JPH11236687A - Method for preventing crevice corrosion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for preventing crevice corrosion which is capable of easily preventing the occurrence of corrosion with simple work in a crevice between members of even a structure in the seawater or the like and is excellent in maintenance. SOLUTION: When butt welding or the like is executed by butting a chill plate 3 to a base metal 11, an air film 7 is formed in a part 6 coated with a coating agent capable of holding the air film when submerged in the water even under an environment immersed with a corrosive liquid 5 and a corroding liquid does not infiltrate this part and, therefor, the corrosion does not occur in the crevice spacing between the members even of the structure wetted with the seawater or the like. In addition, there is no need for a sacrificial electrode which is heretofore necessary for instance, the welded structure in the sea and the repair work in the environment where the crevice corrosion is liable to occur is drastically reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for preventing crevice corrosion, and more particularly to an oil production facility, a cargo handling facility, a storage facility, an offshore airport, a marine storage base, a ship, a buried tunnel, a wave power / tidal current / temperature difference power generation facility. For a crevice corrosion prevention method suitable for preventing crevice corrosion between members in underwater or overwater structures such as marine ranches, oil refinery plants, chemical plants, bridges, etc. exposed to rainwater, water droplets or splashes on land. .

[0002]

2. Description of the Related Art Conventionally, as a method of preventing corrosion such as rust generated in a gap between members of a structure of this type, the following two methods are generally employed. In underwater or overwater structures such as marine structures, anticorrosion using a sacrificial anode is performed. In land-based structures, anticorrosion paints are often applied.

[0003]

However, the above prior art has the following problems. In the cathodic protection method using a sacrificial anode, it is necessary to attach many sacrificial anodes by welding in order to prevent crevice corrosion, and since the sacrificial anode is consumed, it is necessary to periodically perform replacement work in water. Even when the anticorrosive paint is applied, the concentration of the substance involved in the corrosion is high particularly in the gap portion, and the corrosion progresses from a slight scratch or a defective coating. In addition, liquid enters from the wound part, and expansion of gas components generated by corrosion causes
The coating film may come off. In addition, since normal welding cannot be applied to the welded portion, the coating method cannot be used for corrosion prevention of a fillet weld, a lap fillet weld, a socket weld, and the like, which are likely to be gaps in structure.

An object of the present invention is to solve the above-mentioned problem, and is a structure which is completely or partially or continuously or temporarily wetted by seawater, water droplets, splashes, rainwater, or the like. Another object of the present invention is to provide a method for preventing crevice corrosion, which can easily prevent the occurrence of corrosion in a gap between members with a simple operation and is excellent in maintenance.

[0005]

The above-mentioned object is attained as follows. The invention according to claim 1 is a method of preventing corrosion of the gap by forming a coating layer capable of retaining an air film when submerged in water in a gap between members. According to this method, the gap between the members has an air layer holding function by the present coating layer,
Due to the gaps, they are exerted to the fullest extent and form a stable air film layer. The air film does not get wet with moisture and no corrosive liquid can penetrate, so that corrosion does not occur. According to the second aspect of the present invention, since the coating layer has a contact angle with water of 90 degrees or more, it repels water, so that corrosion can be prevented. According to a third aspect of the present invention, the coating layer is formed using a hydrophobic powder, and the hydrophobic powder is obtained by treating the surface of the inorganic oxide particles with a coupling agent having a hydrophobic group. Become. Therefore, a water repellent which forms a coating layer capable of holding an air film when submerged in water can be easily prepared and coated on a member. The invention described in claim 4 is:
The coating layer is formed using a hydrophobic powder, the hydrophobic powder uses silica as inorganic oxide particles,
In addition, since a silane coupling agent having a hydrophobic group is used as the coupling agent, a coating layer having high water repellency and excellent maintenance can be easily formed.

