A pickling and passivation process for stainless steel equipment surfaces

Abstract

The application discloses a pickling and passivation process for a stainless steel equipment surface and relates to the technical field of stainless steel purification, and comprises the following steps: S1, pretreatment, sequentially performing degreasing and water washing operations on the surface of the stainless steel equipment; S2, pickling treatment, immersing the pretreated stainless steel equipment into a pickling solution, pickling for 10-20 min at 40-50 DEG C, and gently stirring at a speed of 30-50 r / min every 5 min during the pickling; S3, passivation treatment, immersing the pickled stainless steel equipment into a passivation solution; and S4, post-detection, detecting the surface of the treated stainless steel equipment, and requiring that the surface roughness Ra is less than or equal to 0.8 mu m, the passivation film adhesion grade is greater than or equal to 1, and there is no rust after a 48-h neutral salt spray test. The pickling solution adopts a phosphoric acid-organic acid composite system, replaces a traditional strong oxidizing acid or a hydrofluoric acid system, and is supplemented with a compounded corrosion inhibitor, so that the corrosion rate of the pickling solution on the stainless steel base material can be significantly reduced, defects such as pitting and etching pits can be avoided, and the flatness of the base material surface can be ensured.

Description

Technical Field

[0001] This invention relates to the field of stainless steel purification technology, specifically to a pickling and passivation process for the surface of stainless steel equipment. Background Technology

[0002] During the rolling, welding, and heat treatment processes of stainless steel equipment, contaminants such as oxide scale and welding slag easily form on the surface. These not only affect the appearance quality but also reduce its corrosion resistance. Therefore, pickling and passivation treatment is necessary to remove these contaminants and form a dense passivation film on the surface. In existing pickling and passivation processes, the acid solutions are highly corrosive and easily cause over-corrosion of the stainless steel substrate, leading to increased surface roughness and affecting the equipment's sealing performance. Traditional strong oxidizing acids or hydrofluoric acid systems tend to react excessively with the stainless steel substrate, damaging the surface smoothness and forming defects such as pitting and corrosion. Especially for precision sealing surfaces, excessive roughness can directly lead to seal failure and increase the risk of media leakage. Summary of the Invention

[0003] To solve the above-mentioned technical problems, a pickling and passivation process for stainless steel equipment surface is provided, which solves the problems existing in the background technology.

[0004] To achieve the above objectives, the technical solution adopted by this invention is: a pickling and passivation process for the surface of stainless steel equipment, comprising the following steps:

[0005] S1. Pretreatment: The surface of the stainless steel equipment is degreased and washed sequentially. The degreasing is performed using an alkaline degreasing agent, and the equipment is immersed at 50-60℃ for 10-15 minutes, supplemented by ultrasonic vibration at 40-60kHz. The alkaline degreasing agent consists of 5-8wt% sodium hydroxide, 3-5wt% sodium carbonate, 2-4wt% trisodium phosphate, 0.5-1wt% surfactant, and the balance being deionized water.

[0006] S2. Pickling treatment: Immerse the pretreated stainless steel equipment in the pickling solution and pickle at 40-50℃ for 10-20 minutes, gently stirring every 5 minutes at a rate of 30-50 r / min to maintain the pH of the acid solution at 1.5-2.5. The pickling solution consists of 8-12 wt% phosphoric acid, 3-5 wt% citric acid, 1-2 wt% oxalic acid, 0.5-1 wt% corrosion inhibitor, and the balance being deionized water. The corrosion inhibitor is a mixture of benzotriazole and hexamethylenetetramine in a 1:2 mass ratio. After pickling, rinse sequentially with deionized water, neutralize with 1-2 wt% sodium carbonate solution, and rinse with deionized water until neutral.

[0007] S3. Passivation treatment: Immerse the pickled stainless steel equipment in a passivation solution and passivate at 20-30℃ for 5-10 minutes. The passivation solution consists of 5-8wt% nitric acid, 2-3wt% sodium nitrite, 1-2wt% sodium nitrate, and the balance deionized water. After passivation, rinse with deionized water, dry at 60-80℃ for 15-20 minutes, and cool naturally to room temperature.

