A cupro-nickel rust removal and passivation solution, cleaning method and shell-and-tube heat exchanger

Abstract

The application discloses a cupronickel rust removal and passivation solution, a cleaning method and a shell-and-tube heat exchanger. The cupronickel rust removal and passivation solution is composed of 8-15% of inorganic acid, 0.1-0.5% of corrosion inhibitor and passivation agent (consisting of copper alloy corrosion inhibitor and black metal corrosion inhibitor) and the rest of B-grade water. After the cupronickel rust removal and passivation solution is circulated for 90-120 minutes, a uniform passivation film can be formed on the surface of BFe1011 or BFe3011 cupronickel, and the corrosion rate of the base material and the weld is less than 0.01 mm / a. The application also provides a corresponding pickling and passivation method, which comprises the steps of circulation treatment, displacement rinsing and drying and inspection. The problems of high corrosion rate, poor passivation film quality and lack of heterogeneous metal protection in the traditional pickling method are solved, and efficient and low-corrosion maintenance is realized.

Description

Technical Field

[0001] This invention belongs to the field of metal rust removal and passivation technology, and particularly relates to a rust removal and passivation solution for white copper, a cleaning method, and a shell-and-tube heat exchanger. Background Technology

[0002] Shell-and-tube heat exchangers are core heat exchange equipment widely used in industries such as power, petrochemicals, and shipbuilding. Their operational reliability directly affects the energy efficiency and safety of the entire system. Among them, shell-and-tube heat exchangers made of cupronickel (copper-nickel alloy) materials such as BFe10-1-1 and BFe30-1-1 are widely used due to their good thermal conductivity and excellent resistance to seawater and industrial cooling water corrosion.

[0003] However, during long-term operation, dissolved oxygen, chloride ions, microorganisms, and various impurities in the cooling water can cause uniform corrosion and pitting on the inner wall of the shell-and-tube heat exchanger, forming a corrosion product layer (rust scale) mainly composed of copper oxide and basic copper carbonate. This rust scale not only greatly reduces heat exchange efficiency, but the active corrosion points hidden beneath it will continue to develop, and in severe cases, it can lead to perforation and leakage of the shell-and-tube heat exchanger, causing unplanned downtime and significant economic losses.

[0004] To ensure the long-term safe and economical operation of equipment, thorough chemical cleaning of shell-and-tube heat exchangers during maintenance to remove rust and scale, and subsequently form a stable and dense passivation protective film on their clean surface, is a crucial maintenance method. Currently, for pickling and passivation treatment of copper and copper alloys, the industry typically uses single or simple mixed acid systems, such as nitric acid, sulfuric acid, or mixtures thereof. While this method can achieve the effect of pickling and passivation, it has the following technical problems.

[0005] First, there is the challenge of corrosion control: While strong oxidizing acids, such as nitric acid, have strong cleaning and passivation capabilities, they also exhibit a high rate of uniform corrosion on the cupronickel substrate, particularly in areas with uneven microstructures, such as welds, where selective corrosion can easily occur, potentially leading to material thinning or even stress corrosion cracking. Using sulfuric acid alone, however, yields poor passivation results.

[0006] Secondly, the quality of the membrane layer is unstable: the passivation membrane formed by traditional acid washing methods is often loose and uneven, and its resistance to pitting corrosion is insufficient in working environments containing chloride ions, resulting in a short protection life. After the equipment is put into operation, the water quality (such as iron ion content) is difficult to quickly and stably meet the standards.

