JP5756414B2 - Heat transfer tube or header tube of open rack type vaporizer - Google Patents

Description

  The present invention relates to a heat transfer tube or a header tube constituting a heat exchange panel of an open rack type vaporizer.

  Liquefied natural gas (hereinafter referred to as LNG as appropriate) is usually a low-temperature and high-pressure liquid that is vaporized before being used as fuel. And in order to vaporize a lot of LNG efficiently, the open rack type vaporizer (henceforth ORV suitably) using the heat of seawater is used.

  As shown in FIGS. 1 (a) and 1 (b), the ORV 10 includes a plurality of arranged heat transfer tubes 2, a lower header tube 3 and an upper header tube (hereinafter referred to as “joint”) that join these heat transfer tubes 2 in parallel at the upper and lower ends. A plurality of heat exchange panels 1 including appropriate header tubes 3 and 4 are provided. The ORV 10 is disposed in the upper part between the heat exchange panels 1 and has a trough (weir) 7 for storing seawater supplied to the outer surface of each heat transfer tube 2, and each header tube 3 of the heat exchange panel 1, 4 further includes a lower manifold 5 and an upper manifold 6 that join the four in parallel. LNG is introduced from the lower manifold 5 through the lower header pipe 3 into the heat transfer pipe 2 from the lower end. On the other hand, seawater stored in the trough 7 by a supply means (not shown) overflows from the side edge of the trough 7 and hangs down while wetting the outer surface of the heat transfer tube 2. The LNG introduced into the heat transfer tube 2 is heated (sealed) by seawater flowing outside the heat transfer tube 2 and vaporizes, and rises in the heat transfer tube 2. The vaporized LNG is led from the upper end of the heat transfer tube 2 to the upper manifold 6 through the upper header tube 4. That is, ORV10 is a kind of heat exchanger, and heats and vaporizes LNG by heat exchange with seawater.

  For the heat exchange panel 1 (heat transfer tube 2 or header tubes 3 and 4), aluminum alloys such as 3000 series, 5000 series, and 6000 series are usually used from the viewpoint of thermal conductivity, workability, and the like. However, since the outer surface of the heat exchange panel 1 is exposed to seawater, in the case of an aluminum alloy material that is easily corroded, once the outer surface starts to be eroded, the portion may be eroded intensively, resulting in pitting corrosion. There is. Therefore, the surface of the aluminum alloy material constituting the heat exchange panel 1 needs to be subjected to anticorrosion treatment. In particular, at the lower part of the heat exchange panel 1, the outer seawater is cooled to about 0 ° C. by LNG at an extremely low temperature (about −160 ° C.), so the dissolved oxygen concentration is high and the environment is more severe. . Further, seawater flowing in a large amount from above collides with the outer surface of the heat exchange panel 1. In the lower part of the heat exchange panel 1 (in the vicinity of the lower header pipe 3 in the heat transfer pipe 2), the flow rate of the seawater is as high as 4 m / s or more, so the outer surface is worn by the seawater. Furthermore, flow-induced corrosion (FAC: Flow Accelerated Corrosion) occurs due to a synergistic effect of corrosion by seawater and erosion (erosion) by the flow of seawater, thereby promoting wear of the anticorrosion treatment layer. Therefore, the anti-corrosion treatment for the heat exchange panel 1 requires not only general corrosion (corrosion resistance) but also resistance to flow-induced corrosion (hereinafter referred to as FAC resistance). Durability is also required so that the ORV10 can be operated continuously for a long time.

  Therefore, a heat exchange panel has been developed in which the outer surface of a base material made of an aluminum alloy material is covered with a sacrificial anode layer made of a base Al-Zn alloy or the like having a lower potential than the base material so that the base material is not brought into contact with seawater. Yes. In this heat exchange panel 1, corrosion of the base material is prevented by preferentially dissolving Zn in the sacrificial anode layer as ions in seawater. In addition, as a method for coating the base material with the sacrificial anode layer, a method for coating the base material with a material for the sacrificial anode layer (Al-Zn alloy or the like) by thermal spraying is carried out from the viewpoint of workability and workability to complex shapes. . However, a film (sprayed film) coated by thermal spraying has a structure including a certain amount of pores inside the film because of its formation method, and there is a possibility that seawater may enter the base material through the pores. In addition, the thermal spray coating is weaker in adhesion to the base material than the clad material, and may be peeled off due to the flow of seawater, and may be peeled off or cracked due to freezing caused by cooling or stress due to a temperature difference. Furthermore, when the thermal spray coating is made thicker to increase the life of the sacrificial anode effect, the thermal spray coating becomes easier to peel off, and there is a limit to the thickness of the thermal spray coating that can be formed by one thermal spray (pass). The number of passes increases and costs increase.

