JP2005213634A - Method for corrosion prevention of ocean rig, coastal structure, and ship - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a material having excellent wear resistance and corrosion preventiveness appropriate for structural members near the sea surface, such as ocean steel rigs and ships and an electrolytic protection method. <P>SOLUTION: The electrolytic protection method for stainless steel members comprises setting the surface roughness (Rz) of the stainless steel at 0.5 to 40 μm in the electrolytic protection method for the ocean rigs or ships using the stainless steel member as at least a part of the structural material. The stainless steel is preferably one kind selected from SUS 317, SUS 317L, SUS 836L, UNS S31254, SUS 329J4L, and JSL 310MoCu and is more preferably stainless clad steel. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an anticorrosion method for offshore structures, coastal structures, ships, and the like, and more particularly to an anticorrosion method for offshore structures, coastal structures, ships, and the like.

  The structural materials of ships (ordinary ships, icebreakers, etc.), offshore structures (floating oil storage ships, oil drilling bases, floating breakwaters, floating piers offshore hotels, etc.) and mooring structures (dolphins, etc.) are resistant to seawater. Construction members made of steel members such as steel plates and steel pipes having excellent corrosivity are often used. However, especially near the sea surface, the corrosiveness is severer than other parts, so the corrosion rate is large, and the useful life of the marine structure is determined by the useful life of this part, ensuring a long service life. It becomes difficult.

  For this reason, the corrosion protection of a ship using steel materials is usually prevented by painting the steel material surface and further using electrocorrosion protection. The coating film formed by coating is anticorrosive by completely covering the steel surface and completely blocking the outside atmosphere.

  In general ships, when mooring, the coating may peel off by rubbing against the mooring structure due to the vertical movement of the waves. In addition, ice breakers and oil drilling bases are coated with ice paint that is harder and thicker than ordinary paint due to contact with ice in the ice sea, but the paint may peel off due to deterioration over time. However, by using it together with cathodic protection, the peeled area is protected.

In steel structures, there are organic lining (polyethylene, polyurethane, petrolatum, plastic cover) and concrete lining as one of the anticorrosion methods besides painting.
There are other metal linings. Metal lining is a method of wrapping metal around the surface of ordinary steel to prevent corrosion, but it has higher mechanical strength, impact resistance and wear resistance than other methods, and the wrapping metal has good corrosion resistance. It has the advantage of being almost maintenance-free and ensuring long-term corrosion protection. The metal lining includes a method of winding a thin plate of corrosion-resistant metal (monel, stainless steel, titanium clad), a method of using a thick plate of clad steel, and a thermal spraying method of spraying a molten metal to form a coating.

Russian and Finnish icebreakers may take a 316 class clad steel approach to contact the ice to prevent corrosion. Recently, a method of lining thin stainless steel to a steel structure may be applied.
For example, Patent Document 1 describes that a portion of the ice sea structure in which the structure main body comes into contact with the ice sea is composed of clad steel having high seawater resistance and high wear resistance. It is described that long-term corrosion resistance is imparted to a harbor / marine steel structure by using clad steel made of seawater-resistant stainless steel as a combination material.

  Due to interference with mooring structures and ice, the part where the paint is peeled off may be above the sea level. If the part where the paint is peeled is above the sea surface, the surroundings will be in the gas phase, so that the anticorrosion current does not flow through the seawater, and the effect of the anticorrosion cannot be obtained. Therefore, although it is necessary to repair the part where the paint has peeled off, it is very expensive to repair the paint by putting it in the ship dock.

When stainless steel is used, it may be worn from the surface depending on the type of stainless steel due to interference with mooring structures and ice. SUS304, which is a typical austenitic stainless steel, is subjected to repeated loads, and the surface undergoes work hardening, the surface layer becomes hard, and a phenomenon called spalling occurs in which metal pieces peel from the surface due to fatigue.
In the case of lining, the lining may peel off due to contact with ice.

