M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
673–682
EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF
AISI 4140, AISI 4340, AND AISI 5140 STEELS EXPOSED TO
3.5 w/% NaCl ENVIRONMENT
VPLIV KOROZIJE NA DINAMI^NO OBNA[ANJE JEKEL VRST
AISI 4140, AISI 4340, IN AISI 5140, IZPOSTA VLJENIH VODNI
RAZTOPINI S 3,5 w/% NaCl
Mesut Yýldýz
*
, Hüsnü Gerengi
Düzce University, Faculty of Engineering, Department of Mechanical Engineering, Düzce, Turkey
Prejem rokopisa – received: 2024-05-22; sprejem za objavo – accepted for publication: 2024-09-11
doi:10.17222/mit.2024.1199
Material loss due to corrosion can weaken the structural integrity of systems and potentially lead to failure if timely action is not
taken. Such deterioration adversely affects the dynamic behavior of metals. This study deals with the post-corrosion changes in
the dynamic behavior of AISI 4140, AISI 4340 and AISI 5140 metals after exposure to 3.5 w/% NaCl for 1, 7, 15 and 30 days.
Electrochemical impedance spectroscopy (EIS) and dynamic electrochemical impedance spectroscopy (dynamic EIS) were used
to elucidate the corrosion mechanisms. Modal analysis and finite element method (FEM) were used to characterize dynamic be-
havior changes. In addition, scanning electron microscopy (SEM-EDS) was used to analyze changes in the surface morphology
after corrosion. The results showed a decrease in the corrosion resistance of AISI 4140, AISI 4340 and AISI 5140 metals over
time, although there was an improvement after 30 days. The electrochemical test results indicated that the metal with the highest
corrosion resistance in the 3.5 w/% NaCl environment was AISI 4340, while AISI 5140 showed the lowest resistance. In partic-
ular, natural frequency values showed a decreasing trend with increasing corrosion exposure time, accompanied by discernible
changes in mode shapes.
Keywords: corrosion, dynamic behavior, steel
Izgube materiala zaradi korozije lahko oslabijo strukturno integriteto sistema in potencialno vodijo do odpovedi, ~e ne
ukrepamo pravo~asno. Tak{ne korozijske po{kodbe lahko {e posebej mo~no vplivajo na dinami~no obremenjene sisteme
oziroma materiale. V tem ~lanku avtorji opisujejo {tudijo, ki obravnava obna{anje izbranih jekel (AISI 4140, AISI 4340 in AISI
5140) po koroziji v slanici (vodni raztopini s 3,5 w/% NaCl) in dinami~nem obremenjevanju. Vzorci izbranih jekel so bili
v slanici razli~no dolgo (1, 7, 15 in 30) dni. Nato so avtorji izvedli elektro-kemijsko impedan~no spektroskopijo (EIS) in
dinami~no elektro-kemijsko impedan~no spektroskopijo (angl.: Dynamic-EIS) zato, da so pojasnili korozijske mehanizme. Za
karakterizacijo sprememb dinami~nega obna{anja materialov so uporabili modalno analizo in metodo kon~nih elementov (FEM;
angl.: finite elements method). Dodatno so uporabili {e vrsti~no elektronsko mikroskopijo in elektronsko disperzijsko
spektroskopijo (SEM-EDS) za analizo morfolo{kih in mikro-kemijskih sprememb povr{ine zaradi korozije. Rezultati analiz so
pokazali zmanj{evanje odpornosti vzorcev iz izbranih jekel AISI 4140, AISI 4340 in AISI 5140 s podalj{evanjem njihovega
zadr`evanja v slanici. Elektro-kemijski testi so pokazali, da ima najbolj{o odpornost proti koroziji jeklo AISI 4340, ki je bilo
izpostavljeno vodni raztopini s 3,5 w/% NaCl medtem, ko je imelo najslab{o odpornost proti koroziji jeklo AISI 5140. [e
posebej so bile o~itne spremembe oblik in trend padanja vrednosti naravnih frekvenc s podalj{evanjem ~asa zadr`evanja vzorcev
izbranih jekel v slanici.
