Passivation treatment method, passivation treatment method for tank, method for manufacturing tank, and tank maintenance method

Abstract

This passivation treatment method involves a passivation treatment step for performing a passivation treatment by applying a passivation treatment liquid to a stainless steel component or by immersing the stainless steel component in the passivation treatment liquid, wherein the passivation treatment liquid contains an organic acid and ozone as an oxidizing agent, with the balance including water.

Description

Passivation treatment method, tank passivation treatment method, tank manufacturing method, and tank maintenance method

[0001] The present disclosure relates to a passivation treatment method, a tank passivation treatment method, a tank manufacturing method, and a tank maintenance method.

[0002] In the manufacturing process of stainless steel parts including tanks, the passive film responsible for the corrosion resistance of the stainless steel parts is removed by assembly processing including welding, cutting or polishing. For this reason, passivation treatment is performed after the assembly processing of the stainless steel parts. Further, when a tank is made of stainless steel, passivation treatment is performed to impart corrosion resistance to the inner side of the tank.

[0003] Patent Document 1 discloses a passivation treatment method comprising a passivation treatment step of applying or immersing a passivation treatment liquid containing 1% by mass of hydrogen peroxide and citric acid at a content rate less than the content that gives the depassivation pH of the stainless steel forming the stainless steel part, with the balance being water, to the stainless steel part to carry out the passivation treatment. This passivation treatment liquid further contains carboxymethyl cellulose, a carboxymethyl cellulose compound, xanthan gum or pectin in place of part of the water as the balance.

[0004] Japanese Unexamined Patent Publication No. 2016-108640

[0005] In the passivation treatment method described in Patent Document 1, hydrogen peroxide solution is used as an oxidizing agent for forming a passive film. However, when disposing of the passivation treatment liquid, since the self-decomposition of hydrogen peroxide takes time, post-treatment using a reducing agent containing sodium sulfate is required.

[0006] The present disclosure has been devised to solve the above problems, and aims to provide a passivation treatment method, a tank passivation treatment method, a tank manufacturing method, and a tank maintenance method that allow passivation treatment using a treatment liquid that facilitates post-treatment.

[0007] To achieve the above objective, the passivation treatment method according to this disclosure is a passivation treatment method for stainless steel parts, comprising a passivation treatment step of applying or immersing the stainless steel parts in a passivation treatment solution containing an organic acid and ozone as an oxidizing agent, with the remainder being water, to perform the passivation treatment.

[0008] According to this disclosure, by using a treatment solution containing ozone that self-decomposes as an oxidizing agent, passivation treatment can be performed using a treatment solution that is easy to post-treat.

[0009] Flowchart showing passivation treatment according to Embodiment 1 of this disclosure Figure showing the structure of a tank according to Embodiment 1 of this disclosure Figure showing the manufacturing process of a tank according to Embodiment 1 of this disclosure Figure showing passivation treatment according to Embodiment 1 of this disclosure Figure Showing passivation treatment according to a modified example Figure Showing passivation treatment according to a modified example Figure Showing passivation treatment according to a modified example Figure Flowchart showing maintenance process according to a modified example Figure Flowchart showing manufacturing method of a tank according to a modified example Figure

[0010] The passivation treatment method according to the embodiment of this disclosure will be described in detail below with reference to the drawings.

[0011] (Embodiment 1) The passivation treatment method according to Embodiment 1 is a passivation treatment method for a stainless steel part R, as shown in Figure 1, and comprises a passivation treatment step (step S101) and a removal step (step S102). The stainless steel part R is not particularly limited as long as it is a part made of stainless steel, and includes a tank for storing hot water.

[0012] Stainless steel parts R are made from austenitic stainless steel, ferritic stainless steel, austenitic-ferritic stainless steel, martensitic stainless steel, and precipitation-hardening stainless steel. The stainless steel used in stainless steel parts R includes Fe-Cr alloys, Fe-Cr-Ni alloys, Fe-Cr-Ni-Mo alloys, Fe-Cr-Ni-Mo-Cu alloys, and Fe-Cr-Ni-Mn alloys, and is an Fe-based alloy containing chromium (Cr).