Hereinafter, a coating method for forming a coating layer capable of retaining an air film when immersed in water and its operation will be described. The present inventors have found that two conditions are required to maintain the air film. One is water repellency of the material, and the other is a fine uneven structure on the surface for holding an air film. The water repellency of the former is preferably 90 degrees or more in terms of the contact angle with water. If it is less than 90 degrees, the air film cannot be maintained for a long time. The fine irregularities on the latter surface are spaces necessary for retaining air in water. Because of the unevenness, it is possible to hold air when immersed in water and to hold an air film on the surface, thereby preventing the material from directly contacting water.

[0007] The shape of the fine irregularities on the surface may be one in which convex or concave portions having a triangular, hemispherical or irregular cross section are arranged continuously or irregularly. As for the size of the unevenness, the interval between the concave portions or the interval between the convex portions (hereinafter simply referred to as the interval between the concave portions) is preferably 2 nm to 200 μm. More preferably, it is 20 nm to 50 μm. If it is less than 20 nm, the size of the space that can hold air is small, and the volume of the air film in water will be small. If it is 200 μm or more, the interval between the irregularities will be large, and the air will not be well held in water. is there. The height of the unevenness is preferably 20 nm or more. Below this, there is a problem that the holding performance of the air film is poor. The upper limit is not particularly limited with respect to the formation of the air film,
This is a range in which work is not difficult to perform during welding and dimensional accuracy does not deteriorate. 1 mm or less is preferable.

As a specific coating method, a coating solution prepared in advance is welded to a grooved portion or the like so that the above-described water repellency and a concavo-convex structure for retaining an air layer are formed after curing. A method of applying to the part surface can be used. The coating liquid is composed of a slurry in which hydrophobic particles and a resin are mixed and dispersed in a solvent.

As the hydrophobic particles, particles obtained by treating the surface of an inorganic oxide particle with a surface treating agent having a hydrophobic group, or polytetrafluoroethylene particles of an organic material can be used.
Since the particles constitute a concavo-convex structure for holding an air film, the average particle size is preferably 2 nm to 200 μm. More preferably, it is 20 nm to 50 μm. If it is less than 20 nm, the size of the space that can hold air is small, and the volume of the air film in water will be small. If it is 200 μm or more, the interval between unevenness will be wide and the retention of air in water will be poor. is there. Examples of the type of the inorganic oxide particles to be treated include silica, alumina, titanium oxide, and iron oxide.

Examples of the surface treating agent having a hydrophobic group include so-called silane compounds such as silane, chlorosilane, and silazane having an alkyl group or a fluorine-substituted hydrophobic group; dimethylpolysiloxane; titanate coupling agents having an alkyl group; Use a ring agent. However, the method is not limited to the above method as long as the method can render the inorganic oxide particles hydrophobic.

As a silane compound having an alkyl group,
Those having a methyl group, for example, methylmethoxysilane CH ₃ Si (OCH ₃ ) ₃ , dimethyldimethoxysilane (CH
₃ ) ₂ Si (OCH ₃ ) ₂ , trimethylmethoxysilane (CH ₃ )
₃ Si (OCH ₃ ), methyl triethoxysilane CH ₃ Si
(OC ₂ H ₅ ) ₃ , dimethyldiethoxysilane (CH ₃ ) ₂ Si
(OC ₂ H ₅ ) ₂ , trimethylethoxysilane (CH ₃ ) ₃ Si
(OC ₂ H ₅ ) and hexamethyldisilazane.

As the silane compounds having an alkyl group, those having an ethyl group include ethyltrimethoxysilane C ₂ H ₅ Si (OCH ₃ ) ₃ and diethyldimethoxysilane (C ₂ H ₅ ) ₂ Si (OCH ₃ ). _2, triethyl silane _{(C 2 H 5) 3 Si} (OCH 3), ethyl triethoxysilane _{C 2 H 5 Si (OC 2} H 5) 3, dimethyl diethoxy silane (C
₂ H ₅ ) ₂ Si (OC ₂ H ₅ ) ₂ , triethylethoxysilane (C
_{2 H 5) 3 Si (OC} 2 H 5) , and the like.