  As a countermeasure, Patent Document 1 discloses an aluminum alloy material for ORV that prevents the sprayed coating from being peeled off due to corrosion at the interface of the base material. And in patent document 1, by performing machining after thermal spraying, the pore area ratio in the region of a predetermined depth from the surface of the thermal spray coating is reduced to 10% or less, thereby preventing seawater from entering the base material. Suppresses corrosion. Patent Documents 2 and 3 disclose ORV heat transfer tubes in which the adhesion of the thermal spray coating is increased by roughening the surface of the substrate to a predetermined surface roughness by blasting before thermal spraying. .

JP 2006-183087 A JP 2005-265393 A JP 2008-111638 A

  According to general knowledge, the adhesion generally depends on the bonding state of the interface, and further, in a corrosive environment, the bonding state of the interface deteriorates due to corrosion. Therefore, as disclosed in Patent Documents 1 and 2, in the thermal spraying method, a thermal spray coating is formed so as to reduce the number of pores in the film that promote corrosion. In the thermal spraying method, the interface, that is, the surface roughness of the base material is adjusted by pretreatment so that no gap is formed at the interface. However, in the prior art disclosed in Patent Documents 1 to 3, the corrosion resistance and peel resistance are improved, but there is room for improvement for ORV. That is, the present inventors have found that under the ORV operating environment, the influence of freezing due to cooling or stress due to a temperature difference is greater than simple corrosion. Specifically, in the configurations of Patent Documents 1 and 2, micro-cracks are generated in the initial stage by receiving stress due to icing or temperature difference in addition to corrosion, and the base material and the thermal-spray film as the micro-cracks progress. Interfacial cracking (local peeling) occurs. As a result, in the configurations of Patent Documents 1 and 2, a non-adhesion region occurs at the interface between the base material and the thermal spray coating, and when the non-adhesion region spreads, the thermal spray coating swells and further interlocks with the progress of microcracks. The sprayed coating will fall off.

  The present invention has been made in view of the above problems, and provides an ORV heat transfer tube or header tube excellent in crack resistance and peeling resistance of a sacrificial anode layer, particularly even in an ORV operating environment. Let it be an issue.

  As a result of diligent research, the present inventors have determined that a sacrificial anode layer that hardly cracks or peels off and does not easily progress even under an ORV operating environment by controlling the sprayed coating structure of the sacrificial anode layer by adjusting the spraying conditions. It was found that it can be obtained.

That is, the heat transfer tube or header tube of the open rack type vaporizer according to the present invention is a heat exchange panel of an open rack type vaporizer that vaporizes liquefied natural gas flowing inside by heat exchange with seawater supplied to the outer surface. A base material made of an aluminum alloy and a thickness of 100 μm or more made of an aluminum alloy that is coated on at least a part of the outer surface of the base material and has a sacrificial anode property with respect to the base material A sacrificial anode layer, wherein the sacrificial anode layer has a laminated structure in which a plurality of sprayed coatings formed by thermal spraying are laminated, and the sacrificial anode layer has a cross section in the thickness direction and the base material. In a state where the average film thickness of the plurality of sprayed coatings is in the range of 3 to 8 μm in the region from the interface to the depth of at least 100 μm and is sandwiched between the laminated structures of the sprayed coatings. Among the grain-shaped structures to be formed, the unit area of coarse grains having an average diameter of 20 μm or more in a pseudo-circular shape with a circularity of 0.5 or more of the grain-shaped structure calculated by the following formula (1) The number density per unit is less than 2.5 pieces / mm ² .
Circularity = 4π × area / (peripheral length) ² (1)
The sacrificial anode layer preferably has a pore area ratio of 1.0 to 5.0% in the cross section in the thickness direction in a region from the interface with the base material to a depth of at least 100 μm. .

