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Factors affecting corrosion behavior of SS316L as bipolar platematerial in PEMFC cathode environments

Ying Yang a,b, Liejin Guo a,*, Hongtan Liu a,c,**a International Research Center for ‘Solar-Hydrogen’ Renewable and Clean Energy, State Key Laboratory of Multiphase Flow in Power

Engineering, Xi’an Jiaotong University, 28 West Xianning Road, Xi’an, Shaanxi 710049, PR Chinab Shaanxi Provincial Key Laboratory of Electroanalytical Chemistry, Institute of Analytical Science, Northwest University, Xi’an, Shaanxi

710069, PR ChinacDepartment 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 11 February 2012

Received in revised form

5 April 2012

Accepted 6 April 2012

Available online 7 May 2012

Keywords:

Bipolar plate

Corrosion model

Proton exchange membrane fuel cell

(PEMFC)

Stainless steel

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.2012.04.026

a b s t r a c t

An empirical corrosion model for SS316L in simulated proton exchange membrane fuel cell

(PEMFC) environments is developed based on systematic experimental data on the effects

of various factors, such as acidity, fluoride ion concentration, temperature and polarization

potential. Correlation parameters under different conditions are provided in tabulated

forms and comparisons of the empirical model with experimental results are shown in

graphical forms. The results show that the empirical model agrees very well with the

experimental data except at the short initial polarization time and the model is applicable

up to a polarization potential of 0.7 V. The results also show that polarization potential is

the most sensitive parameter among all the parameters studied.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction temperature [16] and polarization potential [17] on corrosion

Proton exchange membrane fuel cell (PEMFC), as a promising

new type of power source, has attracted worldwide attention.

Bipolar plates are important components in PEMFC and

stainless steel is popularly used as bipolar plate material [1,2].

Stainless steel bipolar plates will undergo corrosion in acidic

PEMFC environments, leading to lower efficiency and

reducing the lifetime of PEMFC stacks [3,4].

In recent years, corrosion of stainless steel bipolar plates has

been studied by many researchers [1,5,6]. Although there were

some studies on effects of acidity [7e12], fluoride ion [4,13e15],

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

(Y. Yang), lj-guo@mail.xj2012, Hydrogen Energy P

behavior of stainless steel bipolar plates, a systematic study on

the relative effects of the various factors, such acidity, fluoride

ion concentration, temperature, and polarization potential, etc.

is still lacking. Therefore it is the objective of this work to

examine the effects of various factors in depth and develop

empiricalmodels by correlate the experimental results. Stainless

steel 316L (SS316L) is chosenasbipolarplatematerial since itmay

be the most promising and popular metal material for bipolar

plate in PEMFCs. The potentiostatic polarization method is

employed to study the effect of the various factors on the

corrosion behavior of SS316L. In terms of the experimental

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.

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 3 8 2 2e1 3 8 2 8 13823

results, an empirical corrosion model describing the change of

corrosioncurrentdensitywithpolarization time isproposed, and

the validity and applying condition of the model are discussed.

The results could provide guidelines for PEMFC stack operation

and maintenance.

II

ensi

ty /

A. U

. I

2. Experimental

SS316L (the main chemical composition is showed in Table 1)

specimens were machined into cylinders with a diameter of

10 mm. The electrode was sealed with polyethylene heat-

shrink tube and silicone, and working surface was polished

with 800-grit silicon carbide abrasive paper. The detailed

electrode fabrication procedure is described in [13,14]. The

surface of specimens was rinsed with acetone and de-ionized

water before measurements.

The electrochemical experiments were carried out in

a corrosion 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) or

calomel electrode with 0.1 M KCl (CE0.1 M KCl) (used in

measurements of varying temperature [16]) served as the

reference electrode. All the potentials are referred to the SCE

except stated otherwise. The tests were conducted using

a PAR 273A potentiostat (EG&G). The PowerSuite software was

used for electrochemical data acquisition. Each SS316L spec-

imen was polarized cathodically for 10 min before potentio-

static tests, after that it was potentiostatic polarized for 5 h.

