JP2003321090A - Method of preventing corrosion of crude oil tank - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of reducing corrosion occurring on the internal surface of a tank for transporting or storing crude oil. <P>SOLUTION: The method comprises a step of pouring a water-soluble oxygen scavenger into the tank. The step is preferably taken before the crude oil is loaded and/or after the oil is unloaded. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing corrosion occurring on the inner surface of a tank for transporting or storing crude oil (referred to as a crude oil tank).

[0002]

2. Description of the Related Art In crude oil tanks, it has been conventionally believed that the steel material used in the tank does not corrode because the crude oil itself has a corrosion inhibiting effect. However, it has recently become clear that the steel material is severely corroded in the crude oil tank. Corrosion that occurs in this crude oil tank (called crude oil tank corrosion) is
Crude oil, that is, the part in contact with the liquid phase (this is called the liquid phase part)
Then it becomes a bowl-shaped local corrosion, while on the other hand,
At the part in contact with the gas phase of crude oil (this is called the gas phase part),
It is characterized by the general corrosion of unevenness accompanied by delamination rust.

The causes of such crude oil tank corrosion are as follows.
The following items are listed.   Crude oil tank protection film (crude oil type)
The protective film to prevent corrosion in the
Prolapse   Higher concentration of sulfide in crude oil   Inert gas (O₂About 5 vol
%, CO₂About 13 vol%, SO₂About 0.01 vol%, balance N₂
O in the exhaust gas of the engine whose representative composition is gas) ₂,
CO₂, SO₂Concentration of   Microbial involvement However, none of the items are beyond the scope of estimation, and a clear
The cause was unknown.

As measures against such crude oil tank corrosion, at present, measures such as applying a rust preventive coating to shield steel from a corrosive environment, and using high corrosion resistant steel for the crude oil tank have been proposed.

[0005]

However, the application of the anticorrosion paint has a problem in that the application area is enormous and it is necessary to repaint it approximately once every 10 years, which is very expensive. It was On the other hand, as a measure for using a high corrosion resistant steel material for the steel material used in the crude oil tank, for example, JP 2001-107
179, JP 2001-107180, JP 2001-21423
Although it has already been proposed in Japanese Patent No. 6 etc., it is hard to believe that the use of high corrosion resistant steel will stabilize the resistance to corrosion in crude oil tanks and that corrosion resistance can be fully exhibited. Improvement is desired.

[0006] The present invention advantageously solves the above-mentioned problems of the prior art and provides a method for preventing corrosion of a crude oil tank, which is excellent in the environment.

[0007]

In order to achieve the above-mentioned object, the present inventor first extracts each factor involved in corrosion in a crude oil tank, and carries out a corrosion test in a laboratory based on the combination thereof. Was earnestly implemented. As a result, we succeeded in reproducing in the laboratory the same type of corrosion that occurs in an actual crude oil tank, and succeeded in elucidating the controlling factors of crude oil tank corrosion occurring in a crude oil tank and its corrosion mechanism.

That is, in a crude oil tank of an actual machine, the bowl-shaped local corrosion occurring in the liquid phase portion thereof occurs in the presence of salt water separated from the crude oil, and a protective film called an oil coat is formed by Cl ⁻ ions. It was found that the corrosion occurred because the partially broken metal became the anode reaction position where the ground iron in the broken part was fixed, and the oil coating part became the cathode reaction position to form a corrosion battery.

Although this corrosion cell action is originally small, the present inventor has found that, in the presence of a certain amount of O ₂ and H ₂ S, H ₂ S is converted into solid S by an oxidation reaction by O _2. Furthermore, it was found that the solid S is reduced to FeS on the oil coat to promote the cathode reaction, and local corrosion becomes severe. Further, the general corrosion caused by the layered peeling rust with unevenness, which occurs in the vapor phase portion of the crude oil tank of the actual machine, is caused by O ₂ in the inert gas, H ₂ S in the crude oil, and condensed water which are enclosed in the vapor phase portion. I found out that it was related.

