JPS5834300B2 - Steel materials for offshore structures - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a steel material for marine structures that has excellent corrosion resistance.

Steel is widely used as a material for offshore development because it has the advantage of being cheap and easy to process, but in the marine environment, unlike so-called atmospheric corrosion, it is affected by salt, which creates harsh corrosive conditions and makes it prone to corrosion. It has drawbacks and requires some kind of anti-corrosion measure.

The first diagram schematically shows the corrosion status and characteristics of large steel structures that extend continuously from the seabed or underwater to the sea surface.
As shown in the figure, for example, in structures such as piers and sea berths, corrosion is most severe in the spray zone and underwater parts directly below the low tide line.

The reason for the severe corrosion in the splash zone is that seawater is intermittently sprayed by waves, and on top of that, the temperature of the steel rises considerably due to solar heat.

For this reason, due to repeated wet and dry cycles caused by seawater and rise in temperature, corrosion of steel is extremely severe, with an annual average of 0.3 to 0.5
Reach mvr.

Regarding corrosion directly below the low tide line, the tidal area has a greater supply of oxygen than the underwater area, and as a result, an oxygen concentration battery is formed between the underwater area and the tidal area, making the tidal area less susceptible to corrosion, and causing corrosion in the underwater area. This is interpreted to be due to accelerated corrosion, and while the corrosion rate in tidal areas is low at less than 0.1 mm per year, it increases to 0.1 to 0.3 mm in underwater areas just below the low tide line.

A little further down the ocean, the corrosion rate of steel is 0.05 per year.
It is affected by dissolved oxygen, water temperature, current velocity, water quality, marine organisms, bacteria, etc. at a rate of ~0.1%.

Furthermore, it is known that corrosion in seabed soil is slow due to the diffusion of dissolved oxygen, so corrosion of steel materials is also less likely.

Corrosion of steel materials in the above marine environment varies depending on the location, but
For parts located in seawater, cathodic protection is the most effective corrosion protection method, and therefore, protection against splash zone corrosion is the most important for corrosion protection of steel materials for marine structures.

To summarize the conventional knowledge regarding the corrosion protection of splash zones, it can be roughly divided into 1) metal coatings, 2) inorganic coatings, 2) organic coatings, and 2) alloy-added steels.

As for metal coatings mentioned in (2) above, Monel alloy coatings exhibit the most stable corrosion resistance, and it is known that a coating with a thickness of 1.5 mm will have a service life of 20 years, preventing splash zone corrosion in a normal marine environment. Therefore, it is sufficient to apply monel lining to a thickness appropriate to the expected service life of each structure.

In this case, the disadvantages are that the monel alloy is relatively expensive, and if the shape of the structure becomes complex, the method of constructing the monel lining becomes quite complicated and difficult.

In addition, if sacrificial steel is used as a lining, as seen in the example of an underwater island in the United States, the lining thickness required for the same service life of 20 years would be 251 m, and as the service life increases, the plate thickness will increase. However, this is not a good idea because the structure becomes unnecessarily large just for the purpose of corrosion protection, which causes extremely serious problems in terms of the effects of earthquakes, waves, etc., and ensuring the safety of the structure.

Next, according to the U.S. Navy's experimental results, 110 to 12
It has been confirmed that aluminum sprayed to a thickness of 5 μm shows no abnormalities for more than 10 years, while zinc sprayed to the same thickness shows no defects after 3 to 4 years of exposure. It is reported that there was.

In the case of zinc spraying, further heavy coatings are therefore required and a single metal coating is insufficient.

The most common type of inorganic coating described in ■ is concrete lining (mortar lining), but in the case of concrete lining, it is difficult to use fresh water or sand on the ocean, so seawater or sea sand is used. The salt contained in these materials significantly corrodes the reinforcing bars in concrete and the steel materials of offshore structures, and as a result, the adhesion between the concrete and steel materials decreases, peeling occurs, and eventually cracks occur in the concrete, and through these cracks, Furthermore, local corrosion due to salt etc. will be promoted.

Furthermore, as structures grow in size, if they are wrapped in concrete, the shaking of the structure due to an earthquake will become extremely large in the superstructure, which may lead to the destruction of the structure.

(3) Corrosion prevention by organic coating is a commonly used corrosion prevention method, but in general, instead of the above-mentioned zinc spraying, an inorganic zinc coating of about 60 μm is applied, and then a 100 μm top coat of cool epoxy paint is applied. is customary.