[0007] Secondly, there is a lack of targeted protection: shell-and-tube heat exchangers may contain carbon steel supports, flanges, or welds made of dissimilar steels. Existing pickling solutions for copper often neglect the simultaneous protection of these ferrous metal components, which can easily lead to over-corrosion or hydrogen embrittlement, affecting the overall structural integrity. Summary of the Invention

[0008] To address the shortcomings of existing technologies, this invention provides a method for removing rust and passivating white copper, a cleaning solution, and a shell-and-tube heat exchanger. To achieve the aforementioned objectives, this invention employs the following technical solution: A solution for removing rust and passivating white copper, comprising the following components by mass percentage: (A) Inorganic acid combination: 8%~15%, wherein the inorganic acid combination is composed of sulfuric acid, phosphoric acid and nitric acid in a mass ratio of (4~6):(2~4):(2~4); (B) Corrosion inhibitor and passivator combination: 0.1%~0.5%, wherein the corrosion inhibitor and passivator combination is composed of a copper alloy-specific corrosion inhibitor and passivator and a ferrous metal corrosion inhibitor; (C) The remainder is Grade B water, which is chloride ion (Cl) water. - Deionized water or distilled water with a content not exceeding 1 mg / L; The cupronickel rust removal and passivation solution is configured to form a uniform passivation film on the surface of BFe10-1-1 or BFe30-1-1 cupronickel material after 90-120 minutes of cyclic treatment, and the uniform corrosion rate of the base material and weld is less than 0.01 mm / a.

[0009] Furthermore, the inorganic acid combination has a mass percentage of 10% to 12% and is composed of sulfuric acid, phosphoric acid, and nitric acid in a mass ratio of 5:2.5:2.5.

[0010] Furthermore, the copper alloy-specific corrosion inhibitor and passivator is at least one of benzotriazole, methylbenzotriazole, or mercaptobenzothiazole; the ferrous metal corrosion inhibitor is at least one of thiourea, hexamethylenetetramine, or rutin.

[0011] Furthermore, the mass percentage of the corrosion inhibitor and passivator combination is 0.2% to 0.3%, and the mass ratio of the copper alloy-specific corrosion inhibitor and passivator to the ferrous metal corrosion inhibitor is 1:(15 to 25).

[0012] Furthermore, the copper alloy-specific corrosion inhibitor and passivator is benzotriazole, and the ferrous metal corrosion inhibitor is thiourea, with a mass ratio of 1:20.

[0013] According to a second aspect of the present invention, a pickling and passivation method for a shell-and-tube heat exchanger is provided, comprising: S1: Using the aforementioned white copper rust removal and passivation solution, a vertically placed shell-and-tube heat exchanger is subjected to circulating acid pickling treatment at a temperature of 20℃~35℃, with a circulation flow rate of 0.5~2.0 m / s and a treatment time of 90-120 minutes; S2: After the acid washing cycle is completed, immediately pump Class B water at 40℃~50℃ into the shell-and-tube heat exchanger at a flow rate of 1.5~3 times that of the acid washing cycle to replace and rinse until the pH value of the discharged water is ≥6.8 for three consecutive measurements. S3: Use compressed air or nitrogen with a dew point below -20℃ and a temperature of 50℃±5℃ to purge the inner cavity of the shell-and-tube heat exchanger until no water film accumulates on its inner wall.

[0014] Furthermore, in step S1, the cyclic pickling is carried out in a closed loop consisting of an acid-resistant pump, the shell-and-tube heat exchanger, and an external circulation box with a 20μm filter.

[0015] Furthermore, in step S2, after the replacement flushing, a shaking rinsing step is also included: the shell-and-tube heat exchanger is placed horizontally, and gas is pulsed into the shell-and-tube heat exchanger filled with Class B water at a pressure of less than 0.8 MPa, and the overflow state is maintained for continuous rinsing for more than 30 minutes.

[0016] Furthermore, after step S3 is completed, the inner wall of the shell-and-tube heat exchanger is inspected 100% using an endoscope. The passivation film should be a uniform matte silver-white color, without any rust or active corrosion spots remaining.

[0017] According to a third aspect of the present invention, a shell-and-tube heat exchanger subjected to pickling and passivation treatment is also provided, wherein the passivation film on the inner wall is formed by processing with the aforementioned white copper rust removal and passivation solution according to the above method, and the pitting potential of the passivation film in 3.5% NaCl solution is positively shifted by not less than 50mV relative to the untreated surface.