  In this way, by controlling the sprayed coating structure of the sacrificial anode layer, icing due to cooling and cracking due to stress due to temperature differences are suppressed even in the ORV operating environment, preventing corrosion of the sprayed coating in the depth direction. Therefore, no interfacial cracking occurs in the lowermost sprayed coating, and no non-adherent region occurs at the interface between the lowermost sprayed coating and the substrate. As a result, the sacrificial anode layer is hardly cracked or peeled off. Further, by limiting the pore area ratio to a predetermined range, there is no possibility that seawater reaches the surface of the base material through a plurality of pores, so that the base material is more difficult to corrode, and the sacrificial anode layer is cracked. Peeling is less likely to occur.

  According to the ORV heat transfer tube or header tube of the present invention, the sacrificial anode layer is excellent in crack resistance and peel resistance even under the ORV operating environment. As a result, the corrosion resistance and FAC resistance of the ORV can be maintained over a long period of time.

It is the partial schematic explaining an example of an open rack type vaporizer, (a) is a front view, (b) is side sectional drawing. It is a schematic diagram which shows the cross section of the thickness direction of the heat exchanger tube or header pipe | tube of the open rack type vaporizer | carburetor which concerns on this invention.

  Hereinafter, the form for implementing the heat exchanger tube or header tube of the open rack type vaporizer | carburetor which concerns on this invention is demonstrated with reference to drawings.

  As shown in FIGS. 1 (a) and 1 (b), the heat transfer tube 2 or header tube (lower header tube, upper header tube) 3, 4 according to the present invention is used for heat exchange of an open rack type vaporizer (ORV) 10. The panel 1 is constituted. Seawater circulates outside the heat exchange panel 1 (heat transfer tube 2 or header tubes 3 and 4), and liquefied natural gas (LNG) circulates inside. The other structures and functions of the ORV 10 are the same as those described above as an example, and are therefore omitted.

  As shown in FIG. 2, the heat transfer tube 2 or the header tubes 3 and 4 include a base material 21 formed in a tube shape and a sacrificial anode layer 22 covered on at least a part of the outer surface of the base material 21. Prepare. The sacrificial anode layer 22 has a laminated structure in which a plurality of sprayed coatings 22a formed by thermal spraying are laminated on the base material 21, and satisfies the requirements described later. Here, at least a part of the base material 21 is a lower part in the ORV 10 (heat exchange panel 1), in particular, a lower part where the temperature becomes low due to LNG and the flow rate of seawater becomes high, for example, the entire lower header pipe 3 or The lower part of the heat transfer tube 2 in the vicinity thereof. The sacrificial anode layer coated outside this region of the substrate 21 may not necessarily satisfy the requirements of the sacrificial anode layer 22 described later. In the region covering the sacrificial anode layer 22, the heat transfer tube 2 and the header tubes 3 and 4 according to the present invention have the same laminated structure. Below, the element which comprises the heat exchanger tube 2 or the header tubes 3 and 4 is demonstrated.

〔Base material〕
The base material 21 is not particularly limited, but usually a JIS standard 3000 series, 5000 series, or 6000 series aluminum alloy is used, and the shape of the heat transfer pipe 2 or the header pipes 3 and 4 is determined by a known method such as extrusion molding. It is molded. Although the thickness of the base material 21 is not specifically limited, it is shape | molded by the thickness from which required intensity | strength is obtained according to the pipe diameter, length, etc. of the heat exchanger tube 2 (header tubes 3 and 4). The base material 21 is formed into the shape of the heat transfer tube 2 or the header tubes 3, 4, and then welded and assembled into the shape of the heat exchange panel 1. In addition, although the heat exchanger tube 2 or the header tubes 3 and 4 which concern on this invention are each made into the cylindrical shape, it is not limited to this.

  In addition, before the formation (spraying) of the sacrificial anode layer 22 described later, it is preferable to roughen the outer surface of the base material 21 serving as an interface with the sacrificial anode layer 22 by blasting or the like. When the surface of the base material 21 is roughened, the sacrificial anode layer 22 that is a sprayed coating is difficult to peel off. Although the surface property of the base material 21 is not specifically limited, Arithmetic mean roughness Ra: 15-50 micrometers and maximum height roughness Ry: 150-500 micrometers are preferable.