[0013] Stainless steel parts R have high corrosion resistance and are resistant to rust due to the formation of a passivation film by the reaction of chromium in the stainless steel with oxygen. When stainless steel parts R are welded, the area around the weld is exposed to high temperatures, which destroys the passivation film and forms scale, which is an oxide derived from iron. The scale includes welding burns or rust. Therefore, the corrosion resistance of the area around the weld of the stainless steel parts R decreases. The passivation treatment method according to this embodiment 1 can be used to form a passivation film on stainless steel parts R whose passivation film has been destroyed.

[0014] The passivation treatment step (step S101) is a process in which a passivation treatment is carried out by applying or immersing a stainless steel part R with a passivation treatment solution P containing an organic acid and ozone as an oxidizing agent, with the remainder being water. The passivation treatment solution P may contain additives other than the organic acid and ozone as an oxidizing agent.

[0015] Organic acids are acidic organic compounds, including citric acid, acetic acid, and malic acid. Of these, citric acid is preferred as the organic acid. The concentration of the organic acid is preferably 0.1% by mass or more and less than or equal to the saturation solubility amount in the passivation treatment solution P, and the pH is preferably between 1 and 3. When the passivation film is formed, the organic acid forms a complex with the iron (Fe) content (iron ions) contained in the passivation film, thereby dissolving and removing the iron and concentrating the chromium (Cr) content contained in the passivation film. Furthermore, since iron-derived scale or rust has porous voids, the organic acid can penetrate into the voids, making it possible to dissolve and remove iron more effectively. This increases the chromium concentration on the surface of the stainless steel base material, so a denser passivation film is formed, which further improves corrosion resistance. In addition, organic acids also have the effect of dissolving scale through a reduction reaction. Since organic acids are weak acids that are also used in food, it is possible to reduce the environmental impact.

[0016] The passivation treatment solution P contains ozone as an oxidizing agent and has the function of forming a passivation film on the surface of the stainless steel part R. Ozone requires less energy to purify than hydrogen peroxide. Also, the oxidizing power of ozone is greater than that of hydrogen peroxide. Furthermore, ozone self-decomposes in about tens of minutes, whereas, for example, 6% hydrogen peroxide self-decomposes at room temperature at a rate of 2% / year, and decomposition using a reducing agent containing sodium sulfate requires treatment costs. In addition, ozone in the atmosphere is not harmful to the human body at concentrations of 0.2 ppm or less. For this reason, it is preferable to use ozone as an oxidizing agent. When using ozone as an oxidizing agent, the ozone concentration in the passivation treatment solution P is preferably 0.01 ppm or more and 0.2 ppm or less.

[0017] Ozone can be produced by a discharge method, in which an oxygen-containing gas is flowed between a pair of electrodes arranged in a parallel plate shape or coaxial cylindrical shape, and a high AC voltage is applied to cause a discharge in the oxygen-containing gas and generate ozone; an electrolytic method, in which an electrolyte membrane is placed in water, a pair of electrodes are placed in between, and a DC voltage is applied between the two electrodes to cause electrolysis of water, generating ozone simultaneously with oxygen on the oxygen-generating side; or an ultraviolet lamp method, in which ultraviolet light is shone on air to generate ozone. Alternatively, in the passivation treatment step (step S101), ozone gas may be blown into a solution containing an organic acid to produce a passivation treatment solution P.

[0018] The removal step (step S102) is a step to remove the passivation treatment liquid P adhering to the passivation-treated stainless steel part R. Specifically, the passivation treatment liquid P is removed, and the passivation-treated stainless steel part R is washed with tap water or pure water. After washing, the stainless steel part R is dried as needed.

[0019] Next, as an example of a stainless steel part R, a method for manufacturing a tank, as shown in Figure 2, will be described. As shown in Figure 3, the method for manufacturing the tank comprises a top member process (step S201) for forming a dome-shaped part 230, a side member process (step S202) for forming a cylindrical part 240, and an assembly process (step S203) for assembling the dome-shaped part 230 and the cylindrical part 240.

[0020] The tank 200 is a hot water storage tank and is composed of dome-shaped parts 230 that form the upper and lower tops and cylindrical parts 240 that form the sides. These parts are made from sheet stainless steel and are processed by plastic deformation and welding.

[0021] In the top component process (step S201), a plate material is formed into a dome shape by plastic deformation processes including press working, metal spinning, and spinning to obtain a dome-shaped component 230. The connection part with the cylindrical component 240 can be further processed by cutting or plastic deformation as needed. In addition, to form a connection part with the outside, a connection opening is provided in the top or curved part of the dome-shaped component 230, and a joint is connected to this opening by a welding process. The material of the joint is not limited to metal; a resin material can also be used, and in the case of a resin joint, it can be connected to the components of the tank 200 using an adhesive.

