JP4464762B2 - Al alloy member for liquefied gas vaporizer with excellent corrosion resistance and liquefied gas vaporizer - Google Patents

Description

  The present invention relates to an aluminum alloy member excellent in corrosion resistance and applied to a liquefied gas vaporizer using the aluminum alloy member, which is applied as a constituent material (such as a heat transfer tube) for a liquefied gas vaporizer such as an open rack vaporizer. is there.

  Liquefied natural gas (hereinafter referred to as LNG), which is widely used as a liquefied gas for various fuels, is usually transferred and stored in a liquid state at a low temperature and high pressure, and is vaporized in advance when used. In order to vaporize this large amount of LNG, an open rack vaporizer (hereinafter referred to as ORV) has been conventionally used. This ORV is a kind of heat exchanger, and heats and vaporizes LNG by heat exchange with seawater. That is, the seawater is stored in the trough from the watering nozzle, and droops while wetting the outside of the panel (heat transfer tube) from both side edges, while the LNG sent to the panel (heat transfer tube) exchanges heat with the seawater. And is vaporized in the panel (heat transfer tube).

  This ORV member, particularly a panel (heat transfer tube) member, is made of an Al alloy such as JIS 3203, which has good thermal conductivity, can be easily processed into a complicated shape for increasing the heat transfer area, and has good weldability. Yes. However, the Al alloy originally has a drawback that it is easily corroded when immersed in seawater, and once erosion starts, the eroded portion is intensively eroded, and there is a drawback that it is susceptible to so-called pitting corrosion.

  For this reason, anti-corrosion treatment has been actively studied for Al alloys used in the above-mentioned applications, and currently, anti-corrosion methods using sacrificial anti-corrosive action occupy the mainstream. In this sacrificial anticorrosion, the surface of a base material alloy made of an Al alloy is coated with an alloy that is more easily corroded than the base material alloy, that is, has a higher ionization tendency and has a lower corrosion potential in seawater than the base material alloy. . As a result, the corrosion resistance of the base alloy does not directly contact the seawater due to the coating action of the coating alloy at the beginning, and the corrosion resistance is maintained. Even so, the sacrificial anticorrosive action of the remaining coating alloy ensures the long-term corrosion resistance of the base alloy.

  Conventionally, an alloy of Al and Zn is known as a coating alloy having the sacrificial anticorrosive action, and alloys such as Al-2% Zn and Al-3% Zn are used. As a method for coating the base material alloy with this alloy, for example, a method (for example, Patent Document 1) is proposed in which an Al—Zn alloy is clad by a thermal spraying method or the like to extend the surface protection.

  Further, for the purpose of further enhancing the sacrificial anticorrosive action, an Al—Zn-coated alloy containing a trace amount of Hg, Sn, In, Ga, Cd or the like or Al—Zn content of 3.5 to 85.0% is used. An alloy using a Zn-coated alloy (such as Patent Document 2) has been proposed. This technique can further enhance the sacrificial anticorrosive action as compared with the coating alloy such as Al-2% Zn, Al-3% Zn, etc., since the Zn content is increased and the trace element is added.

  However, the ORV has been improved in efficiency in recent years, and tends to be large in size or continuously operated for a long time. For this reason, durability and reliability, such as an increase in the flow velocity of the fall seawater by the enlargement of the member which comprises ORV, and the long-time corrosion resistance which endures the driving | running for continuous prolonged time are required. In this regard, the present inventors have found that the coating alloys such as Al-2% Zn and Al-3% Zn and Al- in which the Zn content is 3.5 to 85.0%. It is recognized that the following two problems occur even in an Al alloy member coated with a Zn coating alloy or the like.

  [1] First, when the Zn composition of the Al—Zn sprayed coating is low, the surface of the sprayed coating is easily passivated, so that the natural potential is increased and the sacrificial anticorrosive effect cannot be sufficiently obtained. Due to the passivation of the surface of the Al—Zn sprayed film, as shown in FIG. 3, when water enters the interface defect portion with the Al alloy member, the interface defect becomes low in potential due to lack of dissolved oxygen. The reversal of the potential occurs between the active surface in the vicinity and the passive state portion on the outermost surface of the sprayed coating where the potential becomes high, and the interface portion is preferentially dissolved, leading to interface destruction. The Zn composition that begins to passivate changes depending on the environment.

