(Chan Yang Kim)
1
(Do hyung Kim)
1
(Won sub Chung)
1*
Copyright © 2021 The Korean Institute of Metals and Materials
Key words(Korean)
ferrite stainless steel, corrosion resistance, oxide layers, passive film, pitting corrosion
1. Introduction
Stainless steel is a type of steel with excellent corrosion resistance compared to
ordinary steel materials, due to a passive film. The passive film is an ultra-thin
film formed during the initial stainless steel oxidation process. This film plays
an important role in the corrosion resistance of stainless steel because it prevents
the exposure of the base metal to the external environment, thereby preventing oxidation
of the surface. A wide variety of stainless steel species are available, depending
on the intended purpose and field of use. These materials are applied extensively,
in petrochemical industries, power plants, plumbing, facilities, home appliances,
and household goods [1,2]. Compared to austenite stainless steel, ferrite stainless steel contains only a
small amount of expensive nickel (Ni), making it relatively inexpensive. In addition,
this steel species has a good surface gloss, formability, and oxidation resistance.
Hence, it can be used for various applications, including heat-resistant appliances,
enclosures, home appliances, and electronic components. Moreover, ferrite stainless
steel is economical and has excellent corrosion resistance, in line with the demand
for inexpensive and efficient steel materials. Accordingly, many studies have been
conducted on ferrite stainless steel, which has been recommended as an alternative
to steel materials in various fields [3].
The corrosion resistance of most stainless steel is generally superior to other steel
species. However, stainless steel is highly susceptible to pitting corrosion due to
destruction of the passive film [4]. Pitting corrosion results when anodic dissolution occurs in a limited area of the
material surface covered with the passive film. In particular, pitting corrosion can
occur when the external environment destroys the passive film locally, and the melting
rate of the film is faster than the recovery rate. Pitting corrosion arises with specific
reactive ionic atmospheres and electrochemical potentials, including chlorine ions
(Cl-), and ultimately shortens the life span of the stainless steel [5]. Although several studies have been conducted on the reduction of the lifespan of
stainless steel by pitting corrosion, not many have reported on the corrosion mechanisms
and their causes. Therefore, further research is necessary to improve the corrosion
resistance of stainless steel and its resistance to pitting corrosion, which is a
globally important problem.
Stainless steel undergoes several heat-treatment processes during its manufacture.
The secondary annealing process, following roll pressing, is performed several times
in a high-temperature atmosphere to effectively remove hydrogen embrittlement inside
the material [6]. However, this process results in rapid iron oxidation on the surface. Although
the oxide produced on the surface is typically eliminated through an acid pickling
process, it is complicated and challenging to eliminate the iron oxidation, and is
thereby inefficient for production. Furthermore, the generation of precipitates, including
chromium (Cr) carbide and titanium (Ti) nitride, can induce galvanic effects in the
material, which is fatal to the corrosion resistance and the lifespan of the material
[1,7]. Accordingly, there are ongoing efforts to study and develop steel species to minimize
the generation of iron (Fe) oxide on the Fe surface, and simultaneously improve the
corrosion resistance of the materials.
AISI 439 ferrite stainless steel typically contains 17-19 wt.% Cr. Chromium is one
of the main constituent elements of stainless steel, essential for improving corrosion
resistance. During the initial oxidation process of stainless steel, the Cr2O3 passive film plays a key role in preventing corrosion. Several studies have been
conducted on how Cr content affects various properties of stainless steel. Generally,
the corrosion resistance increases as the Cr content of stainless steel increases.
Specifically, studies have reported that every 1 wt.% increase in Cr content increases
the value of the pitting potential, a major corrosion resistance indicator of stainless
steel, by approximately 100 mV [8,9]. However, excessive Cr content degrades the mechanical properties of the stainless
steel, such as formability, which limits the use of stainless steel in certain fields.