Each experiment was repeated by using three specimens to

ensure reproducibility of the results.

To examine the effect of acidity on corrosion behavior of

SS316L under PEMFC cathode condition, a solution of

6� 10�4 M F� with different concentrations of H2SO4 (1� 10�5,

1 � 10�4, 1 � 10�3, 0.01, 0.1 and 1 M) was used as corrosion

environment. In the tests of examining the effect of F�

concentration, 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. The polarization potential is at 0.6 V and

the temperature is at 70 �C in the above two group tests. For

testing the effect of temperature (at 25, 50, 70 and 90 �C,respectively; polarized at 0.5 V vs. CE0.1 M KCl) and polarization

potential (at 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 V, respectively; at

70 �C), a solution of 1 � 10�5 M H2SO4 with 6 � 10�4 M F� was

used. Pre-bubbling air was carried out 1 h before each test and

the solution was bubbled with air during the test.

in,t=0

Experimental dataModel

rrosi

on c

urre

nt d

An i

3. Results and discussion

All SS316L specimens were polarized potentiostatically for 5 h

in different simulated PEMFC cathode environments. The data

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

of potentiostatic polarization measurements from 2 h to 5 h

were analysed. Since the trend of the experimental data

shows that the corrosion current density decreases expo-

nentiallywith polarization time, an empirical corrosionmodel

(illustrated in Fig. 1), similar to the one shown by Vermilyea

[18], is used,

in ¼ inN þAne�Rnt (1)

where each n represents a specific parameter, such as acidity,

fluoride ion concentration, temperature, or polarization

potential; in the corrosion current density at time t; inN the

asymptotic corrosion current density as the polarization time

approaches infinity; An is a constant and equals to the corro-

sion current density at the start of polarization minus the

asymptotic corrosion current density, i.e. An ¼ in, t¼0�inN; Rn

the decreasing rate of corrosion current density; t the time for

potentiostatic polarization.

The model proposed by Vermilyea [18] is

i ¼ i1e�Rt (2)

where i represents corrosion current density at time t; i1 the

current density on the surface uncovered by passive film; R�1

the time constant. By contrast, An is a constant in this

empirical model, but i1 is a variable in Vermilyea’s model.

Vermilyea [18] assumed that there was no ions transportation

in the growing film, however, there are ions (such as oxygen)

transport through the film formed on SS316L [13,14].

When the empirical model shown in Eq. (1) is applied to the

experimental data for SS316L polarized at 0.6 V in solution of

6 � 10�4 M F� with different concentrations of H2SO4 at 70 �C,the values of the parameters in Eq. (1) are obtained and listed in

Table 2. Comparison of the model and experimental data

throughout the entirepolarizationare shown inFig. 2(a). It canbe

seen from Table 2, the value of inN increases with increasing

H2SO4 concentration, which indicates that the corrosion of

SS316L increased as acidity increases. The corrosion current

density is the indicator of corrosion behavior of metal bipolar

plates, so that monitoring the change of corrosion current

density is one of the direct approaches to this end. For this

0 1 2 3 4 5Polarization time / h

Co n infinity

Fig. 1 e The illustration of empirical corrosion model for

SS316L in simulated PEMFC cathode environments.

Table 2 e The parameters of corrosion model for SS316L polarized at 0.6 V in solution of 6 3 10L4 M FL with differentconcentrations of H2SO4 at 70 �C.

n Concentration of H2SO4/M inN/mA cm�2 An/mA cm�2 Rn/h�1

Concentration of H2SO4 1 � 10�5 0.361 1.100 0.745

1 � 10�4 0.512 1.734 0.643

1 � 10�3 0.519 1.645 0.600

0.01 0.894 5.016 0.815

0.1 1.091 2.553 0.722

1 3.297 4.408 0.501

Fig. 2 e (a) Comparison of the model and experimental

results for SS316L in environments varying H2SO4

concentration. (b) Dependence of corrosion current density

of SS316L on acidity at different potentiostatic polarization

time.