[2] Next, when the Al-Zn sprayed coating has a high Zn composition, the surface of the sprayed coating is not passivated, so the problem [1] does not occur. However, as shown in FIG. When the Al base material is exposed, the surface of the Al base material near the sprayed film is excessively cathode-polarized, and a large amount of OH-ion is generated due to excessive reduction reaction of dissolved oxygen on the surface of the Al base material. Happens. In addition, as shown in FIG. 5, the passive region of Al is in the range of pH 4 to 8 at 25 ° C., and the passive film disappears when the pH exceeds 8 and therefore cathodic corrosion occurs in seawater at room temperature. Has been shown to do. The potential at which such cathode corrosion occurs varies depending on various conditions such as seawater temperature and flow velocity. Preferential damage to the alloy base material will occur.
Japanese Patent Laid-Open No. 5-16496 JP-A-4-60393 JP-A-6-317392

  The present invention solves the above-mentioned problems [1] and [2], and its purpose is an Al alloy for a liquefied gas vaporizer that can guarantee long-term corrosion resistance even in a corrosive environment such as seawater. The member is to be provided.

  The present invention has been completed to achieve such an object, and the gist of the present invention is as follows.

(1) An Al alloy member for a liquefied gas vaporizer having an Al—Zn alloy film having a thickness of 100 μm or more formed on a surface thereof, wherein the Al—Zn alloy film has a region from the interface with the Al alloy material to a thickness of 50 μm ( Hereinafter, the average Zn composition of the interface layer) is 0.3 to 10%, and the average Zn composition of the region from the outermost surface of the Al—Zn alloy film to a depth of 50 μm (hereinafter referred to as the surface layer) is 3.3%. An Al alloy member for a liquefied gas vaporizer excellent in corrosion resistance, characterized in that the Zn composition of the surface layer is 3% or more larger than the Zn composition of the interface layer.

(2) Average Zn composition from 0.3 to 5% of the interfacial layer of Al-Zn alloy coating, the average Zn composition of the surface layer of the Al-Zn alloy coating 10 to 85%, and Zn composition of the surface layer The Al alloy member for a liquefied gas vaporizer having excellent corrosion resistance according to the above (1) , which is 5% or more larger than the Zn composition of the interface layer.

  (3) The Al alloy member excellent in corrosion resistance according to the above (1) or (2), wherein the liquefied gas vaporizer is an open rack vaporizer.

  (4) A liquefied gas vaporizer comprising the Al alloy member according to any one of (1) to (3) above.

  According to the present invention, it is possible to maintain and continue the effective and stable sacrificial anticorrosive action by forming the Al—Zn alloy film having the unique configuration described in the above solution on the Al alloy member. Therefore, it exhibits excellent corrosion resistance as a constituent material for a liquefied gas vaporizer such as ORV in a corrosive environment in contact with seawater, and can guarantee its long-term use.

As a result of intensive experiments and studies to solve the above problems, the inventors of the present invention are Al alloy members having an Al—Zn alloy sprayed coating having a thickness of 100 μm or more formed on the surface, the Al—Zn alloy coating. The average Zn composition in the region (interface layer) from the interface with the Al alloy member to 50 μm in thickness is 0.3 to 10%, more preferably 0.3 to 5%. The average Zn composition in the region (surface layer) up to 50 μm is 3.3 to 85%, more preferably 10 to 85%, and the Zn composition of the surface layer is 3% or more than the Zn composition of the interface layer. By applying an Al alloy member that is preferably 5% or larger as a constituent material of a liquefied gas vaporizer such as ORV, it exhibits excellent corrosion resistance even in a corrosive environment in contact with seawater and extends the useful life of the vaporizer. Confirmed that it could be.

FIG. 1 is a cross-sectional view illustrating the configuration of an Al alloy member according to the present invention. Here, 1 is an Al alloy base material, and an Al—Zn alloy film 2 having a thickness of 100 μm or more formed by a film forming means such as thermal spraying is formed on the surface of the base material 1. This alloy film 2 is located on the interface side with the Al alloy base material, and is located in the region in the range from the interface to a thickness of 50 μm, that is, the Al—Zn interface layer A, on the outermost surface side, and deeper from the outermost surface. A region in the range of up to 50 μm, that is, an Al—Zn surface layer B, and an Al—Zn intermediate layer C located between the interface layer A and the surface layer B.

  In the present invention, the average Zn composition of the Al—Zn interface layer A is 0.3 to 10%, the average Zn composition of the Al—Zn surface layer B is 3.3 to 85%, and the Al The Zn composition of both layers is adjusted and maintained so that the average Zn composition of the -Zn surface layer B is 3% or more higher than the average Zn composition of the Al-Zn interface layer A.

  Thus, by making the composition of the Al—Zn surface layer B of the Al—Zn alloy film 2 3% or more higher than the Zn composition of the Al—Zn interface layer A, the potential of the outermost surface of the film 2 becomes the interface portion. It is possible to prevent the potential from becoming excessively higher than the potential (= preferential dissolution of the interface portion).

