Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.10, pp.888-903, 2011
jmmce.org Printed in the USA. All rights reserved
888
Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel
in Marine Environment
1
O.O. Daramola,
2
B.O. Adewuyi and
2
I.O. Oladele
1
Ajaokuta Steel Company Limited, Ajaokuta Steel City, Kogi State, Nigeria
2
Metallurgical and Materials Engineering Department, Federal University
of Technology,
Akure, Ondo State, Nigeria.
1
Corresponding Author: ojaythompsoms@yahoo.com
Phone Number: 2348166814002
ABSTRACT
Investigation were carried out to study the corrosion behaviour of heat treated rolled medium
carbon steel and as-rolled medium carbon steel in sodium chloride medium. The as-rolled
medium carbon steel was heated to a temperature of 830
o
C to completely austenize it and water
quenched; it was reheated to the ferrite-austenite dual phase region at a temperature of 745
o
C
below the effective Ac
3
point. The steel was then rapidly quenched in water and tempered at a
temperature of 480
o
C. The corrosion behaviour of the steel in marine medium (NaCl) was
studied by weight loss measurement. The weight loss is between 0.02g-0.11g for the as-rolled
steel and 0.01g – 0.013g for the heat treated steel. The results obtained showed that the as-rolled
medium carbon steel is more susceptible to corrosion than the heat treated rolled medium
carbon steel.
Keywords: Corrosion, Medium carbon steel, sodium chloride.
1. INTRODUCTION
Corrosion is the destruction of a metal by its reaction with the environment; this reaction is an
electrochemical oxidation process that usually produces rust or other metal oxide [1].
Structural steel is used in most metal water front structures because it is strong, readily available,
easily fabricated and not excessively costly. The steel is normally used for such accessories as
bitts, bollards, cleats, and chocks [2]. There are many types of marine corrosion that can occur to
steel water front structure (e.g. galvanic corrosion, stray current, differential environment,
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 889
erosion corrosion, and biological corrosion) and many methods for corrosion control. Heat
treatment of structural steel is one of the way of improving its resistance to corrosion. Heat
treatment involves the application of heat to a material to obtain desired material properties (e.g.
Mechanical, corrosion, electrical, magnetic e-t-c) [3]. During the heat treatment process, the
material usually undergoes phase microstructural and crystallographic changes and this has
effect on the corrosion, mechanical and electrical properties of the steel [4].
Rolled medium carbon steel products are produced through a forming process called rolling. The
process is carried out in a rolling mill which consist of a complex machine for deforming metal
in rotary rolls and performing auxiliary operations such as transportation of stock to rolls,
disposal after rolling, cutting, cooling, piling or coiling e-t-c [5]. To study the corrosion
behaviour of heat treated rolled medium carbon steel and the as-rolled steel in marine medium
(NaCl); 24 specimens were prepared from the as-rolled medium carbon steel sample; 12 out of
the specimens were heat-treated. The corrosion susceptibility of the heat treated specimens and
as-rolled specimens were investigated.
The objective of this work is to investigate the effects of heat treatment on the corrosion
susceptibility of rolled medium carbon steel.
2. MATERIALS AND METHODS
The material used in this study was 12mm diameter rolled medium carbon steel. The
spectrometric analysis of the steel was carried out; The Chemical compositions from the analysis
are shown in Table 1. Twenty four specimens were prepared from this material using lathe
machine; Twelve out of these specimens were heat treated. The corrosion test and metallographic
examination of the as-rolled and heat treated specimens were carried out.
2.1.Determination of Operating Temperature
The lower critical temperature (AC
1
) and upper critical temperature (AC
3
) were determined by
Grange empirical formula [6] as presented here.