Corrosion occurring in the gas phase portion is similar to that occurring in the liquid phase portion. H ₂ S is oxidized by O ₂ and changed to solid S, and the formed solid S is formed on the oil coat. Then, the cathodic reaction is promoted by being reduced to FeS, which causes severe general corrosion due to the delaminated rust with unevenness. In the gas phase, since there is almost no oil coat, the position of the anode is not fixed and the anode reaction occurs at many positions, so that local corrosion does not occur but general corrosion occurs. .

From the above, corrosion in the crude oil tank can be prevented by water, Cl ^- ion, oil coat, O ₂ and H.
It has been found that it can be achieved by excluding any of the corrosion factors of ₂ S and S. However, of these corrosion factors,
Moisture, Cl ^- ions, oil coat and H ₂ S are difficult to remove because they are supplied in large amounts from crude oil, but O ₂ is supplied only from inert gas and has a low partial pressure (about 5 v
ol%), it is relatively easy to remove.

[0012] Therefore, in order to remove oxygen, an experiment was repeated in which a commercially available water-soluble oxygen scavenger was added to confirm the effect,
The present invention has been completed. The present invention has been completed based on the above findings. That is, the present invention is a method for preventing corrosion of the inner surface of a crude oil tank, wherein the above problem is solved by the method for preventing corrosion in a crude oil tank, which comprises a step of injecting a water-soluble oxygen scavenger into the tank. Settled.

Further, according to the present invention, it is preferable that the above step is carried out before the crude oil is loaded and / or after the crude oil is unloaded.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION As the water-soluble oxygen scavenger applied to the present invention, any commercially available oxygen scavenger can be used regardless of powder or solution as long as it is an oxygen scavenger that is soluble in water. For example, a water-soluble oxygen scavenger used for preventing corrosion of steel pipes in water injection in oil wells is one of them.

A method for preventing corrosion in a crude oil tank according to the present invention is
A case of applying to a crude oil tanker will be described. In the present invention, a water-soluble oxygen scavenger, in the oil producing area, before loading crude oil,
At the point of consumption, add after unloading crude oil. Here, in the oil producing area, after the crude oil is loaded, the inert gas (O ₂
However, if an oxygen scavenger is added before the crude oil is loaded, the water-soluble oxygen scavenger will be dispersed in the crude oil due to the stirring action when the crude oil is loaded, and salt water contained in the crude oil will be dispersed. It dissolves in and fixes oxygen. At the time of subsequent transportation, oil and water are separated, and water is rusted on the bottom of the tank, but even in that state, oxygen remains fixed, so that the dissolved oxygen concentration is H ₂
This is a sufficiently low concentration that does not cause the oxidation of S.

On the other hand, crude oil tank corrosion is caused by the
After dropping, even if there is an empty load going to the oil producing area, it will be enclosed there
Inert gas and a small amount of crude oil left
Min, Cl ^-Aeon, oil coat, O₂And H₂S exists
Will be present and corrosion will occur. Therefore, the crude oil load
Add water-soluble oxygen scavenger even when empty after unloading
However, the amount of water added is smaller than when crude oil is installed.
Therefore, a small amount is sufficient.

[0017]

[Examples] Hereinafter, as an example, a verification example using a corrosion test apparatus (referred to as Example 1) and a verification example applied for a long time to a crude oil tank of a crude oil tanker of about 260,000 tons (referred to as Example 2)
Will be described. (Example 1) Two test pieces (10 mm thick x 50 mm) were prepared from a commercially available TMCP steel plate for shipbuilding (16 mm thick) having the composition shown in Table 1.
(mm width × 50 mm length) was cut out and set in the corrosion test apparatus shown in FIG. 1 to carry out the corrosion test.

[0018]

[Table 1]

As the corrosion test apparatus 10 in FIG. 1, a two-layer structure apparatus consisting of a corrosion test tank 2 and a constant temperature tank 3 was used. The test piece 1a simulated the corrosion occurring in the liquid phase portion of the crude oil tank of the actual machine, and was set in the test liquid 6a which is the liquid phase portion of the corrosion test tank 2 (that is, the liquid phase test).

