A. GRAJCAR, A. PLACHCIÑSKA: EFFECT OF SULPHIDE INCLUSIONS ON THE PITTING-CORROSION BEHAVIOUR ...
713–718
EFFECT OF SULPHIDE INCLUSIONS ON THE
PITTING-CORROSION BEHAVIOUR OF HIGH-Mn STEELS IN
CHLORIDE AND ALKALINE SOLUTIONS
VPLIV SULFIDNIH VKLJU^KOV NA JAMI^ASTO KOROZIJO
JEKEL Z VISOKO VSEBNOSTJO Mn V RAZTOPINAH KLORIDOV
IN ALKALIJ
Adam Grajcar, Aleksandra P³achciñska
Silesian University of Technology, Institute of Engineering Materials and Biomaterials, Konarskiego Street 18a, 44-100 Gliwice, Poland
adam.grajcar@polsl.pl
Prejem rokopisa – received: 2015-07-01; sprejem za objavo – accepted for publication: 2015-09-02
doi:10.17222/mit.2015.169
The corrosion behaviour of the 27Mn-4Si-2Al- and 26Mn-3Si-3Al-type austenitic steels were evaluated in chloride 3.5 % NaCl
and alkaline 0.1-M NaOH environments using potentiodynamic polarization tests. The type of non-metallic inclusions and their
pitting-corrosion behaviour were investigated. In the chloride solution, both the steels exhibited a lower corrosion resistance in
comparison to the alkaline solution. The high-Mn steels showed evidence of pitting and uniform corrosion, both in the chloride
and alkaline solutions. SEM micrographs revealed that the corrosion pits are characterized by various shapes and an irregular
distribution at the metallic matrix. Corrosion damage is more numerous in the chloride solution than in the alkaline solution.
EDS analyses revealed that the corrosion pits nucleated on MnS inclusions or complex oxysulphides. The chemical composition
of the steels (change in the Al and Si contents) does not affect the privileged areas of pit nucleation, whereas it influences the
electrochemical behaviour of the steels in the chloride solution.
Keywords: high-Mn steel, austenitic steel, non-metallic inclusion, corrosion resistance, pitting corrosion, potentiodynamic pola-
rization test
Korozijsko obna{anje avstenitnih jekel 27Mn-4Si-2Al in 26Mn-3Si-3Al je bilo ocenjeno v raztopini 3,5 % NaCl in v alkalni
raztopini 0,1 M NaOH, s pomo~jo potenciodinami~nih polarizacijskih preizkusov. Preiskovana je bila vrsta nekovinskih
vklju~kov in njihovo pona{anje pri jami~asti koroziji. V raztopini kloridov sta obe jekli, v primerjavi z alkalno raztopino, poka-
zali manj{o korozijsko obstojnost. Jekla z visoko vsebnostjo Mn so pokazala jami~asto in splo{no korozijo v obeh raztopinah,
tako v kloridni kot v alkali~ni. SEM-posnetki so pokazali korozijske jamice razli~nih oblik in njihovo neenakomerno razpo-
reditev po kovinski osnovi. Korozijske po{kodbe so bolj {tevilne v kloridni raztopini kot pa v alkalni raztopini. EDS-analize so
pokazale, da korozijske jamice nastajajo na vklju~kih MnS ali na kompleksnih oksisulfidih. Kemijska sestava jekel (spremembe
v vsebnosti Al in Si) ni vplivala na prednostna mesta nukleacije jamic, medtem ko je vplivala na elektrokemijsko pona{anje
jekel v raztopini kloridov.
Klju~ne besede: jekla z veliko vsebnostjo Mn, avstenitno jeklo, nekovinski vklju~ki, odpornost na korozijo, jami~asta korozija,
potenciodinami~ni polarizacijski preizkus
1 INTRODUCTION
Pitting corrosion is a type of localized corrosion. It
occurs mainly in the passive state of metals, in environ-
ments containing aggressive ions, i.e., chloride anions.
