N 2 H 2 S Vapor space Sludge Crude oil Oil coat Salt water DEVELOPMENT OF CORROSION RESISTANT STEEL FOR BOTTOM PLATE OF COT Y Inohara, JFE steel corporation, Japan T Komori, JFE s
Trang 1Ex : 13%CO 2 , 5%O 2 , 5%H 2 O, 0.2%H 2 S, 0.01%SO x , Bal N 2
H 2 S Vapor space
Sludge
Crude oil
Oil coat Salt water
Ex : 13%CO 2 , 5%O 2 , 5%H 2 O, 0.2%H 2 S, 0.01%SO x , Bal N 2
H 2 S Vapor space
Sludge
Crude oil
Oil coat Salt water
DEVELOPMENT OF CORROSION RESISTANT STEEL FOR BOTTOM PLATE OF COT
Y Inohara, JFE steel corporation, Japan
T Komori, JFE steel corporation, Japan
K Kyono, JFE steel corporation, Japan
K Ueda, JFE steel corporation, Japan
S Suzuki, JFE steel corporation, Japan
H Shiomi, JFE steel corporation, Japan
SUMMARY
Recently, the corrosion problem in the ship is paid to attention Especially, the pitting corrosion occurred on the inner bottom plate of COT (Cargo Oil Tank) of crude oil tanker to need the inspection and the repair of every dock is one of the big problems The inner bottom plate is being covered with the ‘oil-coat’ that is the crude oil element and protected from corrosion But, the pitting corrosion occurs, and grows up by the ‘oil-coat’ defect part become a local anode site This phenomenon was reproduced in the laboratory, and the laboratory pitting corrosion test method was established And the pitting corrosion decrease effect of the zinc-primer on which it reported before is used, the low alloy corrosion resistant steel to which the number of pitting corrosion was greatly decreased by using it together with the zinc-primer was developed When this corrosion resistant steel is applied to a tanker, the number of pitting corrosion for which the repair at the dock is necessary can be decreased sharply at a low level ¶
1 INTRODUCTION
Recently, the corrosion problem in the ship is paid to
attention Especially, the pitting corrosion occurred on
the inner bottom plate of COT (Cargo Oil Tank) of crude
oil tanker to need the inspection and the repair of every
dock is one of the big problems
In Japan, for three years from 1999, field examination
was carried out to make the corrosion phenomenon of
COT of cargo oil tanker clear As a result, the actual
corrosion environments in COT were clarified Because
of the investigation result, it was presumed that the
pitting corrosion on the inner bottom plate occurred and
grew up under the environment not uniform, such as a
defect of ‘oil coat’, localized salt water or etc., and
existence of oxidizer, such as iron oxide, iron sulfide,
elemental sulfur or etc [1]
This phenomenon was reproduced in the laboratory, and
the laboratory pitting corrosion test method was
established And the pitting corrosion decrease effect of
the zinc-primer on which it reported before is used, the
low alloy corrosion resistant steel to which the number of
pitting corrosion was greatly decreased by using it
together with the zinc-primer was developed
2 PITTING CORROSION
Figure 1 shows the cross section of the crude oil tanker
In operation, the gas part of COT is always filled with
‘inert gas’ for the explosion-proof ‘Inert gas’ is the
exhaust gas of the low oxygen concentration It is
composed of CO2, O2, SO2, N2 and etc The inner bottom
from crude oil In addition, it is presumed that the elements of ‘inert gas’ and H2S volatilized in crude oil merge in this salt water This environment is very severe for corrosion of conventional steel [1]
Figure 1: Cross section of crude oil tanker
2.2 MECHANISM OF PITTING CORROSION GENERATION AND GROWTH
Usually, the inner bottom plate covered with ‘oil coat’ has corrosion protection However, piling up of the
‘sludge’, COW (Crude Oil Washing) and etc cause the defect of ‘oil coat’ If the low protective point such as the defect of ‘oil coat’ occurs on the inner bottom plate, the pitting corrosion is generated in this point In the salt water, the low protective point becomes anode site, the
‘oil coat’ and the ‘sludge’ become cathode site In this area, macro-cell is formed and pitting corrosion grows up [1]
As a result of field examination, it was presumed that the
‘sludge’ is piling up various solid such as solid in the
Trang 2Sludge (cathode) Pitting (anode)
Seawater
Steel plate
Oil coat
Sludge (cathode) Pitting (anode)
Seawater
Steel plate
Oil coat
Seawater
Steel plate
Oil coat
Salt water (10%NaCl) Gas (Simulated inert gas + H 2 S)
Specimen Oil coat
(313K) Salt water (10%NaCl)
Gas (Simulated inert gas + H 2 S)
Specimen Oil coat
(313K)
Figure 2: Image of pitting growth
3 PITTING CORROSION TEST METHOD
Figure 3 shows the laboratory pitting corrosion test