Klju~ne besede: korozija, dinami~no obna{anje in lastnosti jekel
1 INTRODUCTION
Mechanical systems consisting of various mechanical
components such as shafts, rolling bearings and discs op-
erate under many heavy conditions, such as overload and
high speed. The most important feature expected from
these systems is that they operate smoothly under these
working conditions. In the case of metallic materials
used in rotating systems, corrosion caused by environ-
mental conditions can lead to material loss on the shaft
surface, which can reduce the shaft’s strength and affect
its dynamic behavior.
1,2
For this reason, it is of great im-
portance to determine the dynamic behavior of shafts op-
erating at high speeds and to monitor potential problems
that may emerge. Various parameters characterizing the
dynamic behavior of a system are used to investigate the
working conditions and performance of high-speed ma-
chines. Vibration measurements are an effective method
for describing the dynamic properties of rotating ma-
chine systems, detecting manufacturing faults, malfunc-
tions, and mechanical problems.
3
Vibration measure-
ments are an essential tool for monitoring the health of
rotating machinery and equipment. By measuring the vi-
bration of a machine, potential problems can be identi-
fied before they lead to costly downtime or equipment
failures. Therefore, the importance of vibration damping
capacity as an engineering property of materials has been
widely researched in recent years.
4–6
Corrosion, which
occurs due to the interactions between materials and the
surrounding environment, prevents the materials from
Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682 673
UDK 669.1.017:620.193 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek Mater. Tehnol.
*Corresponding author's e-mail:
mesutyildiz@duzce.edu.tr (Mesut Yildiz)

performing their function, causes the metal to lose its
original shape and size, reduces the performance of the
material, and causes a shift in the natural frequency of
the material.
7,8
Corrosion of shafts can be classified into
two types: uniform corrosion, where the surface is cor-
roded evenly, and localized corrosion, which occurs in a
certain area of the surface.
9
Aryayi et al. investigated the
change in the natural frequency of shafts as a result of
pitting corrosion on clamped-free shafts with the finite
element method. As a result of the study, it was deter-
mined that the natural frequencies of the corroded shafts
decreased.
10
Li and Akid investigated the corrosion fa-
tigue behavior of a shaft examined in air and NaCl solu-
tion. The findings indicate that pitting corrosion, which
is more common in the aggressive NaCl environment,
led to a reduced fatigue life of the shaft compared to the
one tested in air.
11
The failure of the rear axle shaft of a
three-wheeled vehicle was investigated by Singh et al.
The analysis results confirmed that it was due to the
presence of corrosion products detected on the surface of
the AISI 4140 shaft that caused it to fail.
12
Zhang et al.
studied the relationship between the corrosion depth and
natural frequency of reinforced concrete test beams.
13
Gillich et al. developed two mathematical relations to de-
termine the natural frequency changes caused by corro-
sion in metallic structures.
7
As a result of corrosion, the natural frequencies of
metals in machine systems are altered, resulting in vibra-
tory wear failures.
10,14,15
When the operating frequency of
a system is in close proximity to the natural frequency of
the material, the highest degree of vibration occurs, re-
sulting in resonance. The operation of a system with res-
onance results in the overlapping of energy waves, which
in turn causes fractures and accidents. For that reason,
the impact of corrosion on metals’ natural frequencies
should be quantified and the likelihood of unfavorable
outcomes should be minimized without compromising
the functionality of the operating system. In this study,
the corrosion behaviors of AISI 4140, AISI 4340 and
AISI 5140 steels exposed to a 3.5 w/% NaCl environ-
ment for (1, 7, 15 and 30) days were investigated with
electrochemical methods, and post-corrosion natural fre-
quency changes were determined with the modal analy-
sis and finite element method (FEM). Although there are
studies on the corrosion behavior of AISI 4140, AISI
4340 and AISI 5140 metals in a 3.5 w/% NaCl environ-
ment in the literature,
16–21
there is no comparable investi-
gation, in which time-dependent experimental studies are
performed. This study will contribute to the literature on
the changes in corrosion resistance, natural frequencies,
and vibration damping rates of AISI 4140, AISI 4340
and AISI 5140 steels over time.
