Effect of chromium content on the corrosion resistance of ferritic stainless steels in sulfuric acid solution

Abstract

Due to recent increases in the price of Ni, steel use is currently undergoing a global shift from austenitic stainless steels to ferritic stainless steels. In this study, the corrosion behavior of four types ferritic stainless steels with different Cr contents was investigated to study the effect of Cr content on the corrosion resistance in a sulfuric acid solution. The polarization curves of the ferritic stainless steel with the highest Cr content indicated the best corrosion resistance. No corrosion was observed for the stainless steel with 24 mass% Cr after a potential sweep based on ex-situ SEM images. Corrosion resistivity was improved for high Cr content (>24 mass%) stainless steel because it is considered to form a stable passivation layer.

Full text

Effect of chromium content on
the corrosion resistance of
ferritic stainless steels in
sulfuric acid solution
Yang Yu
a
, Sayoko Shironita
a
, Kenichi Souma
a,b
, Minoru Umeda
a,∗
a
Department of Materials Science and Technology, Graduate School of Engineering, Nagaoka University of
Technology, 1603-1 Kamitomioka, Nagaoka, Niigata, 940-2188, Japan
b
Hitachi Industrial Equipment Systems Co., Ltd., 3 Kanda Neribei, Chiyoda, Tokyo, 101-0022, Japan
∗
Corresponding author.
E-mail address: mumeda@vos.nagaokaut.ac.jp (M. Umeda).
Abstract
Due to recent increases in the price of Ni, steel use is currently undergoing a global
shift from austenitic stainless steels to ferritic stainless steels. In this study, the
corrosion behavior of four types ferritic stainless steels with different Cr contents
was investigated to study the effect of Cr content on the corrosion resistance in a
sulfuric acid solution. The polarization curves of the ferritic stainless steel with the
highest Cr content indicated the best corrosion resistance. No corrosion was
observed for the stainless steel with 24 mass% Cr after a potential sweep based on
ex-situ SEM images. Corrosion resistivity was improved for high Cr content (>24
mass%) stainless steel because it is considered to form a stable passivation layer.
Keywords: Materials science, Electrochemistry, Metallurgical engineering
1. Introduction
Stainless steels are classified by the three main types: austenitic, ferritic, and
martensitic [1,2,3,4,5]. Among these stainless steels, the corrosion resistances
were superior for the Cr-Ni-type austenitic and Cr-type ferritic stainless steels. These
Received:
13 April 2018
Revised:
25 October 2018
Accepted:
16 November 2018
Cite as: Yang Yu,
Sayoko Shironita,
Kenichi Souma,
Minoru Umeda.Effect of
chromium content on the
corrosion resistance of ferritic
stainless steels in sulfuric acid
solution.
Heliyon 4 (2018) e00958.
doi: 10.1016/j.heliyon.2018.
e00958
https://doi.org/10.1016/j.heliyon.2018.e00958
2405-8440/Ó2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license
(http://creativecommons.org/licenses/by/4.0/).

stainless steels have been used as kitchen tools, architectural materials, mechanical
and chemical-industrial equipment, fuel cell materials, etc. [6,7,8]. However, due to
the increase in Ni price because of resource exhaustion, use of stainless steels is
currently undergoing a global shift from austenitic to Ni-less ferritic stainless steels
[9,10,11,12].
The ferritic stainless steel SUS400 series, especially SUS430 in the 1960s and SUS445
in the 1990s, were developed for high corrosion resistance [13,14]. SUS430 contains
16e18% Cr and SUS445 contains 22% Cr and 1e2% Mo as the corrosion-resistant el-
ements. The corrosion resistance of SUS445 in an acidic solution is as high as that of
SUS316, which is a Ni-containing austenitic stainless steel [15]. However, further
improvement of corrosion resistance is needed for the application of these stainless
steels in acidic and electrochemical environments, such as fuel cell materials.
To realize corrosion resistance of the stainless steels, the natural oxidation of Cr and
Fe on the surface to form passivation films composed of oxides/hydroxides is an
important factor [16,17,18,19]. The Cr content significantly affects the corrosion
resistance; there are a number of papers concerning the relationship between the Cr
content amount and the corrosion resistance of stainless steel [20,21,22,23,24],
although few among them report the use of ex-situ SEM observation.