  Moreover, the area | region which touches Al alloy base material 1 can suppress the cathode corrosion by the electric potential of Al alloy base material 1 falling too much by making Zn composition of the interface layer A 10% or less. In order to exhibit such an anticorrosive effect of the Al—Zn alloy film 2, the thickness of the interface layer A and the surface layer B must be 50 μm or more, respectively, and the total thickness must be 100 μm or more. is there. There is no particular upper limit, but if it is too thick, blisters and cracks are likely to occur, so 1 mm is a guide for the upper limit.

  The vaporizer incorporating the Al alloy member of the present invention suppresses the peeling damage of the coating 2 due to corrosion of the header part, and in the event of peeling, obtains the effect that the erosion of the Al alloy base material 1 is suppressed. Can do.

  On the Al alloy base material 1 side where cathode corrosion may occur, the Zn composition is lowered to suppress this, and the outermost surface of the film may promote preferential dissolution and peeling at the interface of the film 2 by passivation. This surface layer B suppresses this by increasing the Zn composition. At this time, the lower limit of the Zn composition of the interface layer A in the base material side film is set to 0.3% in order to make the corrosion potential lower than that of the base material to exert the sacrificial anticorrosive action. In addition, the Zn concentration of the surface layer B needs to be 3% or more higher than the Zn concentration of the interface layer A on the base material side. However, if there is too much Zn, the elution rate of the surface side layer increases, and the coating takes a short time. Therefore, the upper limit is set to 85%.

  Which Zn composition causes cathodic corrosion and which composition promotes delamination due to passivation takes different values depending on seawater components, flow velocity, and temperature, so the composition range of interface layer A and surface layer B is optimal. Although the values could not be determined uniformly, it was found by various experiments that this effect can be achieved in any environment by setting the Zn composition of the surface layer part to be 3% or more higher than the interface part.

  By the way, for the intermediate layer C sandwiched between the interface layer A and the surface layer portion B of the Al—Zn alloy film 2 having a thickness (or depth) of 50 μm in FIG. It is desirable to be in the intermediate composition range of B. However, the present invention is not limited to this Al—Zn composition, and even if an independent layer having a different type of composition is inserted as the intermediate layer C, it is a conductive material, and is an electrochemical layer between the interface layer A and the surface layer B. Since the effect of the present invention can be obtained as long as it exhibits the desired characteristics, it is included in the technical scope of the present invention.

  Here, the electrochemical characteristics are natural potentials in seawater. For example, when other elements such as Mg and Ti are added to an Al—Zn alloy, the electrochemical characteristics are intermediate between the interface layer A and the surface layer B. What is necessary is just to adjust the composition of the intermediate | middle layer C so that it may become an electric potential. In order to directly confirm this, when the in-plane distribution of the natural potential is directly measured using a scanning vibration electrode (SVET) or a Kelvin probe by cutting a cross section of the sprayed film, the aluminum alloy base material 1-interface layer A- If the potential gradually decreases from the intermediate layer C to the surface layer B, the necessary requirements are satisfied.

  Further, the intermediate layer C is not necessarily essential in the present invention, and it is needless to say that the intermediate layer C is composed only of the interface layer A and the surface layer B within the scope of the present invention.

  Examples are given below to demonstrate the excellent effects of the present invention. In addition, the following Example is a result by a specific seawater component, flow velocity, and temperature to the last, and does not restrict | limit the technical scope of this invention at all.

(Example)
On the Al alloy A5083 base material, a sprayed film with a Zn composition changed at a thickness of 100 μm was laminated at the interface, and a sprayed film with a Zn composition changed at a thickness of 100 μm was laminated thereon. The region from the interface of the sprayed film on the interface side to the 50 μm thick portion, that is, the interface layer, and the surface layer portion, the region from the outermost surface of the sprayed film to the depth of 50 μm, that is, the surface layer is analyzed by SEM-EDX cross-sectional observation The average Zn composition in each region was confirmed. The field of view is preferably 300 μm square, and the thickness is measured on the SEM-EDX cross-sectional image, but if the substrate has irregularities at the interface, as shown in the schematic diagram of FIG. When the surface unevenness is in the range from the intermediate line (AA ′) of the uneven surface of the interface between the heat transfer tube substrate and the sacrificial anticorrosive metal coating layer to the surface direction by 50 μm, The range extends from the center line (AA ′) to a position 50 μm away from the center line. At this time, the composition of the intermediate layer between the interface layer and the surface layer is prepared so that the interface layer side has the same composition as the interface layer, and the surface layer side has the gradient composition that is the same composition as the surface layer. did.

