A treatment method for inhibiting the elution of hexavalent chromium from stainless steel piping

Abstract

This invention relates to the field of stainless steel surface treatment technology, specifically to a method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes. The method includes the following steps: (1) pickling: immersing the stainless steel pipe in a pickling solution, then rinsing it with water; (2) passivation: immersing the pickled stainless steel pipe in a passivation solution, then rinsing it with water; (3) cleaning and testing: neutralizing the passivated stainless steel pipe with a sodium carbonate solution, rinsing it with water, then immersing it in deionized water, cleaning it thoroughly, collecting the immersion solution, testing the hexavalent chromium content in the immersion solution, and allowing the cleaned stainless steel pipe to air dry naturally. This method involves pickling and then passivating the stainless steel pipe, forming a mesh-like passivation film on the surface of the pipe. This passivation film hinders contact between the stainless steel and corrosive media, improving the corrosion resistance of the stainless steel pipe and preventing the precipitation of hexavalent chromium due to corrosion.

Description

Technical Field

[0001] This invention relates to the field of stainless steel surface treatment technology, and specifically to a treatment method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes. Background Technology

[0002] Chromium exists in water in two to six valent forms, with trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) being the most common oxidation states in nature. Hexavalent chromium is highly toxic, mutagenic, and carcinogenic to organisms. The State Environmental Protection Administration classifies hexavalent chromium and total chromium as Class I pollutants. According to its "Integrated Wastewater Discharge Standard" (GB8996-1996), the maximum allowable discharge concentration of hexavalent chromium is 0.5 mg / L. For drinking water, the concentration of hexavalent chromium, as a toxicological indicator, is not allowed to exceed 0.05 mg / L.

[0003] Studies have shown that hexavalent chromium is mainly exposed to human drinking water through natural erosion of soil and rocks and industrial pollution, resulting in an average exposure of 0.2-2 μg of hexavalent chromium. Drinking water poses a health risk due to hexavalent chromium pollution. In addition to pollution at the source of the water, pollution may also occur during the transportation of drinking water through pipelines.

[0004] Welding of stainless steel pipes creates weld spots, which are mainly oxide scale and corrosion products generated at high temperatures during welding. These oxide scale and corrosion products can be removed through pickling and passivation. In 2016, Li Haihua et al. soaked different stainless steel pipes in immersion solutions with residual chlorine and densities of 2 mg / L and 100 mg / L for 24 hours and tested the changes in hexavalent chromium in the water. They found that the treatment of stainless steel pipe welds, grinding and polishing, pickling and passivation processes all affected the migration of hexavalent chromium. To ensure drinking water safety, it is urgent to develop a treatment method to inhibit the precipitation of hexavalent chromium from stainless steel pipes, reduce the precipitation of hexavalent chromium, and decrease chromium pollution in drinking water during pipeline transportation. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to overcome the problems existing in the prior art and provide a treatment method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes. The treatment method is simple and easy to operate, and can improve the corrosion resistance of stainless steel pipes, prevent stainless steel pipes from being corroded, and reduce the amount of hexavalent chromium precipitation.

[0006] To achieve the above-mentioned technical objectives and effects, the present invention is implemented through the following technical solution:

[0007] A method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes includes the following steps:

[0008] (1) Pickling: Immerse the stainless steel pipe in the pickling solution, rinse with water and set aside.

[0009] (2) Passivation: Immerse the pickled stainless steel pipe in the passivation solution, rinse with water and set aside.

[0010] (3) Cleaning and testing: The passivated stainless steel pipe is neutralized with sodium carbonate solution, rinsed with water, and then soaked in deionized water to clean it. The soaking solution is collected and the content of hexavalent chromium in the soaking solution is tested. The cleaned stainless steel pipe is then air-dried.

[0011] Pickling followed by passivation of stainless steel pipes will form a passivation film with a mesh structure on the surface of the stainless steel pipes. The formation of this passivation film can prevent the stainless steel from contacting the corrosive medium, improve the corrosion resistance of the stainless steel pipes, and prevent the stainless steel pipes from being corroded and releasing hexavalent chromium.