AC
1
(
o
C) = (1333 – 25Mn + 40Si +42Cr– 26 Ni)- (32) 5/9 ---------------1.1
AC
3
(
o
C) = (1570 – 323C – 25Mn + 80Si – 3Cr – 32Ni) – (32) 5/9 –----1.2
Table 1. Chemical Composition of As – Rolled Medium Carbon Steel
C Si Mn P S Cr Mo Ni Al Co Cu Nb Ti
0.353 0.290 0.987 0.050 0.057 0.071 0.005 0.11 0.025 0.015 0.185 0.005 0.0037
V W Pb Sn Zn Fe
0.0057 0.010 0.005 0.026 0.0076 97.805
890 O.O. Daramola, B.O. Adewuyi and I.O. Oladele Vol.10, No.10
2.2. Heat Treatment Processes
Representative samples of as-rolled medium carbon steel were subjected to heat treatment
processes
2.2.1. Quenching + Quenching + Lamelarizing + Tempering (Q+Q+L+T)
The steel specimens were heated to the austerizing temperature of 830
o
C, soaked for 20 minutes
and quenched in water, this process was repeated again before the specimens were thereafter
heated to the dual phase region at a temperature of 745
o
C, soaked for 20 minutes again and
quenched in water. The specimens were finally tempered at a temperature of 480
o
C for 30
minutes.
2.2.2 Quenching + Lamelarizing + Tempering (Q + L + T)
The steel specimens were heated to 830
o
C, soaked for 20 minutes and quenched in water, the
specimens were reheated to the dual phase region at a temperature of 745
o
C, soaked for 20
minutes and quenched in water. The specimens were tempered at temperature of 480
o
C for
30minutes.
2.2.3 Lamerlarizing + Tempering (L + T)
The specimens were heated to the dual phase region at a temperature of 745
o
C, soaked for 20
minutes, quenched in water and tempered at 480
o
C for 30 minutes.
T
o
C
Figure 1: Temperature- Time Graph involving Quenching, Quenching, Lamelarizing and
Tempering (Q + Q + L + T)
200
480
718
745
788
840
Quenching (Q)
Quenching (Q)
Lamelarizing (L)
Tempering (T)
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 891
T
O
C
Figure 2: Temperature – Time Graph involving Quenching, Lamelarzing and tempering
(Q + L + T)
Figure 3: Temperature – Time Graph involving Lamelarzing and Tempering (L+T)
2.3 Corrosion Test
The corrosion rates of as-rolled specimen and heat treat treated specimens were measured by
immersing these specimens in a solution of sodium chloride (NaCl). The cross-sectional area of
200
480
718
745
788
840
Quenching (Q)
Lamelarizing (L)
Tempering (T)
Time (minutes
)
200
480
718
745
788
840
T
o
C
Lamelarizing (L)
Tempering (T)
892 O.O. Daramola, B.O. Adewuyi and I.O. Oladele Vol.10, No.10
each of the specimens was calculated; each of the specimens was also weighed on a chemical
balance and the weight recorded. The PH and the electrode potential of the corrosive medium
were measured and recorded. After every 3 days interval (72 hours); specimens were retrieved,
washed properly in water, dried and
weighed on a weighing balance to determine the weight loss during exposure. The laboratory
simulation experiments were carried out in NaCl (O.5M and 1.OM) medium..
Finally, the corrosion rate was calculated using
mpy = 3.45 x 106W
ATD
where W = weight loss in g
D = density of specimen in g/cm
3
A = Area in cm
2
T = Exposure Time in hour
2.4 Metallographic Examination
Samples of as-rolled and heat – treated specimen were mounted in hot phenolic powder and were
ground on a water lubricated hand grinding set-up of abrasive papers, progressing through from
the coarsest to the finest grit sizes. The 240, 320, 400 and 600 grades were used in that order.
Polishing was carried out on a rotating disc of a synthetic velvet polishing cloth impregnated
with micron alumna paste. Final polishing was carried out with diamond paste. The specimens
were then etched with the standard 2% Nital so as to reveal the ferrite grain boundaries.
The optical microscopic examinations were carried out on a metallurgical microscope at a
magnification of 400X. The specimens were illuminated with 100 kilowatts detachable quartz
iodine lamp.