Table 3 e The parameters of corrosion model for SS316L polarconcentrations of FL at 70 �C.

n Concentration of F�/M

Concentration of F� 0

3 � 10�4

6 � 10�4

1 � 10�3

5 � 10�3

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 3 8 2 2e1 3 8 2 813824

reason, inN is an important parameter and provides a corrosion

performance of long-term operation for bipolar plates in PEMFC.

For instance, although the concentration of 0.1MH2SO4 is higher

than thatof 0.01MH2SO4, thecorrosioncurrentdensityofSS316L

in environment containing 0.1 M H2SO4 is lower than that in

environment containing 0.01 M H2SO4 at the initial polarization

for about 2.5 h. Therefore, it is essential to perform long-term, i.e.

more than 3 h, potentiostatic measurement in bipolar plate

research. As for SS316L polarized in strong acid solution (such as

1 M H2SO4), even though the corrosion current fluctuates

significantly, the current density curve still changes along the

model curve (Fig. 2(a)). This result shows that the empirical

model is applicable even in high acidity environment. Note that

An in Eq. (1) is not the true difference at the start polarization

between the realistic corrosion current density and the asymp-

totic corrosion current density, but rather a constant used to

provide an initial condition for the model. Rn indicates the

decreasing rate of corrosion current density. The larger the Rn is,

the faster decreases the corrosion current density. Unfortu-

nately, no monotonic trend of Rn with H2SO4 concentration for

SS316L was found.

In terms of the parameters in Table 2 and Eq. (1), the

corrosion current densities of SS316L at 5 h, 16 h, 24 h, 50 h and

5000 h are calculated and shown in Fig. 2(b). The results show

that all the corrosion current densities of SS316L approach

very close to the asymptotic value after potentiostatic polar-

ized for 24 h. When the concentration of H2SO4 is less than

0.01 M, the corrosion current densities can satisfy the 2015

target of Department of Energy, US (US DOE), lower than

1 mA cm�2 [19]. When the concentration of H2SO4 is higher

than 0.01 M, the corrosion current density of SS316L cannot

satisfy the 2015 target of US DOE, even after a very long time of

potentiostatic polarization.

When the empirical model is applied to the experimental

data for SS316L polarized at 0.6 V in solution of 1 � 10�5 M

H2SO4 with different concentrations of F� at 70 �C, the values

of the parameters in Eq. (1) are obtained and listed in Table 3.

Comparison of the model and experimental data throughout

ized at 0.6 V in solution of 1 3 10L5 M H2SO4 with different

inN/mA cm�2 An/mA cm�2 Rn/h�1

0.198 0.991 0.657

0.212 1.027 0.648

0.216 1.075 0.620

0.233 1.333 0.652

0.349 1.363 0.657

Fig. 4 e (a) Comparison of the model and experimental

results for SS316L in environments varying temperature.

(b) Dependence of corrosion current density of SS316L on

temperature after different potentiostatic polarization

time.

Fig. 3 e (a) Comparison of the model and experimental

results for SS316L in environments varying FL

concentration. (b) Dependence of corrosion current density

of SS316L on FL concentration at different potentiostatic

polarization time.

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 3 8 2 2e1 3 8 2 8 13825

the entire polarization are shown in Fig. 3(a). There are three

points should be noted: 1) The asymptotic corrosion current

density increases with the concentration of F�; 2) The

decreasing of corrosion rate with polarization time is similar,

Rn is 0.639 � 0.019 h�1, for SS316L in all of the different solu-

tions, which indicate that the effect of concentration of F� is

slight on the corrosion rate; 3) The empirical model agree with

the experimental results sooner with the increase of F�

concentration.