[0012] Preferably, the pickling solution comprises the following raw materials in parts by weight: 4-7 parts sulfuric acid, 15-22 parts nitric acid, 2-3 parts polyferric sulfate, 4-6 parts ammonium fluoride, 3-8 parts ammonium bifluoride, 1-2 parts acetic acid, and 50-75 parts water. The pickling solution removes the oxide scale and corrosion products generated at high temperatures during the welding process of stainless steel pipes.

[0013] Preferably, the passivation solution comprises the following raw materials in parts by weight: 8-12 parts nitric acid, 1.2-2.5 parts ethylenediaminetetramethylene phosphoric acid, 0.3-0.6 parts formic acid, 0.3-0.6 parts acetic acid, 0.02-0.05 parts polyether, 1-2 parts sodium dihydrogen phosphate, and 80-90 parts water. The passivation solution washes away the surface oxide layer, exposing the substrate. The substrate reacts in situ with the passivation solution to form a new, dense passivation film, preventing the stainless steel from contacting corrosive media.

[0014] Preferably, the mass concentration of the sodium carbonate solution is 4.5-5.5%.

[0015] Preferably, in step (1), the pickling immersion temperature is 15-30℃. Different pickling solutions with different ratios require different pickling immersion temperatures to ensure that the pickling solution fully removes the corrosion products generated during the welding process of stainless steel pipes.

[0016] Preferably, in step (1), the pickling soaking time is 10-20 minutes. Appropriate pickling soaking time can improve the corrosion resistance of stainless steel. Excessive or insufficient pickling will reduce the corrosion resistance of stainless steel, and reduced corrosion resistance will lead to the precipitation of hexavalent chromium exceeding the acceptable standard.

[0017] Preferably, in step (2), the passivation immersion temperature is 15-30℃. After pickling, a defective oxide layer appears on the surface of the stainless steel pipe. Passivation treatment is performed at a suitable temperature to ensure that the passivation solution fully washes away the defective oxide layer.

[0018] Preferably, in step (2), the passivation immersion time is 10-20 minutes. An appropriate passivation immersion time allows an orderly passivation film to form on the surface of the stainless steel pipe. This passivation film has a tight mesh structure, which prevents the stainless steel pipe from contacting the corrosive medium.

[0019] Preferably, in step (3), the water washing and soaking temperature is 15-20℃, and the water washing and soaking time is 5-10min.

[0020] Preferably, in step (3), the method for detecting hexavalent chromium is the diphenylcarbazide spectrophotometric method. In acidic solution, hexavalent chromium can react with diphenylcarbazide to form a purple-red complex, which is quantified by colorimetry. The minimum detectable mass of this method is 0.2 μg (as Cr). 6+ count).

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] The processing method of the present invention involves pickling and then passivating stainless steel pipes. A passivation film with a mesh structure is formed on the surface of the stainless steel pipe. The formation of this passivation film can prevent stainless steel from contacting corrosive media, improve the corrosion resistance of stainless steel pipes, and prevent the stainless steel pipes from being corroded and releasing hexavalent chromium. Attached Figure Description

[0023] Figure 1 This is a SEM image (5.0k) of the surface of the stainless steel pipe in Example 1;

[0024] Figure 2 This is a SEM image (50.0k) of the surface of the stainless steel pipe in Example 1;

[0025] Figure 3 This is a SEM image (5.0k) of the surface of the stainless steel pipe in Comparative Example 1.

[0026] Figure 4 This is a SEM image (50.0k) of the surface of the stainless steel pipe in Comparative Example 1.

[0027] Figure 5 These are the electrochemical polarization curves of Example 1, Comparative Example 1, and Comparative Example 2. Detailed Implementation

[0028] To facilitate understanding by those skilled in the art, the following description is provided in conjunction with Examples 1-3 and Comparative Examples 1-3 and their accompanying drawings. Figure 1-5 The present invention will be further described below. The content mentioned in the embodiments is not intended to limit the present invention.

[0029] Example 1

[0030] A method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes includes the following steps:

[0031] (1) Pickling: Immerse the stainless steel pipe in the pickling solution, rinse with water and set aside.

[0032] (2) Passivation: Immerse the pickled stainless steel pipe in the passivation solution, rinse with water and set aside.