[0018] Compared with the prior art, the present invention has the following beneficial effects: 1. By employing a ternary composite acid system of sulfuric acid, phosphoric acid, and nitric acid in a specific mass ratio ((4~6):(2~4):(2~4)), the copper rust removal and passivation solution of this invention fully leverages the advantages of each component: sulfuric acid provides strong dissolving power and quickly removes rust; phosphoric acid acts as a corrosion inhibitor and surface leveling agent; and nitric acid serves as an effective passivation promoter. This composite system ensures efficient dissolution of rust layers on the copper surface (typically completely removed within 90-120 minutes), while synergistically using a specific combination of composite corrosion inhibitors (benzotriazoles and thioureas, etc.). This significantly suppresses the uniform corrosion rate of copper base materials such as BFe10-1-1 and BFe30-1-1, as well as welds, during the pickling process to below 0.01 mm / a, solving the technical problems of high corrosion rate, selective corrosion, and material thinning associated with traditional nitric acid pickling methods.

[0019] 2. In this invention, nitric acid, acting as an oxidant in synergy with corrosion inhibitors, promotes the formation of a uniform, dense passivation film, primarily composed of Cu2O, on a clean cupronickel surface. Verification using a 3.5% NaCl solution showed that the pitting potential on the treated cupronickel surface shifted positively by no less than 50 mV compared to the untreated surface. This demonstrates that the passivation film possesses resistance to chloride ion pitting corrosion, solving the problems of loose, uneven, and poorly corrosion-resistant passivation films found in traditional methods, thus extending the equipment's protection life and operating cycle.

[0020] 3. This invention incorporates both copper alloy-specific corrosion inhibitors (such as benzotriazole) and ferrous metal corrosion inhibitors (such as thiourea) in its corrosion inhibitor combination, with optimized proportions. This allows the copper rust removal and passivation solution of this invention to simultaneously and effectively protect carbon steel supports, flanges, or dissimilar steel welds that may exist inside the shell-and-tube heat exchanger when treating the copper body material, preventing hydrogen embrittlement or over-corrosion. This ensures the integrity of the overall structure of the shell-and-tube heat exchanger and solves the safety hazards of existing technologies that protect only a single object and may damage related components. Attached Figure Description

[0021] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a schematic diagram of the BFe30-1-1 cupronickel standard specimen before pickling in Example 1; Figure 2 This is a schematic diagram of the BFe30-1-1 cupronickel standard specimen after pickling in Example 1; Figure 3 This is a schematic diagram of the metallographic structure of the specimen before immersion corrosion in Example 1; Figure 4 This is a schematic diagram of the metallographic structure of the specimen after immersion etching in Example 1; Figure 5 This is a schematic diagram of the metallographic structure of the test plate before immersion etching in Example 1; Figure 6 This is a schematic diagram of the metallographic structure of the test plate after immersion etching in Example 1; Figure 7 This is a schematic diagram comparing the polarization curves of Example 3. Detailed Implementation

[0022] To enable those skilled in the art to better understand the technical solutions of the present invention, the following will provide a detailed description of the copper rust removal, passivation solution, cleaning method, and shell-and-tube heat exchanger described in the present invention, in conjunction with specific embodiments and performance comparison data. It should be understood that the specific embodiments described herein are for illustrative purposes only and do not constitute any limitation on the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0023] Example 1 The rust removal and passivation solution for white copper provided by this invention is a chemical treatment liquid specifically designed for shell-and-tube heat exchangers made of white copper materials such as BFe10-1-1 and BFe30-1-1. Its core lies in achieving a balance between efficient cleaning, low corrosion rate, and high-quality passivation through the synergistic effect of specific components. This rust removal and passivation solution for white copper is composed of the following components by mass percentage: Component (A) Inorganic acid combination: 8% ~ 15%.