[Sacrificial anode layer]
The sacrificial anode layer 22 is made of an aluminum alloy that is suitable as a thermal spray material and has a lower potential (higher ionization tendency) in seawater than the aluminum alloy that forms the substrate 21. Examples of such an aluminum alloy include an Al—Zn alloy, an Al—Mg alloy, and an Al—Zn—Mg alloy. That is, Zn, Mg, or the like may be added alone or in combination of two or more to Al so as to have a base potential as compared with the potential of the aluminum alloy forming the substrate 21. By constituting the sacrificial anode layer 22 with such an aluminum alloy, the sacrificial anode layer 22 is positively subjected to an anode reaction (M → M ^{n +} + ne ⁻ , M: Al and an additive element, n) in a corrosive environment (in seawater). : Valence), so-called sacrificial anodic properties can be obtained, and corrosion of the substrate 21 can be prevented. The sacrificial anode layer 22 is made of an aluminum alloy having the above components, for example, a linear spraying material (welding material). It is formed by thermal spraying on the surface of the substrate 21 by a thermal spraying method. The sacrificial anode layer 22 has a laminated structure in which a plurality of thermal spray coatings 22a formed by thermal spraying are laminated.

(Sacrificial anode layer thickness: 100 μm or more)
In order to provide a sacrificial anticorrosive action for a long time, the thickness of the sacrificial anode layer 22 is set to 100 μm or more, and preferably 250 μm or more. On the other hand, if the sacrificial anode layer 22 is made thicker, the heat exchange efficiency is lowered and it becomes easy to peel off. Therefore, the thickness is preferably 1000 μm or less, and more preferably 600 μm or less.

  The sacrificial anode layer 22 has the following thermal spray coating structure in a region (depth region) from the interface with the substrate 21 to a depth of at least 100 μm, which has a particularly strong influence on the adhesion to the substrate 21. When the thickness of the sacrificial anode layer 22 exceeds 100 μm, the entire region of the sacrificial anode layer 22 preferably has the following thermal spray coating structure.

(Average film thickness of sprayed coating: 3-8 μm)
In the thermal spraying, the aluminum alloy melted by the heat source is ejected as droplets, collides with the object to be sprayed and deforms, and solidifies while being connected to form the thermal spray coating 22a. The sacrificial anode layer 22 has a stacked structure in which a large number of sprayed coatings 22a formed by spraying are stacked. The thermal spray coating 22a becomes denser as the film thickness is thinner, and is difficult to break due to freezing due to cooling or stress due to a temperature difference. Therefore, the sacrificial anode layer 22 has an average film thickness of 3 to 8 μm in the sprayed coating 22a constituting the sacrificial anode layer 22 in the depth region. Here, the average film thickness is calculated from the film thickness of each sprayed coating 22a and refers to the average value of the film thickness of the plurality of sprayed coatings 22a formed in the depth region.

  When the average film thickness of the thermal spray coating 22a exceeds 8 μm, micro-cracks are likely to occur in the thermal spray coating 22a due to freezing or temperature change. Further, it has been found that the corrosion of the thermal spray coating 22a proceeds along the layer. Even when the film thickness is the same, the thermal spray coating 22a has a film thickness of 3 μm. Corrosion can be delayed. Therefore, the sacrificial anode layer 22 has an average film thickness of the sprayed coating 22a of 8 μm or less, preferably 6 μm or less. However, if the average film thickness of the thermal spray coating 22a is too thin, less than 3 μm, it is too dense, and stress relaxation does not occur and cracking easily occurs. Further, corrosion does not proceed along the layer of the thermal spray coating 22a, and corrosion occurs in the depth direction. Is easier to proceed. Therefore, the sacrificial anode layer 22 has an average thickness of the sprayed coating 22a of 3 μm or more, and preferably 4 μm or more.

  The average film thickness of the thermal spray coating 22a is obtained by cutting the depth region of the sacrificial anode layer 22 in the cross-sectional direction and observing it as a mirror surface with an optical microscope or the like. When it is difficult to observe the laminated structure even in the mirror state, the interlayer is dissolved by further etching with an alkaline chemical, and the laminated structure becomes clearer. For example, what is necessary is just to immerse in the NaCl aqueous solution of a density | concentration of 40 g / l for 1 to 3 minutes. If the etching is performed too much, the entire layer is dissolved.