[0033] (3) Cleaning and testing: The passivated stainless steel pipe is neutralized with sodium carbonate solution, rinsed with water, and then soaked in deionized water to clean it. The soaking solution is collected and the content of hexavalent chromium in the soaking solution is tested. The cleaned stainless steel pipe is then air-dried.

[0034] The pickling solution comprises the following raw materials in parts by weight: 4 parts sulfuric acid, 15 parts nitric acid, 2 parts polyferric sulfate, 4 parts ammonium fluoride, 3 parts ammonium bifluoride, 1 part acetic acid, and 50 parts water; the passivation solution comprises the following raw materials in parts by weight: 8 parts nitric acid, 1.2 parts ethylenediaminetetramethylene phosphoric acid, 0.3 parts formic acid, 0.3 parts acetic acid, 0.02 parts polyether, 1 part sodium dihydrogen phosphate, and 80 parts water; the sodium carbonate solution has a mass concentration of 5%.

[0035] The polyferric sulfate has an iron content of 21%, and the polyether is an iminohydroxy polyether.

[0036] In step (1), the pickling soaking temperature is 15°C and the pickling soaking time is 10 min; in step (2), the passivation soaking temperature is 15°C and the passivation soaking time is 10 min; in step (3), the water washing soaking temperature is 15°C and the water washing soaking time is 5 min, and the method for detecting hexavalent chromium is diphenylcarbazide spectrophotometry.

[0037] The test results showed that the amount of hexavalent chromium leached was ≤0.0037 mg / L, which does not exceed the qualified standard of 0.005 mg / L and meets the national standard requirements.

[0038] Example 2

[0039] A method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes includes the following steps:

[0040] (1) Pickling: Immerse the stainless steel pipe in the pickling solution, rinse with water and set aside.

[0041] (2) Passivation: Immerse the pickled stainless steel pipe in the passivation solution, rinse with water and set aside.

[0042] (3) Cleaning and testing: The passivated stainless steel pipe is neutralized with sodium carbonate solution, rinsed with water, and then soaked in deionized water to clean it. The soaking solution is collected and the content of hexavalent chromium in the soaking solution is tested. The cleaned stainless steel pipe is then air-dried.

[0043] The pickling solution comprises the following raw materials in parts by weight: 6 parts sulfuric acid, 18 parts nitric acid, 2.5 parts polyferric sulfate, 5 parts ammonium fluoride, 5 parts ammonium bifluoride, 1.5 parts acetic acid, and 60 parts water; the passivation solution comprises the following raw materials in parts by weight: 10 parts nitric acid, 2 parts ethylenediaminetetramethylene phosphoric acid, 0.5 parts formic acid, 0.5 parts acetic acid, 0.04 parts polyether, 1.5 parts sodium dihydrogen phosphate, and 85 parts water; the sodium carbonate solution has a mass concentration of 5%.

[0044] The polyferric sulfate has an iron content of 21%, and the polyether is an iminohydroxy polyether.

[0045] In step (1), the pickling soaking temperature is 20°C and the pickling soaking time is 15 min; in step (2), the passivation soaking temperature is 20°C and the passivation soaking time is 15 min; in step (3), the water washing soaking temperature is 18°C ​​and the water washing soaking time is 8 min, and the method for detecting hexavalent chromium is diphenylcarbazide spectrophotometry.

[0046] The test results showed that the amount of hexavalent chromium leached was ≤0.0041 mg / L, which does not exceed the qualified standard of 0.005 mg / L and meets the national standard requirements.

[0047] Example 3

[0048] A method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes includes the following steps:

[0049] (1) Pickling: Immerse the stainless steel pipe in the pickling solution, rinse with water and set aside.

[0050] (2) Passivation: Immerse the pickled stainless steel pipe in the passivation solution, rinse with water and set aside.

[0051] (3) Cleaning and testing: The passivated stainless steel pipe is neutralized with sodium carbonate solution, rinsed with water, and then soaked in deionized water to clean it. The soaking solution is collected and the content of hexavalent chromium in the soaking solution is tested. The cleaned stainless steel pipe is then air-dried.