As the silane compounds having an alkyl group, those having a propyl group include propyltrimethoxysilane C ₃ H ₇ Si (OCH ₃ ) ₃ and dipropyldimethoxysilane (C ₃ H ₇ ) ₂ Si (OCH ₃ ) ₂ , tripropylmethoxysilane (C ₃ H ₇ ) ₃ Si (OCH ₃ ), propyltriethoxysilane C ₃ H ₇ Si (OC ₂ H ₅ ) ₃ , dipropyldiethoxysilane (C ₃ H ₇ ) ₂ Si (OC ₂ H ₅ ) ₂ and tripropylethoxysilane (C ₃ H ₇ ) ₃ Si (OC ₂ H ₅ ).

As long-chain alkylsilanes, there are n-octadecyltrimethoxysilane, n-dodecyltriethoxysilane and the like. It is possible to use one or a mixture of chlorosilane and silazane with an alkoxysilane having 1 to 3 alkyl groups having 1 to 20 carbon atoms as described above. However, the metal alkoxide in the present invention is not limited to the above exemplified compounds (the same applies hereinafter).

Examples of the silane compound having a fluorine-substituted hydrophobic group include, for example, perfluorooctylethyltriethoxysilane CF ₃ (CF ₂ ) ₇ (CH ₂ ) ₂ Si (OC ₂ H ₅ ) ₃ , perfluoroisopropylethyl triethoxy Silane (CF ₃ ) ₂
In addition to CF (CH ₂ ) ₂ Si (OC ₂ H ₅ ) _{3 and the} like, perfluoromethylethyltrimethoxysilane, perfluorobutylethyltrimethoxysilane, perfluorooctylethyltrimethoxysilane, perfluorooctylethylmethyldimethoxysilane And alkoxysilane having a perfluoroalkyl group having 1 to 20 carbon atoms, preferably 1 to 10 carbon atoms.

As the titanate coupling agent, isopropyl triisostearoyl titanate, isopropyl tris (dioctyl pyrophosphate) titanate,
Examples thereof include tetraisopropyl bis (dioctyl phosphite) titanate and tetraoctyl bis (ditridecyl phosphite) titanate. Also, examples of the aluminum-based coupling agent include acetoalkoxyaluminum diisopropylate.

Tetrafluoroethylene particles are also suitable as hydrophobic particles. The above-mentioned hydrophobic inorganic oxide particles and tetrafluoroethylene particles can be used alone or in combination. The mixing ratio between the hydrophobic particles and the resin is preferably from 100: 0 to 10:90 by weight, but the contact angle after the hydrophobic treatment may be 90 ° or more, and is not necessarily limited to this range. However, the method is not limited to the above method as long as the method can render the inorganic oxide particles hydrophobic. Further, as a method of coating the member surface with the coating agent in the present invention, spraying, brush coating,
Usual methods such as dipping can be used.

By performing the above-mentioned treatment, the surface of the member has a property of being completely or almost not wet with water. When immersed in water, a thin air film is formed on the surface of the coating layer. This air film can be confirmed by being reflected in silver when exposed to light. In addition, even if immersed in water, the water film is repelled by the air film and does not infiltrate into the inside of the member. for that reason,
Corrosive liquid is prevented from entering and crevice corrosion is prevented.

Here, the property of the present coating layer formed by each of the above surface treatment agents that can retain an air film when immersed in water will be described with reference to FIGS. . As shown in FIG. 8, when a flat plate 50 (a plate made of metal, ceramic, synthetic resin, or the like) having a main coating layer 51 formed on a part of its surface is put into water 53 from the air, the main coating layer 51 The air that was in contact with the air was brought into the water while being held, and an air film 5 was formed on the surface of the layer.
2 are formed. Then, for example, when a small amount of air is sent to the air film 52 through the thin tube 54, the air is taken into the air film 52, and the film 52 expands as shown by a dotted line. As described above, the present coating layer 51 has a specific property of holding a thin air film 52 on the surface in water and taking in air supplied from the outside into the air film 52.

