Repairing method for metal anticorrosive coating of liquefied natural gas open rack gasifier

Abstract

The invention relates to a method for repairing a metal anti-corrosion coating of an open rack liquefied natural gas gasifier, which comprises the following steps of: performing electric spark deposition on the metal anti-corrosion coating in a to-be-repaired area, so that the metal anti-corrosion coating is metallurgically bonded with a base body and covers the exposed base body, and the repairing of the metal anti-corrosion coating of the gasifier is completed. By means of the repairing method, the locally-falling metal coating can be quickly and efficiently repaired, metallurgical bonding of the repaired metal anti-corrosion coating and the base body is achieved, the bonding strength between the coating and the base body is improved, the service life of the overall anti-corrosion coating of the heat exchange part of the open-frame vaporizer is prolonged, the maintenance cost and time are reduced, and the service life of the open-frame vaporizer is prolonged. And the safe operation reliability of the equipment is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of coating repair, and in particular to a method for repairing a metal anti-corrosion coating of a liquefied natural gas open-rack gasifier. Background Art

[0002] Open-rack vaporizers are baseload vaporizers installed at liquefied natural gas (LNG) receiving stations in coastal areas. They use seawater to vaporize the low-temperature LNG at the receiving station, superheating it to above 0°C as natural gas (NG). The gas is then transported to users via the natural gas pipeline network. The open-rack vaporizer consists of several aluminum alloy heat exchange tubes arranged in parallel to form a plate-shaped tube array. The upper and lower ends are welded to the gas and liquid manifolds, forming a plate-shaped tube bundle. Several plate-shaped tube bundles then form the vaporizer. A seawater film flows downward from the upper end along the vertical heat exchange panels, heating and vaporizing the LNG within the heat exchange tubes. Coastal areas have abundant seawater resources, and using seawater as a heat source to heat and vaporize LNG offers low operating costs and zero carbon emissions.

[0003] The heat exchange portion of an open-frame gasifier is made of aluminum alloy. Aluminum alloy has excellent low-temperature mechanical properties and thermal conductivity, but its resistance to seawater corrosion is poor. Therefore, the outer surface in contact with seawater is protected with an Al-Zn alloy anti-corrosion coating. This sacrificial metal coating, which acts as the anode, prevents seawater from corroding the gasifier substrate, thereby protecting the cathode. The Al-Zn alloy anti-corrosion coating is typically prepared using flame thermal spraying, where the alloy is melted and sprayed onto the surface of the open-frame gasifier. The flame spraying process introduces a certain amount of pores into the coating, with a porosity of approximately 5-15%. These porous coatings require sealing with a sealant, resulting in a metal coating thickness of 0.2-0.5 mm. It is worth noting that the flame-sprayed Al-Zn alloy coating forms a non-metallurgical bond with the substrate, resulting in a low bond strength, typically only around 7 MPa. This low bond strength impacts the coating's durability.

[0004] After the open-frame vaporizer has been in operation for a period of time, the Al-Zn alloy coating in the low-temperature zone at the bottom of the heat exchange part (the area within 0.4 meters upward from the bottom of the LNG collecting pipe) will experience problems such as bulging and severe shedding. This will cause many localized shedding areas of the metal coating on the surface of the vaporizer, significantly reducing the local seawater corrosion resistance of the vaporizer's aluminum alloy substrate, threatening the safe operation of the equipment. The reasons for the shedding of the Al-Zn alloy coating include: (1) The temperature of the LNG collecting pipe and heat exchange tube at the bottom of the open-frame gasifier is the lowest (generally -160 to -150°C), while the seawater temperature is 1-30°C. The temperature difference between the inside and outside of the pipe is large, and the high temperature difference stress between hot and cold causes local shedding of the metal coating; (2) The open-frame gasifier is frequently started and stopped during operation, and the frequent hot and cold alternations cause thermal stress fatigue; (3) During the process of flame thermal spraying to form the coating, residual stress exists inside the coating. The thicker the local thickness of the coating, the greater the residual stress and the worse the coating stability; (4) The adhesion of the coating formed by thermal spraying is relatively low in local areas; (5) Seawater scouring accelerates the damage and shedding of the coating. Compared with thinning caused by seawater scouring, coating shedding has the greatest damage to the anti-corrosion coating. Coating shedding causes the open-frame gasifier substrate to come into direct contact with seawater. The equipment operates under high-pressure, low-temperature conditions with LNG. Once seawater corrosion leakage occurs, operational safety will be seriously affected. When the area of ​​local coating shedding is too large, the metal coating around the exposed aluminum alloy substrate cannot provide cathodic protection. Operation must be stopped and the shedding metal coating repaired.

