JPH0734272A - Ballast Tank Anticorrosion Method - Google Patents

Abstract

(57) [Summary] [Purpose] By combining the constituent materials of ballast tanks such as crude oil tankers and coal tanks with environmental control,
Corrosion protection for ballast tanks. [Structure] C: 0.15% or less, Si: 0.02 to 1.5
0%, Mn: 0.2 to 5.0%, P: 0.03 to 0.10
%, S: 0.005% or less, Cu: 0.1 to 1.0%, N
i: A low alloy steel containing 0.2 to 1.0% and the balance being substantially Fe and inevitable impurities was used as a constituent material, and the oxygen gas concentration in the interior was adjusted to a ratio to the value in the atmosphere. It is characterized in that it is 0.5 or less. If necessary, a predetermined amount of one or more of Mo, V, W, Al and Ti is added to improve the physical properties of the constituent materials.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an anticorrosion method for a ballast tank in a ship such as a crude oil tanker or a mining ship, and in particular, it combines the composition of the steel material constituting the ballast tank of the ship with the control of the environment. The present invention relates to a method for preventing the corrosion of ballast tanks.

[0002]

2. Description of the Related Art The inside of a ballast tank is exposed to corrosion of seawater or seawater splashes or condensed water due to condensed water. Therefore, conventionally, the submerged part has been subjected to cathodic protection by a sacrificial anode and further tar epoxy resin coating. Moreover, cathodic protection cannot be applied to the ceiling of the tank, and it is exposed to repeated dry and wet environments, resulting in severe corrosion.

As a countermeasure, there is a method of reducing the oxygen concentration in the tank as disclosed in JP-A-48-73993.

In the case of a conventional crude oil tanker, etc. 1975
The separation ballast method was not used before 1 year, and corrosion of steel materials due to the inclusion of some crude oil in seawater became a problem, and attempts were made to use steel materials to which alloying elements such as Cu were added. The material has not been changed and it is now. After that, the separation ballast system was adopted from 1975, but repair or repainting with tar epoxy coating is the mainstream. However, for safety reasons, the crude oil tank is required to have a double hull, the coating area is increased, and the structure is such that it is difficult to coat. Therefore, it is required to omit the coating process.

[0005]

SUMMARY OF THE INVENTION The problem to be solved by the present invention is to provide a ballast tank which does not require complicated resin coating and its repair during the period of use of a ship and which has excellent durability and long life. To provide.

[0006]

The present invention uses a corrosion resistant low alloy steel mainly containing Cu, Ni and P to meet the above needs, and compares the oxygen concentration in the ballast tank with the atmosphere. By reducing it to less than 0.5,
The ballast tank is extended in life by adopting an anticorrosion method that can omit the painting process.

That is, a low alloy corrosion resistant steel material containing corrosion resistance improving elements such as Cu, Ni and P is used as a constituent material of the ballast tank. The oxygen concentration in the ballast tank is less than that in the atmosphere.
By combining the above and other anti-corrosion methods of 5 or less (oxygen gas concentration of 10% or less), it is possible to extend the life of the ballast tank by eliminating the need for anti-corrosion coating and reducing the corrosion rate. It is intended.

In seawater, the low alloy steel containing Cu, Ni and P has a markedly lower corrosion rate than ordinary steel. on the other hand,
Under dry and wet cyclic conditions of seawater under the atmosphere, Cu,
The corrosion resistance of the low alloy steel containing Ni and P is not so different from that of ordinary steel. However, in the process of reaching the completion of the present invention, the Cu-Ni-P-containing low alloy steel exhibits remarkable corrosion resistance if the oxygen concentration in the vapor phase portion is lowered even under the repeated dry-wet cyclic condition, which is the most problematic problem. It has been found that the corrosion rate is significantly reduced compared to steel.

Therefore, the structure of the present invention as means for solving the above-mentioned problems is as follows.

