.

The thickness of passive film was obtained from the

surface to the profile with highest peak of Cr3þ in XPS scan

spectra. According to this method, it is found that the thick-

ness of passive film formed on 316L stainless steel in solutions

with F� concentrations of 0, 3 � 10�4, 6 � 10�4, 1 � 10�3 and

5 � 10�3 M can be obtained as approximately 3.3 nm, 2.2 nm,

3.5 nm, 1.8 nm and 1.8 nm, respectively. It is noticed that the

passive film thickness is not linearly correlated with F�

concentrations. The change tendency of passive film thick-

ness is similar with passive films formed in PEMFC cathode

environments [9], and there exists a maximum thickness for

passive film formed when F� concentration in solutions is

about 6 � 10�4 M.

From Fig. 4(a)e(e), it is found that the content change of Cr

can be divided into three regions in the depth profile. In the

first region, Cr content increases rapidly from the surface to

a maximum. The change in this region can be regarded as

taking place in the passive film. In the second region, Cr

content decreases gradually to the content in substrates. In

the third region, the Cr contents changes very slight in

substrates. The Fe and Ni contents increase in the depth

profile until reaching the content in substrates.

From Fig. 5, it is clear that the Cr/Fe atomic ratio decreases

continuously from the surface to the substrates for each

specimen. The Cr/Fe atomic ratios in all passive films are

larger than 1 and that in substrate of 316L stainless steel is

about 0.26, which means the relative dissolution rate of Fe in

316L stainless steel is larger than that of Cr in PEMFC anode

environments. It is concluded from Fig. 5 that the F�

concentration in solution is higher, the Cr/Fe atomic ratio in

passive film of 316L stainless steel lower is. It is well known

that chromium oxide in passive film mainly provides corro-

sion resistance [15], thus this result gives an explanation of

why the corrosion resistance of 316L stainless steel decreases

with increasing F� concentration in solutions.

3.4. MotteSchottky measurements

The semiconductor behavior of passive film at the interface of

passive film-electrolyte can be determined by MotteSchottky

method. The potential dependence of space charge layer at

a passive film-electrolyte interface is described by the

MotteSchottky equation [20]:

1C2

¼ 2eND330

�E� Efb � kT

e

�(3)

for an n-type semiconductor and

1C2

¼ 2�eNA330

�E� Efb � kT

e

�(4)

for a p-type semiconductor, where C is the space charge layer;

3 the dielectric constant of the film; 30 the permittivity of free

space; e the electron charge;ND andNA the donor and acceptor

densities, respectively; E the applied potential; Efb the flat

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

Fe Cr Ni O

Atom

ic c

once

ntra

tion

/ %

Sputtering time / s0 100 200 300 400 500

0

10

20

30

40

50

60

70

80

Fe Cr Ni O

Atom

ic c

once

ntra

tion

/ %

Sputtering time / s

a b

e

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

Fe Cr Ni O

Atom

ic c

once

ntra

tion

/ %

Sputtering time / s

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

Fe Cr Ni O

Atom

ic c

once

ntra

tion

/ %

Sputtering time / s

c d

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

Fe Cr Ni O

Atom

ic c

once

ntra

tion

/ %

Sputtering time / s

Fig. 4 e XPS depth profiles of passive films formed on 316L stainless steel polarized at L0.1 V for 5 h at 70 �C in 1 3 10L5 M

H2SO4 solution with different FL concentrations bubbled with hydrogen gas. (a) without FL, (b) 3 3 10L4 M FL, (c)

6 3 10L4 M FL, (d) 1 3 10L3 M FL, (e) 5 3 10L3 M FL.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 7 5e1 8 8 31880

band potential; k the Boltzmann’s constant; and T the absolute

temperature.

In MotteSchottky measurements, it assumed that the

capacitance response is controlled by the band bending and

can be described by the variation of the space charge capaci-

tance under depletion conditions. Hence, MotteSchottky plot

shows linear behavior with positive slope for an n-type

semiconductor and with negative slope for a p-type slope.

Fig. 5 e Cr/Fe atomic ratio for the passive film formed on

316L stainless steel polarized at L0.1 V for 5 h at 70 �C in

13 10L5 M H2SO4 solution with different FL concentrations

bubbled with hydrogen gas.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 7 5e1 8 8 3 1881

Fig. 6 shows the MotteSchottky plots of 316L stainless steel

after potentiostatic polarized at �0.1 V for 5 h at 70 �C in

1 � 10�5 M H2SO4 solution with different concentrations of F�

bubbledwith hydrogen gas. TheMotteSchottkymeasurements

showthat thepassivefilmsformedinall solutionscontainingF�

have a single layer n-type semiconductor and the passive films

formed in solutions without F� have a bi-layer ( pen type)

semiconductor. All the passive films behave as an n-type

semiconductor when the applied potential is in the range from

0.0 V to about �0.3 V. The passive films formed in solutions

without F� behave as a p-type semiconductor when the applied

potential is in the range fromabout�0.3 V to�0.5 V. It is clear to

see from Fig. 4 that the anode potential range of PEMFC (�0.19 V

to �0.04 V [17,18]) lies in the n-type semiconductor behavior

region of passive films. The structure of n-type semiconductor

Fig. 6 e MotteSchottky plots for passive films formed on

316L stainless steel at 70 �C in 1 3 10L5 M H2SO4 solution

with different concentrations of FL (0, 3 3 10L4, 6 3 10L4,

1 3 10L3, 5 3 10L3 M) bubbled with hydrogen gas.