[0005] The main existing method for repairing Al-Zn alloy coatings is flame thermal spraying. Before using thermal spraying for repair, the old coating must be completely removed. However, due to site conditions and the limited workspace (the spacing between two adjacent heat exchange panels is 500 mm), both sandblasting to remove the old coating and flame spraying for repair are difficult and time-consuming, typically requiring several weeks per gasifier. Furthermore, the adhesion between the new and old coatings is poor, making them more susceptible to detachment. The quality and lifespan of the re-sprayed coating are inferior to those of the initial workshop spraying, and the re-spraying cost is higher, increasing equipment maintenance costs and time. To save repair costs, coating repair is typically delayed until a large exposed area of ​​the substrate is present, increasing equipment safety risks. Furthermore, existing flame thermal spraying methods lack sufficient metal coating adhesion, making it impossible to effectively prevent coating detachment. During the construction process, problems such as poor localized coating adhesion, uneven thickness, and excessively high residual stresses inevitably arise, making further detachment of the metal coating unavoidable.

[0006] Therefore, how to repair the peeled coating in a timely and efficient manner, improve the bonding strength between the metal coating and the substrate, and effectively reduce the thermal stress in the coating are important ways to increase the service life of the metal coating in the heat exchange part of the open-frame gasifier and reduce the equipment maintenance cost. Summary of the Invention

[0007] In response to the above problems, the purpose of the present invention is to provide a method for repairing the metal anti-corrosion coating of a liquefied natural gas open-frame vaporizer. Compared with the existing technology, the repair method provided by the present invention can quickly and efficiently repair the metal coating that has partially fallen off the heat exchange part, so that the repaired metal anti-corrosion coating can be metallurgically bonded to the substrate, thereby improving the bonding strength between the coating and the substrate. The bonding strength is much higher than the bonding strength of the coating obtained by the existing flame thermal spraying process, thereby extending the life of the overall anti-corrosion coating of the heat exchange part of the open-frame vaporizer, reducing maintenance costs and time, and improving the safe operation reliability of the equipment.

[0008] In order to achieve the purpose of the invention, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for repairing a metal anti-corrosion coating of a liquefied natural gas open-rack vaporizer, the method comprising the following steps:

[0010] The area to be repaired in the metal anti-corrosion coating on the outer surface of the heat exchange part of the open-frame gasifier is cleaned until the chloride ion content is ≤25ppm, and then dried and polished in sequence to obtain a pre-treated area to be repaired;

[0011] The pre-treated area to be repaired is subjected to electrospark deposition of a metal anti-corrosion coating, so that the metal anti-corrosion coating is metallurgically bonded to the substrate and covers the exposed substrate, thereby completing the repair of the gasifier metal anti-corrosion coating.

[0012] The present invention adopts the electrospark deposition method for coating repair, which can use instantaneous pulse electric energy to melt the anti-corrosion metal and deposit it on the surface of the substrate, covering the exposed metal tube substrate with the anti-corrosion metal layer to form a metallurgically bonded metal anti-corrosion protective layer, which has the advantages of high bonding strength, fast repair speed, and little thermal impact on the surrounding coating.

[0013] In the present invention, the chloride ion content of the surface of the area to be repaired after cleaning is ≤25ppm, for example, it can be 25ppm, 24ppm, 23ppm, 22ppm, 21ppm or 20ppm, but is not limited to the listed values, and other unlisted values ​​within the numerical range are also applicable.

[0014] In the present invention, the washing liquid used for the cleaning may be, for example, fresh water, which is used to flush away salt, marine organisms, etc. in seawater and prevent corrosion caused by chloride ions.

[0015] Preferably, the metal anti-corrosion coating comprises an Al-Zn alloy metal anti-corrosion coating.

[0016] Preferably, the material of the electrode used in the electrospark deposition is the same as that of the metal anti-corrosion coating.

[0017] Preferably, in terms of mass percentage, the Zn content in the Al-Zn alloy metal anti-corrosion coating is 1.65-2.4%, for example, it can be 1.65%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3% or 2.4%; the Si content is ≤0.3%, for example, it can be 0.3%, 0.2% or 0.1%; the Fe content is ≤0.3%, for example, it can be 0.3%, 0.2% or 0.1%; the Cu content is ≤0.05%, for example, it can be 0.05%, 0.04%, 0.03% or 0.02%; the Pb content is ≤0.05%, for example, it can be 0.05%, 0.04%, 0.03% or 0.02%; the balance is Al, which is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0018] Preferably, the electrode material is wire.