[0022] In the side member process (step S202), the plate material is formed into a cylindrical shape by roll forming plastic deformation, and then its ends are joined together by arc welding or laser welding to form a cylindrical shape and obtain a cylindrical part 240. At this time, the connection part with the dome-shaped part 230 can also be cut or plastically deformed as needed. Furthermore, in order to form a connection part with the outside, a connection opening is provided on the side of the cylinder, and a joint is connected to this opening by welding or adhesive.

[0023] In the assembly process (step S203), the molded dome-shaped part 230 is fitted to both ends of the cylindrical part 240. The connection parts 250 of each fitted part are welded around the outer circumference of the cylinder. In this welding process, in order to suppress the occurrence of welding burns, an inert gas containing nitrogen or argon is sprayed onto the outer surface of the tank, and the inner surface of the tank is either filled with an inert gas or sprayed directly onto the back of the weld. This effectively suppresses the occurrence of welding burns.

[0024] The tank 200, constructed by welding a dome-shaped component 230 and a cylindrical component 240, undergoes a passivation treatment, described later, to remove welding discoloration from the inside. After the discoloration of the welded parts is removed by the passivation treatment, a corrosion-resistant passivated coating is formed on the surface. Furthermore, as shown in Figure 2, the tank 200 is equipped with internal piping 260A and 260B inside the tank as extensions of the connecting joints, if necessary. The lower internal piping 260A is made into a downward L-shape to minimize the impact on the hot water supplied to the hot water supply piping. On the other hand, the upper internal piping 260B is made into an upward L-shape to maximize the tank capacity, as shown by the liquid level 270 inside the tank, while providing a hot water outlet joint on the side. Note that the external shape of the tank, the position of the joints, and the configuration of the internal piping 260A and 260B are not limited to the above embodiment and can be appropriately changed according to the application or design conditions.

[0025] Next, we will describe a processing apparatus 100 that performs passivation treatment on stainless steel parts R using a passivation treatment method.

[0026] As shown in Figure 4, the processing apparatus 100 includes a buffer tank 110 for holding the passivation treatment liquid P, an ozone generator 120 for generating ozone, and a heater 130 for heating the passivation treatment liquid P. If the stainless steel part R to be passivated is a tank 200, the processing apparatus 100 further includes pipes 151 and 152 for introducing the passivation treatment liquid P into the tank 200, and a pump 160 for transporting the passivation treatment liquid P.

[0027] The buffer tank 110 is not particularly limited as long as it is a container that can hold the passivation treatment liquid P, and is made of a resin or metal that does not deteriorate with the passivation treatment liquid P. An organic acid is added to the passivation treatment liquid P.

[0028] The ozone generator 120 is not particularly limited as long as it is capable of generating ozone, and specifically generates ozone by a discharge method, an electrolytic method, or an ultraviolet lamp method. Alternatively, instead of using the ozone generator 120, ozone gas may be blown into a solution containing an organic acid to generate a passivation treatment liquid P.

[0029] The heater 130 is not particularly limited as long as it can heat the passivation treatment liquid P, preferably heating the passivation treatment liquid P to a temperature of 0°C or higher but below its boiling point, and more preferably heating it to a temperature of 40°C or higher but below 60°C. Heating the passivation treatment liquid P allows for more efficient passivation treatment of the stainless steel parts R.

[0030] Next, a specific example of performing passivation treatment on a tank 200, which is a stainless steel component R to be subjected to passivation treatment, using a passivation treatment method with a processing device 100 will be described.

[0031] In the passivation treatment process (step S101), first, an organic acid is added to water stored in the buffer tank 110 to prepare a passivation treatment solution P. Next, ozone is added to the passivation treatment solution P using an ozone generator 120.

[0032] Next, the passivation treatment liquid P is introduced into the tank 200, which is the stainless steel part R to be subjected to passivation treatment, using pipes 151 and 152 and a pump 160. In the passivation treatment process (step S101), the passivation treatment may be carried out by introducing the passivation treatment liquid P into the tank 200 and circulating the passivation treatment liquid P within the tank 200 using the pump 160. In this way, the passivation treatment liquid P circulates between the buffer tank 110 and the tank 200 for a certain period of time.