JP-A-62-244910 "Seawater resistant stainless clad steel pipe", Journal of Japan Steel Structure No. 50 (October 31, 2003), p. 19

  It is an object of the present invention to provide a structural member and an anticorrosion method having excellent wear resistance and corrosion resistance suitable for structural members near the sea surface such as marine structures and ships.

In the marine steel structures and ships, the present inventors have a structure using stainless steel or stainless clad steel having excellent wear resistance and corrosion resistance in the whole or in the vicinity of the sea surface, and the surface thereof. The present invention has been completed by finding that it can be solved by setting the roughness within a specific range.
That is, the present invention has a configuration as described below.

(1) In the marine steel structure using the stainless steel member as at least a part of the structural material or the cathodic protection method of the ship, the surface roughness (Rz) of the stainless steel is 0.5 to 40 μm, To prevent corrosion of stainless steel members.
(2) The method for preventing corrosion of a stainless steel member according to (1) above, wherein the stainless steel is stainless steel having excellent wear resistance and corrosion resistance.
(3) The method of cathodic protection of a stainless steel member according to (1), wherein the stainless steel is a stainless clad steel having excellent wear resistance and corrosion resistance.
(4) The above-described stainless steel member electrocorrosion protection method according to (1) to (3), wherein the stainless steel is excellent in wear resistance and corrosion resistance, and is a type selected from SUS317, SUS317L, SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu. .
(5) The cathodic protection method for stainless steel members according to (1) to (3) above, wherein the stainless steel is a kind selected from SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu, which is excellent in wear resistance and corrosion resistance.
(6) The electrocorrosion method uses an external electrode, measures a current and potential at a predetermined position using a probe, controls a power supply voltage using the measured value, and causes a predetermined anticorrosion current to flow (1 ) To (5) a method of cathodic protection of a stainless steel member. (7) A stainless steel member used as at least a part of a structural member constituting an offshore structure, a coastal structure or a hull. A stainless steel structural member having a surface roughness (Rz) of 0.5 to 40 μm and a material selected from SUS317, SUS317L, SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu.
(8) A stainless steel member used as at least part of a structural member constituting an offshore structure, a coastal structure, or a hull, and the surface roughness (Rz) of the stainless steel member is 0.5 to 40 μm. A stainless steel or stainless clad steel structural member used for an offshore structure, a coastal structure, or a hull, characterized in that the material is one selected from SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu.

  According to the present invention, it is possible to solve the problems of wear above the sea surface and corrosion due to peeling of the coating film, which were problems in the past due to interference with mooring structures and ice, collision with driftwood, and the like. Further, when carbon steel is used for a portion other than stainless steel in the sea, corrosion of the carbon steel can be prevented by using electrocorrosion together. Furthermore, when an external power source is used, electric potential and current can be measured using a probe, and the external power source can be controlled based on the measured value, thereby enabling cathodic protection without excess or deficiency.

In the present invention, stainless steel or stainless clad steel excellent in wear resistance and corrosion resistance is used as a structural material of a part requiring corrosion resistance such as a ship or an offshore structure, but stainless clad steel is more preferably used.
Stainless steel clad steel is composed of carbon steel and stainless steel, which is a laminated material, but carbon steel and stainless steel are metal materials that are compatible with each other in the first place. Is reasonable. In particular, clad steel produced by a hot rolling method has very high adhesion between the two materials and behaves as a mechanically integrated piece.

When stainless steel clad steel is used, stainless steel as a laminated material exhibits functions such as corrosion resistance and wear resistance, and carbon steel as a base material ensures high strength and weldability. Therefore, the amount of expensive corrosion resistant alloy can be minimized. Also, by selecting the base steel grade, the plate thickness can be reduced and the equipment weight can be reduced. For this reason, a cheaper product can be made than using pure stainless steel.
Furthermore, unlike clad steel lining of stainless steel thin plate (a stainless steel thin plate having a thickness of about 0.4 to 2 mm is welded to the base steel material), the whole is integrated with the base carbon steel. For this reason, it is strong against mechanical damage as compared with the stainless steel lining.