3. RESULTS AND DISCUSSION
3.1 Corrosion Properties
The results of the corrosion rate of the heat treated specimens and as-rolled specimen is shown in
Tables 2–8. The corrosion rates of the heat treated specimens in marine medium (NaCl) is low
when compared to the as-rolled steel, as shown in Figures 4-9, this is because the as-rolled steel
consist of mainly pearlite in which each crystal consist of alternate layers of ferrite and
cementite, it was understood that ferrite is anodic to cementite and this corrode with moisture as
the electrolyte [7]. This was confirmed from the microstructure of the as-rolled steel shown in
Plates 1–7. In the NaCl environment, the corrosion rate of the as-rolled steel was very high
within the first 2 days and after this, the corrosion decreases with increase in exposure time; this
is because the FeCl
2
formed during the process is insoluble and forms a protective film on the
corroding surface of the steel which effectively prevent corrosive medium from coming into
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 893
contact with the steel and greatly reduces the corrosion rate[8]. After 20 days of exposure, the
corrosion rate becomes uniform, this is because the steel is in the thermodynamically stable
phase, the surface of the steel becomes immuned and no corrosion occurs as shown in
Figures 5 – 16. It could be seen that the same trend of corrosion rate hold for all the specimens
but the steel developed by Q + L + T process have the least corrosion rate followed by L + T
process and Q + Q + L + T process.
Table 2: As – Rolled Steel in NaCl (0.5M). Initial Weight = 8.940g
Exposure
Time (Hrs)
New Weight
(g)
Weight
Loss (g)
Corrosion
Rate (mpy)
pH p.d (v)
0 8.940 0.00 0 8.030 0.776
72 8.920 0.020 18.820 8.250 0.765
144 8.905 0.030 16.470 8.290 0.795
216 8.890 0.040 15.680 8.460 0.801
288 8.880 0.050 14.110 8.470 0.803
360 8.870 0.060 13.170 8.180 0.813
432 8.860 0.070 12.540 8.250 0.814
504 8.850 0.080 12.090 8.360 0.816
576 8.840 0.090 11.760 8.410 0.866
648 8.840 0.0100 10.460 8.470 0.869
720 8.840 0.0100 9.410 8.490 0.876
792 8.830 0.0110 9.410 8.520 0.899
Table 3: Heat Treated Specimen A2 in NaCl (0.5M). Initial Weight = 7.540g
Exposure
Time (Hrs)
New Weight
(g)
Weight
Loss (g)
Corrosion
Rate (mpy)
pH p.d(v)
0 7.540 0 0 8.030 0.776
72 7.530 0.010 10.780 8.250 0.765
144 7.528 0.012 6.740 8.290 0.795
216 7.528 0.012 4.314 8.460 0.801
288 7.528 0.012 2.235 8.470 0.803
360 7.528 0.012 2.588 8.180 0.813
432 7.528 0.012 2.157 8.250 0.814
504 7.528 0.012 1.849 8.360 0.816
576 7.528 0.013 1.753 8.410 0.866
648 7.527 0.013 1.558 8.470 0.869
720 7.526 0.014 1.509 8.490 0.876
792 7.525 0.015 1.471 8.520 0.899
894 O.O. Daramola, B.O. Adewuyi and I.O. Oladele Vol.10, No.10
Table 4: Heat Treated Specimen B2 in NaCl (0.5M). Initial Weight = 8.120g
Exposure
Time (Hrs)
New Weight
(g)
Weight
Loss (g)
Corrosion
Rate (mpy)
pH p.d (v)
0 8.120 0.00 0 8.030 0.776
72 8.110 0.010 9.89 8.250 0.765
144 8.108 0.012 5.936 8.290 0.795
216 8.107 0.013 4.287 8.460 0.801
288 8.106 0.014 2.463 8.470 0.803
360 8.106 0.014 2.770 8.180 0.813