[0033] Furthermore, the ozone generator 120 of the processing device 100 may be configured to continuously supply ozone gas to the passivation treatment liquid P contained in the buffer tank 110. This makes it possible to maintain a constant dissolved ozone concentration in the passivation treatment liquid P. Generally, ozone has the property of decomposing into oxygen through self-decomposition, and its decomposition rate depends on the temperature and pH of the passivation treatment liquid P. In particular, the self-decomposition rate is faster at higher temperatures, and it is known that the half-life is about 10 minutes to several tens of minutes. The ozone concentration in the passivation treatment liquid P converges to a concentration where the ozone supply rate and the self-decomposition rate are in equilibrium after a certain period of time. For this reason, when starting the treatment, after ensuring time for the ozone concentration to reach equilibrium, the circulation of the passivation treatment liquid P between the buffer tank 110 and the tank 200 is started. Furthermore, in order to maintain a constant ozone concentration, the circulation rate of the passivation treatment liquid P circulating between the buffer tank 110 and the tank 200 is preferably higher than the rate of self-decomposition of ozone contained in the passivation treatment liquid P. In this way, the passivation treatment liquid P is replaced at a rate higher than the rate of self-decomposition of ozone. Specifically, the circulation rate is equal to or greater than the tank capacity (L) of tank 200 / half-life (minutes) = rate (L / min). Also, if the tank capacity of tank 200 is 100L and the half-life of ozone in the passivation treatment liquid P is 10 minutes, the desired ozone concentration can be maintained by ensuring a circulation rate of tank capacity (L) / half-life (minutes) = rate (10L / min) or greater. With this configuration, the passivation treatment liquid P maintains a stable ozone concentration even during treatment, and a decrease in cleaning performance can be prevented.

[0034] When the passivation treatment liquid P is introduced into the tank 200, which is a stainless steel component R, the organic acid forms a complex with iron (Fe) components (iron ions), causing the iron ions to dissolve and removing welding burns or rust. It also has the function of concentrating the chromium (Cr) components contained on the surface of the base material. This increases the chromium concentration on the surface of the base material. Ozone oxidizes the Cr on the surface, forming a passivation film and improving corrosion resistance. As described above, a passivation film can be formed inside the tank 200, which is a stainless steel component R, using the treatment device 100.

[0035] Next, in the removal step (step S102), the passivation treatment liquid P adhering to the passivated stainless steel part R is removed. Specifically, the passivation treatment liquid P is removed using pipes 151 and 152 and a pump 160, and then washing water containing tap water or pure water is introduced to the passivated stainless steel part R for cleaning. In the removal step (step S102), the operation of introducing and discharging the washing water may be repeated multiple times. After cleaning, the stainless steel part R is dried as needed. After that, the passivation treatment of the stainless steel part R is completed.

[0036] As described above, the treatment apparatus 100 and passivation treatment method of Embodiment 1 make it possible to perform passivation treatment on stainless steel parts R that have weld burns or rust. Specifically, the organic acid forms a complex with iron (Fe) (iron ions), causing the iron ions to dissolve and removing the weld burns or rust. It also has the function of concentrating the chromium (Cr) contained in the base material surface. This increases the chromium concentration on the base material surface. Ozone can improve corrosion resistance by oxidizing Cr near the surface to form a passivation film. The passivation treatment liquid P contains ozone, which self-decomposes as an oxidizing agent, and does not contain hydrogen peroxide. Compared to hydrogen peroxide, ozone requires less energy to purify and is easier to process afterward. Furthermore, ozone is not harmful to the human body at concentrations of 0.2 ppm or less in the atmosphere. Therefore, it is possible to reduce energy costs and impact on the human body compared to using hydrogen peroxide as an oxidizing agent. As described above, the treatment apparatus 100 and passivation treatment method of Embodiment 1 can perform passivation treatment on stainless steel parts R that have weld burns or rust using organic acid and ozone, thus eliminating the need for pretreatment including mechanical polishing of the stainless steel parts R.