2 EXPERIMENTAL PART
2.1 Materials
The chemical composition of specimens AISI 4140,
AISI 4340 and AISI 5140 procured from Almina
Iron-Steel Co., Turkey, are listed in Table 1. The sam-
ples, used as working electrodes in the electrochemical
corrosion investigation, were cut into a circular shape of
25 mm in diameter and 10 mm in thickness. The working
surfaces of the specimens were mechanically abraded us-
ing SiC paper ranging from 600 grit to 2000 grit. They
were washed with distilled water, subsequently
degreased with alcohol and dried with air before electro-
chemical measurements. The corrosive medium was an
aqueous 3.5 w/% NaCl environment.
Table 1: Chemical compositions of AISI 4140, AISI 4340 and AISI
5140 steels
AISI
Alloy
CS iM nPSC rM oN iF e
4140 0.451 0.317 0.771 0.012 0.010 0.941 0.169 0.082 Bal.
4340 0.380 0.304 0.567 0.012 0.011 1.832 0.199 1.578 Bal.
5140 0.451 0.301 0.757 0.014 0.006 1.020 0.031 0.088 Bal.
2.2. Corrosion measurements
2.2.1 EIS and dynamic EIS
The corrosion resistance of the samples was tested
using EIS on Gamry Reference 600 potentiostat/galvano-
stat/ZRA workstation in a three-electrode cell, which
was composed of a reference electrode of Ag/AgCl, Pt
mesh as the counter electrode and working electrodes of
AISI 4140, AISI 4340 and AISI 5140 steels with ex-
posed surface areas of 0.502 cm
2
. Electrochemical tests
were carried out by connecting the working electrode
immersed in the test solution to the corrosion cell and
stabilizing the OCP (open circuit potential) for2ht oo b -
tain a steady state. EIS measurements were conducted in
a frequency range of 10 mHz to 100 kHz, with a 10 mV
peak-to-peak amplitude signal.
22
The dynamic EIS method that provides instant moni-
toring of the corrosion process on the working electrode
is a better technique for corrosion measurements. Dy-
namic EIS was performed using a National Instruments
PCI-4461 digital-to-analog card for the generation of a
multi-sinusoidal current-perturbation signal. This card
was also used for measuring current-perturbation and
voltage-response signals. The sampling frequency was
12.8 kHz and the perturbation signal was in a frequency
range of 4.5 kHz to 300 mHz.
23
2.3. Surface analysis
Scanning electron microscopy (SEM), a surface anal-
ysis technique, was used for investigating the damage on
the specimens’ surfaces exposed to the corrosive medium
for different periods of time. A Quanta FEG 250 (FEI,
M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
674 Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682

Holland) model device equipped with an energy disper-
sive X-ray spectroscopy detector was used for SEM.
2.4. Modal analysis
Natural frequencies and damping ratios of materials
are determined with the modal analysis, measured with
an impact hammer and accelerometer.
24
The dynamic be-
havior of cylindrical metals under a free-free condition
with a diameter of 23.5 mm and a length of 200 mm
(AISI 4140, AISI 4340 and AISI 5140) before corrosion
and after exposure to the 3.5 w/% NaCl environment for
(1, 7, 15 and 30) days was examined using the modal
analysis. A DYTRAN 5800B3 model force hammer with
a head weight of 100 g was used to apply impact stimu-
lation to the experimental samples. A KS78C100 model
accelerometer was mounted to record the input accelera-
tion using a KRYPTON 8xACC data acquisition device
(DAQ) with a sampling rate of 20 kS/s. The natural fre-
quencies and damping ratios obtained as a result of the
modal analysis were determined using DewesoftX soft-
ware.
2.5. Finite element method (FEM)
A numerical analysis of the dynamic behavior of cor-
roded circular steels was simulated using FEM in
ANSYS. The steels were considered to have a Poisson’s
ratio of 0.29, Young’s modulus of 200 GPa and density
of 7850 kg/m
3
. In this study, the modal analysis results
based on the FEM method were not affected by the
Young’s modulus. In addition, the modal analyses with
FEM were calculated by reducing the thickness of the
uniform corrosion layer formed on the surfaces of the
metals, with the diameters of the metals measured by
SEM, as shown in Table 2.