With these considerations, the goal of the present study is to reduce Cr content to
preserve various mechanical properties while supplementing corrosion resistance by
adding other corrosion resistance-enhancing elements. In this study, the decrease
in corrosion resistance resulting from the Cr reduction was offset by adding silicon
(Si) and tin (Sn). Si acts as a reducing agent and ferrite stabilizer in the manufacturing
process. Si is also reported to inhibit the production of oxides on the iron surface
[10,11]. Moreover, SiO2 layers produced on iron surfaces enhance the stability of the passive film by inhibiting
the iron oxidation of the base metal with a passive film, thereby improving the corrosion
resistance of the material. In addition, an appropriate amount of Si improves the
formability of stainless steel, thus improving its mechanical properties.
Adding small amounts of Sn to stainless steel can also improve its corrosion resistance
and passive film stability. Similar to Si, Sn reacts with oxygen on the surface to
produce SnO2 layers, which has been reported to improve the stability of the passive film [5,12]. Moreover, the electromotive force (EMF) potential of Sn is -0.136 V, higher than
iron (-0.440 V). Elements with higher EMF values will exhibit higher electrochemical
stability. However, when the passive film of the stainless steel is destroyed, the
galvanic effect of the potential difference between the Sn and Fe in the base metal
accelerates the corrosion of the base metal [4,5,13]. Therefore, studies have reported that the addition of Sn in the range of 0.05 to
0.1 wt.% is optimal for effectively improving the corrosion resistance of stainless
steel [12]. Based on the above studies, the present work evaluated corrosion resistance and
optimized the heat-treatment process for Si and Sn alloys that replaced the Cr composition
of AISI 439 ferrite stainless steel.
2. Experimental Methods
The stainless steel samples were acquired from POSCO Co., Ltd. (South Korea). The
samples were produced by roll pressing a thin sheet with a 1 mm thickness. The composition
of AISI 439 ferrite stainless steel samples are illustrated in Table 1, with (#A) containing 17 wt.% Cr and stainless alloys (#B) containing 15 wt.% Cr,
Si, and Sn. Each sample was prepared with dimensions of 20 mm × 20 mm × 1 mm. In addition,
to analyze the surface conditions before and after the heat treatment, grinding and
polishing were performed from #600 grit to #2000 grit using a silicon carbide (SiC)
paper, and micro-grinding was performed using a 1 µm diamond suspension.
Subsequently, heat treatment was conducted for a short time after the roll pressing
process to remove the hydrogen brittleness generated during the production of stainless
steel during the manufacturing process at POSCO. Each heat treatment was performed
for one minute in a high-temperature atmosphere at 900, 950, and 1000 °C, followed
by air-cooling. The obtained specimens was oxidized and etched in Viella’s Reagent
(ethanol + 10 mL/L HCl + 10 g/L picric acid) to analyze surface morphology and microstructure
using an optical microscope (OM, Nikon, ECLIPSE LV150N, Japan). Additionally, a field-emission
scanning electron microscope (FE-SEM, Hitachi S4800, Japan) and energydispersive X-ray
spectrometer (EDS, ELECT PLUS, AMETEK, USA) were employed to obtain highmagnification
images and component changes for each condition. Depth profiles (NEXA Thermophosphorous)
were analyzed by X-ray photoelectron spectrophotometer (XPS) to quantitatively assess
the component changes based on the depth direction of the specimen cross section.
The analysis was conducted for 2500 s at a sputtering energy of 3 kV, sputter rate
of 0.49 nm/s, and sputter size of 2 mm × 2 mm. An electrochemical analysis of each
specimen was performed using a potentiostat (VersaSTAT 4, AMETEK, USA) to conduct
a potentiodynamic polarization test, and an electrochemical impedance spectrometer
(EIS, VersaSTAT 4, AMETEK, USA). A saturated calomel electrode (SCE) was used as the
reference electrode. The relative electrode (CE) used a platinum mesh of 2 cm × 2
cm. Additionally, the gap between the working electrode (WE) and CE was set to 12
cm. The test was conducted at a temperature of 25 °C under the conditions of NaCl
and H2SO4 mixed solution (0.5 M NaCl + 0.5 M H2SO4) according to ASTM G 5-14. The potentiodynamic polarization test was conducted at
a scan rate of 0.167 mV/s under conditions of –0.2 V (vs OC) to 1.0 V (vs SCE) to
quantitatively evaluate the corrosion resistance and electrochemical properties of
the specimen. EIS was conducted to measure the solution resistance (Rs) and polarization resistance (Rp), depending on the conditions of the specimen, and proceeded in the range of 1 Hz
to 100 kHz.
