Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel in Marine Environment

Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.10, pp.888-903, 2011

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)

Lamelarizing (L)

Tempering (T)

Time (minutes)

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)

Time (minutes)

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)