The nature of taking in air will be further described with reference to FIGS. 9 (A) and 9 (B). First, FIG.
As shown in FIG. 5, when the flat plate 50 having the main coating layer 51 formed on the entire surface is inclined in water and air is continuously supplied to the plate surface through the thin tube 54, the air is held on the main coating layer 51 surface. Taken into the air film 52
It rises along the inclined surface of the plate 50 and floats from the end of the air film 52 as a bubble 57. FIG. 9 (B) is a view of the flat plate 50 in FIG. 9 (A) viewed from the front of the upward slope, and as shown, air 55 flows while spreading along the upward slope, and an air reservoir 56 at the upper end. , And floats as a bubble 57 from there. On the other hand, when the flat plate 50 having no coating layer, for example, the surface of the dough is slightly inclined and immersed in water, and air is sent through the thin tube to the surface of the metal plate, as shown in FIG. As soon as it comes into contact with the surface of the plate 50, it separates from the plate surface and floats as bubbles 57.

[0021]

Embodiments of the present invention will be described below with reference to the drawings. Example 1 (see FIGS. 1 to 3) As shown in FIG. 1, a chill plate 3 was attached to base materials 1 and 2 and butt welding was performed to perform a corrosion test. Reference numeral 4 denotes a weld. As shown in FIG. 2, when this test piece was immersed in the etchant 5, an air film 7 was formed on the coating agent application portion 6 in the present invention. FIG. 3 shows a test piece in which this coating agent was not applied, and crevice corrosion 8 occurred.

Hereinafter, the contents of the test will be described. Welding specimen shape / material 16t x 250 x 500 45 degree V groove butt (groove gap 7mm), material SS400 chill plate; 9
t × 50 × 600, material SS400 Coating method of the present coating agent The coating agent was applied by brush application to the entire surfaces of the contact portions of the welded specimen and the chill plate, and then dried for about 2 hours.

Preparation of Test Specimen The test specimen coated with the present coating agent was set so as to have a groove gap of 7 mm, and the restraint plate and the tab plate were attached by welding.
Tack welding stop of 30 mm length was performed at m pitch.

Welding ・ Welding method; CO ₂ automatic welding ・ Welding wire: MG50, 1.2φ ・ Welding condition: first layer: 250A-30V-15cpm 2-4 layers; 300A-32V-20cpm

Corrosion Test of Welded Specimen A specimen having a width of 50 mm and a length of 50 mm was cut out from the above welded specimen and immersed in a crevice corrosion test apparatus (see FIG. 7) to perform a corrosion test. For comparison, a similar test piece was cut out from a test piece welded under the same conditions as the above test piece without applying the coating agent, and immersed in the same corrosion test apparatus for testing. The solution plasticity is defined as 3% NaCl + 1/20 M NaSO ₄ + activated carbon solution (mixing ratio between solution and activated carbon; solution: activated carbon = 5: 2), which accelerates corrosion in seawater.
(Weight ratio, pH 5, 60 ° C.), and air was blown into the solution during the test.

Results of Corrosion Test After immersion for 72 hours, each specimen was taken out and evaluated.
Clearly, corrosion occurred in the gap between the chill plate and the test specimen in the test specimen not coated with this coating agent. The effect was confirmed.

Other Examples (See FIGS. 4 to 6) The same test as in Example 1 was conducted for the following joints where crevice corrosion easily occurs. (A) Fillet weld (see FIG. 4) (B) Lap fillet weld (see FIG. 5) (C) Socket weld (see FIG. 6) Similar to Example 1, the coating layer of the present coating agent Crevice corrosion did not occur in the specimen in which No. 9 was formed, and an excellent effect was confirmed.