Table 2: Diameters of the metals before and after exposure to the cor-
rosive medium (in mm)
Time/Metals AISI 4140 AISI 4340 AISI 5140
Ref 23.500 23.500 23.500
After 1 day 23.496 23.498 23.497
After 7 days 23.495 23.497 23.496
After 15 days 23.495 23.495 23.496
After 30 days 23.494 23.494 23.495
3 RESULTS
3.1. Electrochemical impedance spectroscopy (EIS)
Figure 1 displays the electrochemical behavior of
three steel samples after (1, 7, 15, and 30) days of expo-
sure to 3.5 w/% NaCl. Nyquist diagrams of all the metals
studied show an imperfect capacitive loop. According to
M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682 675
Figure 1: Nyquist diagrams for AISI 4140, AISI 4340 and AISI 5140 steels in 3.5 w/% NaCl
Table 3: EIS test data for AISI 4140, AISI 4340 and AISI 5140 steels in 3.5 w/% NaCl
Metal Day(s)
R s
( cm
2
)
Q
f
(μ s
n
m
–2
)
R
f
( cm
2
)
Q
ct
(μ s
n
m
–2
)
R
ct
( cm
2
)
R
total
( cm
2
)
 2
AISI 4140
1 32 380 1353 175 4088 5441 8.399e-05
7 33 451 222 213 3656 3878 5.104e-05
15 33 1374 503 662 2283 2786 1.251e-04
30 35 889 797 637 2319 3116 1.146e-05
AISI 4340
1 33 345 1883 151 4154 6037 2.374e-04
7 34 447 680 420 3751 4431 7.364e-05
15 34 569 535 519 2606 3141 6.093e-05
30 35 471 549 489 2629 3178 2.430e-05
AISI 5140
1 33 1572 1156 961 3318 4474 1.399e-04
7 34 3028 762 1868 2624 3386 1.852e-05
15 35 3830 222 2620 2065 2287 4.757e-05
30 36 3147 260 2513 2195 2455 6.008e-06

the literature, this imperfection is due to the roughness or
heterogeneity of the electrode surface.
25,26
Figure 1
shows that the semicircle of AISI 4340 steel is larger
than that of other metals, and the smallest semicircles be-
long to AISI 5140 steel. For all the steel types examined,
the capacitive radii decreased up to 15 d and increased
after 30 d. This suggests that there is a time-dependent
behavior of the capacitive radii of steel. A possible rea-
son for his observation could be the formation of a pro-
tective oxide layer on the surfaces of steels. Further, as
the metals exposed to the environment may develop a
corrosion film on their surface over time,
27
the obtained
data were analyzed with ZsimpWin 3.21 software using
the R(Q(R(QR))) equivalent circuit, where R
s
is the solu-
tion resistance, R
f
is the corrosion product film resist-
ance, Q
f
is the capacitance of the film, Q
ct
is the dou-
ble-layer capacitance and R
ct
is the charge-transfer resis-
tance. The results are listed in Table 3.
The  2 values in Table 3 demonstrate that the applied
equivalent circuit model is appropriate. It is clearly seen
that the highest R
f
, R
ct
and R
t
values were obtained for
AISI 4340 steel while the lowest ones were obtained for
AISI 5140 steel. In addition, while resistance values de-
creased for the first 15 d, Q
f
and Q
dl
values increased.
Moreover, after 30 d, the R values increased and Q val-
ues decreased for all three samples. The corrosion resis-
tance of AISI 4340 metal is higher due to its higher ratio
of Cr and Ni in its chemical composition compared to
the other two metals. Alloying elements Cr and Ni form
a protective oxide layer on the surface of the metal,
which acts as a barrier against corrosive agents.
28,29
This
oxide layer helps to prevent the penetration of moisture
M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
676 Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682
Figure 2: Dynamic-EIS plots of AISI 4140, AISI 4340, and AISI 5140 steels obtained in 3.5 w/% NaCl environment: a) 1 d, b) 7 d, c) 15 d,
d) 30 d

and other corrosive substances, thereby reducing the cor-
rosion rate. An addition of a small amount of molybde-
num (less than 0.5 %) was reported to prevent corrosion
in steels.