3. Results and Discussions
3.1 Microstructure (Optical microscope)
Based on the heat-treatment conditions, the surface microstructures of specimens #A
and #B were analyzed using an optical microscope, as shown in Figure 1. Similar precipitates, including Cr carbide and Ti nitride, were formed under heat-treatment
conditions in both #A and #B. These deposits were randomly created inside the grain
boundaries and grains. However, no significant changes were observed in the deposits
produced at varying heat-treatment temperatures, due to the short heat-treatment time.
Conversely, it was confirmed that the grain size of each specimen slightly increased
with increasing heat-treatment temperature. In general, the grain growth occurred
during the annealing process, due to an increase in the heat-treatment temperature
of the stainless steel, thereby increasing the grain size. In addition, studies have
reported that the corrosion resistance of stainless steel is inversely proportional
to the grain size. Thus, the contact area between the base metal and the corrosion
environment increased with the grain size, allowing easy exposure to the corrosion
environment [14,15].
3.2 Surface analysis (SEM and EDS)
The surface morphology of specimens under each condition was analyzed using SEM and
EDS to compare and analyze the surface changes before and after corrosion, as shown
in Figure 2. No significant changes were observed before and after corrosion from the SEM results.
However, the formation of oxide films was common under all conditions, allowing the
identification of non-uniform surfaces from the images. The results of the before-
and after-corrosion specimen surface composition ratio measured by the EDS spectrum
of the surface are shown in Table 2.
An increase in O content from 1 wt.% to approximately 30 wt.% was common for all specimens
after corrosion. This is a common phenomenon, indicating that corrosion creates an
oxide film on the surface. In addition, the contents of Cr and Fe were relatively
reduced, indicating the production of Fe oxide and Cr oxide in the oxide films.
Furthermore, in the case of Si present in specimens #A and #B, the Si content on the
surface increased after corrosion, indicating that the oxide film consisted of Si
oxide. Studies have reported that adding Si to stainless steel resulted in the formation
of SiO2 layers, which increased the stability of the passive film. These layers inhibited
the oxidation of Fe by contributing to the stability of the passive film [16,17]. The amount of Si in specimen #B was approximately twice that in specimen #A, which
was attributed to the difference in Si content. For specimen #B containing Sn, the
Sn content increased by approximately 2 wt.% after corrosion. This indicated the formation
of SnO2 layers on the surface oxide film. The following chemical reaction can account for
the formation of Sn oxide layers on the surface: [2,12,18]
It has been reported that the Si and Sn oxide films formed on stainless steel surfaces
improved the stability of passive films by inhibiting the Fe oxidation of the base
metal. Thus, the decreasing Fe oxidation ratio on the surface lead to the formation
of Si and Sn oxide films, thereby contributing to enhancement of the passive film
stability.
3.3 Cross-sectional analysis (Depth profiles)
SEM images of the cross-section of each specimen based on the heat treatment conditions
are shown in Figure 3. No significant differences in the Cr, Si, and Sn contents were observed on the surfaces
of the base metal. Thus, to quantitatively clarify the differences based on the conditions,
depth profiles using XPS analysis were obtained, as shown in Figure 4. These profiles determined the changes in the content of each element based on the
depth direction of the specimen. Etch level indicated the length, measured in the
direction of depth from the surface.
