Method for improving corrosion resistance of stainless steel plate welded joint and application thereof

Abstract

This invention relates to the field of home appliance technology, providing a method and its application for improving the corrosion resistance of welded joints of stainless steel sheets. The welded joint is formed by welding two stainless steel sheets with a gap size of 0-1 mm and a misalignment size of 0-1.5 mm under conditions where the oxygen concentration is below 1%. An anode material is placed on the stainless steel sheets within 30 mm of the welded joint. The pitting potential of the anode material is lower than the lowest pitting potential of the stainless steel sheet, and the potential difference between the two is greater than 0.15V. This invention designs a specific anode material and installs it at the aforementioned specific welded joint location, achieving a protective effect far exceeding that of magnesium rods without passivation. It has great application potential in heating water tanks and heat pump water heaters.

Description

Technical Field

[0001] This invention relates to the field of home appliance technology, and specifically to a method for improving the corrosion resistance of welded joints in stainless steel sheets and its application, particularly in stainless steel heating water tank assemblies and heat pump water heaters. Background Technology

[0002] Electrochemical corrosion has always been a key factor affecting equipment lifespan. Currently, the common methods to solve potential corrosion are passivation and sacrificial anode methods. Although passivation can improve the overall corrosion resistance of equipment, it is extremely costly. Using magnesium rods as sacrificial anodes is more common. Magnesium rods have high activity and a wide protection range. However, magnesium rods are consumed too quickly, about every six months, resulting in a high replacement frequency.

[0003] Therefore, it is of great significance to study a low-cost method that can effectively improve the corrosion resistance of metallic materials and thus extend the service life of equipment. Summary of the Invention

[0004] This invention provides a method for improving the corrosion resistance of welded joints of stainless steel plates and its application, in order to solve the defects of existing stainless steel products such as stainless steel heating water tanks in solving potential corrosion problems, such as the need for subsequent pickling and passivation treatment, high cost, frequent replacement of anodes, and limited protection effect.

[0005] Taking a heat pump water heater with a stainless steel tank as an example, it includes main components such as end caps, tank body, and coil. The end caps are made by stamping stainless steel sheets, and the tank body is made by rolling stainless steel sheets and then welding them with straight seams. The end caps are directly welded to the tank body or welded after flaring to form the water tank body. The coil can be located inside or outside the tank body and mainly includes the coil body and clamps.

[0006] In the process of researching the aforementioned specific structure of the heating water tank, this invention discovered that key factors affecting the service life of this type of heat pump water heater and improving user experience mainly include the potential corrosion problems at the welded joints, flared sections, and coils. Specifically, the coil body, in order to maintain heating efficiency, generally has a relatively low thickness, ranging from 0.6mm to 1.2mm, resulting in small corrosion channels. Welded joints are more prone to corrosion, and protecting these joints has always been a challenge in this field. Furthermore, different welding conditions in different fields lead to variations in the technical problems encountered. For example, in the development of the aforementioned stainless steel heating water tank assembly, it was found that while there are multiple welded joints in the heating water tank, the annular welded joint formed by the stainless steel plate end cap and the stainless steel plate body has significantly lower corrosion resistance than other welded joints. This poses a significant technical obstacle to the development of a long-lasting heat pump water heater incorporating the aforementioned stainless steel heating water tank.

[0007] Based on this, the present invention conducts an in-depth study on the technical problem that the above-mentioned specific weld joints and flared positions are more prone to potential corrosion. By monitoring the processing conditions of different weld seams, it was found that although the stainless steel plate end cap and the stainless steel plate barrel body are welded into a water tank, the ventilation protection of the weld is difficult to achieve the oxygen content of less than 0.2% required by conventional welding, and even reaches 1%. Moreover, the difference in the forming process of the end cap and the barrel body results in a large gap and misalignment of the parts to be welded. The gap size is distributed in 0-1mm and the misalignment size is distributed in 0-1.5mm. This is also significantly different from conventional welding conditions. It is speculated that this is the main reason why the weld between the end cap and the barrel body becomes the area most susceptible to corrosion, thus greatly reducing the service life of the water tank.

[0008] In this invention, the oxygen concentration refers to the atmosphere at the weld joint monitored by a monitoring device during the formation of the weld joint. There is a spatial misalignment between the two stainless steel plates to be welded. The welding points of the two stainless steel plates are placed in a spatial rectangular coordinate system. The misalignment includes the distance in the x-axis direction and the distance in the z-axis direction. In this invention, the gap refers to the distance in the x-axis direction, and the misalignment refers to the distance in the z-axis direction. The gap size and the misalignment size are not both zero.

[0009] This invention, through comparison of various anti-potential corrosion schemes, found that placing an anode material with a pitting potential lower than the lowest pitting potential of the stainless steel sheet constituting the welded joint, and with a potential difference greater than 0.15V, within 30mm of the annular welded joint can significantly improve the corrosion resistance of the welded joint. This has significant technical implications for products containing such welded joints, such as stainless steel heating water tank assemblies. If the anode is placed beyond 30mm of the annular welded joint, the corrosion resistance of the welded joint deteriorates.