The pits are often invisible during the formation stage,
but their progressive local damage can lead to an element
perforation.1 It is well known that various factors – the
chemical composition, microstructure, heat treatment
and plastic deformation – affect the pitting potential.2–4
There are many reports that confirm the negative
impact of non-metallic inclusions on the corrosion resis-
tance of steel.5–7 High-manganese austenitic steels have
different types of inclusions, which form during melting
and casting. These steels contain Mn, which combines
with sulphur, and Si and Al additions with a high che-
mical affinity for oxygen (Al also to nitrogen).8–10 There-
fore, the presence of various sulphide and oxide inclu-
sions in these steels can be expected.11,12 I. J. Park et al.7
observed MnS, AlN, Al2O3, MnAl2O4 and other complex
inclusions in Mn-Al steels. ^. Donik et al.5 and A. Pardo
et al.6 reported that the Mn additions to stainless steels
have a detrimental effect on the pitting-corrosion
resistance in a NaCl medium. Manganese favours the
formation of MnS inclusions, which are vulnerable to the
initiation of corrosion pits. Moreover, its presence
drastically increases the corrosion current density of
steel and displaces the Ecorr values towards less noble
potentials. K. J. Park and H. S. Kwon13 found that the
size of the MnS inclusions increased with an increase in
the Mn concentration in Fe–18Cr–6Mn and
Fe-18Cr-12Mn steels. The shape, composition and
distribution of inclusions have significant effects on the
corrosion resistance too.
The high-manganese alloys belong to a new, ad-
vanced group of steels that combine successively high
strength and high plasticity due to the austenitic micro-
structure. Because of the homogeneous ductile micro-
Materiali in tehnologije / Materials and technology 50 (2016) 5, 713–718 713
UDK 67.017:549.3:620.193 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)713(2016)
structure they can be used for numerous elements of the
energy-absorbing structures of cars.14,15 These steels are
used as a cheaper substitute for austenitic stainless steels.
Manganese (austenite stabilizer) and aluminium can
replace expensive nickel and chromium additions. The
potential applications also include construction materials
to transport different liquid gases with various pH values.
However, their real application also depends on the co-
rrosion behaviour. Therefore, the effect of non-metallic
inclusions on the corrosion properties of the 27Mn-4Si-
2Al and 26Mn-3Si-3Al steels in two environments, i.e.,
3.5 % NaCl (neutral) and 0.1-M NaOH (alkaline), have
been investigated in this study using electrochemical
polarization tests.
2 EXPERIMENTAL PROCEDURE
The investigated materials were high-Mn austenitic
steels with the chemical composition shown in Table 1.
Both steels were treated using the same conditions. The
steel ingots were prepared by vacuum melting, then they
were hot-forged and roughly rolled to a thickness of
4.5 mm. The next step was their thermomechanical
processing, consisting of the hot rolling of flat samples
in three passes (relative reductions: 25 %, 25 % and
20 %) to a final sheet thickness of approximately 2 mm,
obtained at 850 °C. Subsequently, the samples were
rapidly cooled in water to room temperature.
The flat samples of the 27Mn-4Si-2Al and 26Mn-
3Si-3Al steels with a 0.38 cm2 exposed surface area were
prepared for the electrochemical tests in 3.5 % NaCl
(neutral) and 0.1-M NaOH (alkaline) solutions. The
samples were mechanically ground with SiC paper up to
1200 grit. Prior to the experiments, all the samples were
washed in distilled water and rinsed in acetone. The
solutions were prepared using deionised water. The
electrochemical cell comprised three electrodes. A stain-
less steel and a silver/silver chloride (Ag/AgCl) electrode
(SSE) were used as the counter and the reference
electrodes, respectively. The electrochemical measure-
ments were performed using an Atlas 0531
Electrochemical Unit potentiostat/galvanostat driven by
AtlasCorr05 software. The potentiodynamic polarisation
measurements were conducted at a scan rate of 1 mV/s.
The potentiodynamic scan data were collected to deter-
mine the electrochemical parameters: corrosion potential
Ecorr and corrosion current density Icorr.
The samples after the corrosion tests were polished
using Al2O3 with a granularity of 0.1 μm for the scanning
electron microscopy (SEM). To reveal the corrosion pits
the cover formed on the pit surface has to be removed.