As described above, these embodiments of the present invention have the following functions and effects. (1) The coating agent of 90 ° or more in the present invention is applied in advance to a portion that becomes a gap in structure. As a result, the gap is not wetted by moisture and retains the air layer, so that a corrosive liquid cannot enter, and therefore no corrosion occurs. (2) The coating agent of the present invention is applied in advance to the fillet weld, the overlap fillet weld, the socket weld, and the like, which are likely to be gaps in the structure, and welding is performed. The infiltration of the corrosive liquid can be prevented by the air film holding performance of the peripheral portion other than the heat affected zone. (3) Even if there is a small scratch on the coating film of this coating agent,
Because of the gap, the air film holding performance of the present coating layer is further exhibited, and the infiltration of liquid can be prevented by the air film of the surrounding coating layer, and the probability of occurrence of crevice corrosion is reduced. (4) As described above, the present coating agent used in the present invention has good adhesion to metal and has no adverse effect. (5) As a method of coating a mixture of a hydrophobic particle, a resin, a solvent, and the like on the surface of the molded inorganic material, a usual method such as spraying or brushing can be used.

[0029]

As described above, according to the present invention, the following remarkable effects are obtained. Even in the case of a structure that is wet with seawater or the like, the occurrence of corrosion can be easily prevented with a simple operation in the gap between members, and the maintenance is excellent. Since crevice corrosion can be prevented, a sacrificial electrode conventionally required for a welded structure in the underwater portion of an offshore structure can be eliminated. Since a material that does not adversely affect welding is selected, it can be applied in advance before welding and the welding can be performed as it is, so that the welding operation is simple. The service life of the weld is improved and the repair work is greatly reduced in high temperature places and in environments where crevice corrosion is likely to occur, such as seawater.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a butt-welded portion of a test piece with a chill plate.

FIG. 2 is an enlarged view of FIG. 1 when the present coating agent in the present invention is applied.

FIG. 3 is an enlarged view of FIG. 1 when the present coating agent is not applied in the present invention.

FIG. 4 is a cross-sectional view for explaining socket welding.

FIG. 5 is a sectional view for explaining lap fillet welding;

FIG. 6 is a cross-sectional view for explaining fillet welding;

FIG. 7 is a sectional view showing a crevice corrosion test apparatus.

FIG. 8 is a diagram illustrating a coating layer that holds an air film on the surface in water according to the present invention.

FIG. 9 is a diagram illustrating that a coating layer formed on a plate surface holds an air film, and that air taken into the air film moves along the surface of the coating layer.

FIG. 10 is a view showing a state in which, when a plate that has not been subjected to the water-repellent treatment is immersed in water and air is sent to the plate surface, the air immediately floats as bubbles.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 2 Base material 3 Chill plate 4 Weld part 5 Corrosion liquid 6 This coating agent application part 7 Air film 8 Crevice corrosion 9 This coating agent coating layer 50 Flat plate (plate of metal, ceramic, synthetic resin, etc.) 51 This coating layer 52 Air film 53 Water 54 Capillary tube 55 Air 56 Air pool 57 Bubbles θ Contact angle of water

Claims (4)

[Claims] 1. A crevice corrosion prevention method for preventing corrosion of said crevice by forming a coating layer capable of retaining an air film when submerged in water in a crevice between members. 2. The crevice corrosion prevention method according to claim 1, wherein the coating layer has a contact angle with water of 90 degrees or more. 3. The coating layer is formed using a hydrophobic powder, and the surface of the inorganic oxide particles is treated with a coupling agent having a hydrophobic group. Crevice corrosion prevention method described in the above. 4. The coating layer is formed using a hydrophobic powder, and the hydrophobic powder uses silica as inorganic oxide particles and a silane coupling agent having a hydrophobic group as a coupling agent. The method for preventing crevice corrosion according to claim 1, wherein the method is used.