[0010] Based on this, in a first aspect, the present invention provides a method for improving the corrosion resistance of welded joints of stainless steel plates, wherein the welded joint is formed by welding two stainless steel plates with a gap size of 0-1 mm and a misalignment size of 0-1.5 mm under conditions where the oxygen concentration is less than 1%, and an anode material is provided on the stainless steel plates in an area within 30 mm of the welded joint, wherein the pitting potential of the anode material is lower than the lowest pitting potential of the stainless steel plate and the potential difference between the two is greater than 0.15 V.

[0011] The method described above in this invention is highly effective in addressing the problem of potential corrosion in welded joints formed by welding two stainless steel plates with a gap size ranging from 0.3 to 1 mm and a misalignment size ranging from 0.2 to 1.5 mm under conditions where the oxygen concentration is above 0.3%.

[0012] According to the method for improving the corrosion resistance of welded joints of stainless steel plates provided by the present invention, the two stainless steel plates have different pitting potentials, and the stainless steel plate with a lower pitting potential is a low-potential stainless steel, and the anode material is disposed on the low-potential stainless steel.

[0013] According to the method for improving the corrosion resistance of welded joints of stainless steel plates provided by the present invention, the stainless steel plate is made of austenitic stainless steel or duplex stainless steel; the anode material includes one or more of 430 stainless steel, 304 stainless steel, aluminum alloy, 201 stainless steel, and carbon steel.

[0014] Preferably, the stainless steel sheet is made of 316L or 2205; the anode material is 430 stainless steel.

[0015] According to the method for improving the corrosion resistance of welded joints of stainless steel plates provided by the present invention, the anode material is an anode sheet.

[0016] Preferably, the welding anode material is produced using resistance spot welding, argon arc spot welding, or laser spot welding.

[0017] Secondly, the present invention also provides the application of the method described above in a stainless steel heating water tank, wherein the stainless steel heating water tank contains the welded joint.

[0018] Thirdly, the present invention also provides a stainless steel heating water tank assembly, comprising:

[0019] The barrel body is made of austenitic stainless steel or duplex stainless steel.

[0020] The end cap is made of austenitic stainless steel or duplex stainless steel.

[0021] The end cap includes a first end cap and a second end cap, the first end cap being located at one end of the barrel body and the second end cap being located at the other end of the barrel body; the end cap is directly welded to the barrel body or welded after flaring to form an annular welded joint; when the end cap and the barrel body are welded, the gap between the surfaces to be welded is distributed between 0 and 1 mm and the misalignment is distributed between 0 and 1.5 mm; the annular welded joint is formed under conditions where the oxygen concentration is less than 1%;

[0022] An anode material is disposed in the area within 30 mm of the annular welded joint on the head and / or the barrel body. The pitting potential of the anode material is lower than the lowest pitting potential of the stainless steel plate, and the potential difference between the two is greater than 0.15V.

[0023] According to the stainless steel heating water tank assembly provided by the present invention, the end cap and the tank body are made of stainless steel plates with different pitting potentials, and the stainless steel plate with a lower pitting potential is a low-potential stainless steel; the anode material is disposed on the low-potential stainless steel.

[0024] Preferably, the stainless steel sheet is made of one or more of 304, 304L, 316, 316L, and 2205; the anode material is made of one or more of 430 stainless steel, 304 stainless steel, aluminum alloy, 201 stainless steel, and carbon steel, preferably 430 stainless steel.

[0025] According to the stainless steel heating water tank assembly provided by the present invention, the anode material is an anode sheet;

[0026] Preferably, the anode sheet is welded using resistance spot welding, argon arc spot welding, or laser spot welding.

[0027] Preferably, the spacing between adjacent anode plates is within 400 mm, and more preferably 150–400 mm.

[0028] To further address the problem of insufficient service life of stainless steel heating water tank assemblies caused by the small corrosion channels in the coil, the stainless steel heating water tank assembly provided by the present invention further includes: a coil, which may be made of the same or different material as the tank body, and is disposed on the inner and / or outer wall of the tank body;

[0029] In addition, a clamp is provided, wherein at least a portion of the surface of the pipe sidewall constituting the coil is in close contact with the clamp, the pitting potential of the clamp being lower than that of the coil and the potential difference between the two being greater than 0.15V, thereby improving the corrosion resistance of the coil.

[0030] According to the stainless steel heating water tank assembly provided by the present invention, the clamp is a long strip structure with a width of 15 to 40 mm formed by bending and deforming 430 stainless steel sheet. The clamp includes a number of concave surfaces, which are used to fit tightly with the pipe sidewall of the coil.

[0031] Preferably, a gap is formed between each layer of pipe constituting the coil, and a pair of clamps are located on the inner and outer sides of the coil. Fasteners are used to pass through the clamps and the gap to fix the pair of clamps and to achieve a tight fit between the concave surface of the clamps and the side wall of the pipe.