[0008] S4. Post-inspection: Inspect the surface of the treated stainless steel equipment. The surface roughness Ra is required to be ≤0.8μm, the passivation film adhesion grade is ≥1, and there is no rust after 48 hours of neutral salt spray test.

[0009] Compared with existing technologies, the advantages of this invention are as follows: the pickling solution uses a phosphoric acid-organic acid composite system to replace the traditional strong oxidizing acid or hydrofluoric acid system, and is supplemented with a compound corrosion inhibitor, which can significantly reduce the corrosion rate of the acid solution on the stainless steel substrate, avoid the formation of defects such as pitting and corrosion pits, and ensure the surface smoothness of the substrate. The composite pickling solution can efficiently remove surface oxide scale, welding slag and other contaminants. The subsequent passivation treatment can form a dense and strongly adherent passivation film, which improves the corrosion resistance of stainless steel equipment. By precisely controlling the pickling temperature, time, pH value and the amount of corrosion inhibitor added, the pickling process can be controlled, reducing the impact of process fluctuations on the treatment effect. It is suitable for large-scale production. The pickling solution does not contain highly toxic components such as hydrofluoric acid, and the nitric acid concentration in the passivation solution is low. The subsequent neutralization treatment can effectively reduce wastewater pollution. The treated wastewater can meet the discharge standards after simple treatment, which meets environmental protection requirements. Detailed Implementation

[0010] The following description is intended to disclose the invention and enable those skilled in the art to implement it. The preferred embodiments described below are merely examples, and other obvious variations will occur to those skilled in the art.

[0011] Example 1

[0012] This embodiment describes the surface pickling and passivation treatment of a 304 stainless steel sealing valve (including a precision sealing surface). The specific steps are as follows:

[0013] S1. Pretreatment: Prepare an alkaline degreasing agent: 5wt% sodium hydroxide, 3wt% sodium carbonate, 2wt% trisodium phosphate, 0.5wt% sodium dodecyl sulfate, with the remainder being deionized water; immerse the valve in the degreasing agent and soak at 50℃ for 10 minutes while simultaneously turning on 40kHz ultrasonic vibration; after removal, rinse twice with deionized water for 3 minutes each time, and blow dry the surface moisture with compressed air;

[0014] S2. Pickling treatment: Prepare the pickling solution as follows: 8wt% phosphoric acid, 3wt% citric acid, 1wt% oxalic acid, 0.5wt% corrosion inhibitor (benzotriazole: urotropine = 1:2), with the balance being deionized water. Immerse the valve in the pickling solution and pickle at 40℃ for 10 minutes, gently stirring every 5 minutes (30 rpm), and monitor the pH value in real time to maintain it at 1.5-2.5. After pickling, rinse three times with deionized water for 2 minutes each time, then neutralize with 1wt% sodium carbonate solution for 1 minute, and rinse with deionized water until the pH reaches 6.5-7.5.

[0015] S3. Passivation treatment: Prepare a passivation solution: 5wt% nitric acid, 2wt% sodium nitrite, 1wt% sodium nitrate, with the balance being deionized water; Immerse the valve in the passivation solution and passivate at 20℃ for 5min; After removal, rinse twice with deionized water for 3min each time, place in a 60℃ drying oven for 15min, and allow to cool naturally to room temperature.

[0016] S4. Post-inspection: The valve surface was found to be free of pits and corrosion, with a surface roughness Ra=0.6μm, passivation film adhesion grade of 1, no rust after 48 hours of neutral salt spray test, and qualified sealing performance test with no media leakage.

[0017] Example 2

[0018] This embodiment describes the surface pickling and passivation treatment of a 316L stainless steel reactor (including the flange sealing surface). The specific steps are as follows:

[0019] S1. Pretreatment configuration: Alkaline degreasing agent: 8wt% sodium hydroxide, 5wt% sodium carbonate, 4wt% trisodium phosphate, 1wt% Tween-80, with the remainder being deionized water; Immerse the reactor flange and inner wall in the degreasing agent and soak at 60℃ for 15 minutes, then turn on 60kHz ultrasonic vibration; After removal, rinse with deionized water 3 times, 5 minutes each time, and blow dry the surface moisture with compressed air;