28
Furthermore, the chemical composition of
AISI 4140 steel differs from that of AISI 5140 steel due
to the presence of molybdenum. Therefore, the increased
corrosion resistance of AISI 4140 steel compared to
AISI 5140 steel can be attributed to its higher molybde-
num content.
3.2. Dynamic electrochemical impedance spectroscopy
(dynamic EIS)
Dynamic EIS has gained popularity in corrosion
studies as a viable alternative to EIS.
30
One of the key
advantages of dynamic EIS is its ability to simulta-
neously excite a system with all perturbation frequen-
cies. This simultaneous excitation allows for a more
comprehensive analysis of the corrosion process, provid-
ing valuable insights into the electrochemical behavior of
the system under investigation. The corrosion of steels in
the 3.5 w/% NaCl environment was studied using this
method for (1, 7, 15 and 30) days. Results of dynamic
EIS of AISI 4140, AISI 4340 and AISI 5140 steels ob-
tained from the experiments in the 3.5 w/% NaCl envi-
ronment are displayed in Figure 2. Dynamic EIS results
were analyzed with the same software as used for the
EIS results (Figure 1) and its parameters are presented in
Table 4. According to the dynamic-EIS data, the imped-
ance semicircles of all steels were largest at the end of
the first day and gradually decreased to their smallest
size by the 15
th
day. However, the graphs show that the
size of the semicircles increased at the end of the 30
th
day. This phenomenon can be attributed to the formation
of a protective oxide layer on the surface of steel, as
mentioned in the EIS section.
Table 4 shows that the applied equivalent circuit
model is suitable for analyzing the dynamic-EIS data, as
indicated by the  2 values. The results of the dy-
namic-EIS experiments yielded parameters comparable
to those obtained through the EIS method. This indicates
that both techniques provide consistent and reliable data.
The AISI 4340 steel has the highest R
total
values of 5760,
4009, 3256, and 3394  cm
2
for (1, 7, 15, and 30) days,
respectively. On the other hand, the AISI 5140 steel has
the lowest R
total
values of 4212, 3246, 2278, and 2377  cm
2
for (1, 7, 15, and 30) days, respectively. The varia-
tion in the R
total
values among the examined steels can be
attributed to the presence of different alloying ele -
ments.
28,29
3.3 Surface analysis
SEM images (1000× magnification) of metals AISI
4140, AISI 4340 and AISI 5140 after exposure to 3.5%
NaCl for 1, 7, 15 and 30 days are shown in Figure 3.
Corrosion products began forming on the surfaces of the
metals from day one. Different oxide types that may
form on a steel surface are: haematite ( -Fe
2
O
3
), magne-
tite (Fe
3
O
4
), maghemite ( -Fe
2
O
3
), goethite ( -FeOOH),
lepidocrocite ( -FeOOH), akaganeite ( -FeOOH),
feroxyhite (ä-FeOOH), iron hydroxide [Fe(OH)
2
], and
iron trihydroxide [Fe(OH)
3
].
31
SEM images show that the
surfaces of the metals corrode, resulting in the formation
of goethite and lepidocrocite morphologies. These
morphologies are indicative of the corrosion products
that have developed over time. During the corrosion pro-
cess, the oxide layers on the metal surfaces became more
distinct and cracks occurred due to the breakage of the
layers. During corrosion in the 3.5 w/% NaCl environ-
ment, iron oxide and hydroxide are formed on the sur-
face of steel, as illustrated by the following equations (1
and 2):
32
Fe + (1/2)O
2
+H
2
O  Fe(OH)
2
(1)
3Fe(OH)
2
+ (1/2)O
2
 Fe
3
O
4
+3 H
2
O (2)
In SEM images, a detailed examination of the metal
surfaces reveals intriguing morphologies. The presence
of globular structures suggests the existence of a goethite
structure. These globular morphologies could be indica-
tive of the presence of goethite, a common iron hydrox-
ide. Additionally, flower-petal and spider-web morphol-
M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682 677
Table 4: Dynamic-EIS data of AISI 4140, AISI 4340 and AISI 5140 steels in 3.5 w/% NaCl
Metal Day(s)
R s
( cm
2
)
Q
(μ s
n
m
–2
)
R
f
( cm
2
)
Q
(μ s
n
m
–2
)
R
ct
( cm
2
)
R
total
( cm
2
)
 2
4140
1 32 362 1154 210 4033 5187 2.735e-04
7 34 492 412 343 3091 3503 5.528e-06
15 35 682 258 458 2283 2541 1.894e-05
30 37 516 471 399 2331 2802 3.245e-04
4340
1 32 226 1722 189 4038 5760 3.315e-04
7 34 356 692 305 3317 4009 5.760e-05
15 34 573 502 456 2754 3256 3.844e-04
30 35 484 518 353 2876 3394 2.443e-04
5140
1 32 1126 1038 1087 3174 4212 2.249e-06
7 33 1679 361 1401 2885 3246 4.573e-04
15 34 2236 291 1712 1987 2278 3.776e-06
30 35 2129 307 1692 2070 2377 4.498e-06

ogies observed on the surfaces suggest the presence of
lepidocrocite.