Stainless steel is required to have excellent wear resistance and corrosion resistance. In particular, the corrosion resistance is a PI value (Pitting Index = Cr + 3) obtained by multiplying the content (wt%) of chromium, molybdenum, and nitrogen that contribute to corrosion resistance by a coefficient. .3Mo + 16N) is 27 or more (equivalent to SUS317L). Examples of such a material include SUS317, SUS317L, SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu.
More preferable are SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu, which are also excellent in rust resistance. Especially when wear resistance is required, SUS329J4L having a PI value of 38 or more is preferable. Further, when corrosion resistance is required, SUS836L, UNSS31254, and JSL310MoCu are preferable.

  When the anticorrosion is applied to carbon steel or stainless steel, an electrocoat is formed on the surface. This electrocoat has a function of reducing the anticorrosion current, and also exhibits an effect of preventing the base material by keeping the surface alkaline.

  However, since the surface of stainless steel is inactive compared to carbon steel, the electrocoat is less likely to adhere unless the surface is uneven. In general, the surface of the steel material is also blasted to provide unevenness.

On the other hand, from the viewpoint of anticorrosion, if the surface has irregularities, the seawater adhering to the surface is dried to increase the salt concentration and rusting may occur.
Therefore, it is necessary to increase the surface roughness from the surface of electrocoat adhesion and to reduce the surface roughness from the surface of anticorrosion. Therefore, it is necessary to control the surface roughness to an appropriate value.
For this reason, in the present invention, the surface roughness of stainless steel or stainless clad steel is 0.5 to 40 μm, preferably 0.5 to 30 μm.
In addition, the roughness (Rz) in this invention means the 10-point average roughness defined by JIS.

The reason why the lower limit is set to 0.5 μm is that the minimum value to which the electrocoat is attached is below the electrocoat. Further, the upper limit is set to 40 μm because, when exceeding 40 μm, SUS317L having the lowest corrosion resistance forms a small gap structure on the surface and causes rusting. If the thickness is 30 μm or less, the degree of rusting is reduced even with SUS317L having the lowest corrosion resistance.
As a method for adjusting the roughness, methods such as polishing, blasting and grinding can be employed. For polishing, use a grinder that uses abrasive paper with abrasive particles (silicon carbide, silicon dioxide, garnet, etc.) fixed to the paper, or a disc with abrasives hardened with an adhesive, or polish the surface with a stainless steel brush. Can be done by. Blasting is a method of creating irregularities on the surface by spraying an abrasive together with compressed air. As a grinding method, there is a method of grinding with a grinder using a disk whose surface is thin and a tool or an abrasive is hardened with an adhesive.

Next, the method of cathodic protection is described.
For corrosion protection, a sacrificial anode such as an aluminum alloy or zinc is electrically connected to the part that touches the seawater to be protected by welding, etc. The galvanic anode method that keeps the anticorrosion state, and the insoluble electrode is immersed in the sea and electrically connected to the steel material to be anticorrosion via the external power supply. There is an external power supply method.

  In cathodic protection, an alloy composed of Mg, Zn, and Al is generally used as a galvanic anode. However, in the ocean, aluminum alloys having a large amount of effective generated current per unit weight are mainly used. In the case of the galvanic anode method, construction is easy, and construction is possible in a relatively short period of time. However, when the anticorrosion period is long, the construction cost tends to be higher in the galvanic anode method than in the external power supply method.

Examples of the anode material used in the external power system include silicon cast iron, magnetic iron oxide (Fe3O4), cast iron, carbon steel, lead, lead alloy, silver, silver alloy, platinum, platinum-plated titanium, and graphite.
Since the potential control can be easily performed in the external power supply method, a part of the structure can be the external power supply method and the rest can be the galvanic anode method.