According to the results of an actual experiment on an artificial island in the United States, the service life of this system is 8 to 10 years in the ocean atmosphere, which can be said to be insufficient for the spray zone.

In this experiment, the items that showed a service life of 10 years or more were 60
It is coated with 150 to 250 μm of vinyl, saran, epoxy, etc. after μ zinc thermal spraying, and it has been shown that a considerable amount of heavy coating is required.

As shown in Table 1, the coating system recommended by NACE for marine structures requires a considerable number of coatings, and moreover,
The long-term maintenance of the structure requires huge amounts of effort, as it has a service life of less than a year and must be repainted and repaired under operating conditions that are subject to highly unstable weather conditions in the marine environment. Costs are required.

The anti-corrosion method using alloy-added steel aims to be maintenance-free and does not require repainting or repair, but a typical example is ``Mariner Steel,'' which has a small amount of Cu-N1-P added. It has about twice the corrosion resistance of ordinary carbon steel against band corrosion.

However, since 0.1% P is added to this Mariner steel, high temperature cracking is likely to occur during welding, and it cannot be used as a structural steel.

In order to overcome this drawback, research has been conducted on various low alloy steels over the past 10 years, and it has been found that low carbon steel phosphorus-molybdenum steel is superior (Japanese Patent Application Laid-open No. 70911/1983).
However, even in this case, the corrosion resistance is only 2 to 3 times higher, so the steel material may be used in marine structures for a limited lifespan and is good for purposes, but for general use. It's not really a deciding factor.

To summarize the above, conventionally known corrosion prevention methods for spray zones on offshore structures include the need to consider maintenance using methods such as painting or aluminum spraying for oil drilling rigs and the like, which are expected to have a service life of about 10 years. However, for semi-permanent structures with a service life of 20 to 50 years, it must be said that monel lining is the most stable, except for the fact that it is expensive. Considering the development of ocean exploration as the third giant science following space exploration, there is an urgent need to establish a new comprehensive corrosion protection method that fully covers the fateful drawback of steel materials, such as rust, and aims to be maintenance-free for a long period of time. has been done.

The present invention relates to a comprehensive corrosion prevention method that is extremely resistant to the above-mentioned splash zone corrosion, and its basic idea is to coat and paint metal with the minimum necessary thickness using structural steel for welding. The objective is to ensure the required long-term corrosion resistance.

That is, in the present invention, a thickness of 400 to 1000 mm is applied to the steel material.
Thickness 2.8~ having the oxygen-containing manganese compound of A in the upper layer
A steel material for marine structures characterized by forming a manganese plating layer of 11μ, and further having a zinc-rich paint layer with a thickness of 50 to 100μ as an undercoat on the upper layer of the steel material, and further has a zinc-rich paint layer on the upper layer as a topcoat. thickness 2
Any one of 00 to 900μ epoxy, cool epoxy, urethane, vinyl, or phenol
The upper layer of the steel material has a coating layer consisting of seeds, or has a thickness of 20 to 60 mm mainly composed of polyvinyl butyral resin.
This is a steel material for marine structures that has a rust stabilizing coating of μ.

The reason why the inventors came to the above-mentioned idea will be explained below.

Generally, steel materials are coated with other metals to prevent corrosion.
When improving the corrosion resistance of steel, there are two main methods: chrome plating, which coats metals that are more harmful than iron from an electrochemical perspective, and galvanization, which coats metals that are less base than iron. However, when there are pinholes in the coating metal, or when the plating layer becomes thick, cracks are likely to occur in the plating layer, such as in chrome plating, and in either case, there are defects in the metal coating layer, which causes the coating to fail. Since iron is electrochemically more base than other metals, steel corrodes first and is more likely to cause pitting corrosion, unlike zinc, which has a sacrificial anode effect. .

On the other hand, when coating a steel material with a zinc or alloyed zinc layer, the zinc disappears very quickly in a severe corrosive environment such as splash zone corrosion, and the zinc disappears in a short period of time, causing the corrosion of the steel material to progress. As mentioned above, a long-term corrosion prevention effect cannot be expected.

However, it can be said that metals such as zinc, which have a sacrificial anode effect on steel materials, are more advantageous in preventing corrosion of steel materials.