432 8.106 0.014 1.308 8.250 0.814
504 8.106 0.014 1.979 8.360 0.816
576 8.106 0.014 1.731 8.410 0.866
648 8.106 0.014 1.539 8.470 0.869
720 8.105 0.015 1.484 8.490 0.876
792 8.105 0.015 1.349 8.520 0.899
Table 5: Heat Treated Specimen C2 in NaCl (0.5M). Initial Weight = 7.750g
Exposure
Time (Hrs)
New Weight
(g)
Weight
Loss (g)
Corrosion
Rate (mpy)
pH p.d(v)
0 7.750 0 0 8.030 0.776
72 7.750 0.0 0 8.250 0.765
144 7.750 0.0 0 8.290 0.795
216 7.740 0.010 3.423 8.460 0.801
288 7.740 0.010 2.567 8.470 0.803
360 7.740 0.010 2.054 8.180 0.813
432 7.740 0.010 2.712 8.250 0.814
504 7.739 0.011 1.614 8.360 0.816
576 7.739 0.011 1.412 8.410 0.866
648 7.739 0.011 1.522 8.470 0.869
720 7.738 0.012 1.232 8.490 0.876
792 7.737 0.013 1.213 8.520 0.899
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 895
Table 6: Heat Treated Specimen A1 in NaCl (0.5M). Initial Weight = 6.890g
Exposure
Time (Hrs)
New Weight
(g)
Weight
Loss (g)
Corrosion
Rate (mpy)
pH p.d (v)
0 6.980 0.00 0 8.030 0.776
72 6.970 0.010 11.275 8.250 0.765
144 6.970 0.010 5.637 8.290 0.795
216 6.970 0.010 3.758 8.460 0.801
288 6.969 0.011 3.101 8.470 0.803
360 6.969 0.011 2.481 8.180 0.813
432 6.969 0.011 2.057 8.250 0.814
504 6.969 0.011 1.771 8.360 0.816
576 6.968 0.012 1.691 8.410 0.866
648 6.968 0.012 1.503 8.470 0.869
720 6.967 0.013 1.353 8.490 0.876
792 6.967 0.013 1.333 8.520 0.899
Table 7: Heat Treated Specimen B1 in NaCl (0.5M). Initial Weight = 9.200g
Exposure
Time (Hrs)
New Weight
(g)
Weight
Loss (g)
Corrosion
Rate (mpy)
pH p.d (v)
0 9.200 0.00 0 8.030 0.776
72 9.190 0.010 9.409 8.250 0.765
144 6.190 0.010 4.705 8.290 0.795
216 9.189 0.011 3.450 8.460 0.801
288 9.188 0.012 2.832 8.470 0.803
360 9.188 0.012 2.258 8.180 0.813
432 9.187 0.013 2.839 8.250 0.814
504 9.187 0.013 1.748 8.360 0.816
576 9.187 0.013 1.529 8.410 0.866
648 9.187 0.013 1.359 8.470 0.869
720 9.187 0.013 1.223 8.490 0.876
792 9.186 0.014 1.198 8.520 0.899
896 O.O. Daramola, B.O. Adewuyi and I.O. Oladele Vol.10, No.10
Table 8: Heat Treated Specimen C1 in NaCl (0.5M). Initial Weight = 6.680g
Exposure
Time (Hrs)
New Weight
(g)
Weight
Loss (g)
Corrosion
Rate (mpy)
pH p.d (v)
0 6.680 0.000 0 8.030 0.776
72 6.680 0.000 0 8.250 0.765
144 6.680 0.000 0 8.290 0.795
216 6.670 0.010 3.492 8.460 0.801
288 6.670 0.010 2.619 8.470 0.803
360 6.670 0.010 2.095 8.180 0.813
432 6.669 0.011 2.920 8.250 0.814
504 6.669 0.011 1.646 8.360 0.816
576 6.668 0.012 1.571 8.410 0.866
648 6.668 0.012 1.397 8.470 0.869
720 6.668 0.012 1.257 8.490 0.876
792 6.668 0.012 1.143 8.520 0.899
0
5
10
15
20
25
30
35
40
0200 400 600 8001000
Exposure Time (Hrs)
Corrosion Rate (mpy)
As-Rolled Steel
Heat Treated Steel A2
(Q+Q+L+T)
Fig. 4: Corrosion Rate of Heat-Treated Specimen A2(Q+Q+L+T) and As-Rolled Steel in 0.5M
NaCl Environment
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 897
0
10
20
30
40
50
60
70
0200 400 600 8001000
Exposure Time (Hrs)
Corrosion Rate (mpy)
As-Rolled Steel
Heat Treated Specimen B2 (Q+L+T)
Fig.5: Corrosion Rate of Heat-Treated Specimen B2(Q+L+T) and As-Rolled Steel in 0.5M NaCl
Environment
0
10
20
30
40
50
60
70
0200 400 600 8001000