[0024] The inorganic acid combination is composed of sulfuric acid (H2SO4), phosphoric acid (H3PO4), and nitric acid (HNO3) mixed in a specific mass ratio, ranging from (4~6):(2~4):(2~4). Each of the three acids has a specific function. Sulfuric acid provides the primary acidification and dissolving power, rapidly removing oxide scale and rust products; phosphoric acid has a mild complexing and polishing effect, contributing to a cleaner, smoother surface; and nitric acid, as a strong oxidizing agent, is a key component in promoting the formation of a dense passivation film, primarily composed of Cu2O, on the surface of the cupronickel. This specific ratio avoids problems such as over-corrosion, hydrogen embrittlement, or a loose passivation film that can result from using a single acid or improper proportions.

[0025] Preferably, the inorganic acid combination has a mass percentage of 10% to 12%, and the mass ratio of sulfuric acid, phosphoric acid and nitric acid is 5:2.5:2.5.

[0026] Component (B) Corrosion inhibitor and passivator combination: 0.1%~0.5%.

[0027] This combination is an additive that ensures the safety of the processing and the final performance. It is a compound of a copper alloy-specific corrosion inhibitor and a ferrous metal corrosion inhibitor.

[0028] The copper alloy-specific corrosion inhibitor and passivator is selected from at least one of benzotriazole (BTA), methylbenzotriazole (TTAA), or mercaptobenzothiazole (MBT). These molecules can form stable adsorption films or surface complexes with copper ions through their heteroatoms, effectively inhibiting the active dissolution of copper during the pickling stage and integrating into the passivation film in subsequent processes, significantly improving the film's density and stability.

[0029] The ferrous metal corrosion inhibitor is selected from at least one of thiourea, hexamethylenetetramine (urotropine), or the commercial corrosion inhibitor "Rudin" (whose main component is di-o-toluenethiourea, etc.). The addition of such corrosion inhibitors aims to protect carbon steel supports, flanges, or weld areas (such as dissimilar steel welds) that may exist in the shell-and-tube heat exchanger, preventing uneven corrosion or hydrogen damage from acid solutions and ensuring that the overall equipment corrosion rate meets standards.

[0030] Preferably, the mass percentage of the corrosion inhibitor and passivator combination is 0.2% to 0.3%. Furthermore, the mass ratio of the copper alloy-specific corrosion inhibitor and passivator to the ferrous metal corrosion inhibitor should be controlled within the range of 1:(15~25). This ratio optimizes the adsorption and synergistic effect of the two types of corrosion inhibitors on the surfaces of copper and iron substrates.

[0031] In a more preferred embodiment, benzotriazole is used as a corrosion inhibitor for copper alloys, and thiourea is used as a corrosion inhibitor for ferrous metals, and the two are compounded at a mass ratio of 1:20. This combination exhibits excellent corrosion inhibition efficiency and synergistic passivation effect.

[0032] Component (C) Grade B water: balance.

[0033] As a solvent and diluent, it must be Grade B water (i.e., deionized or distilled water) that meets relevant standards (such as GB / T6682 or ASTM D1193), with extremely low chloride ion (Cl) levels as a key indicator. - The content and conductivity of chlorine. The purpose of using high-purity water is to avoid introducing Cl. - Corrosive ions such as chloride ions interfere with the formation of the passivation film and induce pitting corrosion at film defects, fundamentally impairing the treatment effect. Specifically, in this embodiment, the chloride ions (Cl...) in the B-grade water... - The content is not higher than 1 mg / L.

[0034] Preparation method: In an acid-resistant container, first take the required amount of Grade B water according to the calculated mass. While continuously stirring, slowly add the pre-mixed inorganic acid combination (A) to the Grade B water. It should be noted that the acid should be added to the water during the preparation process to prevent fluid splashing and violent exothermic reactions. After the acid solution is evenly mixed and cooled to room temperature, add the pre-weighed and evenly mixed corrosion inhibitor and passivating agent combination (B) to the acid solution, and continue stirring until it is completely dissolved to obtain a homogeneous and clear pickling and passivation working solution.

[0035] It should be noted that before the prepared white copper rust removal and passivation solution is put into practical application, its core performance must be verified through standardized laboratory experiments: rust (scale) removal ability and corrosion control level of the base material.