1. C: 0.15% or less, Si: 0.02
~ 1.50%, Mn: 0.2-5.0%, P: 0.03-
0.10%, S: 0.005% or less, Cu: 0.1 to 1.0
%, Ni: 0.2-1.0% and the balance is low alloy steel consisting essentially of Fe and unavoidable impurities as a constituent material, and the internal oxygen gas concentration is the ratio to the value in the atmosphere. The anticorrosion method for a ballast tank is characterized by setting the ratio to 0.5 or less.

2. In the above 1, the constituent materials are Mo: 0.05-1.0%, V: 0.05-1.0%,
W: The anticorrosion method for a ballast tank, which comprises one or more of 0.05 to 1.0%.

3. In the above 1 or 2, the constituent material further contains Al: 0.02 to 1.0%, and a ballast tank anticorrosion method.

4. In any one of 1 to 3 above, a corrosion resistant method for a ballast tank having excellent durability, characterized in that the constituent material further contains Ti: 0.01 to 0.5%.

[0014]

[Operation] The reason why the content of each alloying element of the Cu-Ni-P-containing low alloy steel used in the present invention is limited will be described below.

1. C has the effect of increasing the strength of steel, but it is not preferable to add a large amount of C because it deteriorates the weldability of steel. Further, it is preferable that C is not too high from the viewpoint that it becomes a cathode of corrosion and reduces the amount of solid solution Cr effective for corrosion resistance to suppress the formation of chromium carbide that accelerates corrosion. Therefore, the content of C is set to 0.15% or less.

2. Si is necessary as a deoxidizing element and is also an effective element for improving seawater resistance. Therefore, it is necessary to add 0.02% or more, but if it exceeds 1.5%, the hot workability deteriorates, so the addition amount was made 0.02 to 1.5%.

3. Mn is an effective element that secures mechanical properties of steel and improves seawater resistance. However, when the amount is less than 0.2%, the effect is small, and
On the contrary, if it exceeds 0%, the seawater resistance decreases, so 0.2 to 0.2
The amount added was 5.0%.

4. P is an element that deteriorates weldability,
If the added amount exceeds 0.10%, the adverse effect becomes remarkable. On the other hand, P is also a basic effective element for improving seawater resistance, and its effect is not exhibited if it is less than 0.03%. Therefore, the added amount is 0.03 to 0.10%. P
Is absorbed into iron rust as PO ₄ ³⁻ , and by making the rust layer have a cation-selective property, it has the effect of suppressing the penetration of Cl ⁻ , which is harmful to corrosion, into the steel surface and reducing the corrosion rate.

5. S is an element that adversely affects the corrosion resistance. This is because non-metallic inclusions MnS, which are the starting points of corrosion, are generated. In order to prevent the formation of MnS, the smaller the content of S, the better. Therefore, the content should be 0.005% or less. MnS becomes S ²⁻ , HS ⁻ , and H ₂ S when it dissolves in seawater, and accelerates the anodic dissolution of steel.

6. Cu is a basic effective element that enhances the seawater resistance of steel and suppresses the growth of local corrosion such as pitting corrosion. If the added amount is less than 0.1%, the effect is not exhibited,
On the other hand, if it exceeds 1.0%, the hot workability deteriorates. Especially C
u is also effective for improving the corrosion resistance of welded parts. Therefore 0.
It is necessary to add 1% or more. For these reasons, Cu
Was 0.1 to 1.0%.

7. Ni is an element that exhibits the same effect as Cu, and particularly enhances the corrosion resistance at the growth stage of pitting corrosion with a lowered pH such as a local anode, and suppresses the progress of local corrosion. Further, coexistence with Cu is extremely effective.
If the amount is less than 0.20%, the effect is insufficient, while if it exceeds 1.0%, the hot workability deteriorates. Therefore, the amount of Ni added is set to 0.20 to 1.0%.