Recently, we found that the ferritic stainless steel can show higher corrosion resis-
tance than SUS316 by increasing the Cr content, without any surface treatment,
such as nitriding or coating [25,26,27]. Ferritic stainless steel without surface treat-
ment should be studied from the viewpoint of materials cost for application as bipo-
lar plates which constitute one of the key components in fuel cells. Investigation of
this high-Cr-containing ferritic stainless steel in greater detail revealed the possibility
that materials with high corrosion resistance and appreciable electrical conductivity
in even harsher environments than usual can be supplied without any surface treat-
ment [28,29,30,31,32]. In this study, corrosion evaluation experiments were con-
ducted in an acidic solution using a proprietary high-Cr-content ferritic stainless steel
in addition to the three SUS 400 series types with the different Cr contents of
commercially-available products. The bulk properties were evaluated by X-ray
diffraction (XRD) and glow discharge spectroscopy (GDS), and a surface scientific
approach was conducted by ex-situ scanning electron microscopy (SEM). Corrosion
resistance was highly improved at 24 mass% Cr without Ni, while exhibiting a useful
level of electrical conductivity (e.g. in fuel cells).
2. Experimental
2.1. Materials
The four types stainless steels used in this study were SUS410 (t¼3.9 mm),
SUS430 (t¼1.0 mm), SUS445 (t¼0.1 mm) and 24 mass% Cr-content stainless
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steel (24CrSS, t¼0.1 mm). The chemical compositions (in mass%) of these four
types of stainless steels are shown in Table 1. The SUS445 stainless steel contains
small amounts of aluminum (Al), molybdenum (Mo), titanium (Ti) and niobium
(Nb) elements compared to the other stainless steels. All the four types of stainless
steels contain no Ni and contain different Cr amounts.
2.2. Electrochemical measurements
The electrochemical measurements were carried out at room temperature using the
Electrochemical Analyzer Model 802B (ALS/[H] CH Instruments). To evaluate
the corrosion resistance of the SUS410, SUS430, SUS445 and 24CrSS stainless
steels, corrosion behavior was studied using an electrochemical three-electrode glass
cell. The working, counter, and reference electrodes were the stainless steel spec-
imen, a platinum coil, and an Ag/Ag
2
SO
4
electrode, respectively. All measured elec-
trode potentials were converted from Ag/Ag
2
SO
4
to the SHE in this study. A 0.5 mol
dm
-3
H
2
SO
4
solution was used as the electrolyte. The stainless steels were washed
with acetone and distilled water for 5 min during sonication prior to the linear sweep
voltammetry (LSV) measurement. A 30-min Ar gas bubbling was also performed.
Next, the cathodic treatment for 1 min was conducted at a potential of -0.47 V vs.
SHE, then kept for 5 min at the rest potential in the cell. During this holding
time, Ar bubbles nucleated on the surface of the sample to remove generated H
2
gas. The potential sweep was conducted from the rest potential to 1.1 V vs. SHE
at a scan rate of 0.33 mV s
-1
[15]. After the electrochemical measurement, the sam-
ples were carefully removed from the cell and cleaned with ethanol. The corrosion
resistance of all the samples was evaluated based on the Japanese Industrial Stan-
dards (JIS) G0579: 2007 measurement method [33].
2.3. Characterization
To evaluate the depth profiles of the elements on the surface of the four types of
stainless steels, glow discharge optical emission spectroscopy (GDS) was carried
Table 1. Composition of the SUS410, SUS430, SUS445 and 24CrSS stainless
steels (mass%) studied.
Sample Fe C Si Mn Cr Mo Nb Ti Al Ni
SUS410
a
Bal. 0.15 0.50 1.0 11.5w13.0 
SUS430
a
Bal. 0.12 0.75 1.0 16.0w18.0 
SUS445
b
Bal. 0.01 0.18 0.20 22.10 1.20 0.23 0.19 0.09 
24CrSS
c
Bal. 0.05 0.15 0.30 24.0 0.15 
a
From The Nilaco Corporation.
b
From Nisshin Steel Co., Ltd.
c
From Hitachi Metals, Ltd.
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out using a Horiba GD-Profiler 2 instrument. The measured elements included Fe, C,
N, Cr, Al, etc. In this study, the Cr contents of the four types of stainless steels is the
main data to be analyzed.
The X-ray diffraction (XRD) experiments were performed by an XRD-6100 made
by Shimadzu. The measurements were carried out in reflection geometry using Cu
Karadiation (l¼1.5406

A) generated at 40 kV and 30 mA; 2qwas scanned
from 20to 110at a rate of 2$min
1
.
Scanning electron microscopy (SEM) (JSM-6060A, JEOL Ltd.) was used to observe
the surface morphology of the stainless steels before and after the LSV measure-
ments were stopped at the peak of the polarization curves. Micrographs of were ob-
tained at 15 kV.