facilitates the growth of passive films due to promoting metal

ions migrating from substrate to the films, however, the struc-

ture cannot completely prevent harmful ions (such as F� and

SO2�4 ) from penetrating themetal substrate leading to localized

corrosion,according topreviousstudy [9]. The results imply that

it is possible for localized corrosion occurring on the surface of

Fig. 7 e Surface morphology of 316L stainless steel

polarized at L0.1 V for 5 h at 70 �C in 1 3 10L5 M H2SO4

solution with different FL concentrations bubbled with

hydrogen gas. (a) without FL, (b) 6 3 10L4 M FL and (c)

5 3 10L3 M FL.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 8 7 5e1 8 8 31882

316L stainless steel, although the corrosion current of 316L

stainless steel is tiny in PEMFC anode environments.

3.5. Surface morphology

The surface morphology of 316L stainless steel potentiostatic

polarized at different temperature is examined by scanning

electronmicroscope (SEM) and atomic force microscope (AFM).

TheSEMexaminations showthatnopitting corrosionoccurson

thesurfaceof316Lstainlesssteel so that the imagesdonot list in

thispaperandtheAFMmeasurementresultsareshowninFig.7.

Fig. 7 displays the surface morphology of 316L stainless

steel polarized in simulated PEMFC anode environments with

different F� concentration (a) without F�, (b) 6� 10�4 M and (c)

5 � 10�3 M. In Fig. 7, the stretching ridges and valleys are

corresponding to the polishing direction of the specimens,

and there are some different height and wide peaks distrib-

uting on the surface. According to the root-mean-square

(RMS) roughness analyses by software, the RMS roughness

of 316L stainless steel polarized with increasing F� concen-

tration is 31.0 nm, 33.6 nm and 34.6 nm, respectively. The AFM

examination results show that the roughness of 316L stainless

steel increases slightly with increasing F� concentration in

simulated PEMFC anode environment and provide another

evidence on that the corrosion resistance of 316L stainless

steel decreases as the F� concentration increases.

The potentiostatic results indicate that the currents of 316L

stainless steel in all solutions stabilize after about 3000 s and

a stable passive state is established. During the passive film

formation of 316L stainless steel, the relative dissolution rates

of Cr and Fe are different in environment with different F�

concentration. Consequently, 316L stainless steel undergoing

long-term potentiostatic polarization forms stable passive

film with different composition and structure under different

simulated PEMFC anode condition. The XPS analysis results

show that the fraction of chromium oxide in passive film

decreases with the increase of F� concentration, thus results

in the difference of corrosion resistance of stainless steel

under long-term running condition. With regard to all these

stable passive films, the rates of film growth and dissolution

are equilibriumaccording to the theory of steady-state passive

films [14]. The XPS and MotteSchottky analysis results indi-

cate the passive films formed in PEMFC anode environments

behave an n-type semiconductor and the semiconductor

structure facilitates the growth of passive film.

The PEMFC’s anode working potential range (�0.19 V to

�0.04 V) lies in the passive region of passive films in every

corrosion solution presented in this paper, which means the

films will protect the metal substrate. For 316L stainless steel

in 1 � 10�5 M H2SO4 solution with different concentrations of

F� (0, 3 � 10�4, 6 � 10�4, 1 � 10�3, 5 � 10�3 M), the corrosion of

316L stainless steel will be severe in solution containing

higher F� concentration, and this analysis is consistent with

the AFM results.

4. Conclusions

The corrosion performance of 316L stainless steel in simu-

lated PEMFC anode environment with different F�

concentrations are studied by electrochemical techniques,

scanning electronmicroscope (SEM), atomic force microscope

(AFM) and X-ray photoelectron spectroscopy (XPS) analysis.

316L stainless steel shows typical passive behavior in all

simulated PEMFC anode environments. The open circuit

potential of 316L stainless steel decreases with increasing F�

concentration in the solutions. It is found that corrosion

currents of 316L stainless steel increases with F� concentra-

tions under PEMFC anode working condition, while all current

densities satisfy the US DOE’s 2015 corrosion criterion. No

localized corrosion occurs, however, a single layer n-type

semiconductor passive film formed on 316L stainless steel

indicates there is a potential risk of localized corrosion. The

XPS analysis results show that the Cr/Fe atomic ratio in

passive film of 316L stainless steel decreases with increasing

F� concentration in the solutions, which provides an expla-

nation for the result that corrosion resistance of 316L stainless

steel decreases with the increase of F� concentration in the

simulated PEMFC anode environments.

Acknowledgments

The financial supports of Chang Jiang Scholars Program of

Ministry of Education of China, the National Natural Science

Foundation of China (No. 50821064) and National Basic

Research Program of China (No. 2009CB220000) are gratefully

acknowledged.

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