The present inventors conducted systematic research from this point of view and found that a material in which an oxygen-containing manganese compound was formed on the upper layer of manganese had extremely excellent corrosion resistance.

In other words, manganese was expected to be inferior to zinc, as is clear from the natural electrode potential sequence in an aqueous solution of the metal, but since manganese is chemically reactive, by heating and drying it immediately after plating, An oxygen-containing manganese compound is formed on the upper layer of the plating layer, and since this thin film of the oxygen-containing manganese compound is poorly soluble in water, it contributes as a kind of passive film, unlike pure metallic manganese.

That is, when metal manganese is deposited by electroplating using a normal sulfate bath, the metal manganese combines with oxygen in the air and, for example, a thin film of manganese hydroxide formed during electroplating is oxidized in the air to 2M n. (OH)2 + 0242 H2Mn Oa
H2MnOa+ Mn (OH)24Mn a Mn
The reaction of 03+ 2H20 forms an oxygenated manganese compound.

As mentioned above, the corrosion resistance of manganese does not lie in the metallic manganese itself, but in response to the gradual disappearance of the oxygen-containing manganese compound formed on top of it in a corrosive environment, this protection is self-repairing and continuous. The goal is to regenerate the film.

The corrosion resistance of a composite steel material in which a manganese plating layer containing an oxygen-containing manganese compound as an upper layer is formed on the surface of the steel material is tested using, for example, a galvanized steel sheet and a salt spray test (J I 5-Z = 2371), which is very similar to splash zone corrosion. When compared with
Compared to the disappearance rate of hr, 8 InI? /m/hr
It was confirmed that the corrosion resistance was only about 1/125, and it became clear that it had amazing corrosion resistance.

From this, the above-mentioned experimental results in the U.S. Navy indicate that 1
Judging from the actual case where steel materials coated with 10~125μ (785~892g/rrU') zinc became defective within 3~4 years.
(Although hot-dip galvanizing and electrogalvanizing have better corrosion resistance than zinc spraying.) Assuming that approximately 2509/m of zinc is lost per year, manganese plating loses 1/12 of that amount.
5, that is, 2 g/lri: disappearance.

Therefore, if 20g (a little more than 2μ) of manganese plating is applied, the manganese plating layer alone will be sufficient for 10 years, and compared to the case of aluminum thermal spraying, a metal coating layer of less than 1/40 will be sufficient. Become.

Furthermore, in the salt spray test, there is a linear relationship between the amount of manganese lost and the test time, so if the thickness of the oxygen-containing manganese compound and the manganese plating layer is increased, the corrosion resistance will increase proportionally, and the service life will increase. The thickness of the coating layer may be determined accordingly.

As mentioned above, the purpose of corrosion protection against splash zone corrosion can be almost achieved by coating several micrometers of oxygen-containing manganese compound-manganese. For this purpose, use the NACE recommended coating system shown in Table 1, apply a wash primer or zinc rich paint as an undercoat, and then apply a 250 μm coat of epoxy, vinyl, or chlorinated rubber-based topcoat. It is said that it is effective against splash zone corrosion for equipment with a service life of about 10 years, such as oil drilling rigs, if it is painted to a certain extent. Polyvinyl butyral resin was developed by some of the present inventors for the purpose of chemical treatment and proposed in Japanese Patent Publication No. 53-22530, with iron oxide, phosphoric acid, and zinc chromate as an undercoat, and an acrylic resin paint as a topcoat. It has been confirmed that this coating system exhibits excellent corrosion resistance in the marine environment, especially against splash zone corrosion.If such a coating system is further applied to the above-mentioned oxygenated manganese compound/manganese layer, it will be possible to improve the corrosion resistance of marine structures. It is possible to further significantly improve the corrosion resistance of steel materials.

From the detailed description above, the characteristics of the steel materials for marine structures related to the present invention are clear, but in the present invention, the thickness of the metallic manganese layer and the oxygen-containing manganese compound layer and the coating system for the purpose of rust stabilization treatment are The thickness of each is limited to the following ranges for the reasons described below.

First, the oxygen-containing compound is formed by water washing and forced oxidation after manganese plating, and its thickness is determined by the plating electrodeposition conditions and the degree of (air) oxidation.