[0032] Fourthly, the present invention also provides a heat pump water heater, including the stainless steel heating water tank assembly as described above.

[0033] This invention provides a method for improving the corrosion resistance of welded joints of stainless steel plates and its application. By studying the corrosion characteristics of welded joints formed by stainless steel plates under specific working conditions, it proposes to significantly improve the corrosion resistance of welded joints by strictly controlling the material and position of the anode for specific welded joints. Specifically, the anode material with a pitting potential lower than the lowest pitting potential of the stainless steel plates constituting the welded joint and a potential difference greater than 0.15V is placed in an area within 30mm of the welded joint. This can significantly improve the corrosion resistance of welded joints formed by welding two stainless steel plates with a gap size of 0-1mm and a misalignment size of 0-1.5mm under conditions where the oxygen concentration is less than 1%.

[0034] Furthermore, the aforementioned methods for improving corrosion resistance are applied to the corrosion-prone areas of stainless steel heating water tanks. By structurally designing the anodic material at a specific potential and installing it at the aforementioned specific welded joint position, a protective effect far exceeding that of magnesium rods can be achieved without passivation. During use, the corrosion rate is low, and the corrosion matrix shows no volume reduction after 500 hours, with only some pitting corrosion (compared to the complete consumption of magnesium rods in 130 hours). Moreover, the safety of water quality is ensured, and the cost of the heating water tank is greatly controlled.

[0035] In addition, this invention provides the first specific contact protection for the coil, which increases the lifespan of the coil by more than 50% (compared to passivation), and the corrosion resistance lifespan of the water tank is superior to that of the acid-washed and passivated water tank. Attached Figure Description

[0036] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0037] Figure 1 This is a partial schematic diagram of the anode plate installation position in the stainless steel heating water tank assembly provided in Embodiment 1 of the present invention.

[0038] Figure label:

[0039] 1: Anode plate; 2: Flared end; 3: Upper end cap. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0041] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field, or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased through legitimate channels.

[0042] The following is combined with Figure 1 This invention describes a method for improving the corrosion resistance of welded joints in stainless steel sheets and its applications.

[0043] Specifically, in this embodiment of the invention, a method for improving the corrosion resistance of welded joints of stainless steel plates is first provided. The welded joint is formed by welding two stainless steel plates with a gap size of 0-1 mm and a misalignment size of 0-1.5 mm under conditions where the oxygen concentration is less than 1%. An anode material is provided on the stainless steel plates in an area within 30 mm of the welded joint. The pitting potential of the anode material is lower than the lowest pitting potential of the stainless steel plate and the potential difference between the two is greater than 0.15V.

[0044] The pitting potential in this invention is measured based on the GB / T 17899 test method.

[0045] In some embodiments of the present invention, the two stainless steel plates have different pitting potentials, and the stainless steel plate with a lower pitting potential is a low-potential stainless steel, and the anode material is disposed on the low-potential stainless steel.

[0046] In some embodiments of the present invention, the stainless steel sheet is made of austenitic stainless steel (such as 304, 304L, 316, 316L) or duplex stainless steel (such as 2205); the anode material includes one or more of 430 stainless steel, 304 stainless steel, aluminum alloy, 201 stainless steel, and carbon steel.

[0047] Preferably, the stainless steel sheet is made of 316L or 2205; the anode material is 430 stainless steel.

[0048] As mentioned earlier, there are many factors that determine the corrosion resistance effect. Experiments have shown that for welds formed by welding 316L and 2205 stainless steel plates, the best comprehensive effect in terms of corrosion resistance improvement, anode consumption rate, and cost control is achieved when the anode material is 430.

[0049] In some embodiments of the present invention, the anode material is an anode sheet. Sheet-shaped materials have a large contact area and small volume, which is preferred in the present invention.

[0050] In some embodiments of the present invention, the anode material is welded onto a stainless steel plate using resistance spot welding, argon arc spot welding, or laser spot welding.

[0051] The larger the contact area between the anode material and the stainless steel sheet, the better the corrosion resistance is improved.

[0052] When argon arc spot welding is used, the length of the argon arc spot weld is 4 to 15 mm.

[0053] When resistance spot welding, argon arc spot welding or laser spot welding is used, the higher the density of the weld points, the higher the processing cost. Correspondingly, the effect of improving corrosion resistance will also be improved, but not necessarily in a direct proportion. When applying the method of this invention, the density of the weld points should be reasonably designed in combination with the specific equipment application environment and corrosion resistance requirements.

[0054] When welding, select self-fusion welding wire or welding wire that matches the base material grade as the welding material. For example, select ER316L grade welding wire when welding 316L, and select ER2209 grade welding wire when welding 2205.

[0055] The present invention also provides the application of the method described above in a stainless steel heating water tank, wherein the stainless steel heating water tank contains the welded joint.