[0036] Experiment 1: Verification of the effect of pickling for rust removal Specimen preparation: such as Figure 1 As shown, a BFe30-1-1 cupronickel standard specimen was selected, and a uniform artificial rust layer (such as oxide scale produced by high-temperature oxidation or salt spray test to simulate service) was prepared on its surface.

[0037] Subsequently, the rusted specimens were completely immersed in a freshly prepared cupronickel rust removal and passivation solution prepared according to the above ratio (12% inorganic acid combination, benzotriazole:thiourea = 1:20, total concentration 0.25%). The specimens were then left to stand in a constant temperature water bath at 25°C, or subjected to low-speed magnetic stirring to simulate slight circulation.

[0038] Finally, observe the surface changes of the specimen regularly. Typically, changes should occur within 90-120 minutes. Figure 2 As shown, the rust layer on the surface of the specimen could be completely dissolved and removed, exposing the metal substrate. The treated surface had a uniform color, was in an activated state, and showed no signs of over-corrosion such as localized bright or dull areas, nor any obvious tendency for intergranular corrosion. This indicates that the formulation of this invention has a mild, efficient, and uniform rust removal capability, laying the foundation for subsequent uniform film formation.

[0039] Experiment 2: Verification of Uniform Corrosion Rate Specimen preparation: such as Figure 3 , 5 As shown, a BFe10-1-1 white copper polished specimen with a polished and cleaned surface and a precisely measured surface area (S), as well as a butt weld test plate of the same material (simulating actual working conditions), were prepared. Their initial mass (m1) was accurately weighed using an analytical balance.

[0040] Corrosion test: The specimen and test plate were completely immersed in a copper rust removal and passivation solution with the same ratio as in Experiment 1, and placed in a constant temperature shaking water bath at 30°C to simulate dynamic conditions. The test duration was strictly controlled at 120 minutes.

[0041] Result calculation: After the experiment, the specimen should be removed immediately, such as... Figure 4 , 6 As shown, the treatment was carried out according to the standard procedure (e.g., neutralizing residual acid with dilute Na₂CO₃ solution, then rinsing with Grade B water and anhydrous ethanol, and drying). The treated mass (m²) was then accurately weighed on the same analytical balance. The uniform corrosion rate (v, unit: mm / a) was calculated using the following formula: v = (m1- m2) / (ρ S t) K Where ρ is the density of cupronickel (g / cm³), and t is the processing time (years, here 120 / (365)). twenty four (60) years), K is a conversion constant.

[0042] Experimental data show that, using the copper rust removal and passivation solution of this invention, the uniform corrosion rate v of the copper base material and weld area is consistently below 0.01 mm / a, fully meeting the stringent safety standards set in the technical solution. This confirms the excellent protective performance of the compound corrosion inhibitor system on the copper substrate.

[0043] In addition, such as Figure 3 , 4 Images 5 and 6 show metallographic photographs of the specimens and test plates before and after corrosion, respectively. They demonstrate that the edges of the specimens and test plates after immersion in the white copper rust removal and passivation solution of this invention show no obvious corrosion marks.

[0044] Example 2 Based on the favorable results of laboratory verification, the copper rust removal and passivation solution and supporting methods of this invention can be applied to the online chemical cleaning and passivation of actual shell-and-tube heat exchangers, including the following steps: Step S1: System setup and cyclic pickling and passivation System preparation: The shell-and-tube heat exchanger to be treated is included as part of the cleaning loop. It is connected to an acid-resistant pump (usually made of plastic, such as PP or PVDF) and a mobile circulating cleaning tank (or fixed tank) with a 20μm precision filter via temporary piping, forming a closed forced circulation system. The filter is used to capture solid particles that fall off during the cleaning process, preventing them from scratching the cleaned surface or clogging the pipes.