  When an external power supply is used, the potential is fed back by the reference electrode and the current output from the anode is controlled by the control device. This current control can cope with an increase in the anticorrosion current due to the peeling of the coating over time, and a sudden change in the anticorrosion current that causes an increase in dissolved oxygen and a rise in the water temperature.

  As materials used for anti-corrosion coatings for general ships, epoxy (tar epoxy, ultra-thick film epoxy, solventless epoxy, etc.), urethane (tar urethane, etc.) and the like are used. In the ice sea area, a paint composed mainly of solvent-free epoxy called ice sea paint is used.

  For the marine environment, high-grade stainless steels such as SUS329J4L, SUS836L, and UNS S31254 are commercially available as seawater resistant stainless steel. However, these steel types are expensive because they contain many additive elements such as nickel, chromium and molybdenum. Therefore, a steel type such as SUS316 or SUS316L may be used in a portion where surface rusting is allowed.

  FIG. 1 conceptually shows an example in which the present invention is applied to an offshore structure. A pile material 1 is composed of a stainless steel clad steel pipe 3 near the sea surface, and the other part is composed of a general steel pipe 2. ing. In the figure, the sacrificial electrode and the like are not shown.

Below, this invention is demonstrated based on an Example.
First, the test methods used in the examples will be described.
<Abrasion test>
The shape of the test piece was a cylindrical shape shown in FIG. 2, and all were collected from a rod-shaped material by machining. Prior to the test, the sample was degreased with acetone and weighed.
A Nishihara type abrasion tester was used as a test apparatus. An outline of the test apparatus is shown in FIG.
The test surface was a side surface of a cylinder, and two cylinders were rotated in the circumferential direction, and slip was given by changing the number of rotations between the upper and lower test pieces. The number of rotations was 800 rpm on the high speed side and 560 rpm on the low speed side.

<Corrosion resistance test>
A 250 × 300 × 6 mmt test piece was attached to the ridge and an exposure test was conducted. The exposure position was 200 mm above the sea level. The test pieces exposed for 2 years were collected and photographed, and then evaluated according to the relationship between the corrosion area ratio of JIS Z2371 and the rating number (RN).

[Abrasion test]
A wear test was performed on various steels shown in Table 1. Table 1 shows the composition of each steel type.
The wear test was performed using SUS329J4L on the high speed side and SUS304, SUS316, SUS316L, SUS317, SUS317L, SUS329J4L, SUS836L, UNS S31254, SUS310S, JSL310MoCu on the low speed side. The experimental results are shown in Table 2.

FIGS. 4A to 4C are views showing the appearance (a) of the test piece after the test, the peeled piece (b) peeled from the test piece, and the cross-sectional microstructure (c), respectively.
In an experiment using SUS304, SUS304L, SUS316, and SUS316L, peeled pieces as shown in FIG. In particular, SUS304 and SUS304L have a large amount of wear. Next, SUS316L had the largest wear amount, and SUS316 was the next.
No exfoliation pieces were produced in other steel types.

[Corrosion resistance test]
An exposure test was conducted by attaching test pieces of each steel type shown in Table 1 to a cage. The exposure position was 200 mm above the sea level. The test pieces exposed for 2 years were collected and photographed, and then evaluated according to the relationship between the corrosion area ratio of JIS Z2371 and the rating number (RN). The results are shown in FIG. The larger the RN, the smaller the corrosion area rate.

  From these results, SUS304, SUS304L, and SUS310S rusted severely. SUS316, SUS316L, SUS317, and SUS317L have some rusting. SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu did not rust.

[Anti-corrosion current measurement test]
Of various steel types shown in Table 1, SUS317L and SUS836L were selected, and the corrosion resistance current measurement experiment was performed by changing the surface roughness. The experiment was carried out in seawater at 30 ° C., kept at −1000 mV using a saturated calomel electrode (SCE) as a reference electrode at a flow rate of 2 m / sec, and the current flowing through the test piece was measured. The current density was calculated by dividing the measured current by the surface area.
A 6 × 20 × 50 mm steel plate was adjusted to a predetermined surface roughness, and then degreased with acetone for use in experiments. The surface roughness was adjusted by polishing with abrasive paper for 0.5 μm or less and blasting for 10 μm or more.
Table 4 shows the test results.
At the beginning of the experiment, a large amount of current flows because the electrocoat does not adhere to the surface, but when the electrocoat adheres, the current significantly decreases. Therefore, the measured current value is a value one month after the start of the experiment.