Exposure test 1
The spray coating sample was immersed in artificial seawater at 0 ° C., ph 8.2, and a flow rate of 3 m / sec for 3 months. The obtained results are indicated by symbols (◎, ○, Δ, and x) in the upper part of each sample in Tables 1 and 2. Here, each symbol represents that the thermal spray interface peeling occurrence area ratio after the test is in the following range.
◎: 0% ○: 10% or less △: 10-30% ×: 30% or more

  Tables 1 and 2 show that it is effective that the Zn composition of the surface layer of the sprayed film is 3% or more, preferably 4% or more, more preferably 5% or more higher than the interface layer. When the surface layer Zn is 90%, the film dissolution rate is remarkably high and the surface layer film peels off and disappears. Therefore, Zn is preferably 85% or less.

Exposure test 2
100 holes with a diameter of 2 mm reaching the surface of the aluminum base material through the surface layer and the interface layer are formed in the sprayed film at 0 ° C., pH 8.2, flow rate 3 m / sec. After being immersed in the artificial seawater for 3 months, the number of holes in which pits proceeding to a depth of 0.5 mm or more into the aluminum base material was examined. The obtained results are indicated by symbols (◎, ○, Δ, and x) at the bottom of each sample in Tables 1 and 2. Here, each symbol represents that the number of holes having a depth of 0.5 mm or more to the inside of the aluminum base material after the test is in the following range.
◎: None ○ 10 or less △: 10 to 30 ×: 30 or more From Tables 1 and 2, when the Zn composition is 15%, the dissolution of the base material proceeds due to cathodic corrosion, so the Zn composition of the interface layer is 10% or less. Preferably, 5% or less, more preferably 2% or less is effective. However, if the Zn composition is 0.3% or less, the sacrificial anticorrosive effect of the sprayed film cannot be obtained, so that the base material is dissolved. Therefore, the Zn composition of the interface layer is 0.3% or more, preferably 0.5% or more.

  In the present invention, the effect cannot be obtained unless the thicknesses of the interface layer having a Zn composition of 10% or less and the surface layer having a Zn composition of 3% Zn or more larger than the interface layer are ensured over 50 μm.

  Next, as shown in Table 3, when the thickness of the interface layer maintaining the average Zn composition at 2% is 40 μm, 30 μm, and 20 μm, the Zn composition of the surface layer exceeds 2% than this thickness. Since the portion approaches the base material, the effect of suppressing damage to the base material is reduced due to the electrochemical influence on the base material.

  On the other hand, as shown in Table 3, when the thickness of the surface layer that maintains the Zn 7% composition 5% larger than the Zn composition 2% of the interface layer is 40 μm, 30 μm, and 20 μm, Although the activation can be prevented, the portion having a high Zn composition of the surface layer is worn out at an early stage in the corrosion stage where the peeling of the sprayed coating becomes a problem, so that the peeling resistance effect cannot be exhibited.

It is sectional drawing explaining the structure of the Al alloy member concerning this invention. In the measurement of the thickness of each layer in the Example of this invention, it is a schematic diagram for demonstrating the measurement reference | standard when there exists an unevenness | corrugation. It is a schematic diagram explaining the mechanism of corrosion in the member which formed the thermal spray coating with low Zn composition concentration in Al alloy base material. It is a schematic diagram explaining the mechanism of corrosion in the member which formed the thermal spray coating with high Zn composition concentration on Al alloy base material. Is a graph showing the relationship between potential and pH of the al-H ₂ O system.

Explanation of symbols

1: Al alloy base material 2: Al—Zn alloy film A: Al—Zn interface layer B: Al—Zn surface layer C: Al—Zn intermediate layer

Claims (4)

An Al alloy member for a liquefied gas vaporizer having an Al—Zn alloy film having a thickness of 100 μm or more formed on a surface thereof, wherein the Al—Zn alloy film has a region from the interface with the Al alloy material to a thickness of 50 μm (hereinafter referred to as an interface). The average Zn composition of the layer is 0.3 to 10%, and the average Zn composition in the region from the outermost surface of the Al—Zn alloy film to a depth of 50 μm (hereinafter referred to as the surface layer) is 3.3% to 85%. An Al alloy member for a liquefied gas vaporizer excellent in corrosion resistance, wherein the Zn composition of the surface layer is 3% or more larger than the Zn composition of the interface layer. Al-Zn average Zn composition of the interfacial layer of the alloy film is 0.3 to 5%, Al-Zn average Zn composition of the surface layer of the alloy coating is 10 to 85%, Zn composition the interface layer of the surface layer of those 5% or more greater than the Zn composition, corrosion resistance superior liquefied gas vaporizing dexterity Al alloy member according to claim 1.   The Al alloy member excellent in corrosion resistance according to claim 1 or 2, wherein the liquefied gas vaporizer is an open rack vaporizer.   The liquefied gas vaporizer provided with the Al alloy member in any one of Claims 1-3.

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