[0045] Method Execution: Inject a pre-prepared, sufficient amount of copper rust removal and passivation solution into the circulation system. Start the circulation pump, controlling the solution temperature within a mild range of 20℃ to 35℃, and maintain the circulation flow rate at 0.5~2.0 m / s to ensure sufficient fluid exchange and chemical action in all parts of the shell-and-tube heat exchanger, especially complex areas such as welds and corners. The circulation treatment time is strictly controlled within 90-120 minutes. During this stage, the chemical dissolution of oxide scale and dirt, as well as the passivation pretreatment of the fresh metal surface, are simultaneously completed.

[0046] Step S2: Residual liquid replacement and deep rinsing Hot Displacement Flushing: Immediately after step S1, switch the pipeline and pump preheated Class B water (40°C to 50°C) into the shell-and-tube heat exchanger at a flow rate 1.5 to 3 times higher than that in step S1. This provides a rapid and continuous displacement flushing of the system. The high-temperature, high-speed water flow effectively dilutes and removes residual acid, minimizing the contact time between the metal and the dilute acid.

[0047] Determining the endpoint of rinsing: Continuously sample at the system discharge port and measure the pH value with a pH meter. When the pH value is consistently ≥6.8 for three consecutive measurements, it is determined that the residual acid has been basically removed, and the replacement rinsing can be stopped.

[0048] Agitation rinsing: Preferably, agitation rinsing is performed to further remove trace amounts of residual liquid and loose particles that may remain in corners, crevices, or adhere to surfaces. Specifically, during this process, the shell-and-tube heat exchanger is adjusted to a horizontal position and filled with Grade B water. Then, clean compressed air or nitrogen is pulsed (i.e., intermittently) introduced at a pressure less than 0.8 MPa. Specifically, the pressure is set to 0.1-0.8 MPa. In this way, the agitation force generated by the gas agitates the water flow, enhancing the cleaning effect. The process is maintained in an overflow state, and rinsing continues for at least 30 minutes.

[0049] Step S3: Drying and Final Quality Inspection Deep drying: After draining the rinse water, use dry compressed air or nitrogen with a dew point temperature below -20℃ (ensuring extremely dry air) and a temperature controlled at 50℃ ± 5℃ to thoroughly purge the inner cavity of the shell-and-tube heat exchanger. Purge should continue until the inner wall is completely dry, with no visible or tactile water stains or water film buildup. This method prevents incomplete passivation film or flash rust caused by a humid environment.

[0050] Final acceptance: After drying, use an industrial endoscope to conduct a 100% visual inspection of the inner wall of the shell and tube heat exchanger (including all areas such as tube sheet, support plate, and weld seams of the shell and tube heat exchanger).

[0051] Acceptance criteria: The entire inner surface after treatment should form a uniform, complete, and continuous matte silver-white passivation film. The film should have a consistent color, with no reflective highlights or dark areas. No rust (including yellow rust and red rust), watermarks, active corrosion spots, untreated original surfaces, or obvious contaminant residues should be found during inspection.

[0052] Example 3 Performance characterization: electrochemical testing According to standard electrochemical testing methods (such as GB / T 17899 or ASTM G5), in a 3.5% (mass fraction) NaCl solution (simulating seawater or chlorine-containing cooling water environment). Figure 7 As shown, potentiodynamic polarization curves were scanned for both the untreated raw cupronickel sample (black line) and the sample treated by the method of this invention (red line) (scanning rate typically 0.5-1 mV / s). By comparing the polarization curves of the two samples, the key parameter—pitting potential (Eb, or breakdown potential)—was extracted.

[0053] Typical test results: The pitting potential (Eb_treated) of the treated sample shows a significant positive shift compared to the untreated sample (Eb_untreated). According to the requirements of this invention and a large amount of experimental data, this positive shift (ΔEb = Eb_treated - Eb_untreated) is not less than +50 mV. This significant positive shift in pitting potential is direct, quantitative evidence of the enhanced resistance of the passivation film to localized corrosion (especially to chloride-induced pitting), indicating a longer service life for the product in corrosive media.