  The film thickness of the thermal spray coating 22a depends on the type of thermal spray material, the thermal source of the thermal spray, the construction conditions, and the like. The film thickness of the sprayed coating 22a is considered to be larger as the molten droplet is larger and the molten droplet is connected. Moreover, it is thought that it becomes thick, so that the deformation | transformation on a spraying target object is small. Therefore, it is considered that the film thickness can be reduced by controlling the thermal spraying conditions to reduce the amount and size of the molten droplets and to suppress the solidification before colliding with the object to be sprayed. For example, when the temperature of the heat source is lower, the melted droplets are smaller and the film thickness becomes thinner. Further, the film thickness is reduced by increasing the spray gun speed and decreasing the supply amount of the spray material. Alternatively, by increasing the injection speed and shortening the spraying distance, the film thickness is reduced by suppressing solidification until it collides with the object to be sprayed. Further, if the collision angle is too shallow with respect to the object to be sprayed, the deformation on the object to be sprayed is small and the film thickness is increased.

  Such knowledge is greatly different from the concept of construction conditions in normal thermal spraying. For example, with respect to the spraying distance, in particular, when the object to be sprayed is large and the shape is complicated, for example, the base material 21 assembled in the shape of the heat exchange panel 1 as in the present invention is uniform over a wide area. In general, it has been common to increase the spraying distance, specifically, at least about 300 mm. Moreover, since thermal distortion occurs in the object to be sprayed in short-distance spraying, it is necessary to provide a certain spraying distance. However, an aluminum alloy sprayed coating having a spraying distance of about 300 mm or more increases the film thickness. Alternatively, for example, the higher the temperature of the heat source and the larger the supply amount of the spray material, the higher the spraying speed, so that a desired film thickness can be obtained in a short time. Therefore, it is common to construct with a high temperature heat source at a high supply rate.

  Although the heat transfer tube 2 and the header tubes 3 and 4 of the ORV 10 according to the present invention do not limit the thermal spraying conditions in the formation of the sacrificial anode layer 22 (thermal spray coating 22a) for the manufacture, the thickness of the sacrificial anode layer 22 It is desirable to set the spray distance, the heat source, the spray gun speed, and the supply amount of the spray material so that the average film thickness of the spray coating 22a falls within the range of the present invention. Specifically, the spraying distance is preferably 200 mm or less, propane is preferably used as the heat source, and the spraying angle is preferably in the range of 45 ° to 90 °. Further, the spray gun speed and the amount of spray material supplied are determined in an appropriate range in consideration of the number density of coarse particles and the pore area ratio described later. In order to easily control the spray gun speed and spray angle, the heat exchange panel 1 can be flattened rather than placed in a vertical position and the spray gun moved up and down and sprayed horizontally. It is preferable to place the spray gun horizontally and spray it in the downward direction.

(Number density of coarse particles: less than 2.5 / mm ² )
In the thermal spraying, the aluminum alloy molten droplets that have been solidified by the time when they are insufficiently melted or collide with the object to be sprayed are insufficiently deformed on the object to be sprayed, and are close to grains without forming the sprayed coating 22a. It becomes a tissue of shape. And this grain-shaped structure | tissue is formed in the state pinched | interposed between the laminated structures of the thermal spray coating 22a, and becomes a part of thermal spray coating 22a. Stress tends to concentrate between the thermal spray coating 22a and the grain-shaped structure, and the thermal spray coating 22a is likely to break due to freezing due to cooling or stress due to a temperature difference. As a result, pores are formed between the thermal spray coating 22a and the grain-shaped structure, and serve as a seawater infiltration path, which promotes corrosion in the depth direction. In particular, among the grain-shaped structures, the coarse grains 23 having a quasi-circular shape with a roundness of 0.5 or more in the grain-shaped structure and an average diameter of 20 μm or more easily form microcracks and form pores. Cheap. Here, the circularity means that calculated by the following formula (1).
Circularity = 4π × area / (peripheral length) ² (1)
In the sacrificial anode layer 22 according to the present invention, the number density of the coarse particles 23 having the shape and size in the depth region is set to less than 2.5 / mm ² to suppress pore formation due to cracking. Preferably it is less than 2 pieces / mm ² , more preferably less than 1.5 pieces / mm ² .

  The number density of the coarse particles 23 can also be obtained by cutting the depth region of the sacrificial anode layer 22 in the cross-sectional direction and observing it as a mirror surface with an optical microscope or the like. Similarly, when it is difficult to observe the laminated structure even in the mirror state, it may be further etched with an alkaline chemical. From the observed image, it is possible to determine whether it corresponds to the corresponding coarse particle 23 by analyzing the average diameter and the circularity with image software.