Exposure Time (Hrs)
Corrosion Rate (mpy)
As-Rolled Steel
Heat Treated Specimen C2 (L+T)
Fig.6: Corrosion Rate of Heat-Treated Specimen C2(L+T) and As-Rolled Steel in 0.5M NaCl
Environment
898 O.O. Daramola, B.O. Adewuyi and I.O. Oladele Vol.10, No.10
0
5
10
15
20
25
0200 400 600 8001000
Exposure Time (Hrs)
Corrosion Rate (mpy)
As-Rolled Steel
Heat Treated Specimen A1 (Q+Q+L+T)
Fig. 7: Corrosion Rate of Heat-Treated Specimen A1(Q+Q+L+T) and As-Rolled Steel in 0.5M
NaCl Environment
0
5
10
15
20
25
0200400600800 1000
Exposure Time (Hrs)
Corrosion Rate (mpy)
As-Rolled Steel
Heat Treated Specimen B1 (Q+L+T)
Fig. 8: Corrosion Rate of Heat-Treated Specimen B1(Q+L+T) and As-Rolled Steel in 0.5M
NaCl Environment
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 899
Fig. 9: Corrosion Rate of Heat-Treated Specimen C1(L+T) and As- Rolled Steel in 0.5M NaCl
Environment
3.2 Microstructure
The microstructures obtained are shown in Plates 1-7. The microstructure produced by the as-
rolled steel consist of pearlite while the microstructure produced by Q + Q + L + T, Q + L + T
and L + T processes consist of a duplex ferrite martensite but may contain bainite and retained
austenite [9].
The corrosion rates of the heat treated specimens in marine medium (NaCl) is low when
compared to that of as-rolled steel. This is because the as-rolled steel which consist of mainly
alternate layers of ferrite and cementite, ferrite is anodic to cementite and this corrode with
moisture as electrolyte [9-10].
0
2
4
6
8
10
12
14
16
0
200
400
600
800
1000
Exposure
Time (Hrs)
Corrosion
Rate
(
mpy
)
As-
Rolled
Steell
Heat
Treated
Specimen C1(L+T)
900 O.O. Daramola, B.O. Adewuyi and I.O. Oladele Vol.10, No.10
Plate 1: Microstructure of As – Rolled 5SP Steel etched in 2% Nital (400X) (Pearlitic Structure)
Plate 2: Microstructure of Heat Treated Specimen A2 (Q+Q+L+T) (400X)
(Duplex Ferrite – Martensite Microstructure)
Plate 3: Microstructure of Heat Treated Specimen B2 (Q+L+T) (400X)
(Duplex ferrite – Martensite Microstructure)
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 901
Plate 4: Microstructure of Heat Treated Specimen C2 (L+T) (400X)
(Duplex ferrite – Martensite Microstructure)
Plate 5: Microstructure of Heat Treated Specimen A1 (Q+Q+L+T) (400X)
(Duplex ferrite – Martensite Microstructure)
902 O.O. Daramola, B.O. Adewuyi and I.O. Oladele Vol.10, No.10
Plate 6: Microstructure of Heat Treated Specimen B1 (Q+L+T) (400X)
(Duplex ferrite – Martensite Microstructure)
Plate 7: Microstructure of Heat Treated Specimen C1 (L+T) (400X)
(Duplex ferrite – Martensite Microstructure)
4. CONCLUSION
From the findings, the heat treated medium carbon steel have better corrosion properties than the
as-rolled medium carbon steel. The steel developed by Quenching + Lamelarizing + Tempering
Vol.10, No.10 Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel 903
(Q+L+T) process has the best corrosion properties followed by Lamelarizing + Tempering (L+T)
process and Quenching + Quenching + Lamelarzing + Tempering (Q+Q+L+T) process.
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