In addition, the XPS analysis showed that the film thickness of the material was approximately
1 µm [19]. The oxide film thickness for each condition was relatively comparable through the
intersection of Fe and O. For specimen #A, the increasing temperature increased the
Fe content and decreased the O content (etch levels 200 nm, 500 nm, and 600 nm). This
suggests that the amount of Fe oxide produced on the surface increased as the heat-treatment
temperature increased.
In contrast, the amount of oxide was constant at etch level 350 nm until 950 °C and
subsequently increased to 750 nm from 1000 °C. Thus, the production of Fe oxide was
inhibited up to 950 °C, and the effect of Si and Sn was relatively halved at 1000
°C. The (c, d) graph shows the change in the contents of Cr and Ti as the temperature
increases. The contents of Cr and Ti on the surface decreased with the direction of
the base metal, implying that the oxidation ratio of Cr and Ti on the surface increased
with temperature.
The (e, f) graph shows the change in content based on the depth direction of Si and
Sn [20]. For specimen #B with a high Si content, the ratio of oxidized Si on the surface
was high. Moreover, Sn exhibited behavior similar to Si. This suggests that Si and
Sn reacted with O to form oxide layers on the surface of specimen #B, reducing the
production rate of Fe oxide. Thus, the addition of Si and Sn contributed to an improvement
in the passive film stability of the stainless steel.
3.4 Corrosion test
Graphs and data were obtained from the potentiodynamic polarization test results of
each specimen for the various heat-treatment conditions, as shown in Figure 5 and Table 3. The overall corrosion resistance data of specimen #B, including Si and Sn, were
relatively superior to specimen #A. For the corrosion potential (Ecorr) indicating the corrosion resistance of the material, #A and #B were measured in
the range of -580 to -570 mV and -540 to -530 mV, respectively. The corrosion potential
of #B was approximately 40 mV higher than that of #A, implying an improved resistance
to corrosion on the surfaces of the stainless steel, due to the effect of Si and Sn
[2,21]. In addition, the Ecorr of both specimens decreased when the temperature increased. This is considered to
be a slight decrease in corrosion resistance, attributed to an increase in the material
grain size.
The Nyquist plot of the EIS analysis allows comparison of the corrosion resistances
of the #A and #B specimens, as shown in Figure 6 and Table 4. The Rs was measured at 4.0 ohms for all specimens. However, the Rp for #A and #B were measured in the range 280-325 ohms and 505-510 ohms, respectively,
indicating that #B has better corrosion resistance than #A in the same corrosion environment
[13,21]. Furthermore, the polarization resistance of specimen #B showed a relatively smaller
decrease with increasing temperature than specimen #A, implying that specimen #B has
higher stability due to the temperature changes. Thus, the addition of Si and Sn contributed
to the improvement in the corrosion resistance of the materials [6].
The corrosion current density (Icorr) was less than 100 µA/cm2 for specimen #B compared to specimen #A, indicating that the corrosion rate in the
same environment was reduced. In addition, the Icorr increased with temperature. Furthermore, the increase in Icorr with temperature was similar to the decrease in Ecorr. Thus, the decrease in corrosion resistance increased the corrosion rate [22].
The measured passive film maintenance current values (Id) of specimen #B were between 7 and 20 µA/cm2, lower than that of specimen # A (45-53 µA/cm2). This implies that the passive film of specimen #B was produced in an electrochemically
stable region, due to the improvement in passive film stability of the SiO2 and SnO2 layers.
The pitting potential (Epit) of specimen #B was measured to be approximately 100 mV, lower than that of specimen
#A. This indicates that the destruction of passive film in #B was faster, leading
to the onset of pitting corrosion. Such behavior could have two causes. First, the
Cr content of specimen #B was approximately 2 wt.% lower than that of #A. However,
the amount of added Si and Sn was only about 1 wt.%. Studies have reported that the
value of Epit decreased by approximately 100 mV each time the Cr content of the stainless steel
decreased by 1 wt.% [8,23]. However, the decrease in the Cr content of the stainless steel led to a decrease
in the Cr2O3 content, which is the main component of the passive film. This reduces the Fe oxidation
inhibiting effect on the surface; therefore, an appropriate content of Cr is essential
for corrosion resistance [24,25].