method Test solution was 10% NaCl solution, and it was
saturated with
13%CO2-5%O2-0.01%SO2-0.2%H2S-bal.N2 gas The temperature of the solution was
maintained 313K with a double cell Specimen size was
75 x 50 x 4 millimeters Surface of specimen was
covered with the seal tape expect one test surface Test
surface of specimen was coated with the crude oil residue
gathered from COT, and at the center of test surface, no
coated area (diameter: 5mm) imitated the defect of oil
coat’ was made The specimens were soaked upward in
the solution After these specimens were soaked from 28
for 36 days, the shape of the pitting corrosion that
occurred on the surface was measured, and each
specimen was evaluated by each maximum pitting
corrosion depth The shape (ratio of average diameter
and depth) of the pitting corrosion that occurred by this
test was corresponding to the shape of the pitting
corrosion observed on the inner bottom plate of COT
well
Figure 3: Simulated pitting corrosion test for COT
4 CHARACTTERISTICS OF DEVELOPED STEEL
4.1 EFFECT OF DECREASE OF PITTING CORROSION OF ZINC-PRIMER
As a result of field examination, the number of pitting corrosion occurred on the inner bottom painted zinc-primer was clearly lower than the number of pitting corrosion occurred on the non-painted inner bottom plate
In the first dock, the number of pitting corrosion occurred on the inner bottom painted zinc-primer was from one-fifteenth to one-thirtieth as compared with the number of pitting corrosion occurred on the non-painted inner bottom plate Various addition elements that strengthened the effect of the zinc-primer of the pitting corrosion decrease were examined, and the corrosion resistant steel was developed
4.2 PITTING CORROSION RESISTANCE The zinc-primer painted specimen of conventional steel and that of developed steel were prepared, and they were evaluated by the above-mentioned pitting corrosion test method Figure 4 shows the result of pitting corrosion test The maximum pitting corrosion depth of the developed steel decreased by about 35% compared with that of conventional steel
When this result is applied to the distribution of the depth
of the pitting corrosion measured by field examination of the zinc-primer specification tanker, the number of pitting corrosion in need of repair of developed steel that painted the zinc-primer is provisionally calculated that it
is possible to decrease to one third or less of the conventional steel painted the zinc-primer
Trang 30.5
1.0
1.5
a decrease of about 35%
with Zinc-primer, Test period: 36days
Figure 4: Maximum pitting depth of conventional steel
and developed steel
Table 1 shows an example of the typical mechanical
properties of base metal of the developed steel The
developed steel was satisfied with the specification of
32D grade of IACS Table 2 shows the results of tensile
test and Charpy V notch impact test of the welded joint
of the developed steel The welded joint of the developed
steel was satisfied with the specification of 32D grade of
IACS, too The mechanical properties of developed steel
are equal to them of conventional steel, and in building
of the tanker, the welding and construction performance
similar to conventional steel are usually possible
Table 1: Mechanical properties of developed steel
Charpy Impact Test
at 253K
(N/mm2)
TS (N/mm2)
EL (%) Energy (J) Developed
IACS, 32D > 315 440/590 > 18 > 31
Table 2: Mechanical properties of welded joint
Charpy Impact Test
at 273K
(N/mm2)
Notch position
Energy (J)
WM 106
FL 149 HAZ 1mm 247 HAZ 3mm 273
Developed steel (FCB welding method, Heat input: 108(kJ/cm))
515
HAZ 5mm 317 IACS, 32D > 440 - > 34
5 CONCLUSIONS
• The pitting corrosion test method that was able to simulate the pitting corrosion that occurred on the inner bottom plate of COT was established
• The corrosion resistant steel that strengthened the effect of the zinc-primer of the pitting corrosion decrease was developed The maximum pitting corrosion depth has decreased by about 35% compared with conventional steel
• The developed steel has mechanical properties and construction performance equal with conventional steel as steel plate for shipbuilding
6 REFERENCES
1 Ship Research Panel 242, ‘Study on Cargo Oil Tank
Corrosion of Oil Tanker’, Ship Research Summary
Report No.431, Tokyo, JSRA, 2002
7 AUTHORS’ BIOGRAPHIES Yasuto Inohara holds the current position of senior
researcher at Corrosion Protection Research Department, Steel Research Laboratory, JFE Steel Corporation He is responsible for development of corrosion resistant steel