[0019] Preferably, the diameter of the electrode material is 0.8-4 mm, for example, it can be 0.8 mm, 1 mm, 1.2 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2 mm, 2.2 mm, 2.4 mm, 2.6 mm, 2.8 mm, 3 mm, 3.2 mm, 3.4 mm, 3.6 mm, 3.8 mm or 4 mm, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0020] Preferably, the voltage of the electrospark deposition is 30-100V, for example, 30V, 40V, 50V, 60V, 70V, 80V, 90V or 100V, and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0021] Preferably, the pulse width of the electrospark deposition is 10-70 μs, for example, 10 μs, 20 μs, 30 μs, 40 μs, 50 μs, 60 μs or 70 μs, and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0022] Preferably, the frequency of the electrospark deposition is 40-160 Hz, for example, it can be 40 Hz, 50 Hz, 60 Hz, 70 Hz, 80 Hz, 90 Hz, 100 Hz, 110 Hz, 120 Hz, 130 Hz, 140 Hz, 150 Hz or 160 Hz, but is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0023] Preferably, the thickness of the metal anti-corrosion coating is 0.2-1 mm, for example, it can be 0.2 mm, 0.25 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.7 mm, 0.8 mm, 0.9 mm or 1 mm, but is not limited to the listed values. Other unlisted values ​​within the numerical range are also applicable, preferably 0.25-0.6 mm.

[0024] Preferably, the electrospark deposition is performed under a protective atmosphere.

[0025] Preferably, the protective atmosphere comprises argon.

[0026] Preferably, the bonding strength between the metal anti-corrosion coating and the substrate is greater than 35 MPa, for example, it can be 36 MPa, 37 MPa, 38 MPa or 39 MPa, and is not limited to the listed values. Other values ​​not listed within the numerical range are also applicable.

[0027] As a preferred technical solution of the present invention, the repair method comprises the following steps:

[0028] Clean the area to be repaired in the metal anti-corrosion coating on the outer surface of the heat exchange part of the open-frame vaporizer, rinse to remove seawater components, marine organisms, etc., and ensure that the chloride ion content on the surface is ≤25ppm after cleaning. Then dry it in the air or blow it dry to prevent chloride ion corrosion;

[0029] Polish the area to be repaired to remove the residual coating in the area to be repaired, so as to prevent the old coating from affecting the bonding strength of the repaired coating, and obtain the pre-treated area to be repaired;

[0030] The pretreated area to be repaired is subjected to electrospark deposition of a metal anti-corrosion coating under the conditions of 30-100V voltage, 10-70μs pulse width and 40-160Hz in an argon atmosphere to complete the repair of the gasifier metal anti-corrosion coating. The electrode material used is a wire with a diameter of 0.8-4mm. The material of the electrode material is the same as that of the metal anti-corrosion coating. In terms of mass percentage, the Zn content is 1.65-2.4%, the Si content is ≤0.3%, the Fe content is ≤0.3%, the Cu content is ≤0.05%, the Pb content is ≤0.05%, and the balance is Al. The thickness of the metal anti-corrosion coating is 0.2-1mm, and the bonding strength between the metal anti-corrosion coating and the substrate is greater than 35MPa.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The repair method provided by the present invention uses instantaneous pulsed electric energy to melt and deposit the anti-corrosion metal onto the surface of the substrate within microseconds. Diffusion and re-alloying occur during the electrospark deposition process, and the coating and the substrate form a metallurgical bond. The bonding force is much greater than that of the existing thermal spraying method. The coating repaired by the present invention can effectively resist the stress of alternating hot and cold temperatures and will not fall off again.

[0033] (2) In the repair method provided by the present invention, the discharge duration is short, the heat generated during discharge is conducted and diffused in a very small range, the substrate is not easily heated, and the thermal impact on the changes in the structure and performance of the coating around the gasifier substrate and the repair area can be ignored.

[0034] (3) The repair method provided by the present invention is simple and easy to implement, and the required repair time is greatly shortened. The detached coating can be repaired and production can be resumed on the same day, which greatly reduces the equipment maintenance cost and time.

[0035] (4) The repair method provided by the present invention can be repaired on site without the need for sandblasting or gas combustion heating, and the operation process is more completely environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 1 is a schematic structural diagram of the heat exchange portion of the open-frame vaporizer in Example 1 of the present invention;

[0037] Among them, 1-heat exchange tube; 2-liquid collecting tube; 3-gas collecting tube; 4-metal anti-corrosion coating peeling area. DETAILED DESCRIPTION

[0038] The technical solution of the present invention is further described below by way of specific embodiments. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0039] Example 1

[0040] This embodiment provides a method for repairing a metal anti-corrosion coating of a liquefied natural gas open-rack vaporizer, the method comprising the following steps:

[0041] The structural diagram of the heat exchange part of the open-frame vaporizer in this embodiment is as follows: Figure 1 As shown, the heat exchange part of the open-frame gasifier includes a number of heat exchange tubes 1 made of aluminum alloy, which are arranged in parallel to form a plate-like tube array. The upper and lower ends of the plate-like tube array are respectively connected to the gas collecting pipe 3 and the liquid collecting pipe 2. There is a metal anti-corrosion coating peeling area 4 on the lower part of the plate-like tube array and the liquid collecting pipe 2, which is the area to be repaired.