On the other hand, the test piece 1b simulates the corrosion occurring in the gas phase portion of the crude oil tank of the actual machine, and was set in the gas phase 6b of the corrosion test tank 2 (that is, the gas phase test). The test liquid 6 used was artificial seawater specified in ASTM D1141 as the test mother liquor, and water-soluble oxygen scavengers A and B commercially available in the test mother liquor were added at a rate of 1 g / liter. ing.

By the way, as the water-soluble oxygen scavenger A,
Product name CRW9151 (BAKER HU containing Na ₂ SO ₄ as the main component)
GHES), and as the water-soluble oxygen scavenger B, a trade name VpCl-120 containing NH ₄ HSO ₃ as a main component.
(Made by CORTEC) was adopted. And 5 vol% O ₂ + 1
The mixed gas 4 adjusted to the partial pressure ratio of 0 vol% H ₂ S was introduced. N ₂ gas was used as the inert gas for adjusting the balance of the mixed gas. The temperature of the test solution 6a is
The temperature of the water 7 in the constant temperature bath 3 was adjusted to maintain a constant value of 50 ° C. The test period was one month.

After one month, the rust formed on the surfaces of the test pieces 1a and 1b was removed, and the corrosion morphology was visually observed in the liquid phase test, while the test pieces were weighed in the gas phase test to determine the corrosion rate. Calculated and calculated. The results are shown in Table 2. The corrosion rate in the vapor phase test is shown as a ratio when the corrosion rate of the test piece in the comparative example without addition of the oxygen scavenger is 1.

[0023]

[Table 2]

From Table 2, none of the examples of the present invention showed local corrosion in the liquid phase test, and the corrosion rate in the gas phase test was smaller than that of the comparative example (without addition of the oxygen scavenger). It turns out that it is excellent. (Example 2) Approximately 26 built from TMPC shipbuilding steel plate
The water-soluble oxygen scavenger A used in Example 1 was added to the third center tank (25 m width x 60 m length x 30 m height) of the 10,000-ton class crude oil tanker before the crude oil was loaded in the oil producing area. In addition, the addition of crude oil after unloading was repeated at the point of consumption, and verification was continued for approximately 2.5 years to investigate the actual corrosion status.

The amount of the water-soluble oxygen scavenger A added is about 50 kg / cycle before loading crude oil, and about 3 kg / cycle after unloading crude oil.
It was time. This amount of addition is about 0.01 m higher when the crude oil is loaded, which means that salt water equivalent to a height of about 0.2 m separates from the crude oil at the bottom of the tank, and that it accumulates at the bottom of the tank after unloading the crude oil. It was decided in consideration of the fact that a considerable amount of condensed water will be generated.

In a survey after 2.5 years, in the tank to which the water-soluble oxygen scavenger A was not added, local corrosion was observed at the tank bottom, and the tank ceiling had a general corrosion rate equivalent to about 0.2 mm / year. Was recognized. On the other hand, no local corrosion was observed at the tank bottom of the tank containing the water-soluble oxygen scavenger A, and corrosion of the tank ceiling was about 0.05 mm.
/ Year, and the degree of corrosion was extremely slight.

[0027]

According to the present invention, it is possible to provide a method for preventing corrosion in a crude oil tank simply and inexpensively, and it is possible to achieve a marked industrial effect.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a corrosion test apparatus used for verification of the present invention.

[Explanation of symbols]

1a, 1b test piece 2 Corrosion test tank 3 constant temperature bath 4 mixed gas 5 gas outlet 5a exhaust gas 6a Test solution 6b gas phase 7 water 10 Corrosion test equipment

Claims (2)

[Claims] 1. A method for preventing corrosion of the inner surface of a crude oil tank, comprising a step of injecting a water-soluble oxygen scavenger into the tank. 2. The method for preventing corrosion in a crude oil tank according to claim 1, wherein the step is performed before loading crude oil and / or after unloading crude oil.