According to the parameters in Table 3 and Eq. (1), the

corrosion current densities of SS316L at 5 h, 16 h, 24 h, 50 h and

Table 4e The parameters of corrosionmodel for SS316L polarizwith 6 3 10L4 M FL at 0.5 vce with 0.1 M KCl.

n Temperature/�C inN

Temperature 25

50

70

90

5000 h are calculated and shown in Fig. 3(b). The results show

that all the corrosion current densities of SS316L approach

very close to the asymptotic value after 24 h potentiostatic

polarization. Furthermore, even if the concentration of F�

goes up to 5 � 10�3 M, the corrosion current densities of

SS316L still satisfy the 2015 target of US DOE.

The parameters in Eq. (1) for varying temperature are listed

in Table 4, and the comparison between the model and

experimental data throughout the entire polarization are

shown in Fig. 4(a). It can be seen fromTable 4, that the value of

ed at different temperatures in solution of 13 10L5MH2SO4

/mA cm�2 An/mA cm�2 Rn/h�1

0.042 0.297 0.485

0.128 0.732 0.750

0.198 1.274 0.551

0.307 1.203 0.620

Fig. 5 e (a) Comparison of the model and experimental

results for SS316L in environments varying polarization

potential. (b) Dependence of corrosion current density of

SS316L on potential at different potentiostatic polarization

time.

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 3 8 2 2e1 3 8 2 813826

inN increases with increasing environmental temperature,

indicating that the raising of PEMFC stack temperature

accelerates the corrosion of stainless steel bipolar plates.

The corrosion current densities of SS316L at 5 h, 16 h, 24 h,

50 h and 5000 h are obtained from Table 4 with Eq. (1) and

shown in Fig. 4(b). Note that even at a temperature up to 90 �C,the corrosion current density of SS316L still satisfies the 2015

target of US DOE.

When the empirical model is applied to the experimental

results for SS316L polarized at different potential in solution

of 1 � 10�5 M H2SO4 with 6 � 10�4 M F� at 70 �C, the values of

the parameters are obtained and listed in Table 5. Compar-

ison of the model and experimental results throughout the

entire polarization are shown in Fig. 5(a). It can be seen from

Table 5 and Fig. 5(a), that when the polarization potential

ranges from 0.3 V to 0.7 V, the value of inN increases with

increasing potential and the empirical model agrees well

with experimental results. There is not any visible change on

the surface of SS316L in measurements. However, when the

polarization potential goes up to 0.8 V or 0.9 V, the corrosion

current density does not follow the trend of decreasing

exponentially (inset in Fig. 5(a)). Moreover some visible

pitting or crevice corrosion appears on the surface of SS316L

specimen (Fig. 6).

According to the parameters in Table 5 and eq. (1), the

corrosion current densities of SS316L at 5 h, 16 h, 24 h, 50 h and

5000 h are calculated and shown in Fig. 5(b). As long as the

potential is no more than 0.8 V, the corrosion current density

of SS316L after 5 h polarization can satisfy the 2015 target of

US DOE.

As for long-term operations of PEMFC stacks, high acidity

only appears initially in a new stack and actually the pH value

is greater than 4 in most of running period [20e23]. The

corrosion current densities of SS316L are in the range of

0.4e0.6 mA cm�2 as the concentration of H2SO4 ranging from

1� 10�5 to 1� 10�3 M (pH value around 5 to around 2.8, SS316L

polarized at 0.6 V, 6 � 10�4 M F�, at 70 �C). In addition, the

situation rarely happens for concentration of F� higher than

1 � 10�3 M [4,22e25]. According to the empirical model and

experimental results for concentration of F� lower than

1 � 10�3 M (SS316L polarized at 0.6 V, 1 � 10�5 M H2SO4, at

70 �C), corrosion current density of SS316L ranges

0.2e0.4 mA cm�2. When the operation temperature is in the

range of 25e90 �C (SS316L polarized at 0.5 V vs. CE0.1 M KCl,

1� 10�5 M H2SO4 þ 6� 10�4 M F�), corrosion current density of

SS316L ranges 0.1e0.4 mA cm�2. In terms of the empirical

model and experimental results for polarization potential not

Table 5 e The parameters of corrosion model for SS316L polarwith 6 3 10L4 M FL at 70 �C.