Thus, the samples’ surfaces were polished to obtain a
uniform surface with pit-initiation sites. The corrosion
damage was examined based on SEM observations and
EDS techniques. Additionally, the depth of the corrosion
damage on the cross-sectioned specimens was evaluated
using a light microscope.
3 RESULTS AND DISCUSSION
Typical microstructures of the 27Mn-4Si-2Al and
26Mn-3Si-3Al steel specimens are shown in Figures 1a
and 1b, respectively. Both micrographs exhibit relatively
coarse austenite grains elongated according to the direc-
tion of hot rolling. The mean grain size is approximately
80 μm. The microstructures reveal the presence of
annealing twins, deformation effects and elongated
sulphide inclusions.
Potentiodynamic curves of the 27Mn-4Si-2Al and
26Mn-3Si-3Al steels registered in 3.5 % NaCl (neutral –
pH 7) and 0.1-M NaOH (alkaline – pH 14) solutions are
illustrated in Figures 2 and 3. The average calculated
values of the corrosion potential Ecorr and the corrosion
current density Icorr determined by the Tafel extrapolation
are shown in Table 2. Both steels show lower corrosion
resistance in the 3.5 % NaCl solution than in 0.1-M
NaOH solution. The corrosion current density registered
in the chloride solution was higher in comparison to the
alkaline solution (Table 2). The obtained data are
supported by the similar results of other authors16,17, who
reported that the high-Mn austenitic steels show a lower
A. GRAJCAR, A. PLACHCIÑSKA: EFFECT OF SULPHIDE INCLUSIONS ON THE PITTING-CORROSION BEHAVIOUR ...
714 Materiali in tehnologije / Materials and technology 50 (2016) 5, 713–718
Table 1: Chemical composition of investigated steels in mass fractions (w/%)
Tabela 1: Kemijska sestava preiskovanih jekel v masnih dele`ih (w/%)
Grade Mn Si Al S P Nb Ti N O Fe
27Mn-4Si-2Al 27.5 4.18 1.69 0.017 0.004 0.033 0.010 0.0028 0.0006 bal.
26Mn-3Si-3Al 26.0 3.08 2.87 0.013 0.002 0.034 0.010 0.0028 0.0006 bal.
Table 2: Average values of the electrochemical polarization data for the 27Mn-4Si-2Al and 26Mn-3Si-3Al steels obtained in the 3.5 % NaCl and
0.1-M NaOH solutions
Tabela 2: Srednje vrednosti podatkov elektrokemijske polarizacije 27Mn-4Si-2Al in 26Mn-3Si-3Al jekel, dobljene v raztopinah 3,5 % NaCl in
0,1 M NaOH
Grade Statistics
3.5 % NaCl 0.1 M NaOH
Ecorr/(mV) Icorr/(mA/cm2) Ecorr/(mV) Icorr/(mA/cm2)
27Mn-4Si-2Al
average value –788 0.090 –392 0.007
standard deviation 6.5 0.007 3.2 0.002
26Mn-3Si-3Al
average value –785 0.009 –395 0.005
standard deviation 11.2 0.005 4.3 0.003
corrosion resistance in the chloride medium than in the
alkaline solution.