[0020] S2. Pickling treatment: Prepare the pickling solution as follows: 12wt% phosphoric acid, 5wt% citric acid, 2wt% oxalic acid, 1wt% corrosion inhibitor (benzotriazole: urotropine = 1:2), with the remainder being deionized water. Immerse the reactor components in the pickling solution and pickle at 50℃ for 20 minutes, gently stirring every 5 minutes (50 rpm). Monitor the pH value in real time and maintain it between 1.5 and 2.5. When the pH is lower than 1.5, add 0.1wt% corrosion inhibitor. After pickling, rinse with deionized water 4 times for 3 minutes each time, neutralize with 2wt% sodium carbonate solution for 2 minutes, and rinse with deionized water until the pH reaches 6.5-7.5.

[0021] S3. Passivation treatment: Prepare a passivation solution: 8wt% nitric acid, 3wt% sodium nitrite, 2wt% sodium nitrate, with the remainder being deionized water. Immerse the reactor components in the passivation solution and passivate at 30℃ for 10 minutes. After removal, rinse with deionized water 3 times, 5 minutes each time, and place in an 80℃ drying oven to dry for 20 minutes. Allow to cool naturally to room temperature.

[0022] S4. Post-inspection: The oxide scale and welding slag on the surface of the reactor were completely removed, there were no over-corrosion defects, the surface roughness Ra=0.7μm, the passivation film adhesion level was 1, there was no rust after 48h of neutral salt spray test, and the flange sealing surface was well sealed after assembly with no leakage.

[0023] Comparative Example 1

[0024] This comparative example uses a traditional pickling and passivation process to treat a 304 stainless steel sealing valve of the same specifications as in Example 1. The specific steps are as follows:

[0025] S1. Pretreatment, which is the same as the pretreatment steps in Example 1: Prepare an alkaline degreasing agent with the same formula, soak at 50°C for 10 minutes and supplement with 40kHz ultrasonic vibration, rinse twice with deionized water and then blow dry with compressed air.

[0026] S2. Pickling treatment: Prepare a traditional pickling solution (wt%): 10% nitric acid, 2% hydrofluoric acid, and the balance is deionized water, without adding corrosion inhibitors; Immerse the valve in the pickling solution and pickle at 45°C for 15 minutes, during which it is left to stand without stirring and the pH value is not monitored; After pickling, rinse with deionized water 3 times, 2 minutes each time, without neutralization step, and rinse directly until neutral.

[0027] S3. Passivation treatment: Prepare a traditional passivation solution (wt%): 15% nitric acid, the balance being deionized water; immerse the valve in the passivation solution and passivate at 25°C for 8 minutes; after removal, rinse twice with deionized water for 3 minutes each time, dry at 60°C for 15 minutes, and allow to cool naturally to room temperature.

[0028] S4. Post-inspection revealed that the valve surface had obvious pitting and other corrosion defects, with a surface roughness Ra=1.5μm, a passivation film adhesion level of 3, slight rusting in the 24-hour neutral salt spray test, and failed the sealing performance test, indicating media leakage.

[0029] Performance Comparison Table of Examples and Comparative Examples:

[0030] Comparison Summary of Comparative Examples and Examples

[0031] The pickling and passivation process of this invention has significant advantages over traditional processes: This invention uses a hydrofluoric acid-free phosphoric acid-organic acid composite pickling system, combined with a compound corrosion inhibitor, and precise parameter control (40-50℃, pH 1.5-2.5), which can effectively avoid over-corrosion of the substrate. After treatment, the surface of 304 / 316L stainless steel equipment is free of pits and corrosion, with a roughness Ra≤0.7μm, meeting the requirements for precision sealing surfaces, and the sealing performance is qualified with no leakage. Its low-concentration composite passivation solution can form a dense passivation film with adhesion level 1, showing no rust in a neutral salt spray test for 48 hours, exhibiting excellent corrosion resistance. Moreover, the process is stable, wastewater is easy to treat, and it is more environmentally friendly. In contrast, the traditional nitric acid-hydrofluoric acid system, due to the lack of corrosion inhibition measures and uncontrolled parameters, leads to obvious over-corrosion defects on the equipment surface, with a roughness of up to 1.5μm, a passivation film adhesion of only level 3, rust appearing after 24 hours, unqualified sealing performance, and the presence of highly toxic components, posing high environmental and safety risks. In summary, this invention is superior to traditional processes in terms of controlling over-corrosion, ensuring sealing performance, improving corrosion resistance and environmental friendliness, and is suitable for the processing needs of precision stainless steel equipment, making it more practical and valuable for promotion.