[0042] First, the area to be repaired within the metal anti-corrosion coating on the outer surface of the heat exchange portion of the open-frame vaporizer is cleaned to remove residual marine organisms and seawater components. After cleaning with fresh water, the chloride ion concentration is tested. If the chloride ion concentration is ≤25ppm, the next step is entered. If the chloride ion concentration is >25ppm, the cleaning process is continued until the test is passed, thereby preventing chloride ion corrosion. After passing the cleaning test, the area to be repaired is air-dried or blown dry.

[0043] Then, the area to be repaired is polished to remove the residual coating in the area to be repaired to prevent the old coating from affecting the bonding strength of the repaired coating, thereby obtaining a pre-treated area to be repaired.

[0044] Finally, the pretreated area to be repaired is repaired by electrospark deposition, that is, electrospark deposition of metal anti-corrosion coating is performed under argon atmosphere, voltage of 40V, pulse width of 30μs, and frequency of 70Hz to complete the repair of the gasifier metal anti-corrosion coating. The electrode material is Al-Zn alloy metal wire with a diameter of 3.0mm. The material of the electrode material is the same as that of the Al-Zn alloy metal anti-corrosion coating. In terms of mass percentage, the Zn content is 2.0%, the Si content is 0.1%, the Fe content is 0.16%, the Cu content is 0.006%, the Pb content is 0.0001%, and the balance is Al. Under argon protection, instantaneous pulse electric energy is used to melt and deposit the Al-Zn alloy metal anti-corrosion coating onto the surface of the gasifier aluminum alloy substrate. The exposed substrate is covered with the Al-Zn alloy anti-corrosion metal layer to form a metallurgical bond. The thickness of the Al-Zn alloy anti-corrosion metal layer is 0.3-0.5mm, and the bonding strength is 39MPa.

[0045] In summary, the repair method provided by the present invention can quickly and efficiently repair locally detached metal coatings, so that the repaired metal anti-corrosion coating can be metallurgically bonded to the substrate, thereby improving the bonding strength between the coating and the substrate, extending the life of the overall anti-corrosion coating of the open-frame gasifier heat exchange part, reducing maintenance costs and time, and improving the safe operation reliability of the equipment.

[0046] The applicant declares that the above is only a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for repairing the metal anti-corrosion coating of a liquefied natural gas open-rack vaporizer, characterized in that: The repair method comprises the following steps: The area to be repaired in the metal anti-corrosion coating on the outer surface of the heat exchange part of the open-frame gasifier is cleaned until the chloride ion content is ≤25ppm, and then dried and polished in sequence to obtain a pre-treated area to be repaired; The pre-treated area to be repaired is subjected to electrospark deposition of a metal anti-corrosion coating, so that the metal anti-corrosion coating is metallurgically bonded to the substrate and covers the exposed substrate, thereby completing the repair of the gasifier metal anti-corrosion coating.

2. The repair method according to claim 1, characterized in that: The metal anti-corrosion coating comprises an Al-Zn alloy metal anti-corrosion coating.

3. The repair method according to claim 2, characterized in that: The electrode material used in the electrospark deposition is the same as that of the metal anti-corrosion coating; Preferably, in terms of mass percentage, the Zn content in the Al-Zn alloy metal anti-corrosion coating is 1.65-2.4%, the Si content is ≤0.3%, the Fe content is ≤0.3%, the Cu content is ≤0.05%, the Pb content is ≤0.05%, and the balance is Al.

4. The repair method according to any one of claims 1 to 3, characterized in that: The electrode material used in the electrospark deposition is wire; Preferably, the diameter of the electrode material is 0.8-4 mm.

5. The repair method according to any one of claims 1 to 4, characterized in that: The voltage of the electrospark deposition is 30-100V.

6. The repair method according to any one of claims 1 to 5, characterized in that: The pulse width of the electrospark deposition is 10-70 μs.

7. The repair method according to any one of claims 1 to 6, characterized in that: The frequency of the electrospark deposition is 40-160 Hz.

8. The repair method according to any one of claims 1 to 7, characterized in that: The thickness of the metal anti-corrosion coating is 0.2-1 mm, preferably 0.25-0.6 mm.

9. The repair method according to any one of claims 1 to 8, characterized in that: The electrospark deposition is carried out under a protective atmosphere; Preferably, the protective atmosphere comprises argon.

10. The repair method according to any one of claims 1 to 9, characterized in that: The bonding strength between the metal anti-corrosion coating and the substrate is greater than 35 MPa.

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