In the 3.5 % NaCl solution, the 27Mn-4Si-2Al steel
specimens showed a much higher corrosion current
density (0.09 mA/cm2) than the 26Mn-3Si-3Al steel
(0.009 mA/cm2). This confirms our earlier results from
the potentiodynamic polarisation tests.3 It is related to
the higher Al and lower Si contents in 26Mn-3Si-3Al
steel in comparison to the steel containing 2 % Al (Table
1). It is reported18 that a silicon addition decreases the
corrosion resistance of steel. On the other hand, alumi-
nium improves the corrosion resistance due to its tenden-
cy to form a protective Al2O3 passive layer on the steel
surface in solutions of pH ~7 (Pourbaix diagrams).19 All
the specimens polarized in the 3.5 % NaCl solution show
Ecorr values shifted to less noble potentials (Table 2)
when compared to the specimens polarized in the 0.1-M
NaOH solution. The Ecorr shift was about 400 mV
towards the cathodic direction. The values of the corro-
sion-current density obtained in the 0.1-M NaOH were
quite similar for both steels. The 27Mn-4Si-2Al steel
specimens showed a corrosion current density of
approximately 0.007 mA/cm2, whereas it was 0.005
mA/cm2 for the second steel. The better corrosion
resistance of both steels in 0.1-M NaOH is related to the
fact that in alkaline solutions, manganese precipitates as
Mn(OH)2, which is slightly soluble in solutions with
pH>13, whereas in solutions of pH ~ 7 the manganese
dissolves as Mn2+ (Pourbaix diagrams).19
The morphology of the corrosion pits after the
electrochemical tests were studied using the SEM and
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Materiali in tehnologije / Materials and technology 50 (2016) 5, 713–718 715
Figure 3: Potentiodynamic polarization curves of the: a) 27Mn-4Si-
2Al and b) 26Mn-3Si-3Al steels obtained in 0.1-M NaOH solution
Slika 3: Krivulje potenciodinami~ne polarizacije jekel v raztopini
0,1 M NaOH: a) 27Mn-4Si-2Al in b) 26Mn-3Si-3Al
Figure 2: Potentiodynamic polarization curves of the: a) 27Mn-
4Si-2Al and b) 26Mn-3Si-3Al steels obtained in 3.5 % NaCl solution
Slika 2: Krivulje potenciodinami~ne polarizacije jekel v raztopini
3,5 % NaCl: a) 27Mn-4Si-2Al in b) 26Mn-3Si-3Al
Figure 1: Austenitic microstructure of the thermomechanically
processed: a) 27Mn-4Si-2Al and b) 26Mn-3Si-3Al steels
Slika 1: Avstenitna mikrostruktura termomehansko izdelanih jekel:
a) 27Mn-4Si-2Al in b) 26Mn-3Si-3Al
EDS techniques. The SEM images of the corrosion
damage in both steels after the corrosion tests in 3.5 %
NaCl are shown in Figures 4 and 5. The corrosion pits
are characterized by various shapes and an irregular
distribution at the metallic matrix (Figures 4a and 5a).
They are formed both at the grain boundaries and within
the austenite grains. It is apparent that the pits are
initiated at non-metallic inclusions. The EDS analysis
revealed the variation of the chemical composition in the
interior of the individual pits. For instance, the chemical
composition of the particle inside the corrosion pit in
Figure 4b showed a high content of manganese and
sulphur (Figure 4d). This indicates that the privileged
A. GRAJCAR, A. PLACHCIÑSKA: EFFECT OF SULPHIDE INCLUSIONS ON THE PITTING-CORROSION BEHAVIOUR ...
716 Materiali in tehnologije / Materials and technology 50 (2016) 5, 713–718
Figure 5: a) SEM micrograph of the 26Mn-3Si-3Al steel surface,
b) the individual pit interior, c) EDS analysis from point C after
corrosion test in 3.5 % NaCl
Slika 5: a) SEM-posnetek povr{ine jekla 26Mn-3Si-3Al, b) izgled
posamezne jamice, c) EDS-analiza to~ke C po korozijskem preizkusu
v 3,5 % NaCl
Figure 4: a) SEM micrograph of the 27Mn-4Si-2Al steel surface, b)
the individual pit interior, c) EDS analysis from point D, d) EDS
analysis from point C after corrosion test in 3.5 % NaCl
Slika 4: a) SEM-posnetki povr{ine jekla 27Mn-4Si-2Al, b) izgled
posamezne jamice, c) EDS-analiza v to~ki D, d) EDS-analiza to~ke C
po korozijskem preizkusu v 3,5 % NaCl
places for the pit initiation are MnS inclusions. There are
many reports in the literature5-7,20 that confirm that MnS
inclusions are vulnerable for the initiation of corrosion
pits. Their presence increases the corrosion current
density and displaces the Ecorr values towards less noble
potentials. The chemical analysis of the corrosion
damage shown in Figure 4b also revealed the presence
of oxides containing Al and Si (Figure 4c).