  Further, the number density of the coarse particles 23 of the thermal spray coating 22a also depends on the type of thermal spray material, the thermal source of the thermal spray, construction conditions, and the like. It is considered that the smaller the deformation on the object to be sprayed, the higher the probability of becoming coarse particles 23. In addition, droplets that bounce off the object to be sprayed also become coarse particles 23. Therefore, by shortening the spraying distance, the coarse particles 23 can be reduced by suppressing solidification until they collide with the object to be sprayed. In addition, when the spray angle is shallow, more droplets collide and bounce back, but they are rarely taken into the sprayed coating 22a. On the other hand, when the spray angle is deep, few droplets bounce off and collide, The film 22a is often taken in. The spraying angle is set to 20 to 70 degrees, the rebounding direction of the spraying gun is set to the opposite direction, the rebounding droplet is captured by a dummy plate, the rebounding droplet is removed with high-pressure water or the like for each pass, etc. Is effective.

  In addition, when the spray angle is constant, if the spray gun speed is too high or the supply amount of the spray material is too small, the spray coating 22a is formed in a mottle, and a step is generated in the gradually formed spray coating 22a. A part that becomes a lizard with respect to the spray gun is formed, and coarse pores are formed, or coarse particles 23 are easily formed by spraying at a very shallow spray angle. This can be suppressed by spraying at two or more spray angles. Thus, in order for both the average film thickness of the sprayed coating 22a and the number density of the coarse particles 23 to fall within the scope of the present invention, it is necessary to determine the optimum range of spraying conditions.

  Such knowledge is also very different from the concept of construction conditions in normal thermal spraying. In a member used in a normal environment, even if there are coarse particles 23 in the sprayed coating, the thermal stress on the member is small, so it does not crack and become pores. Therefore, it is common to increase the spraying distance. In addition, spraying is generally performed at an angle close to 90 degrees to reduce rebound, and spraying at two or more spraying angles, inserting dummy plates, and cleaning removal between spraying passes Is unnecessary.

  Next, the preferable form for implementing the heat transfer pipe | tube or header pipe | tube of ORV which concerns on this invention is demonstrated. In the heat transfer tube 2 or the header tubes 3, 4, the pore area ratio of the pores 24 in the cross section in the thickness direction of the sacrificial anode layer 22 is preferably within a predetermined range.

(Porosity area ratio of pores in cross section in thickness direction: 1.0 to 5.0%)
Since the sacrificial anode layer 22 is formed by thermal spraying, it has a structure including a certain amount of pores 24 inside. In the cross-sectional view of FIG. 2, each of the pores 24 is shown as being divided, but in reality, there are many that communicate with other pores 24. When the number of pores 24 is large or the size of each pore 24 is increased, the surface of the sacrificial anode layer 22 is connected to the interface with the substrate 21 through the plurality of pores 24, so that seawater is a substrate. There is a risk of reaching the surface of 21. Therefore, in order to suppress the progress of corrosion, the sacrificial anode layer 22 preferably has the upper limit value of the pore area ratio of the pores 24 in the cross section in the thickness direction in the depth region being 5.0% or less.

  On the other hand, since the pores 24 have a function of relaxing the volume change of the sacrificial anode layer 22 due to temperature change, if the sacrificial anode layer 22 has an extremely small number of pores 24 in the vicinity of the interface with the base material 21, It becomes easy to produce a microcrack on the contrary with a temperature change. Therefore, in the sacrificial anode layer 22, in the depth region, the lower limit value of the pore area ratio of the pores 24 in the cross section in the thickness direction is preferably 1.0% or more.

  The pore area ratio of the pores 24 in the sacrificial anode layer 22 is in a region (depth region) from the interface with the base material 21 to a depth of at least 100 μm. A pore area ratio in the range is more preferable. Moreover, the sacrificial anode layer 22 may be filled with the pores 24 by impregnating an epoxy resin or the like from the surface as a sealing agent after formation (thermal spraying).