[0056] This invention also provides a stainless steel heated water tank assembly, comprising:

[0057] The barrel body is made of austenitic stainless steel or duplex stainless steel.

[0058] The end cap is made of austenitic stainless steel or duplex stainless steel.

[0059] The end cap includes a first end cap and a second end cap, the first end cap being located at one end of the barrel body and the second end cap being located at the other end of the barrel body; the end cap is directly welded to the barrel body or welded after flaring to form an annular welded joint; when the end cap and the barrel body are welded, the gap between the surfaces to be welded is distributed between 0 and 1 mm and the misalignment is distributed between 0 and 1.5 mm; the annular welded joint is formed under conditions where the oxygen concentration is below 1%;

[0060] An anode material is disposed in the area within 30 mm of the annular welded joint on the head and / or the barrel body. The pitting potential of the anode material is lower than the lowest pitting potential of the stainless steel plate, and the potential difference between the two is greater than 0.15V.

[0061] Although existing technologies such as magnesium rods and pickling passivation processes can provide varying degrees of corrosion resistance, they suffer from problems such as frequent replacement, material and process costs associated with adding new magnesium rods, and excessively high costs associated with pickling passivation. In fact, this traditional sacrificial anode method ignores the intrinsic causes of corrosion in specific welded joints.

[0062] The tank body is generally made of cut stainless steel plates welded together, forming a straight weld. Although it is also made of stainless steel plates, actual tests have shown that this type of weld is made by cutting and welding the same plate. Its structural regularity and the degree of corrosion of the weld are much less than those of the circumferential weld. For the heating water tank, the present invention uses the above-mentioned specific anode material to give priority protection to the circumferential weld, which can significantly improve the service life of the heating water tank.

[0063] Due to the differences in processing methods between the end cap and the barrel body, flaring is generally required for easier welding. The flared area becomes a corrosion-prone zone. Therefore, the technical difficulty of forming a circumferential weld after flaring is further increased. This is because, in addition to protecting the flared area, the welding position of the anode material is also affected by the flared area. Experiments have shown that welding the anode plate to the flared area while ensuring that the distance between the anode plate and the circumferential weld is less than 30mm provides the best protection for the circumferential weld, the flared area, and the stainless steel on the low pitting potential side.

[0064] Meanwhile, considering the addability of the anode sheet and the protective effect of the added anode sheet on the flared area, the flared width is preferably 5mm to 30mm larger than the width of the anode sheet.

[0065] In some embodiments of the present invention, the end cap and the barrel body are made of stainless steel plates with different pitting potentials, and the stainless steel plate with a lower pitting potential is low-potential stainless steel; the anode material is welded onto the low-potential stainless steel.

[0066] The present invention also found that the welds formed by welding stainless steel plates with different pitting potentials have a greater impact on potential corrosion, including local defects, oxidation areas, and the weld on the side of the stainless steel plate with a lower pitting potential as well as the stainless steel body itself.

[0067] Considering that in electrochemical corrosion, materials with more negative pitting potentials are required as sacrificial anodes, and that the shape and structure of the anode affect its protective effect under different corrosive environments, and further taking into account the uncertainty of application scenarios and replacement time, as well as increased costs, this invention, through extensive experimental comparison of the corrosion resistance effects of different anodes, concludes that: for welds formed by welding stainless steel plates with different pitting potentials, it is necessary to strictly control the anode material, shape, contact method with the stainless steel plate, and anode position. This can significantly improve the corrosion resistance of this type of weld, resulting in a corrosion resistance life superior to that of stainless steel after welding acid pickling and passivation, and a service life far exceeding that of magnesium rods, eliminating the need for replacement during the product's service life.

[0068] In some embodiments of the present invention, the stainless steel sheet is made of one or more of 304, 304L, 316, 316L, and 2205; the anode material is made of one or more of 430 stainless steel, 304 stainless steel, aluminum alloy, 201 stainless steel, and carbon steel, preferably 430 stainless steel.

[0069] In some embodiments of the present invention, the anode material is an anode sheet.

[0070] In some embodiments of the present invention, the anode sheet is welded using resistance spot welding, argon arc spot welding, or laser spot welding.

[0071] In some embodiments of the present invention, the spacing between adjacent anode plates is within 400 mm, preferably 150 to 400 mm.

[0072] In some embodiments of the present invention, the anode sheet has a length of 30–150 mm, a width of 5–20 mm, and a thickness of 0.5–1.5 mm. By employing a welding process, the contact area between the anode sheet and the stainless steel plate can be made as close as possible to the projected area of ​​the anode sheet, thereby achieving the technical effect of improved corrosion resistance.

[0073] For conventional stainless steel heating water tank components, the corrosion resistance requirement is that the easily corroded areas of the water tank will not leak due to corrosion after 130 hours of operation under test conditions. The size design of the 430 stainless steel anode plate of the present invention, based on test data, can achieve no reduction in the volume of the anode plate after 500 hours of operation under test conditions, and there will be no leakage problem in the easily corroded areas of the water tank.