[0032] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention. The scope of protection claimed by the appended claims and their equivalents is defined.

Claims

1. A pickling and passivation process for the surface of stainless steel equipment, characterized in that, Includes the following steps: S1. Pretreatment: The surface of the stainless steel equipment is degreased and washed sequentially. The degreasing is performed using an alkaline degreasing agent, and the equipment is immersed at 50-60℃ for 10-15 minutes, supplemented by ultrasonic vibration at 40-60kHz. The alkaline degreasing agent consists of 5-8wt% sodium hydroxide, 3-5wt% sodium carbonate, 2-4wt% trisodium phosphate, 0.5-1wt% surfactant, and the balance being deionized water. S2. Pickling treatment: Immerse the pretreated stainless steel equipment in the pickling solution and pickle at 40-50℃ for 10-20 minutes, gently stirring every 5 minutes at a rate of 30-50 r / min to maintain the pH of the acid solution at 1.5-2.

5. The pickling solution consists of 8-12 wt% phosphoric acid, 3-5 wt% citric acid, 1-2 wt% oxalic acid, 0.5-1 wt% corrosion inhibitor, and the balance being deionized water. The corrosion inhibitor is a mixture of benzotriazole and hexamethylenetetramine in a 1:2 mass ratio. After pickling, the equipment is rinsed sequentially with deionized water, neutralized with a 1-2 wt% sodium carbonate solution, and rinsed with deionized water until neutral. S3. Passivation treatment: Immerse the pickled stainless steel equipment in a passivation solution and passivate at 20-30℃ for 5-10 minutes. The passivation solution consists of 5-8wt% nitric acid, 2-3wt% sodium nitrite, 1-2wt% sodium nitrate, and the balance deionized water. After passivation, rinse with deionized water, dry at 60-80℃ for 15-20 minutes, and cool naturally to room temperature. S4. Post-inspection: Inspect the surface of the treated stainless steel equipment. The surface roughness Ra is required to be ≤0.8μm, the passivation film adhesion grade is ≥1, and there is no rust after 48 hours of neutral salt spray test.

2. The pickling and passivation process for stainless steel equipment surface according to claim 1, characterized in that, The surfactant mentioned in S1 is sodium dodecyl sulfate or Tween-80.

3. The pickling and passivation process for stainless steel equipment surface according to claim 1, characterized in that, In S2, when the pH of the acid solution is below 1.5, add an appropriate amount of corrosion inhibitor to maintain the pH in the range of 1.5-2.

5.

4. The pickling and passivation process for stainless steel equipment surface according to claim 1, characterized in that, The stainless steel equipment is 304 or 316L stainless steel equipment with precision sealing surfaces.

5. The pickling and passivation process for stainless steel equipment surface according to claim 1, characterized in that, The corrosion inhibitor also contains 0.1-0.3 wt% thiourea, wherein the mass ratio of thiourea to benzotriazole is 1:3-5.

6. The pickling and passivation process for stainless steel equipment surface according to claim 1, characterized in that, The passivation solution also contains 0.5-1 wt% disodium EDTA, and the mass ratio of disodium EDTA to sodium nitrite is 1:2-3.

7. The pickling and passivation process for stainless steel equipment surface according to claim 1, characterized in that, The drying described in S3 uses hot air circulation drying, with an air velocity of 0.5-1m / s inside the drying chamber, and the temperature fluctuation inside the chamber during the drying process is ≤±2℃.

8. The pickling and passivation process for stainless steel equipment surface according to claim 1, characterized in that, The post-test described in S4 also includes a sealing performance test, which uses a pneumatic sealing test with a test pressure of 0.6-1.0 MPa, a pressure holding time of 30 minutes, and a leakage rate of ≤0.01 MPa / h.

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