Materials Science, Vol. 43, No.

3, 2007

DEGRADATION OF STEELS IN THE HULLS OF RIVER SHIPS

O. I. Balyts’kyi,

1

J. Chmiel,

2

and J. Trojanowski

2

UDC 621.181:669.018

The surface corrosion of skin plates of the hulls of river ships is caused by the interaction of oxy-
gen and hydrogen with the iron surface at the sites of damage to the protective lacquer coatings
on the outer surface of the hull. The inner surface of the hull is subjected to the action of the
condensates of water vapor with different degrees of acidity. Furthermore, near the fuel tanks,
hydrogen-containing media (mainly, the hydrocarbon fuel) also affect the inner surface of the
hull. As a result of hydrogenation, the plasticity of the skin plates of the fuel tanks sharply de-
creases, which is confirmed by the data of measurements of their impact toughness and micro-
hardness, as well as by the results of microfractographic examinations. The contemporary hull
plates subjected to thermomechanical treatment reveal a significant improvement of their plastic
characteristics as compared with the requirements of the existing standard specifications.

The process of operation of river ships includes a fairly long period of demurrage caused by the navigation

conditions in the waterways (low or high levels of water, winter interruptions, etc.). A significant part of staying
ships is concentrated in shallow waters characterized by the repeated and direct contacts with bottom sediments.
Quite often the ships are not preliminary prepared for the period of demurrage (e.g., the underwater part of the
hull is not conserved by protective coatings). Hence, the risk of damage to the protective coatings in the under-
water part of the river ships is much higher than for the sea-going ships [1, 2].

Unlike the sea-going ships whose hulls must have elevated operating characteristics due to the permanent

contact with more aggressive media and higher mechanical loads [3], the hulls of the river ships are designed for
operation under extremely variable navigation conditions but in the freshwater relatively shallow basins. There-
fore, it is necessary to guarantee the possibility relatively small depths of immersion of the hull (but under fairly
high loads). Thus, large surfaces of the flat-bottomed hulls of the river ships with low curvature are located un-
der the water surface [1].

The thickness of the skin of the hulls of river ships is, as a rule, much smaller than for the sea-going ships.

Indeed, it is necessary to minimize the mass of the hull in view of much lower operating stresses caused by the
waves than in the case of sea-going ships. Since the thickness of the skin is smaller, it is more susceptible to the
influence of harmful mechanical and corrosion factors [2].

Materials and Experimental Procedure

The skin of the hulls is made of killed and semikilled steels of category A with the following chemical

composition (recommended by the standard specifications [4])

:

C

max

≈

0.21%,

Mn

min

= 2.5

×

C,

Si

max

= 0.50%,

P

max

= 0.040%,

and

S

max

= 0.040%. The required impact toughness of specimens cut out in the direction of

forge-rolling is equal to

27

J

and the plasticity limit is equal to

235

MPa

[4]. The typical thickness of the skin

of the hulls of river ships is approximately equal to

5

mm.

1

Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, Lviv.

2

Szczecin Marine Academy, Szczecin, Poland.

Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol.

43, No.

3, pp.

117–120, May–June, 2007. Original article submitted

January 31, 2007.

434

1068–820X/07/4303–0434 © 2007 Springer Science+Business Media, Inc.

D

EGRADATION

OF

S

TEELS

IN

THE

H

ULLS

OF

R

IVER

S

HIPS

435

We determine the impact toughness of specimens

(5

mm

in thickness)

cut out from the underwater part of

the skin for the fuel tanks of the river ships with different periods of operation. These data are compared with
the results obtained for specimens cut out from the regular part of the hull and with the data accumulated for the
new hull steel subjected to thermomechanical treatment.

Since it is difficult to perform the direct comparison of the quantitative dependences, we focus our attention

on some trends in the variations of the properties of these materials. Note that it is impossible to get specimens

with different thicknesses and, hence, in our experiments, we use KV-type specimens

10

×

5

×

55

mm

in size.