n Polarization potential/VSCE

Polarization potential 0.3

0.4

0.5

0.6

0.7

exceeding 0.7 V (SS316L polarized in solution of 1 � 10�5 M

H2SO4 þ 6 � 10�4 M F�, at 70 �C), the corrosion current

densities of SS316L are less than 0.4 mA cm�2. When the

polarization potential is higher than 0.7 V, the corrosion

current densities of SS316L can reach several mA cm�2 or tens

of mA cm�2, and severe localized corrosion happens. Thus,

when SS316L bipolar plates are used, efforts should be made

ized at different potential in solution of 1 3 10L5 M H2SO4

inN/mA cm�2 An/mA cm�2 Rn/h�1

0.121 0.659 0.666

0.141 0.697 0.634

0.195 0.912 0.606

0.197 0.862 0.603

0.343 1.192 0.792

Fig. 6 e The localized corrosion appears on the surface of

SS316L specimen after potentiostatic polarization

measurement.

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 3 8 2 2e1 3 8 2 8 13827

to avoid operating PEMFC stacks at very high cell voltage

region even though operating in this region results in high

efficiencies. Besides, un-necessary start-ups and shut-downs

should be avoided to minimizing the operating time in the

high cell voltage regions.

From the Figs. 2e5, it is noticed that the model agrees with

the experimental results from 1 h to 2 h in most cases, even

though the model is obtained from the experimental data

from 2 h to 5 h. The results indicate the empirical corrosion

model is applicable in this region. However, there are signifi-

cant differences between the model and experimental results

at the initial polarization time with the measured corrosion

current densities much higher than those predicted by the

empirical model. The phenomena can be attributed to the

oxidation of fresh metal on the surface of SS316L. Before the

potentiostatic polarization, the SS316L specimens were

polarized cathodically to remove the oxides layer to expose

fresh metal on the surface. After the initial period of poten-

tiostatic polarization, the freshmetal on the specimen surface

is oxidized, i.e. the passive film begin to grow on the stainless

steel surface to impede the corrosion of metal substrates [26].

Consequently the corrosion current density decreases. When

a layer of passive film covers the stainless steel, the metal

dissolves into the solution by the process of passive film dis-

solving/formation, so that the corrosion current density is

much lower. In terms of the analyses, the potentiostatic

polarization behavior of SS316L in simulated PEMFC cathode

environments is divided into two stages (Fig. 1): stage I and

stage II. In stage I passive film is being formed until it covers

entire stainless steel surface, thus in this stage the metal on

the stainless steel surface can be dissolved into the solution

directly; while in stage II the passive film has been formed and

covers the entire surface, in this stage the metal is dissolved

into the solution through the dissolving/formation of the

passive film. Therefore, the empirical model proposed in this

paper only can be applied in stage II, i.e. when a layer of

passive film has formed and covered the entire the stainless

steel surface.

4. Conclusions

In this study, the corrosion behavior of SS316L bipolar plates

are examined systematically by potentiostatic polarization

method in simulated PEMFC cathode environments. The

effects of various factors, such as acidity, fluoride ion

concentration, temperature and polarization potential are

studied. The results show that the polarization potential is the

most sensitive factor affecting the corrosion behavior of

SS316L bipolar plate in long-term PEMFC operation. An

empirical corrosion model describing the corrosion current

density decreasing exponentially with polarization time is

proposed, and the model agrees well with experimental

results except at short initial polarization time. The empirical

model is applicable to a polarization potential of 0.7 V or lower.

Acknowledgments

The financial supports from Open Fund of the State Key

Laboratory of Multiphase Flow in Power Engineering of China,

the National Basic Research Program of China (No.

2009CB220000) and National Science Foundation of China (No.

50821064) are gratefully acknowledged.

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