[0074] In practical applications, the material, size, and installation density of the anode plates can be adjusted according to the working environment of the equipment.

[0075] In addition, when welding the anode sheet, one side of the anode sheet may be parallel to or not parallel to the weld to be protected; the present invention does not impose strict restrictions on this.

[0076] In some embodiments of the present invention, it further includes: a coil, which is made of the same or different material as the tank body, and is disposed on the inside and / or outer wall of the tank body for heating the water in the water tank;

[0077] In addition, a clamp is provided, wherein at least a portion of the surface of the pipe sidewall constituting the coil is in close contact with the clamp, the pitting potential of the clamp being lower than that of the coil and the potential difference between the two being greater than 0.15V, thereby improving the corrosion resistance of the coil.

[0078] As mentioned earlier, the coils used for heating water in heating water tanks are generally made by bending thin-walled stainless steel pipes with a thickness of only 0.6mm to 1.2mm, such as non-welded coils or integrally formed coils. Although the thickness is thinner and the corrosion channels are smaller, there are fewer structural defects. This invention has found that it is not necessary to use the methods for the aforementioned weld seams. Instead, by optimizing the structure and material of the anode material, the corresponding protection can be achieved by using a contact fixing method.

[0079] In some embodiments of the present invention, the clamp is a long strip structure with a width of 15 to 40 mm formed by bending and deforming 430 stainless steel sheet. The clamp includes several concave surfaces, which are used to fit tightly with the pipe sidewall of the coil.

[0080] Preferably, a gap is formed between each layer of pipe constituting the coil, and a pair of clamps are located on the inner and outer sides of the coil. Fasteners are used to pass through the clamps and the gap to fix the pair of clamps and to achieve a tight fit between the concave surface of the clamps and the side wall of the pipe.

[0081] The outer side refers to the outer side of the space enclosed by the pipe, and the inner side refers to the inner side of the space enclosed by the pipe.

[0082] Commonly used fasteners include bolts. When using bolts, holes can be drilled between adjacent concave surfaces, and the bolt passes through the holes and gaps to achieve a tight fit between the concave surfaces and the pipe sidewall. Preferably, the bolts are made of 430 stainless steel and / or 304 stainless steel.

[0083] If the pipe diameter is large, the number of clamps can be increased so that multiple concave surfaces fit on the side wall of each pipe layer; generally, the interval between adjacent concave surfaces on a pipe side wall is within 600mm;

[0084] Based on the aforementioned research, this invention designs a specific clamp using 430 stainless steel anode material according to the structural characteristics of the coil. When this clamp is used in conjunction with the coil, the service life of the coil can be increased by more than 50% at a very low cost.

[0085] If other fixing methods are used, such as welding, burn-through is likely to occur, and welding efficiency will be significantly reduced, significantly increasing processing costs.

[0086] This invention also provides a heat pump water heater, including the stainless steel heating water tank assembly described above.

[0087] The stainless steel heating water tank assembly of the present invention has great application value in the field of household heat pump water heaters, avoiding the drawbacks of consumers having to frequently replace magnesium rods. Moreover, the heating water tank of the present invention can ensure the safety of water quality after long-term use.

[0088] Specifically, taking the aforementioned stainless steel heating water tank assembly and heat pump water heater as examples, this invention further describes the application of the method for improving the corrosion resistance of stainless steel sheet welds in the processing of stainless steel heating water tank assemblies and heat pump water heaters, including:

[0089] (1) Component preparation;

[0090] Stainless steel barrel body: A stainless steel plate with a thickness of 1-3mm is rolled into a circle and then welded to form a stainless steel barrel body with open ends. During welding, the gap between the two sides of the stainless steel plate being welded is distributed in the range of 0-0.3mm, and the misalignment is distributed in the range of 0-0.2mm. The oxygen concentration at the weld joint is below 0.3% due to the ventilation protection.

[0091] Upper and lower end caps: The upper and lower end caps are formed by stamping stainless steel sheets with a thickness of 1-3mm using a stamping process. Further, the upper and lower end caps are flared using a flaring process. The flared upper end cap is used for welding to the top of the barrel body, and the flared lower end cap is used for welding to the bottom of the barrel body.

[0092] Coiled pipe with clamps: First, the pipes are bent to form a coil, with gaps between each layer of pipes. Then, stainless steel sheets (generally 430 stainless steel sheets with a thickness of 0.6-1.2mm and a width of 15-40mm) are bent and deformed to form long strip-shaped clamps. Each clamp includes concave surfaces that correspond one-to-one with the aforementioned coiled pipes, and these concave surfaces are distributed along the length of the long strip structure. Finally, bolts are used to tightly fit each concave surface of a set of clamps (two clamps are considered a set of clamps) to the sidewalls of each layer of pipes in the coil, and each clamp is installed side by side on the pipe sidewall in the same way, so that adjacent concave surfaces on one pipe sidewall are spaced a certain distance (e.g., 250-600mm), thus obtaining the coiled pipe with clamps.