[0037] (Modification) In the above-described embodiment 1, an example was described in which, in the passivation treatment process (step S101), a passivation treatment liquid P is introduced into the tank 200, which is a stainless steel part R, using pipes 151 and 152 and a pump 160. In the passivation treatment process (step S101), as shown in Figure 5, the passivation treatment liquid P may be sprayed onto the treatment target area S of the stainless steel part R from a nozzle 153 attached to the end of the pipe 151. The treatment target area S is an area that requires passivation treatment and is an area from which the passivation film, including the welded parts of the stainless steel part R, has been removed. In this case, by rotating the tank 200 while spraying the passivation treatment liquid P onto the treatment target area S from the nozzle 153, it is possible to efficiently perform the passivation treatment. Alternatively, a camera (not shown) may be attached to the nozzle 153, and the passivation treatment liquid P may be sprayed onto the treatment target area S from the nozzle 153 while observing the treatment target area S. In this way, by spraying the passivation treatment liquid P onto the treatment area S that requires passivation treatment, passivation treatment can be performed using a small amount of passivation treatment liquid P. In addition, the water pressure of the passivation treatment liquid P sprayed from the nozzle 153 can promote scale removal.

[0038] Furthermore, in the passivation process (step S101), as shown in Figure 6, ozone gas OG may be introduced into the tank 200, which is a stainless steel part R, to perform the passivation treatment. In this case, ozone gas OG, which is an oxidizing agent, is filled into the sealed tank 200, and a solution containing an organic acid is sprayed using a spray 154 to generate a passivation treatment liquid P. By further rotating the tank 200, the passivation treatment can be performed inside the tank 200. By spraying a solution containing an organic acid, the surface area of ​​the solution is increased, and the low concentration of ozone gas OG can be efficiently dissolved. In this way, passivation treatment can be performed with a low concentration of ozone gas OG and a small amount of passivation treatment liquid P.

[0039] Furthermore, in the above-described embodiment 1, a configuration was described in which a passivation treatment liquid P is introduced into the tank 200, which is a stainless steel component R, during the passivation treatment process (step S101). Specifically, the passivation treatment liquid P is supplied into the tank 200 using pipes 151, 152 and a pump 160. As shown in Figure 7, the buffer tank 110 containing the passivation treatment liquid P may be equipped with a lid 170 that seals the buffer tank 110, and may have a sealed structure. Furthermore, a configuration in which ozone gas is supplied into the buffer tank 110 may be adopted. By making the buffer tank 110 a sealed structure, leakage of ozone gas to the outside can be suppressed. In addition, the partial pressure of the ozone gas component in the buffer tank 110 increases, creating a pressurized state. This pressurized state makes it possible to improve the ozone dissolution concentration in the passivation treatment liquid P. As a result, even when using a low-concentration and low-power ozone generator, a high-concentration ozone solution can be efficiently produced. Therefore, it becomes possible to stably supply a passivation treatment liquid P having the desired ozone concentration while miniaturizing or saving energy in the device.

[0040] Furthermore, in the removal process described above (step S102), a configuration was described in which washing water containing tap water or purified water is introduced into the passivated stainless steel parts R to remove the passivation treatment liquid P remaining inside the tank 200 and then washed. In addition, in this embodiment, an alkaline solution capable of neutralizing the acidic passivation treatment liquid P may be used as the washing water. When sodium bicarbonate is used, it is widely used as a food additive, and by preparing it as a powder material and dissolving it in a neutral solution containing tap water, alkaline washing water of the desired concentration can be easily produced. Concentration control is also simple, and it offers excellent workability. When sodium bicarbonate is used, the reaction products with the passivation treatment liquid P are only sodium citrate and carbon dioxide, which are also available as food additives. Therefore, the environmental impact of the washing water wastewater is low, and wastewater treatment is easy. In addition, the pH state inside the tank 200 can be easily confirmed by measuring the pH of the wastewater. By using such alkaline cleaning water, the amount of wastewater can be reduced compared to multiple dilution washes with tap water, and the work cycle time of the cleaning process can be shortened. Therefore, a cleaning process that balances reduced environmental impact with improved work efficiency can be achieved. Furthermore, the material used to generate the alkaline solution is not limited to sodium bicarbonate; for example, materials such as sodium hydroxide, potassium hydroxide, and sodium carbonate can be selected to achieve the desired pH adjustment.

[0041] Furthermore, the passivation treatment liquid P can be used for the maintenance of the tank 200, which is a stainless steel component R. The tank 200 is used to store hot water for use in showers, underfloor heating, and radiant heating. When used in Europe, the hot water is hard water, so the passivation film inside the tank 200 may be damaged by ions contained in the hot water. If the passivation film inside the tank 200 is damaged, it is preferable to maintain the tank 200. As shown in Figure 8, the maintenance method for the tank 200 is performed by a maintenance process comprising a passivation treatment step (step S301) and a removal step (step S302).