  The pore area ratio of the sacrificial anode layer 22 may be obtained by slicing the depth region of the sacrificial anode layer 22 in the cross-sectional direction, treating it appropriately, such as mirror polishing, as a mirror surface, and observing it with an optical microscope. Even if the pores 24 are hollow or filled with a sealing agent, the color looks different from the aluminum alloy portion of the sacrificial anode layer 22, so a photograph taken at a magnification of 100 times with an optical microscope, for example, is an image By analyzing, the area ratio can be calculated. The observation of the pores 24 is performed without performing the above-described etching.

  The pore area ratio of the sacrificial anode layer 22 also changes depending on the spraying conditions. For example, the pore area ratio decreases when the spraying distance is small, and the pore area ratio increases when the spray angle is small. In addition, by performing shot blasting on the surface of the sprayed coating, it is possible to reduce the pore area ratio by mechanically crushing the pores 24 in the vicinity of the surface, so by performing shot blasting etc. after one spraying pass, The pore area ratio in the depth region of the sacrificial anode layer 22 can also be reduced.

  Next, an example in which the effect of the present invention has been confirmed will be specifically described in comparison with a comparative example that does not satisfy the requirements of the present invention.

[Sample preparation]
Instead of the heat transfer tube or the header tube, the following specimens were produced.
As a base material, a 5 mm thick plate material of A5083 alloy was cut into 35 mm × 100 mm and used. One side of the substrate was roughened with shot blasting (blast particles: alumina # 16 to 20) to an arithmetic average roughness Ra of 20 to 40 μm, and the blast particles were removed until it could not be visually confirmed. On the roughened surface of this base material, a sacrificial anode layer having a thickness of 500 μm composed of a plurality of sprayed coatings is formed by a wire-type flame spraying method using an Al-2 mass% Zn alloy wire. did. Here, as the spraying conditions, the spraying distance is 150 mm, and as shown in Table 1, the heat source is propane or acetylene, the spraying angle is constant 30 degrees, or the repetition of 45 degrees and 30 degrees, the wire feed rate (spraying) The material supply speed was 3000 mm / min, 2000 mm / min, and 1000 mm / min. Some test materials were washed with 7.5 MPa high-pressure water and shot blasted for each thermal spray pass.

  About each obtained test material, the porosity area ratio of a sacrificial anode layer, the average film thickness of a sprayed coating, and the number density of a coarse grain were measured with the following method, and the result is shown in Table 1.

(Pore area ratio)
The test material was cut out, the cut surface in the thickness direction was polished, and this cut surface was photographed at five locations with an optical microscope at a magnification of 100 (imaging range: 1 mm × 1 mm). For this photograph, the image is binarized using image analysis software (ImageJ), and the area ratio of the pores in the region from the interface with the base material of the sacrificial anode layer to the depth of 100 μm is calculated, and 5 fields of view are calculated. Was the pore area ratio.

(Average film thickness of sprayed coating, number density of coarse particles)
Etching was performed by immersing the same cut surface as described above in a NaCl aqueous solution having a concentration of 40 g / l. Etching was repeatedly performed while observing at a magnification of 100 times with an optical microscope, and five locations were photographed at a magnification of 100 (imaging range: 1 mm × 1 mm) in a state in which the laminated structure of the sprayed coating was easily confirmed. The total etching time was 1 to 3 minutes. For this photograph, using image analysis software (ImageJ), for each image, measure the film thickness of the sprayed coating at five points in the region from the interface with the substrate to a depth of 100 μm, and average The average film thickness was determined. In addition, for 5 images, the area and circumference were analyzed for all grain-shaped structures in the region from the interface with the substrate to a depth of 100 μm, the circularity was 0.5 or more, and the average diameter Was determined to be a coarse particle having a particle size of 20 μm or more, the number of coarse particles was counted, and the number per 1 mm length was calculated to obtain the number density.

  Next, the following thermal cycle tests were performed on the obtained test materials, and the crack resistance and peel resistance were evaluated as follows. The results are shown in Table 1.

(Thermal cycle test)
In order to reproduce the environment including the heat cycle when operated in seawater as an ORV, the following tests were performed on five specimens of each specification. The surface of the sacrificial anode film of the test material is sprayed with artificial seawater (aquamarine for metal corrosion test manufactured by Yashima Co., Ltd.) adjusted to pH 8.2 and liquid temperature of 35 ° C., and once a day by LNG In order to simulate the cooling of the base material, the process of immersing and cooling only the base material in liquid nitrogen was performed for a total of 8 weeks. After the test, the test material was cut out, the cut surface in the thickness direction was polished, and the following evaluation was performed using this surface as an observation surface.