[0054] To more intuitively demonstrate the technical effects of the present invention, the following non-limiting embodiments and comparative examples are provided: Example 4 Formula: By mass ratio, take 12% of inorganic acid combination (H2SO4:H3PO4:HNO3=5:2.5:2.5), 0.25% of corrosion inhibitor and passivator combination (BTA:thiourea=1:20), and the balance is grade B water.

[0055] Objects to be processed: BFe10-1-1 cupronickel base material and welding test plate.

[0056] Method: 30℃, flow rate 1.2 m / s, circulate for 110 minutes, and strictly follow steps S2 and S3 thereafter.

[0057] Results: The surface was a uniform matte silver-white. The uniform corrosion rate was 0.0068 mm / a. In 3.5% NaCl solution, the pitting potential shifted positively by +72 mV.

[0058] Example 5 Formula: By mass ratio, take 10% of inorganic acid combination (H2SO4:H3PO4:HNO3=4:3:3), 0.3% of corrosion inhibitor and passivator combination (methylbenzotriazole:hexamethylenetetramine=1:18), and the balance is grade B water.

[0059] Object to be processed: BFe30-1-1 cupronickel specimen.

[0060] Method: 28℃, flow rate 0.8 m / s, treatment time 115 minutes.

[0061] Results: The passivation film was dense and uniform. The uniform corrosion rate was 0.0079 mm / a. The pitting potential shifted positively by +61 mV.

[0062] Comparative Example 1 (without compound corrosion inhibitor): Formula: Contains only 12% inorganic acid combination (same ratio as in Example 1), without any corrosion inhibitors.

[0063] Method and object: Same as in Example 4.

[0064] Results: The surface was rough and blackened with obvious corrosion grooves. The uniform corrosion rate was as high as 0.41 mm / a. The pitting potential shifted positively by +5 mV. This indicates that the film was unstable.

[0065] Comparative Example 2 (Traditional Nitric Acid Washing): Formula: 10% industrial nitric acid solution (by mass).

[0066] Method and object: Same as in Example 4.

[0067] Results: The surface was bright but uneven, with visible "copper patina" areas and selective corrosion. The uniform corrosion rate was 0.31 mm / a. The pitting potential shifted positively by +18 mV.

[0068] The stark contrast between the examples and comparative examples fully demonstrates that the present invention, through the design of a composite acid system and a targeted dual-effect corrosion inhibitor, successfully achieves the expected goal of controlling substrate corrosion to an extremely low level while forming a high-performance passivation film, thus enabling efficient cleaning.

[0069] Those skilled in the art should understand that the above description is merely a preferred embodiment of the present invention. Without departing from the core concept of the present invention—namely, "using a specific ratio of sulfuric acid-phosphoric acid-nitric acid composite acid system, and synergistically using corrosion inhibitors such as benzotriazole and thiourea to simultaneously protect cupronickel and potentially coexisting ferrous metals, thereby achieving low-corrosion, high-quality passivation treatment of shell-and-tube heat exchangers"—some details can be adaptively adjusted or known equivalent substitutions can be used, for example: The total acidity and proportion of the inorganic acid combination can be finely adjusted within the range described in the claims according to the degree of contamination and material condition of the surface to be treated.

[0070] Corrosion inhibitors such as "Ruoding" can be replaced by pure products of their main active ingredients in an equivalent proportion.

[0071] The specific configuration of the circulating cleaning system (such as the pump model and filtration accuracy) can be selected according to the on-site equipment conditions and cleaning specifications.

[0072] The selection of the drying gas source (instrument gas, nitrogen) should be based on meeting the core requirements of "low dew point and appropriate temperature".

[0073] Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention shall be included within the scope of protection of this invention. The scope of protection claimed in this invention is defined by the appended claims.