8. Mo is an effective element that improves the seawater resistance of steel. Mo dissolves as MoO ₄ ²⁻ ions and acts as an inhibitor in the local anode, and also improves the corrosion resistance by reducing the permeability of Cl ⁻ ions contained in the rust. If the addition amount is less than 0.05%, the effect is not sufficient, and if the addition amount exceeds 1.0%, the effect is saturated and it is not economical. Therefore, the addition amount of Mo is set to 0.05 to 1.0%.

9. V is an element that improves the seawater resistance of steel by the action similar to that of Mo. As with Mo,
Due to the formation of oxygen acid anions (VO ₄ ³⁻ , V ₂ O ₇ ⁴⁻ ),
Exert its effect. Therefore, like Mo, 0.05-
The amount added was 1.0%.

10. W is also an element that improves the seawater resistance of steel by the action similar to that of Mo. Similar to Mo, it exerts its effect by generating oxygenate anion (WO ₄ ²⁻ ). Therefore, the addition amount is set to 0.05 to 1.0% like Mo.

11. Al, like Si, is an element that enhances the seawater resistance of steel, and if it is less than 0.02%, its effect is not sufficient, and if it is added in excess of 1.0%, hot working of steel occurs. Since the property is impaired, the addition amount is set to 0.02 to 1.0%.

12. Due to the addition of Ti, most of S is T
It is fixed as iS to increase the seawater resistance by eliminating the formation of MnS, which is the starting point of seawater corrosion. If it is less than 0.01%, the effect does not occur, and even if it is added in excess of 0.5%, no further improvement of the seawater resistance improving effect is recognized, so 0.01% is not observed.
The amount added was ˜0.5%.

Next, the reason for limiting the oxygen gas concentration inside the ballast tank will be described. The oxygen gas concentration inside the ballast tank is set to 0.5 or less (oxygen gas concentration 10% or less) as a ratio to the value in the atmosphere by replacing the inert gas with air or evacuating. The reason is as follows. The dissolution reaction of steel in seawater

[Chemical Formula 1] The reaction proceeds by an electrochemical oxidation reaction of Fe → Fe ²⁺ + 2e, while the counter reaction is

[Image Omitted] An oxygen reduction reaction of O ₂ + 2H ₂ O + 4e → 40H ⁻ occurs.

Although both redox reactions proceed in equal amounts,
The latter oxygen reduction reaction is a slow reaction in natural seawater, and the diffusion of oxygen to the steel surface is rate-determining. Therefore, the corrosion rate is determined by the oxygen reduction reaction rate, which is a slow reaction. That is, the corrosion rate depends on the dissolved oxygen concentration. On the other hand, the dissolved oxygen concentration in seawater or condensed water is proportional to the oxygen gas concentration in the gas phase portion according to Henry's law. Therefore, the lower the oxygen gas concentration, the lower the corrosion rate of steel. Further, in the low alloy steel of the present invention containing Cu-Ni-P, a rust layer made of an iron oxide containing Cu-Ni-P is generated in a seawater environment, and diffusion of oxygen through the rust layer to the steel surface is suppressed. Has the effect of Therefore, in the steel of the present invention, the effect of suppressing corrosion due to the decrease in the dissolved oxygen concentration is more remarkable than that of ordinary steel.

The ratio of the oxygen gas concentration in the ballast tank to the value in the atmosphere is 0.5 or less (oxygen gas concentration is 10% or less), preferably 0.3 or less (oxygen gas concentration is 6% or less). By so doing, the corrosion inhibition effect of steel becomes remarkable. Therefore, the oxygen gas concentration is limited to 0.5 or less (the oxygen gas concentration is 10% or less) as a ratio to the value in the atmosphere.

As described above, according to the present invention, Cu-Ni-P is used.
By using a low alloy steel containing, the seawater resistance is enhanced, and further, the oxygen gas concentration in the tank is reduced so that the seawater resistance can be effectively acted on.By combining both, corrosion of the ballast tank is reduced. It is possible to prolong its life. Furthermore, there is a great practical advantage that steel can be used naked and the painting process can be omitted.