33
Lepidocrocite is another iron hydroxide
that often exhibits intricate and delicate patterns resem-
bling flower petals or spider webs when viewed under
high magnification. When steels are exposed to a Cl en-
vironment, a lepidocrocite formation is observed as the
primary corrosion product on the metal surface. As the
exposure time increases, a goethite structure is formed
due to the deposition of Cl ions on the lepidocrocite
structure.
34
The goethite structure is a more stable and
protective form of corrosion compared to lepidocrocite.
The SEM-EDS images in Figure 4 illustrate the
cross-section morphologies of AISI 4140, AISI 4340,
and AISI 5140 steel specimens after exposure to the
3.5 % NaCl medium for (1, 7, 15, and 30) days. The
cross-section morphologies show that the corrosion layer
becomes more compact and thicker as the exposure time
to the environment increases for all three metals. An
analysis of the EDS spectra shows that the corrosion
layer on the metal surface contains the highest amount of
Cl after 1 day of exposure. As the exposure time in-
creases, the Cl content decreases and reaches its lowest
level after 30 days. As mentioned above, the decrease in
Cl is a result of the structural change in the corrosion
layer.
M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
678 Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682
Figure 4: EDS images of AISI 4140, AISI 4340 and AISI 5140 metals
after corrosion in 3.5 w/% NaCl environment: a) 1 d, b) 7 d, c) 15 d, d)
30 d
Figure 3: SEM images of AISI 4140, AISI 4340 and AISI 5140 metals after corrosion in 3.5 w/% NaCl environment: a) 1 d, b) 7 d, c) 15 d,
d) 30 d

3.4 Modal analysis and FEM
Table 5 presents the changes in the natural frequency
and modal damping ratios of the first two mode shapes
resulting from the time-dependent corrosion of AISI
4140, AISI 4340, and AISI 5140 steels in the 3.5 w/%
NaCl environment, as obtained with the FEM and modal
analysis. From the examination of the changes in natural
frequencies, it is evident that the frequency values for the
three metals, obtained with the FEM and modal analysis,
decrease due to corrosion. This decrease in the frequency
values indicates that the effect of corrosion on the stiff-
ness of metals is greater than on the mass loss. Corrosion
typically results in material deterioration and structural
changes, leading to a reduction in stiffness and, conse-
quently, a decrease in the natural frequencies of materi-
als. Modal damping ratios, which represent the damping
property of materials, are related to the dissipation or
loss of energy within the system.