A significant difference in Epit of approximately 100 mV was observed for specimens #A and #B. This suggests that
the Si and Sn oxide layers added to #B were equivalent to the corrosion resistance
effect of approximately 1 wt.% Cr. Therefore, the corrosion resistance of specimen
#B might decrease because Cr was reduced less. The addition of Si and Sn could also
supplement the corrosion resistance. Consequently, we were able to reduce the Cr content
and confirm improvements in the corrosion resistance of the stainless steel by Si
and Sn. Second, Sn has a higher potential than Fe. Thus, the acceleration in pitting
corrosion could be attributed to the galvanic effect based on the difference in electromotive
force (EMF). Other studies have confirmed that when the passive film was destroyed
and the pitting corrosion began, the galvanic effect between the Sn layers and the
base metal accelerated the pitting corrosion of the base metal.
4. Conclusions
This study analyzed changes in the corrosion resistance of AISI 439 ferrite stainless
steel with heat-treatment temperature and the amount of added Cr, Si, and Sn contents,
which changed the components of the surfaces and cross sections. The following conclusions
were drawn from this study:
1. It was confirmed that adding Si and Sn to AISI 439 ferritic stainless steel increased
its corrosion resistance.
2. As the heat-treatment temperature of stainless steel increased, the grain size
increased slightly due to grain growth. This change is attributed to the reduced corrosion
resistance of the materials. In addition, precipitates such as Cr carbide and Ti nitride
were produced during the heat treatment, which commonly affect corrosion resistance.
3. As the heat-treatment temperature increased from 900 to 1000 °C, the production
of Fe oxide on the surface increased. This is the leading cause of reduced corrosion
resistance. Simultaneously, the production of Si and Sn oxides on the surface increased.
This inhibited production of the Fe oxide on the surface, thereby improving the passive
film stability.
4. Although the suppression of Fe oxide up to 950 °C was confirmed, the oxide generation
increased at temperatures above 1000 °C. This suggests that the heat treatment temperature
of 950 °C is most effective in the manufacturing process.
5. The Si and Sn added specimens showed improvement in corrosion potential and corrosion
current density. It was also confirmed that a passive film was produced in electrochemically
stable areas by reducing the passive maintenance current.
6. The pitting potentials of the specimens containing Si and Sn were relatively low.
This is believed to have affected the reduction in the passive film properties due
to a 2 wt.% Cr reduction. Thus, further studies on the appropriate contents of Cr,
Si, and Sn could simultaneously improve mechanical properties, formability, and corrosion
resistance.
Acknowledgements
This work was supported by the Korea Institute for Advancement of Technology (KIAT)
grant funded by the Korea Government (MOTIE) (P0002019, Human Resource Development
Program for Industrial Innovation).
This work was supported by the National Research Foundation of Korea (NRF) [grant
number 2020R1A5A 8018822], which is funded by the Korean government (MSIT)
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Figures and Tables
Fig. 1.
Optical microscope image of surface microstructure according to heat treatment conditions.
Fig. 2.
SEM images before and after corrosion of surfaces by heat treatment conditions of
#A and #B specimen.
Fig. 3.
Scanning electron microscope (SEM) analysis results for cross sections of #A and #B
specimens under heat-treatment conditions.
Fig. 4.
Results of Depth profiles for each cross section by heat treatment conditions for
#A and #B specimens.
Fig. 5.
Results of potentiodynamic polarization test by heat treatment conditions of #A and
#B specimens.
Fig. 6.
Nyquist plot curves of EIS analysis results by heat treatment conditions for #A and
#B specimens.
Table 1.