Claims

1. A solution for removing rust and passivating white copper, characterized in that, It consists of the following components by mass percentage: (A) Inorganic acid combination: 8%~15%, wherein the inorganic acid combination is composed of sulfuric acid, phosphoric acid and nitric acid in a mass ratio of (4~6):(2~4):(2~4); (B) Corrosion inhibitor and passivator combination: 0.1%~0.5%, wherein the corrosion inhibitor and passivator combination is composed of a copper alloy-specific corrosion inhibitor and passivator and a ferrous metal corrosion inhibitor; (C) The remainder is Grade B water, which is chloride ion (Cl) water. - Deionized water or distilled water with a content not exceeding 1 mg / L; The cupronickel rust removal and passivation solution is configured to form a uniform passivation film on the surface of BFe10-1-1 or BFe30-1-1 cupronickel material after 90-120 minutes of cyclic treatment, and the uniform corrosion rate of the base material and weld is less than 0.01 mm / a.

2. The copper rust removal and passivation solution according to claim 1, characterized in that, The inorganic acid combination has a mass percentage of 10% to 12% and is composed of sulfuric acid, phosphoric acid and nitric acid in a mass ratio of 5:2.5:2.

5.

3. The copper rust removal and passivation solution according to claim 1 or 2, characterized in that, The copper alloy-specific corrosion inhibitor and passivator is at least one of benzotriazole, methylbenzotriazole, or mercaptobenzothiazole; the ferrous metal corrosion inhibitor is at least one of thiourea, hexamethylenetetramine, or rutin.

4. The copper rust removal and passivation solution according to claim 3, characterized in that, The mass percentage of the corrosion inhibitor and passivator combination is 0.2% to 0.3%, and the mass ratio of the copper alloy-specific corrosion inhibitor and passivator to the ferrous metal corrosion inhibitor is 1:(15~25).

5. The copper rust removal and passivation solution according to claim 4, characterized in that, The copper alloy-specific corrosion inhibitor and passivator is benzotriazole, and the ferrous metal corrosion inhibitor is thiourea, with a mass ratio of 1:

20.

6. A pickling and passivation method for shell-and-tube heat exchangers, characterized in that, include: S1: Using the white copper rust removal and passivation solution as described in any one of claims 1 to 5, a vertically placed shell-and-tube heat exchanger is subjected to circulating acid pickling treatment at a temperature of 20℃ to 35℃, with a circulation flow rate of 0.5 to 2.0 m / s and a treatment time of 90 to 120 minutes. S2: After the acid washing cycle is completed, immediately pump Class B water at 40℃~50℃ into the shell-and-tube heat exchanger at a flow rate of 1.5~3 times that of the acid washing cycle to replace and rinse until the pH value of the discharged water is ≥6.8 for three consecutive measurements. S3: Use compressed air or nitrogen with a dew point below -20℃ and a temperature of 50℃±5℃ to purge the inner cavity of the shell-and-tube heat exchanger until no water film accumulates on its inner wall.

7. The pickling and passivation method for shell-and-tube heat exchangers according to claim 6, characterized in that, In step S1, the cyclic pickling is carried out in a closed loop consisting of an acid-resistant pump, the shell-and-tube heat exchanger, and an external circulation box with a 20μm filter.

8. The pickling and passivation method for shell-and-tube heat exchangers according to claim 6, characterized in that, In step S2, after the replacement flushing, a shaking rinsing step is also included: the shell-and-tube heat exchanger is placed horizontally, and gas is pulsed into the shell-and-tube heat exchanger filled with Class B water at a pressure of less than 0.8 MPa, and the overflow state is maintained for rinsing for more than 30 minutes.

9. The pickling and passivation method for shell-and-tube heat exchangers according to claim 6, characterized in that, After step S3 is completed, use an endoscope to perform a 100% inspection of the inner wall of the shell-and-tube heat exchanger. The passivation film should be a uniform matte silver-white color, without any rust or active corrosion spots.

10. A shell-and-tube heat exchanger subjected to pickling and passivation treatment, characterized in that, The passivation film on its inner wall is formed by using a white copper rust removal and passivation solution as described in any one of claims 1 to 5, and by the pickling and passivation method for shell-and-tube heat exchangers as described in any one of claims 6 to 9. The pitting potential of the passivation film in 3.5% NaCl solution is positively shifted by not less than 50mV relative to the untreated surface.

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