[0042] The passivation treatment step (step S301) is a process in which a passivation treatment liquid P containing an organic acid and ozone as an oxidizing agent, with the remainder being water, is introduced into the tank 200, and passivation treatment is performed on the inner wall of the tank 200. As shown in Figure 9, the passivation treatment liquid P is prepared in a buffer tank 110 and introduced into the tank 200. The passivation treatment liquid P is also introduced from the buffer tank 110 through the hot water inlet 210 of the tank 200. Furthermore, the passivation treatment liquid P may be returned to the buffer tank 110 from the hot water outlet 220 of the tank 200 and circulated for a certain period of time. By introducing the passivation treatment liquid P into the tank 200, passivation treatment is performed on the inner wall of the tank 200, and a passivation film can be regenerated inside the tank 200.

[0043] The removal process (step S302) is a process for removing the passivation treatment liquid P. Specifically, in the removal process (step S302), the passivation treatment liquid P is discharged from the inlet 210 of the tank 200 through the piping 155 by switching valve 161. Then, valve 161 is switched to introduce cleaning water from piping 156 to the inlet 210 of the tank 200, and the introduced cleaning water is discharged through piping 155. The cleaning water includes tap water. By repeating the introduction and discharge of cleaning water multiple times, the passivation treatment liquid P is removed from inside the tank 200. As a result, even if the passivation film inside the tank 200 is destroyed by the use of the tank 200, it is possible to regenerate the passivation film inside the tank 200. The passivation treatment liquid P contains organic acids and ozone as an oxidizing agent. Ozone decomposes or is released into the atmosphere in a short period of time. Organic acids are weak acids that are also used in food, so they have little effect on the human body. Furthermore, since the maintenance method for this tank 200 does not use nitric acid or hydrofluoric acid, it can be easily applied to the maintenance of the tank 200.

[0044] In the above-described first embodiment, an example was explained in which the passivation treatment liquid P is introduced into the tank 200, which is the stainless steel component R, in the passivation treatment step (step S301). The passivation treatment step (step S301) only needs to be capable of performing the passivation treatment on the stainless steel component R, and the stainless steel component R may be immersed in the passivation treatment liquid P stored in the buffer tank 110. In this case, the shape of the stainless steel component R is not particularly limited, and may be a shape other than a tank. Further, even when the stainless steel component R is the tank 200, the passivation treatment can be performed by immersing it in the passivation treatment liquid P stored in the buffer tank 110.

[0045] In the above-described embodiment, the passivation treatment method for the stainless steel component R was explained. This passivation treatment method can be used in a tank manufacturing method. As shown in FIG. 10, the tank manufacturing method includes a welding step (step S401) of manufacturing the tank 200 by welding the stainless steel component R, and a passivation treatment step (step S402) of performing a passivation treatment by applying or immersing the tank 200 manufactured in the welding step (step S401) in a passivation treatment liquid containing an organic acid and ozone as an oxidizing agent, with the balance including water, and a removal step (step S403). The passivation treatment step (step S402) and the removal step (step S403) are the same as the passivation treatment step (step S101) and the removal step (step S102) of the above-described embodiment. In the welding step (step S401), by welding the stainless steel component R, the passive film is destroyed, and scale, which is an iron-derived oxide, is formed. This scale is removed by the passivation treatment step (step S402), and the stainless steel component R is subjected to the passivation treatment. By doing so, even if weld discoloration or rust occurs in the welding step (step S401), a passivation-treated tank can be manufactured. In the passivation treatment step (step S402), the passivation treatment may be performed only on the inside of the tank. In this way, the inside of the tank that requires the passivation treatment can be passivated with a small amount of the passivation treatment liquid.