Finally, the electrical conductivity of the bipolar plate is very important, so the elec-
trical conductivity of the four types of stainless steels was measured by a Mitsubishi
Chemical Loresta HP (MCP-T410) electrometer using a four-point probe resistivity
technique. As a standard measurement, four-point probe characterization is used to
measure the electrical properties of solids and thin films [24,34,35,36]. The resis-
tivity reported here for each stainless steel of this study is the average data of five
measurements in different locations.
3. Results and discussion
3.1. GDS depth profiles
Fig. 1 shows the GDS depth profiles of the SUS410, SUS430, SUS445 and 24CrSS
stainless steels. The main compositions of these four types of stainless steels are Fe
and Cr; a small amount of C was also detected. The GDS depth profiles of the
SUS410, SUS430, SUS445 and 24CrSS stainless steels show different Cr contents
in the bulk. The depth profiles of the Cr element of the SUS410, SUS430, SUS445
and 24CrSS samples are shown in Fig. 2. The GDS results show that the Cr contents
are increased in the order of SUS410, SUS430, SUS445 and 24CrSS stainless steels.
The 24CrSS stainless steel contains the highest content of corrosion-resisting Cr.
3.2. X-ray diffraction analysis
Fig. 3 shows the XRD patterns of the SUS410, SUS430, SUS445, and 24CrSS stain-
less steels. The XRD patterns of all the stainless steels show the same diffraction
peaks at 44.66, 64.82, 82.13and 98.18, relative to a-Fe. The peak intensity ratio
for (110), (200), (211), and (220) are different at each stainless steel, because of the
different rolling directions when they are produced [37]. The XRD results also show
that the four types of stainless steels have the ferritic structure, which is a body-
centered cubic (BCC) structure.
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3.3. Corrosion behavior
The polarization curves of the SUS410, SUS430, SUS445 and 24CrSS stainless
steels in the Ar-saturated 0.5 mol dm
-3
H
2
SO
4
electrolyte are shown in Fig. 4. For
the SUS410, SUS430 and SUS445 stainless steels, although the polarization charac-
teristics for these three specimens are almost similar in shape, the polarization curve
of the SUS445 stainless steel shows the same current density as SUS316 stainless
steel in the potential region of 0.25e0.6 V vs. SHE [15]. The formation of
passive-current peak shifts due to a negative potential by increasing the Cr content,
in increasing order; SUS410, SUS430, and SUS445. Lower current densities are
observed at higher Cr-content levels. The SUS410 stainless steel shows the highest
current densities due to it having the lowest Cr content.
In the case of the 24CrSS stainless steel, the onset potential shifts toward the positive
direction, and there is no active current peak compared to SUS410, SUS430 and
Fig. 1. GDS depth profiles of the SUS410, SUS430, SUS445 and 24CrSS stainless steels. (a) SUS410;
(b) SUS430; (c) SUS445; (d) 24CrSS stainless steel.
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SUS445 [24]. This can be explained by the existence of a stable passivation layer
even after the cathodic treatment of 24CrSS. The current densities of the 24CrSS
sample also decreased indicating a better corrosion resistance of the 24CrSS stainless
steel. Because the stainless steel contains a higher Cr content, the best corrosion
resistance was observed [27].
Fig. 2. Depth profiles of Cr element of the SUS410, SUS430, SUS445 and 24CrSS stainless steels.
Fig. 3. XRD patterns of the SUS410, SUS430, SUS445, and 24CrSS stainless steels.
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3.4. Surface morphology
To investigate how the corrosion occurred on the surface of the four types of stain-
less steels, SEM measurements were carried out to observe the surface morphology
of the four types of stainless steels. Fig. 5 shows the SEM images of the four types of
stainless steels before and after the LSV measurements stopped at the peak (Fig. 4)
and the end of the polarization curves. Compared to before the LSV measurement
(Fig. 5(A-1)), the LSV measurement of SUS410 after the rest potential shifts to
-0.02 V vs. SHE (Fig. 5(A-2)), we can clearly see that the grain of SUS410 was
corroded, so the corrosion occurred on the surface of the SUS410 sample during
the active region. For the SUS430 stainless steel, compared to before the LSV mea-
surement (Fig. 5(B-1)), the LSV measurement after the rest potential to -0.13 V vs.
SHE (Fig. 5(B-2)), the surface of the SUS430 stainless steel was corroded during the
active region and the grain boundaries became visible due to the chemical attack of
the acid solution.