[0093] Anode plates: in several quantities, preferably made of 430 stainless steel, each anode plate having a length of 30-150mm, a width of 5-20mm, and a thickness of 0.5-1.5mm.

[0094] (2) The parts prepared in step (1) are further processed to obtain a stainless steel heating water tank assembly. The specific process is as follows:

[0095] (2.1) Welding anode protective sheet

[0096] Let the side of the upper end cap after flaring be side L1, the side of the lower end cap after flaring be side L3, and the sides of the two ends of the barrel body for welding be side L2 and side L4.

[0097] Argon arc spot welding is used to weld the two sides of the anode sheet along its length to the flared positions of the upper and lower end caps one by one, so that the projected surface of the anode sheet is in complete contact with the flared position. The anode sheet is located inside the box, close to the side L1 and side L3. The distance between the anode sheet and the edge of the annular welded joint is 5 to 30 mm, and the spacing between adjacent anode sheets is 150 to 400 mm.

[0098] (2.2) Assembly and welding of coil, end caps and barrel body

[0099] Place the clamp inside the barrel body and pass the clamp inlet and outlet connectors through the upper end cap pipe connector. Assemble the upper end cap, lower end cap, and barrel body. Measure the gap between the side L1 to be welded and the side L2 of the barrel body, and between the side L3 to be welded and the side L4 of the barrel body. The gap should be within the range of 0 to 1 mm, and the misalignment should be within the range of 0 to 1.5 mm.

[0100] Using a welding device with ventilated protection, side L1 is simultaneously welded to side L2, and side L3 to side L4. The oxygen concentration at the weld joint is monitored to be below 1% under ventilated protection. Upon completion of the welding, a ring-shaped welded joint is obtained. This yields a stainless steel heating water tank assembly, including the tank cavity and a coil containing clamps.

[0101] (3) Assemble the stainless steel heating water tank assembly into a heat pump water heater.

[0102] The following examples illustrate the preferred embodiments proposed in this invention.

[0103] Example 1

[0104] A method for manufacturing a heat pump water heater includes:

[0105] (1) Component preparation;

[0106] Stainless steel barrel body: A welding device with ventilation protection is used to roll 1.5mm thick 316L stainless steel sheet into a circle and weld it to form a stainless steel barrel body with open ends. During welding, the size between the two sides of the stainless steel sheet being welded is distributed in the range of 0 to 0.3mm, and the misalignment size is distributed in the range of 0 to 0.2mm. The oxygen concentration at the weld joint is below 0.3% due to ventilation protection.

[0107] Upper and lower end caps: The upper and lower end caps are formed by stamping 1.5mm thick 316L stainless steel sheets. Further, the upper and lower end caps are flared. The flared upper end cap is used for welding to the top of the barrel body, and the flared lower end cap is used for welding to the bottom of the barrel body.

[0108] The coiled pipe with clamps is made by first bending 316L austenitic stainless steel pipes with a diameter of 22mm and a thickness of 0.8mm to form a coil containing 21 layers of pipes with gaps between each layer. Next, 1mm thick and 30mm wide 430 stainless steel sheets are bent and deformed to form long, strip-shaped clamps. Each clamp includes concave surfaces corresponding to the pipes in the coil, distributed along the length of the strip structure. Finally, 430 stainless steel bolts are used to tightly fit the concave surfaces of each set of clamps (two clamps are considered a set) to the sidewalls of each layer of pipes in the coil. A total of three sets of clamps are installed side-by-side on the pipe sidewall in the same manner, ensuring that adjacent concave surfaces on a single pipe sidewall are evenly spaced, thus obtaining the coiled pipe with clamps.

[0109] Anode plates: Several in number, made of 430 stainless steel, each anode plate is 100mm long, 10mm wide, and 1mm thick.

[0110] (2) The parts prepared in step (1) are further processed to obtain a stainless steel heating water tank assembly. The specific process is as follows:

[0111] (2.1) Welding anode protective sheet

[0112] Let the side of the upper end cap after flaring be side L1, the side of the lower end cap after flaring be side L3, and the sides of the two ends of the barrel body for welding be side L2 and side L4.

[0113] like Figure 1 As shown, the two sides of the anode plate 1 along its length direction are welded one by one to the flared positions 2 of the upper head 3 and the lower head using argon arc spot welding, so that the projected surface of the anode plate is in complete contact with the flared position. The anode plate is located inside the box, close to the side L1 and side L3. The distance between the anode plate and the edge of the annular welded joint is 10mm, and the spacing between adjacent anode plates is 300mm.

[0114] (2.2) Assembly and welding of coil, end caps and barrel body

[0115] Place the clamp inside the barrel body and pass the clamp inlet and outlet connectors through the upper end cap pipe connector. Assemble the upper end cap, lower end cap, and barrel body. Measure the gap between the side L1 to be welded and the side L2 of the barrel body, and between the side L3 to be welded and the side L4 of the barrel body. The gap should be within the range of 0 to 1 mm, and the misalignment should be within the range of 0 to 1.5 mm.