The current density is 0.2 mA / m ² or less for a surface roughness of 0.5 μm or more. If the surface roughness is small, the attached electrocoat peels off due to the influence of the flow velocity, and thus the current density is large. When the current decreases in this way, the amount of the sacrificial anode that dissolves per unit time is reduced in the galvanic anode method, and thus corrosion protection can be achieved for a long period of time. In the external power supply method, the power cost can be reduced by reducing the current from the anode.
From this result, it is desirable that the surface roughness of the stainless steel used for the marine steel structure and the ship which are cathodic protected is 0.5 μm or more.

  From the above, as stainless steel having both wear resistance and corrosion resistance, SUS317, SUS317L, SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu can be cited. Furthermore, it is preferable to use SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu from the viewpoint of corrosion resistance.

  The present invention is not limited to ships, but can be applied to bottomed marine structures, steel sheet piles, steel pipe sheet piles, and floating structures that come into contact with driftwood and ships, or in the ice sea area. Nor.

The cathodic protection method of the present invention can be applied as an anticorrosion method for portions that are susceptible to wear and corrosion in offshore structures, coastal structures, or hulls.
In addition, the structural member of the present invention can be applied as a wear-resistant and corrosion-resistant structural material in a marine structure, a coastal structure, or a ship body that is susceptible to wear and corrosion.

It is a figure which shows the example which applied this invention to the offshore structure. It is a figure which shows the shape of the test piece used in the Example. It is a figure which shows the outline | summary of the abrasion tester used in the Example. It is a figure which shows the external appearance of a test piece after a test, a peeling piece, and a cross-sectional microstructure. It is a figure which shows the relationship between a rating number and a rust generation | occurence | production area ratio.

Claims (8)

  In a marine steel structure or marine vessel anti-corrosion method using a stainless steel member as at least a part of a structural material, the stainless steel has a surface roughness (Rz) of 0.5 to 40 μm. Electrocorrosion protection method for members.   2. The method of cathodic protection of a stainless steel member according to claim 1, wherein the stainless steel is stainless steel having excellent wear resistance and corrosion resistance.   2. The method of cathodic protection of a stainless steel member according to claim 1, wherein the stainless steel is stainless clad steel having excellent wear resistance and corrosion resistance.   The stainless steel member according to any one of claims 1 to 3, wherein the stainless steel is one type selected from SUS317, SUS317L, SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu, which is excellent in wear resistance and corrosion resistance. Galvanic protection method.   The stainless steel member electrocorrosion protection method according to any one of claims 1 to 3, wherein the stainless steel is a kind selected from SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu, which is excellent in wear resistance and corrosion resistance. .   The above-mentioned cathodic protection method uses an external electrode, measures a current or potential at a predetermined position using a probe, controls a power supply voltage using the measured value, and flows a predetermined anticorrosive current. Item 6. Electrocorrosion protection method for stainless steel member according to any one of items 1 to 5   A stainless steel member used as at least a part of a structural member constituting an offshore structure, a coastal structure or a hull, and the surface roughness (Rz) of the stainless steel member is 0.5 to 40 μm; A stainless steel structural member characterized in that the material is a kind selected from SUS317, SUS317L, SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu.   A stainless steel member used as at least a part of a structural member constituting an offshore structure, a coastal structure or a hull, and the surface roughness (Rz) of the stainless steel member is 0.5 to 40 μm; A stainless steel or stainless clad steel structural member used for an offshore structure, a coastal structure, or a hull, characterized in that the material is one selected from SUS836L, UNS S31254, SUS329J4L, and JSL310MoCu.

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