[0031]

[Examples] The steel materials shown in Table 1 were melted in an atmospheric high-frequency furnace, then forged and hot-rolled to produce a steel plate with a thickness of 7 mm, and 50 × 20 × 3 (mm)
A test piece having the dimensions of was produced and subjected to a corrosion test.

[0032]

[Table 1]

The test was conducted on artificial seawater (ASTM-D-1141).
The soaking test and the dry-wet repeated test are conducted by injecting air or N ₂ —O ₂ mixed gas and setting the O ₂ gas concentration to 20%, 15%, 10%, 8%, 5%, 1%. went. The temperature was kept at 35 ° C. in a constant temperature bath. Also, for comparison, the test was performed under an air blowing condition. The test time was 720 hours. The dry and wet cycle conditions were wet 0.5 hour-dry 3 hour cycle.

[0034]

[Table 2]

The results of the corrosion test are shown in Table 2 above. A steel within the composition range of the present invention was used, and the oxygen gas concentration was 0.
If it is 5 or less, the corrosion rate is 0.2 g / m ² h or less and no pitting corrosion occurs even under the condition of immersion in seawater and further severe dry and wet conditions. On the other hand, even if the steel within the composition range of the present invention has an oxygen gas concentration of more than 0.5,
Significantly exceeds the corrosion rate under repeated dry and wet conditions and C
When r is high, pitting corrosion also occurs. From such corrosion test results, it is understood that the life of the ballast tank can be extended only when the steel within the composition range of the present invention and the low oxygen use atmosphere are combined.

Further, when the steel composition and the oxygen concentration are out of the ranges of the present invention, the corrosion rate becomes high as compared with the examples of the present invention, and exceeds 0.2 g / m ² h under repeated dry and wet conditions.
However, as shown in the comparative example, even in the case of steel having a composition outside the range of the present invention, when the oxygen gas concentration becomes low, the Cu-
Although not as high as that of the Ni-P added steel, the corrosion rate is reduced accordingly. For example, when the oxygen gas concentration is 1%, the corrosion rate under the repeated dry and wet conditions is 0.22 to 0.25 g / m ² h. However, in an actual ballast tank, it is quite difficult to keep the oxygen gas concentration as low as 1% or less, and even if the oxygen gas concentration is about 5-8% as in the present invention, the corrosion rate is sufficient. It is not practical unless it drops to.

[0037]

The corrosion resistant low alloy steel containing Cu-Ni-P is used as the constituent material of the tank, and the oxygen gas concentration in the tank is kept at 10% or less, which is sufficient on site, to protect the ballast tank from corrosion. However, it is possible to prolong its life. In particular, by reducing the oxygen gas concentration, the excellent properties of the low alloy corrosion resistant steel can be continuously utilized. Even in a shipbuilding maker, the steel material can be used naked, so that the painting process can be omitted, which saves labor and at the same time makes the ballast tank maintenance-free.

Claims (4)

[Claims] 1. C: 0.15% or less, Si: 0.02 to
1.50%, Mn: 0.2 to 5.0%, P: 0.03 to 0.0.
10%, S: 0.005% or less, Cu: 0.1 to 1.0
%, Ni: 0.2-1.0% and the balance is low alloy steel consisting essentially of Fe and unavoidable impurities as a constituent material, and the internal oxygen gas concentration is the ratio to the value in the atmosphere. The anticorrosion method for a ballast tank is characterized by setting the ratio to 0.5 or less. 2. The composition material according to claim 1, further comprising: Mo: 0.05 to 1.0%, V: 0.05 to 1.0%,
W: The anticorrosion method for a ballast tank, which comprises one or more of 0.05 to 1.0%. 3. The ballast tank anticorrosion method according to claim 1 or 2, wherein the constituent material further contains Al: 0.02 to 1.00%. 4. A corrosion resistant method for a ballast tank having excellent durability according to claim 1, wherein the constituent material further contains Ti: 0.01 to 0.5%. .