Similar results were obtained for corrosion pits
created in the steel containing the higher Al content
(26Mn-3Si-3Al steel). The pits are preferentially ini-
tiated along the grain boundaries (Figure 5a). The EDS
analysis showed the presence of corrosion pits at parti-
cles with the high concentrations of manganese and
sulphur, too (Figures 5b and 5c). The resistance to
pitting corrosion strongly depends on the quantity, size
and type of non-metallic inclusions in the metallic
matrix.11 Park et al.7 found that the size of the MnS
inclusions increased with an increase in the Mn content
from 6 to 12 % in the high-Cr steel. This is why both
high-Mn steels contain a lot of corrosion damage.
The SEM images of both high-Mn steels after the
corrosion tests in 0.1-M NaOH show good agreement
with the results of the potentiodynamic tests. The obser-
vation of the steel surfaces (Figure 6a) confirmed a
substantial reduction in the amount of corrosion damage.
The EDS analyses of the individual pit in Figures 6b and
6c revealed a high content of Mn, S, Al, Si and O, which
indicates that complex oxysulphides containing Mn, Al
and Si are also preferential sites for pit formation in the
alkaline solution. According to I. J. Park et al.21 Al2O3
particles have a higher resistance to pit formation than
MnS particles.
The nature of the corrosion damage was evaluated on
cross-sectioned specimens. In the chloride solution, the
specimens showed evidence of uniform corrosion. In
addition to uniform corrosion, pitting corrosion was also
A. GRAJCAR, A. PLACHCIÑSKA: EFFECT OF SULPHIDE INCLUSIONS ON THE PITTING-CORROSION BEHAVIOUR ...
Materiali in tehnologije / Materials and technology 50 (2016) 5, 713–718 717
Figure 7: Light micrographs of the cross-section of 27Mn-4Si-2Al
steel potentiodynamically polarized in: a) chloride solution and b)
alkaline solution
Slika 7: Posnetek preseka jekla 27Mn-4Si-2Al potenciodinami~no
polariziranega v: a) kloridni raztopini in b) alkalni raztopini
Figure 6: a) SEM micrograph of the 27Mn-4Si-2Al steel surface,
b) the individual pit interior, c) EDS analysis from point C after
corrosion test in 0.1-M NaOH
Slika 6: a) SEM-posnetek povr{ine jekla 27Mn-4Si-2Al, b) izgled
posamezne jamice, c) EDS-analiza iz to~ke C po korozijskem preiz-
kusu v 0,1 M NaOH
observed (Figure 7). Other authors5–7,16 observed corro-
sion pits in different high-manganese steels after polari-
zation tests in chloride solution too. After the corrosion
tests in 3.5 % NaCl the maximum depth of the corrosion
pits in the steel containing 2 % Al was evaluated to be
15 μm (Figure 7a). Similar corrosion pits were also
identified for the 26Mn-3Si-3Al steel. The corrosion
damage formed in both steels in the alkaline medium is
characterized by a small depth of 5 μm (Figure 7b).
However, the quantity of corrosion damage was much
lower when compared to the samples investigated in the
chloride medium.
4 CONCLUSIONS
The morphology of corrosion damage after electro-
chemical tests in 3.5 % NaCl and 0.1-M NaOH supports
the data registered in potentiodynamic tests. The
high-Mn steels are characterized by a low corrosion
resistance, especially in a chloride solution, where
corrosion damage is more numerous. The corrosion pits
are characterized by various shapes and are distributed
irregularly both at grain boundaries and within the
grains. EDS analyses confirmed that the corrosion pits
nucleated preferentially on the MnS inclusions and
complex oxysulphides containing Mn, Al and Si. The
low density of corrosion damage in the alkaline solution
is related to the fact that Mn precipitates as Mn(OH)2,
which is slightly soluble in solutions of pH>13, whereas
in solutions of pH ~ 7 the manganese dissolves as Mn2+.
The concentration of the individual alloying elements
was not strongly related to the corrosion behaviour of the
steels in 0.1-M NaOH, in contrast to the 3.5 % NaCl
solution. The increased contents of Mn and Si and the
smaller content of Al are reflected in the lower corrosion
resistance of the 27Mn-4Si-2Al, as registered during the
potentiodynamic tests.
Acknowledgment
This work was financially supported with statutory
funds of the Faculty of Mechanical Engineering of the
Silesian University of Technology in 2015.
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718 Materiali in tehnologije / Materials and technology 50 (2016) 5, 713–718