The minimum value of their work of fracture (according to [4]) is equal to

18

J. The directions of forge-rolling

were identified according to the results of metallographic examinations. The surface of the specimens operating
in contact with fuel was untouched (to the maximum possible degree). The fracture surfaces of the specimens
were studied by the method of scanning electron microscopy with the help of an ISM-6100 Scanning Microscope
(Fig.

1).
The microhardness of the specimens was studied by the Vickers method with the help of a Hannemann de-

vice mounted on a Carl Zeiss Neophot-2 microscope. The points of measurements were located with steps of
0.1

mm

across the thickness of the specimen beginning from the surface operating in contact with fuel.

Results and Discussion

The results of investigations of the skin of the hull (Figs.

1 and 2) on the macro-, micro-, and mesolevels re-

veal evident significant changes in the properties of the material in the course of its operation as compared with
its properties in the as-delivered state.

The macrostructure of the fracture surfaces (Fig.

1a) reveals the signs of ductile fracture under dynamic

loads (when the period of failure of the specimen is equal to fractions of a second). The microstructure (Fig.

1b)

is formed by ferrite-pearlitic grains of the tenth level according to the ASTM

E-112

PCM

12-501120-10. The

bulk content of pearlite is, on the average, equal to

5%,

and the balance is ferrite. The fractographic examina-

tions (Fig.

1c) carried out in the central part of the specimen in the zone of stable and fast crack propagation re-

veal its ductile character accompanied by the formation of tongues of different sizes with disordered locations.
This feature serves as an indication of the high fracture energy of the material.

After

10

5

h

of operation, the macrostructure of the fracture surfaces becomes fibrous. We detect micro-

cracks originating from the pores between some fibers (Fig.

1d). At the same time, we reveal zones with mixed

types of fracture (Fig.

1g). The microstructure of this material (Fig.

1e) strongly differs from the original micro-

structure by larger ferrite grains and a higher concentration of pearlite (of about

10%)

due to the difference be-

tween the original chemical composition of the investigated steel (confirmed by the certificate) and its chemical
composition after long-term operation.

As a result of the application of the classical technology of hot forge-rolling, pearlite becomes striated

(Fig.

1h). This type of the fracture surface is also observed in the microfractograms (Fig.

1f), and the “river” pat-

tern is repeated every

10

µm. As the resolution increases, we detect secondary microcracks at the bottom of

these “rivers” (up to

20

µm

in size)

propagating into the bulk of the material formed by the layers of pearlitic

grains.

In this case, it is typical to observe the formation of micropores

(2–3

µm

in diameter) in front of the micro-

crack. This is the classical picture of crack initiation and propagation including the appearance of microvoids,
their coalescence, and changes in the paths of microcrack propagation.

The character of the fracture surfaces of the specimens taken from the skin of the hull (after a period of

operation greater than

3

⋅

10

5

h)

is absolutely different. Despite the fibrous macrostructure of the fracture sur-

faces described above (Fig.

1g), we also observe large areas with macrocracks propagating toward the lateral

surfaces of the specimen (Figs.

1j and m).

436

O. I. B

ALYTS

’

KYI

, J. C

HMIEL

,

AND

J. T

ROJANOWSKI

Period of

operation, 10

5

h

Macrostructure

Microstructure

Fractograms

0

1

3

Fig. 1.

Microstructure and fractograms of steel of category A for the hulls of river ships

(C

max

≈

0.21%,

Mn

min

= 2.5

×

C,

Si

max

=

0.50%,

P

max

= 0.040%,

and

S

max

= 0.040%)

in the intact state (a–c) and after operation (d–o) in river water and from the sur-

face operating in contact with fuel.

In this case (Figs.

1k and n), the microstructure differs by the sizes of ferritic and pearlitic grains, as well as

by the lower volume fraction of pearlite observed as a result of a certain hydrogen-induced decarburization of the
grains.