[0046] Furthermore, in the passivation treatment process (step S402), as shown in Figure 11, passivation treatment may be carried out using a line production method with a passivation treatment solution P' containing ozone. The passivation treatment solution P' is prepared considering the self-decomposition rate of ozone. Generally, the half-life of ozone concentration in a solution is 10 minutes to several tens of minutes, so when the treatment time is 30 minutes, the dissolved ozone concentration is set to the range of 0.1 to 1.0 ppm, and a high concentration of ozone is included at the start of the pickling process to compensate for the decrease in concentration during the treatment. For the tank 200 that has completed the welding process (step S401), the passivation treatment solution P' is filled from the lower joint upstream of the line in the passivation treatment process (step S402) until it is full up to the outlet of the hot water pipe. This filling method makes it possible to pickle all welded parts in the manufacturing process with the passivation treatment solution P'. Tanks 200 filled with passivation treatment liquid P' are transported along the line with their fittings sealed, and after a few minutes to several tens of minutes, which is the pickling treatment time, they move to the removal process (step S403). By adjusting the number of tanks on the line, the production cycle can be flexibly adjusted, and no additional processing equipment is required compared to the cell production system. In order to maintain the temperature within a certain range during the passivation treatment process (step S402), a heat retention mechanism equipped with insulation material or a heater may be provided on the exterior of the tank. This configuration prevents a drop in the temperature of the processing liquid and stabilizes the processing effect. After the passivation treatment process (step S402) is completed, the tanks 200 have the passivation treatment liquid P' discharged via the lower piping by pressurizing air from the upper piping. The lower piping has an L-shape, and is structured to minimize the amount of passivation treatment liquid P' remaining at the bottom of the tank. Since the inside of the tank is acidic after the passivation treatment liquid P' is discharged, a neutral solution of washing water containing tap water is filled in from the lower piping and similarly drained by air. By repeating this washing process multiple times, the inside of the tank can be neutralized and the pickling reaction can be stopped. According to this embodiment, by adopting a line production system, the production cycle can be flexibly adjusted and additional equipment is not required.Furthermore, since the passivation treatment liquid P' can be treated in a completely sealed state, the risk of ozone gas leakage can be minimized, and the treatment conditions can be stabilized by adding a temperature control mechanism.

[0047] Hereinafter, the present disclosure will be described in more detail based on Examples, but the present disclosure is not limited to these Examples.

[0048] The passivation treatment of stainless steel part R is shown by Examples. The material of the stainless steel samples used in Example 1 and Comparative Examples 1 to 2 was Duplex 2304. In addition, the sample of Example 1 before the passivation treatment had weld discoloration and rust. Using the passivation treatment liquid under the conditions shown in Table 1, the passivation treatment was performed on the samples of Example 1 and Comparative Example 2. Citric acid was used as the organic acid, and ozone was used as the oxidizing agent. Comparative Example 1 is a sample before the passivation treatment. The passivation treatment time for each sample was 30 minutes.

[0049]

[0050] (Corrosion Resistance) The corrosion resistance of the samples of Example 1 and Comparative Examples 1 to 2 was evaluated by measuring the pitting potential. The measurement of the pitting potential was performed in accordance with JIS G 0577:2014, Method for measuring pitting potential of stainless steels. Taking the pitting potential of the sample of Comparative Example 1 as the reference potential, the pitting potential of the samples of Example 1 and Comparative Example 2 was rated as Poor when less than 1.35 times the reference potential, Good when 1.35 times or more and less than 1.6 times the reference potential, and Excellent when 1.6 times or more the reference potential. The results are shown in Table 2 below. As shown in Table 2, for Example 1, since the passivation treatment liquid containing citric acid as the organic acid and ozone as the oxidizing agent was used, the corrosion resistance was Excellent. For Comparative Example 1, since no passivation treatment was performed, the corrosion resistance was Poor. For Comparative Example 2, since the passivation treatment liquid containing only citric acid as the organic acid was used, the corrosion resistance was Poor.

[0051] (Oxide film removal) Oxide film removal was performed by visual inspection. The oxide film included rust or welding burns. If the oxide film was removed, it was rated as excellent; if a small amount remained, it was rated as good; and if it was not removed, it was rated as poor. The results of the visual inspection for oxide film removal are shown in Table 2. For Example 1, since a passivation treatment solution containing citric acid as an organic acid and ozone as an oxidizing agent was used, the oxide film removal was excellent. For Comparative Example 1, since no passivation treatment was performed, the oxide film removal was poor. For Comparative Example 2, since a passivation treatment solution containing citric acid as an organic acid was used, the oxide film removal was good.