[0116] Using a welding device with ventilated protection, side L1 is simultaneously welded to side L2, and side L3 to side L4. The oxygen concentration at the weld joint is monitored to be below 1% under ventilated protection. Upon completion of the welding, a ring-shaped welded joint is obtained. This yields a stainless steel heating water tank assembly, including the tank cavity and a coil containing clamps.

[0117] (3) Assemble the stainless steel heating water tank assembly obtained in step (2) into a heat pump water heater.

[0118] The heat pump water heater was circulated in an aqueous solution with a concentration of 0.25% FeCl3·6H2O at 75℃ for corrosion resistance testing. The test showed that after 500 hours of operation, the anode plates and clamps inside the water tank did not decrease in volume, only some pitting corrosion was present, and no corrosion was found in the water tank. The coil showed dew point after 468 hours. The water quality was tested and found to meet the requirements of GB4806.9-2016 "National Standard of the People's Republic of China / Hygienic Standard for Stainless Steel Food Utensils and Containers".

[0119] Example 2

[0120] A heat pump water heater is basically the same as that in Example 1, except that the material of the anode plate is completely replaced with 304 stainless steel in step (1).

[0121] The results showed that, under the same test conditions as in Example 1, both the first and second annular welded joints leaked after 250 hours of operation.

[0122] Example 3

[0123] A heat pump water heater is basically the same as that in Example 1, except that in step (1), the material of the anode plate is completely replaced with Q235 carbon steel stainless steel.

[0124] The results showed that, under the same test conditions as in Example 1, after 300 hours of operation, the Q235 carbon steel was completely consumed and rust appeared in the water.

[0125] As can be seen from Examples 1-3, the anolyte material of 430 stainless steel significantly improves the corrosion resistance of the water tank compared to 304 stainless steel and Q235 carbon steel. Furthermore, it ensures the safety of the water in the tank.

[0126] Example 4

[0127] A heat pump water heater is basically the same as that in Example 1, except that in step (1), the 1.5mm thick 316L material tank body is replaced with a 1.2mm thick duplex stainless steel 2205 stainless steel tank body (the anode plate is still installed on the end cap, the same as in Example 1).

[0128] The results showed that, under the same test conditions as in Example 1, after 500 hours of operation, the internal anode plates and clamps did not decrease in volume, only some pitting corrosion was present, and no corrosion was found in the water tank and coils. The water quality was tested and found to meet the requirements of GB4806.9-2016 "National Standard of the People's Republic of China / Hygienic Standard for Stainless Steel Food Containers".

[0129] Comparative Example 1

[0130] A heat pump water heater is basically the same as that in Example 1, except that step (2) does not include the process of welding the anode plate (i.e., the anode plate is not provided in the heating water tank).

[0131] The results showed that, under the same test conditions as in Example 1, both the first and second annular welded joints experienced corrosion leakage after 160 hours of operation.

[0132] Comparative Example 2

[0133] A heat pump water heater is basically the same as that in Example 1, except that: the anode plate is not welded and the clamp is not installed, and the magnesium rod is installed on the tank body in a conventional manner.

[0134] The results showed that, under the same test conditions as in Example 1, the magnesium rod was completely consumed after 140 hours of operation.

[0135] As can be seen from the above Examples 1, Comparative Examples 1 and 2, in the absence of anodic material, the first and second annular welded joints will quickly corrode and have poor corrosion resistance. Although the magnesium rod can prevent the corrosion of the first and second annular welded joints, it will be consumed quickly.

[0136] Comparative Example 3

[0137] A heat pump water heater is basically the same as Comparative Example 1, except that the material of the clamp is replaced with 316L austenitic stainless steel in step (1) (i.e. the same material as the coil).

[0138] The results showed that, under the same test conditions as in Example 1, after about 70 hours of operation, the coil leaked and the time it took for the water tank to reach dew point was basically the same as in Comparative Example 1.

[0139] Comparative Example 4

[0140] A heat pump water heater is basically the same as Comparative Example 3, except that in step (1), the 316L stainless steel plate is replaced with 2205 duplex stainless steel plate to obtain a stainless steel tank body.

[0141] The results showed that, under the same test conditions as in Example 1, the time it took for the coil to leak after 60 hours of operation was basically the same as in Comparative Example 3, and after 200 hours of operation, leakage occurred at the flared end cap.

[0142] This indicates that after using dissimilar steels, the corrosion resistance of the weld is improved because the pitting potential of 2205 duplex stainless steel is higher than that of 316L stainless steel. The weak points in corrosion are on the 316L side and the heat-affected zone of the weld on that side.