D

EGRADATION

OF

S

TEELS

IN

THE

H

ULLS

OF

R

IVER

S

HIPS

437

Fig. 2.

Dependence of the impact toughness on the period of operation of steels whose microstructures are shown in Fig.

1.

Fig.

3.

Variations of microhardness

(HV0.1)

across the thickness of the hull in the process of long-term operation of steels whose

microstructures are shown in Fig.

1.

The microfractograms presented in Figs.

1l, o show that the width of valleys of the river pattern almost tri-

ples and the diameters of the pores also increase (Fig.

1o). In our opinion, the pores are filled with the products

of decarburization (methane and hydrogen) and initiate the process of formation of microcracks [5–7].

The impact toughness rapidly decreases (Fig.

2), mainly for the first

(0.5–1.0)

⋅

10

5

h

of operation. After

1.0

⋅

10

5

h

of operation, it is lower than the value given by the standard specifications [4]. The spread of the data

of measurements is close to

6%.

The results of measurements of the microhardness of specimens by the Vickers method reveal the existence

of a hardened layer

0.6

mm

in thickness from the surface of the plate operating in contact with fuel. The depen-

dence of the increment of microhardness on the duration of the period of operation is obvious. The recorded in-

crement constitutes about

5 HV0.1

after

1.0

⋅

10

5

h

of operation and

18 HV0.1

after

3.0

⋅

10

5

h

(Fig.

3).

CONCLUSIONS

The surface corrosion of skin plates of the hulls of river ships is caused by the interaction of oxygen and

hydrogen with the iron surface at the sites of damage to the protective lacquer coatings of the outer surface of the
hull. The inner surface of the hull is subjected to the action of the condensates of water vapor with different de-

438

O. I. B

ALYTS

’

KYI

, J. C

HMIEL

,

AND

J. T

ROJANOWSKI

grees of acidity. Furthermore, near the fuel tanks, hydrogen-containing media (mainly, the hydrocarbon fuel)
also affect the inner surface of the hull. As a result of hydrogenation, the plasticity of skin plates of the fuel
tanks sharply decreases, which is confirmed by the data of measurements of their impact toughness and micro-
hardness, as well as by the results of microfractographic examinations. Hence, the investigations of the degree
of their degradation under the conditions of intense hydrogenation in the process of long-term operation seem to
be quite urgent. The process of degradation of the skin of fuel tanks leads to the necessity of replacement of
their fragments in the course of regular maintenance. The contemporary hull plates subjected to thermomechani-
cal treatment reveal a significant improvement of their plastic characteristics as compared with the requirements
of the existing standard specifications.

REFERENCES

1. J. Chmiel and J. Trojanowski, “Badania zużycia kadłuba śródlądowej jednostki pływającej,” Probl. Eksploat., No.

1(60), 145–

154 (2006).

2. J. Chmiel and J. Trojanowski, “Degradacja blach kadłubowych statków śródlądowych,” Ochrona przed Korozją, No.

11S/A/2006,

297–300 (2006).

3. H. Liebowitz (editor), Fracture. An Advanced Treatise, Vol.

1: Microscopic and Macroscopic Fundamentals, Academic Press,

New York–London (1968).

4. Polski Rejestr Statków, Przepisy, IX. Materiały i Spawanie, Gdańsk (1995).
5. M. Śmiałowski, Wodór w Stali, Wydawnictwa Naukowo Techniczne, Warszawa (1961).
6. O. I. Balyts’kyi, “Influence of hydrogen on the long-term strength of tire steels,” Fiz.-Khim. Mekh. Mater., 35, No.

4, 37–42

(1999).

7. A. H. Medvid’, V. V. Fedorov, and O. I. Balyts’kyi, “Dependence of the physical properties of

Fe–Ni

alloys on the contents of

reversed and hydrogen-induced austenite,” Fiz.-Khim. Mekh. Mater., 35, No.

2, 75–80 (1999).