Comparing the SEM images of SUS445 before (Fig. 5(C-1)) and after the LSV mea-
surement from the rest potential to -0.22 V vs. SHE (Fig. 5(C-2)), after the LSV mea-
surement, the grain boundaries can still be observed, but not as clearly as that of
SUS430 (Fig. 5(B-2)). The surface of the SUS445 stainless steel was also corroded
during the active region. The corrosion damage to SUS445 was less than that of the
SUS430 stainless steel. At the end of the LSV measurements (Fig. 5(A-3), (B-3), (C-
3)), these three stainless steels were significantly corroded. As for the 24CrSS stain-
less steel, with better corrosion resistance, there was no change to the surface
morphology before (Fig. 5(D-1)) and after the LSV measurements (Fig. 5(D-2),
(D-3)). No apparent corrosion occurred on the 24CrSS stainless steel. The conjec-
tured reason is that the 24CrSS stainless steel contains the highest content of the
corrosion-resisting Cr element, and a passive layer already existed on the surface
of the 24CrSS stainless steel before the experiments, thus the polarization curve
of the 24CrSS stainless steel has no active region.
Fig. 4. Polarization curves of SUS410, SUS430, SUS445 and 24CrSS stainless steels in Ar-saturated 0.5
mol dm
3
H
2
SO
4
electrolyte. Circle symbols mean the point of SEM observation.
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3.5. Electrical conductivity
It is important for the application of fuel cells to evaluate the materials’electrical con-
ductivity. Therefore, electrical conductivities of SUS410, SUS430, SUS445 and
24CrSS were measured using a Mitsubishi Chemical “Loresta HP”(MCP-T410)
Fig. 5. SEM images of (A) SUS410, (B) SUS430, (C) SUS445and (D) 24CrSS stainless steels before/
after LSV measurements stopped at the peak of the polarization curves. (A-1, B-1, C-1, and D-1) before
LSV. (A-2, B-2, C-2, and D-2) LSV stopped at the point of circle symbols in Fig. 4. (A-3, B-3, C-3, and
D-3) after LSV stopped at 1.1 V vs. SHE.
Table 2. Electrical conductivity of the four types of stainless steels, as well as
graphite carbon.
Sample name Thickness
(t/mm)
Surface Resistivity
(Rs/U$,
L1
)
Volume Resistivity
(Rs3t: R/U$mm
L1
)
SUS410 3.9 2.97E-04 1.16E-03
SUS430 1 2.32E-03 2.32E-03
SUS445 0.1 3.17E-02 3.17E-03
24CrSS 0.1 3.61E-02 3.61E-03
Graphite carbon
(Bipolar plate of JARI standard cell)
18 6.29E-04 1.13E-02
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electrometer by the four-point probe resistivity technique, and the results are listed in
Table 2. The volume resistivity of the four types of stainless steels decreased one order
of magnitude compared to that of the graphite carbon. The 0.1 mm-thick 24CrSS
stainless steel (with a high corrosion resistance) showed the same electrical conduc-
tivity as that of the 0.1 mm thick SUS445 stainless steel. Therefore, we conjecture
that it can be used as a bipolar plate, replacing the current graphite carbon.
4. Conclusions
In this study, the corrosion behavior of the four types of ferritic stainless steels with
different Cr contents have been experimentally investigated to study the effect of the
Cr content on the corrosion resistance of ferritic stainless steels in a sulfuric acid
environment. The results revealed the following:
(i) The ferritic stainless steel containing a higher amount of Cr (24 mass%) showed
the best corrosion resistance based on its stable passivation layer.
(ii) Based on the SEM images, no corrosion occurred on the 24CrSS stainless steel
after the LSV measurement from the rest potential to 0.26 V vs. SHE due to its
having the highest Cr content of the stainless steels tested in the sulfuric acid
solution.
Declarations
Author contribution statement
Yang Yu: Performed the experiments; Analyzed and interpreted the data; Wrote the
paper.
Sayoko Shironita: Analyzed and interpreted the data; Wrote the paper.
Kenichi Souma: Analyzed and interpreted the data; Contributed reagents, materials,
analysis tools or data.
Minoru Umeda: Conceived and designed the experiments; Analyzed and interpreted
the data.
Funding statement
This work was supported by the Cross-ministerial Strategic Innovation Promotion
Program (SIP), Cabinet Office, Government of Japan. This work was supported
by JSPS KAKENHI Grant Number JP16K06770 and JP18K19127.
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Competing interest statement
The authors declare no conflict of interest.
Additional information
No additional information is available for this paper.
Acknowledgements
The authors would also like to thank Hitachi Metals, Ltd. for supplying the 24CrSS
stainless steel. The GDS measurement was performed at the Nagaoka University of
Technology Analysis and Instrumentation Center.
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