[0143] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for improving the corrosion resistance of welded joints of stainless steel sheets, characterized in that, The welded joint is formed by welding two stainless steel plates with a gap size of 0~1mm and a misalignment size of 0~1.5mm under conditions where the oxygen concentration is less than 1%. An anode material is placed on the stainless steel plates within 30mm of the welded joint. The pitting potential of the anode material is lower than the lowest pitting potential of the stainless steel plates and the potential difference between the two is greater than 0.15V. The stainless steel sheet is made of austenitic stainless steel or duplex stainless steel; the anode material is one or a combination of two or more of 430 stainless steel, 304 stainless steel, 201 stainless steel, and carbon steel.

2. The method for improving the corrosion resistance of welded joints of stainless steel plates according to claim 1, characterized in that, The two stainless steel plates have different pitting potentials. The stainless steel plate with the lower pitting potential is the low-potential stainless steel, and the anode material is placed on the low-potential stainless steel.

3. The method for improving the corrosion resistance of welded joints of stainless steel plates according to claim 1 or 2, characterized in that, The stainless steel sheet is made of 316L or 2205; the anode material is 430 stainless steel.

4. The method for improving the corrosion resistance of welded joints of stainless steel plates according to claim 3, characterized in that, The anode material is an anode sheet.

5. A stainless steel heated water tank assembly, characterized in that, include: The barrel body is made of austenitic stainless steel or duplex stainless steel. The end cap is made of austenitic stainless steel or duplex stainless steel. The end cap and the barrel body are made of stainless steel sheet; The end cap includes a first end cap and a second end cap, the first end cap being located at one end of the barrel body and the second end cap being located at the other end of the barrel body; the end cap is directly welded to the barrel body or welded after flaring to form an annular welded joint; when the end cap and the barrel body are welded, the gap between the surfaces to be welded is distributed between 0 and 1 mm and the misalignment is distributed between 0 and 1.5 mm; the annular welded joint is formed under conditions where the oxygen concentration is less than 1%; The anode material is disposed in the area within 30mm of the end cap and / or the barrel body and at a distance of 30mm from the annular welded joint. The pitting potential of the anode material is lower than the lowest pitting potential of the stainless steel plate and the potential difference between the two is greater than 0.15V. The anode material includes one or more of 430 stainless steel, 304 stainless steel, 201 stainless steel, and carbon steel.

6. The stainless steel heating water tank assembly according to claim 5, characterized in that, The end cap and the barrel body are made of stainless steel plates with different pitting potentials, and the stainless steel plate with a lower pitting potential is low-potential stainless steel; the anode material is disposed on the low-potential stainless steel. The stainless steel sheet material includes one or more of 304, 304L, 316, 316L, and 2205; the anode material is 430 stainless steel.

7. The stainless steel heating water tank assembly according to claim 5 or 6, characterized in that, The anode material is an anode sheet; The spacing between adjacent anode plates is within 400mm.

8. The stainless steel heating water tank assembly according to claim 7, characterized in that, The spacing between adjacent anode plates is 150~400mm.

9. The stainless steel heating water tank assembly according to any one of claims 5, 6, and 8, characterized in that, Also includes: The coil, made of the same or different material as the barrel body, is located on the inside and / or outer wall of the barrel body; In addition, a clamp is provided, wherein at least a portion of the surface of the pipe sidewall constituting the coil is in close contact with the clamp, the pitting potential of the clamp being lower than that of the coil and the potential difference between the two being greater than 0.15V, thereby improving the corrosion resistance of the coil.

10. The stainless steel heating water tank assembly according to claim 7, characterized in that, Also includes: The coil, made of the same or different material as the barrel body, is located on the inside and / or outer wall of the barrel body; In addition, a clamp is provided, wherein at least a portion of the surface of the pipe sidewall constituting the coil is in close contact with the clamp, the pitting potential of the clamp being lower than that of the coil and the potential difference between the two being greater than 0.15V, thereby improving the corrosion resistance of the coil.

11. The stainless steel heating water tank assembly according to claim 9, characterized in that, The clamp is a long strip structure with a width of 15-40mm formed by bending and deforming 430 stainless steel sheet. The clamp includes several concave surfaces, which are used to fit tightly with the pipe side wall of the coil. A gap is formed between each layer of pipe that constitutes the coil. A pair of clamps are located on the inner and outer sides of the coil. Fasteners are used to pass through the clamps and the gaps to fix the pair of clamps and to achieve a tight fit between the concave surface of the clamps and the side wall of the pipe.

12. The stainless steel heating water tank assembly according to claim 10, characterized in that, The clamp is a long strip structure with a width of 15-40mm formed by bending and deforming 430 stainless steel sheet. The clamp includes several concave surfaces, which are used to fit tightly with the pipe side wall of the coil. A gap is formed between each layer of pipe that constitutes the coil. A pair of clamps are located on the inner and outer sides of the coil. Fasteners are used to pass through the clamps and the gaps to fix the pair of clamps and to achieve a tight fit between the concave surface of the clamps and the side wall of the pipe.

13. A heat pump water heater, characterized in that, Includes the stainless steel heated water tank assembly as described in any one of claims 5 to 12.

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