Components of ferritic stainless steel #A and #B
|
Specimen
|
Cr
|
Si
|
Sn
|
Ti
|
Mn
|
Fe
|
|
#A
|
17.6
|
0.15
|
-
|
0.35
|
0.1
|
bal
|
|
#B
|
15.2
|
0.98
|
0.057
|
0.35
|
0.1
|
bal
|
Table 2.
Results of EDS analysis of components before and after corrosion of surfaces by heat
treatment conditions of #A and #B specimen
|
wt. %
|
O
|
Cr
|
Fe
|
Si
|
Sn
|
Ti
|
|
#A Bal
|
Before
|
0.9
|
32.9
|
56.1
|
1.3
|
-
|
1.9
|
|
After
|
34.1
|
12.8
|
38.3
|
2.5
|
-
|
13.3
|
|
#A 900 °C
|
Before
|
0.7
|
34.8
|
54.9
|
1.3
|
-
|
1.9
|
|
After
|
32.9
|
12.7
|
38.6
|
2.7
|
-
|
13.0
|
|
#A 950 °C
|
Before
|
1.0
|
33.5
|
55.7
|
1.4
|
-
|
1.8
|
|
After
|
34.4
|
15.1
|
38.9
|
3
|
-
|
8.6
|
|
#A 1000 °C
|
Before
|
0.9
|
33.0
|
55.9
|
1.3
|
-
|
2
|
|
After
|
33.2
|
13.6
|
38.7
|
3.2
|
-
|
11.4
|
|
#B Bal
|
Before
|
1.2
|
31.3
|
59.5
|
2.8
|
1.1
|
1.7
|
|
After
|
30.4
|
13.0
|
38.8
|
3.9
|
3.2
|
10.7
|
|
#B 900 °C
|
Before
|
1.0
|
29.3
|
57.4
|
2.6
|
1.0
|
1.5
|
|
After
|
29.6
|
11.1
|
40.3
|
4.0
|
3.7
|
11.3
|
|
#B 950 °C
|
Before
|
0.7
|
29.4
|
57.9
|
2.8
|
1.4
|
1.5
|
|
After
|
28.0
|
11.0
|
41.6
|
4.3
|
4.1
|
11.0
|
|
#B 1000 °C
|
Before
|
0.7
|
30.6
|
56.7
|
2.4
|
1.5
|
1.8
|
|
After
|
29.3
|
11.8
|
40.4
|
3.9
|
3.4
|
11.3
|
Table 3.
Results of potentiodynamic polarization test by heat treatment conditions of #A and
#B specimens
|
Condition
|
Corrosion resistance
|
Passive Range
|
|
Ecorr (mV)
|
Icorr (µA/cm2)
|
Id (µA/cm2)
|
Epit (mV)
|
|
#A |
900 °C |
-569 ± 5
|
160 ± 20
|
45.5 ± 5
|
391 ± 5
|
|
950 °C |
-572 ± 5
|
280 ± 20
|
53.5 ± 5
|
388 ± 5
|
|
1000 °C |
-577 ± 5
|
276 ± 20
|
48.3 ± 5
|
395 ± 5
|
|
#B |
900 °C |
-535 ± 5
|
34 ± 5
|
15.8 ± 10
|
295 ± 5
|
|
950 °C |
-520 ± 5
|
67 ± 5
|
10.7 ± 10
|
293 ± 5
|
|
1000 °C |
-541 ± 5
|
82± 5
|
7.9 ± 10
|
290 ± 5
|
Table 4.
Rs and Rp data of EIS analysis results by heat treatment conditions for #A and #B
specimens.
|
Condition
|
Rs (Ohms)
|
Rp (Ohms)
|
|
#A 900 °C
|
4.0
|
325.9
|
|
#A 950 °C
|
4.0
|
302.4
|
|
#A 1000 °C
|
4.0
|
287.4
|
|
#A 900 °C
|
4.0
|
508.7
|
|
#A 950 °C
|
4.0
|
507.5
|
|
#A 1000 °C
|
4.0
|
506.1
|