M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682 679
Figure 5: First natural bending mode shapes of AISI 4140, AISI 4340 and AISI 5140 steels: a) before corrosion, b) 1 d, c) 7 d, d) 15 d, e) 30 d
Table 5: Natural frequency and modal damping ratios of AISI 4140, AISI 4340 and AISI 5140 steels in 3.5 w/% NaCl environment
4140 5140 4340
f (Hz)
 (%)
f (Hz)
 (%)
f (Hz)
 (%)
Mod FEM MA FEM MA FEM MA
Ref
1 2550,6 2677,24 2,087 2550,4 2670,09 1,922 2550,6 2653,45 1,019
2 6646,9 6818,45 0,118 6645,3 6911,22 0,022 6646,9 6886,83 0,011
1
1 2550,2 2676,21 2,139 2550,1 2669,62 1,927 2550,4 2652,8 1,040
2 6645,9 6818,03 0,192 6644,5 6910,91 0,023 6646,5 6886,49 0,025
7
1 2550,1 2674,54 2,158 2550 2669,07 1,928 2550,3 2652,12 1,045
2 6645,7 6817,92 0,223 6644,4 6910,26 0,023 6646,2 6886,21 0,025
15
1 2550,1 2673,74 2,161 2550 2668,44 1,932 2550,1 2651,6 1,053
2 6645,7 6816,23 0,234 6644,4 6909,85 0,034 6645,8 6885,97 0,028
30
1 2550 2672,6 2,179 2549,9 2668,16 1,938 2550 2650,87 1,054
2 6645,5 6815,29 0,240 6644,1 6909,43 0,041 6645,4 6885,78 0,031

If the system is undamaged, the excitation force pro-
vides free energy to the system, which is used only for
the vibrating of the system. In a damaged system, how-
ever, some of the free energy will be trapped and dissi-
pated by defects, resulting in less free energy being
available for the vibrational motion of the system. The
undamaged material takes longer to damp the vibration
than the damaged material when the excitation force is
stopped and the system is released to damp the vibration.
As the level or severity of corrosion increases, the damp-
ing ratios obtained generally increase.
35
Therefore, as
demonstrated in Table 5, the modal analysis showed that
the damping ratios of the materials increased with the
corrosion time. The mode shapes obtained with FEM for
AISI 4140, AISI 4340, and AISI 5140 steel specimens
before and after corrosion exposure are displayed in Fig-
ure 5 and 6. In both frequency modes the bending modes
were obtained for AISI 4140, AISI 4340 and AISI 5140
steels. Due to the corrosion of the three metals, it can be
observed that both mode shapes change on day 1. De-
spite the increase in the corrosion exposure time of AISI
4140 metal, there is no change in the two mode shapes.
This suggests that the corrosion did not significantly af-
fect the metal’s structural integrity. The shapes of the
first bending mode of metals AISI 4340 and AISI 5140
remain unchanged after the 7
th
day, but a change is ob-
served in the shapes of the second bending mode. This
suggests that the second bending mode shapes of these
metals are more susceptible to change over time com-
pared to their first bending mode shapes.
4 CONCLUSIONS
In this study, changes in the natural frequencies of
three different steels corroded in a 3.5 w/% NaCl envi-
ronment were investigated. The inferences obtained with
the analysis are as follows:
1. Based on the results obtained with electrochemical
corrosion methods, it was found that the corrosion resis-
tance of the three metals decreased by the 15
th
day, but
increased on the 30
th
day due to the protection provided
by the oxide layer formed on the surface. Based on the
EIS and dynamic-EIS results, it can be concluded that
4340 metal has the highest corrosion resistance, while
5140 metal has the lowest resistance.
2. Surface analyses showed that the structure of the
oxide layer on the surface changed and the thickness of
the oxide layer increased with increasing time of expo-
sure to the 3.5 w/% NaCl environment.
M.YILDIZ, H. GERENGI: EFFECT OF CORROSION ON THE DYNAMIC BEHA VIOUR OF AISI 4140, AISI 4340, AND ...
680 Materiali in tehnologije / Materials and technology 58 (2024) 6, 673–682
Figure 6: Second natural bending mode shapes of AISI 4140, AISI 4340 and AISI 5140 steels: a) before corrosion, b) 1 d, c) 7 d, d) 15 d, e) 30 d

3. The results obtained with the FEM and modal
analysis indicate that the corrosion of metals leads to a
reduction in their natural frequencies. When the modal
damping ratios were examined, it was found that the
damping ratios increased with the increase in the oxide
layer formed on the surface, in parallel with the corro-
sion exposure time.
4. Corrosion caused changes in the mode shapes of
the metals. The mode shape of AISI 4140 metal re-
mained unchanged after day 1, while the mode shapes of
AISI 4340 and AISI 5140 metals continued to change.
Acknowledgment
This study was supported by the Scientific and Tech-
nological Research Council of Turkey (TUBITAK) under
Grant Number 123M915. The authors thank TUBITAK
for their support. The authors also thank Prof. Dr. Hamit
SARUHAN for his contributions.
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