When using a normal sulfuric acid bath and performing forced oxidation at a temperature of 40°C to 250°C after washing with water, an oxygen-containing manganese compound with an interference color is formed in the thickness range of 400 to 1,000 people,
If it is less than 400λ, it may cause non-uniformity, and if it exceeds 100OA, it may peel off during processing, transportation or due to mechanical impact.
Since a thickness of 0 Å or less is sufficient, the thickness of the oxygen-containing manganese compound is set in the range of 400 to 1,0 OOA.

As mentioned above, the function of the manganese plating layer is to maintain corrosion resistance by self-repairing replenishment commensurate with the gradual disappearance of the oxygen-containing manganese compound due to corrosive environmental factors. The minimum coating thickness required for continuous and uniform coating is approximately 0.3μ at most, and the thicker the plating, the better.

However, assuming that the service life of the steel material of the present invention used in the marine environment is 20 to 50 years, the lower limit thickness is 2.8 μm and the upper limit thickness is 11 μm for the above-mentioned reasons.

Therefore, the thickness range of the manganese plating layer is 2.8~
It was set to 11μ.

Next, regarding the paint system, the undercoat system is zinc rich paint 50 to 100μ, the topcoat system is epoxy system, cool epoxy system, urethane system, vinyl system, etc.
If the steel material is coated with 200 to 900μ of any one of the phenol-based materials, the anticorrosive effect of this coating will reduce the corrosion resistance to 8 to 1
A service life extension of 0 years is achieved.

In addition, when polyvinyl butyral resin paint is coated on the steel material, a thickness of 20 to 60μ will ensure sufficient corrosion resistance for about 10 years. The range of coating thickness was limited.

The above-mentioned painting, plating, and oxygen-containing manganese compound formation processes can be carried out regardless of the strength, toughness, weldability, and corrosion resistance of the steel material. ), so it can be applied to all steel materials.

For example, plate thickness 25 to 1, which is usually used for marine structures.
After plating a 50 mm thick plate in a sulfate plating bath, washing with water, drying, cutting, and welding, a manganese plating layer was formed locally on only the welded part using a portable electroplating device, and then hot air was applied. A dryer is used to form an oxygen-containing manganese compound, which is the same as that of the base material, on the upper layer.

Furthermore, after processing and welding assembly, it goes without saying that an oxygen-containing manganese compound and a metallic manganese layer can be easily formed using a portable electroplating device and a heating device.

It is even better if the steel material is painted.

As mentioned above, the steel material for marine structures of the present invention has an extremely thin plating layer (having an oxygen-containing manganese compound layer on the upper layer) and The thin coating imparts amazing corrosion resistance against splash zones, and the effects of the present invention will be explained in more detail below with reference to Examples.

Example: Structural steel for welding with a plate thickness of 50 mm was used.
Manganese plating was applied to various thicknesses using Pb-8n (5%) as a counter electrode at a bath temperature of 30°C and a current density of 25 A/di'' in a plating bath of ammonium 609/l). After washing with water, it was further heated and dried at 40'C to 250C to form an oxygen-containing manganese compound.

Furthermore, various paints were applied on top of this, and comparative materials were applied with each structural steel ridge material and painted, and salt water spray tests (JIS
-Z-2371) and a marine environment exposure test (Hirohata Higashihama).

The test results are shown in Table 2.

As is clear from this table, the results of the salt spray test over 2000 hours and the exposure test over 5 years confirmed that the steel for marine structures of the present invention exhibits extremely excellent corrosion resistance.

Claims (1)

[Scope of Claims] 1. For marine structures, characterized in that a manganese plating layer having a thickness of 2.8 to 11 μm and having an oxygen-containing manganese compound of 400 to 1000 μm thick as an upper layer is formed on a steel material. Steel material. 2 After forming a manganese plating layer with a thickness of 2.8 to 11μ with an oxygen-containing manganese compound on the top layer of 400 to 1000 people on the steel material, further apply a zinc rich paint with a thickness of 50 to 100μ as an undercoat. layer, and the top layer is coated with a thickness of 200 to 900 μm.
A steel material for marine structures characterized by having a paint layer made of any one of epoxy, tar epoxy, urethane, vinyl, and phenol. 3 After forming a 2.8-11μ thick manganese plating having an upper layer of an oxygen-containing manganese compound having a thickness of 400-1000 on the steel material, a 20-60μ thick manganese plating mainly composed of polyvinyl butyral resin is formed. A steel material for marine structures characterized by having a rust-stabilizing coating film.