[0052]

[0053] As described above, it was found that the passivation treatment method of the examples makes it possible to perform passivation treatment on stainless steel samples that have weld burns or rust. Specifically, citric acid forms a complex with iron (Fe) (iron ions), causing the iron ions to dissolve and removing the weld burns or rust. It also has the function of concentrating the chromium (Cr) content contained in the base material surface. This increases the chromium concentration on the base material surface. It is thought that ozone oxidizes the Cr near the surface to form a passivation film, thereby improving corrosion resistance. In the above examples, citric acid was used as the organic acid, but it is thought that similar effects can be obtained by using acetic acid or malic acid instead of citric acid. Furthermore, in the passivation treatment method of the above examples, since passivation treatment can be performed on stainless steel parts that have weld burns or rust using an organic acid and ozone, pretreatment including pickling with nitrohydrofluoric acid solution or mechanical polishing is unnecessary for stainless steel parts.

[0054] This disclosure allows for various embodiments and modifications without departing from the broad spirit and scope of this disclosure. Furthermore, the embodiments described above are for illustrative purposes only and do not limit the scope of this disclosure. In other words, the scope of this disclosure is indicated by the claims, not by the embodiments. Various modifications made within the scope of the claims and the equivalent significance of the disclosure are considered to be within the scope of this disclosure.

[0055] This application is based on Japanese Patent Application No. 2025-068624, filed on 18 April 2025. The entire specification, claims, and drawings of Japanese Patent Application No. 2025-068624 are incorporated herein by reference.

[0056] 100... Processing device, 110... Buffer tank, 120... Ozone generator, 130... Heater, 151, 152, 155, 156... Piping, 153... Nozzle, 154... Spray, 160... Pump, 161... Valve, 170... Lid, 200... Tank, 210... Inlet, 220... Outlet, 230... Dome-shaped part, 240... Cylindrical part, 250... Connection part, 260A, 260B... Internal piping, R... Stainless steel part, P... Passivation treatment liquid, OG... Ozone gas

Claims

1. A method for passivating stainless steel parts, comprising a passivation step of applying or immersing the stainless steel parts in a passivation solution containing an organic acid and ozone as an oxidizing agent, with the remainder being water, to perform the passivation treatment.

2. The passivation treatment method according to claim 1, wherein the organic acid is citric acid.

3. The passivation treatment method according to claim 2, wherein the concentration of the citric acid is 0.1% by mass or more and less than or equal to the saturation dissolution amount.

4. The passivation treatment method according to any one of claims 1 to 3, wherein the concentration of ozone is 0.01 ppm or more and 0.2 ppm or less.

5. The passivation treatment method according to any one of claims 1 to 3, wherein the temperature of the passivation treatment solution containing the organic acid and the ozone is 0°C or higher and below the boiling point.

6. The passivation treatment method according to any one of claims 1 to 3, wherein in the passivation treatment step, ozone gas is blown into a solution containing an organic acid to produce a passivation treatment liquid.

7. The passivation treatment method according to any one of claims 1 to 3, wherein in the passivation treatment step, the passivation treatment liquid is sprayed from a nozzle onto the area to be treated of the stainless steel part.

8. The passivation treatment method according to any one of claims 1 to 3, wherein the stainless steel component is a tank, and in the passivation treatment step, the passivation treatment liquid is introduced into the tank from a buffer tank containing the passivation treatment liquid, and the passivation treatment is carried out while circulating the passivation treatment liquid between the buffer tank and the tank.

9. The passivation treatment method according to claim 8, wherein, in the passivation treatment step, the circulation rate at which the passivation treatment liquid is circulated between the buffer tank and the tank is greater than the rate at which ozone contained in the passivation treatment liquid self-decomposes.

10. A method for passivating a tank made of stainless steel, comprising a passivation step of applying or immersing the tank in a passivation solution containing an organic acid and ozone as an oxidizing agent, with the remainder being water, to carry out the passivation treatment.

11. The method for passivating a tank according to claim 10, wherein in the passivation process, ozone gas is filled into the tank and the solution containing the organic acid is sprayed into the tank to generate the passivation treatment liquid.

12. A method for maintaining a tank made of stainless steel, comprising: a passivation treatment step of introducing a passivation treatment liquid containing an organic acid and ozone as an oxidizing agent, with the remainder being water, into the inside of the tank and performing a passivation treatment on the inner wall of the tank; and a removal step of removing the passivation treatment liquid.

13. A method for manufacturing a tank, comprising: a welding step of manufacturing a tank by welding stainless steel parts; and a passivation treatment step of applying or immersing the tank manufactured in the welding step with a passivation treatment solution containing an organic acid and ozone as an oxidizing agent, with the remainder being water, to perform a passivation treatment.

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