(Crack resistance: Observation and evaluation of cracks)
The cut surface of the specimen was observed at 400 times with an optical microscope (observation range: 1 mm × 1 mm), and the presence or absence of what could be identified as a crack was evaluated. ○ (excellent) when there is no crack with a linear distance of 2.5 μm or more and less than 10 μm, △ (good) when there is a crack with a linear distance of 2.5 μm or more and less than 10 μm, and there is a crack with a linear distance of 10 μm or more. The thing to do was made into x (defect).

(Peeling resistance: observation and evaluation of interfacial peeling)
The visual field range of 1 mm × 1 mm including the interface between the sacrificial anode layer and the base material was observed 400 times with an optical microscope. With the interface parallel to the field of view, 100 observation points were set at an interval of 10 μm for an interface length of 1 mm, and adhesion or non-adhesion of the interface was determined at each observation point. Here, when there is a non-contact observation point up to three observation points next to the non-contact observation point, the close observation point between the two non-contact points is also counted as a non-contact point. The number of non-contact points / the total number of observation points (100 points) was defined as the non-contact interface ratio (%). For each specification, 5 random fields of view were observed, and the interfacial peeling was evaluated by the average value of the non-adhesive interface rate (%). When the non-adhesion interface ratio exceeds 30%, it is known that the sacrificial anode layer exfoliates at an early stage even in the actual machine. Therefore, when the non-adhesion interface ratio exceeds 30%, x (defect), the non-adhesion interface ratio is 20-30% was set as Δ (good), and less than 20% was set as ○ (excellent).

  As shown in Table 1, the sacrificial anode layer satisfying the requirements of the present invention is formed by controlling the spraying conditions to control the average film thickness of the sprayed coating, the number density of coarse grains, and the pore area ratio of the sacrificial anode layer. We were able to. And test material No. whose average film thickness and number density satisfy the requirements of this invention. 1-5 (Examples) showed good crack resistance and peel resistance. Among them, the test material No. Nos. 1 to 3 (Examples) exhibited excellent peel resistance because the pore area ratio also satisfied the requirements of the present invention. In contrast, at least one of the average film thickness and number density does not satisfy the requirements of the present invention. 6-11 (comparative example) were cracked and peeled in the vicinity of the interface with the substrate after the thermal cycle test, and were inferior in crack resistance and peel resistance.

  As mentioned above, although the form for implementing this invention and its Example have been described, this invention is not restrict | limited by the said description, It implements by adding a change suitably within the range shown to the claim. All of which are within the scope of the present invention.

1 Heat Exchange Panel 2 Heat Transfer Tube 3 Lower Header Tube 4 Upper Header Tube 10 ORV
21 Substrate 22 Sacrificial anode layer 22a Thermal spray coating 23 Coarse grain 24 Pore

Claims (2)

In the heat transfer tube or header tube of the open rack type vaporizer that constitutes the heat exchange panel of the open rack type vaporizer that vaporizes the liquefied natural gas flowing inside by heat exchange with the seawater supplied to the outer surface,
A base material made of an aluminum alloy, and a sacrificial anode layer having a thickness of 100 μm or more made of an aluminum alloy coated on at least a part of the outer surface of the base material and having a sacrificial anode property with respect to the base material,
The sacrificial anode layer has a laminated structure in which a plurality of thermal spray coatings formed by thermal spraying are laminated,
The sacrificial anode layer has a thickness in a range of 3 to 8 μm in average in the region from the interface with the base material to a depth of at least 100 μm in a cross section in the thickness direction, and
Among the grain-shaped structures formed in a state of being sandwiched between the laminated structures of the thermal spray coating, the circularity of the grain-shaped structure calculated by the following formula (1) is a pseudo-circular shape having a circularity of 0.5 or more, and the average A heat transfer tube or a header tube of an open rack type vaporizer, wherein the number density per unit area of coarse particles having a diameter of 20 μm or more is less than 2.5 pieces / mm ² .
Circularity = 4π × area / (peripheral length) ² (1)   The sacrificial anode layer has a pore area ratio of 1.0 to 5.0% in the cross section in the thickness direction in a region from the interface with the base material to a depth of at least 100 μm. The heat transfer tube or header tube of the open rack type vaporizer according to claim 1.

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