[0052] The pickling solution comprises the following raw materials in parts by weight: 7 parts sulfuric acid, 22 parts nitric acid, 3 parts polyferric sulfate, 6 parts ammonium fluoride, 8 parts ammonium bifluoride, 2 parts acetic acid, and 75 parts water; the passivation solution comprises the following raw materials in parts by weight: 12 parts nitric acid, 2.5 parts ethylenediaminetetramethylene phosphoric acid, 0.6 parts formic acid, 0.6 parts acetic acid, 0.05 parts polyether, 2 parts sodium dihydrogen phosphate, and 90 parts water; the sodium carbonate solution has a mass concentration of 5%.

[0053] The polyferric sulfate has an iron content of 21%, and the polyether is an iminohydroxy polyether.

[0054] In step (1), the pickling soaking temperature is 30°C and the pickling soaking time is 20 min; in step (2), the passivation soaking temperature is 30°C and the passivation soaking time is 20 min; in step (3), the water washing soaking temperature is 20°C and the water washing soaking time is 10 min, and the method for detecting hexavalent chromium is diphenylcarbazide spectrophotometry.

[0055] The test results showed that the amount of hexavalent chromium leached was ≤0.0045mg / L, which does not exceed the qualified standard of 0.005mg / L and meets the national standard requirements.

[0056] Comparative Example 1

[0057] Compared to Example 1, only acid washing was performed.

[0058] The test results showed that the amount of hexavalent chromium leached was ≥0.0128 mg / L, exceeding the qualified standard of 0.005 mg / L, and thus failing to meet the national standard requirements.

[0059] Comparative Example 2

[0060] Compared to Example 1, no pickling and passivation treatment was performed.

[0061] The test results showed that the amount of hexavalent chromium leached was ≥0.0510 mg / L, exceeding the qualified standard of 0.005 mg / L, and thus failing to meet the national standard requirements.

[0062] Comparative Example 3

[0063] Compared to Example 1, only pickling was performed, and the pickling solution used was a conventional pickling solution (i.e., 20% nitric acid + 5% hydrofluoric acid).

[0064] The test results showed that the amount of hexavalent chromium leached was ≥0.0420 mg / L, exceeding the qualified standard of 0.005 mg / L, and thus failing to meet the national standard requirements.

[0065] Tensile tests were conducted on the stainless steel pipes of Examples 1-3 and Comparative Examples 1-3 using a high-temperature electronic universal testing machine ET04 DEM / 20W at room temperature in accordance with GB / T228.1-2010. The results are shown in Table 1.

[0066] Table 1 Tensile Test Results

[0067]

[0068]

[0069] Analysis of the data in Table 1 shows that Examples 1-3 used pickling solutions and passivation solutions with different ratios, and set corresponding pickling temperatures, pickling times, passivation temperatures, and passivation times to pickle and passivate stainless steel pipes. Compared with Comparative Examples 1-3, the tensile strength, yield strength, and elongation after fracture of the stainless steel pipes were significantly improved. Compared with Comparative Examples 1 and 3, Comparative Example 1 used the pickling solution of the present invention, while Comparative Example 3 used a traditional pickling solution. The tensile strength, yield strength, and elongation after fracture of the stainless steel pipe in Comparative Example 1 were all better than those in Comparative Example 3, indicating that the pickling solution of the present invention can improve the mechanical properties of stainless steel pipes more effectively than the traditional pickling solution.

[0070] The morphology of the stainless steel pipes in Example 1 and Comparative Example 1 was examined using a field emission scanning electron microscope (SU8020, Hitachi, Japan) and an X-ray energy dispersive spectrometer (EX-350, HORIBA, Japan) (see [reference]). Figure 1-4 ) and chemical composition analysis (see Table 2).

[0071] Table 2. EDS component analysis of Example 1 and Comparative Example 1

[0072] <![CDATA[w c ]]> <![CDATA[w o ]]> <![CDATA[w Cr ]]> <![CDATA[w Mn ]]> <![CDATA[w Fe ]]> <![CDATA[w Ni ]]> <![CDATA[w Mo ]]> Example 1 / / 16.56 1.17 70.07 9.55 2.65 Comparative Example 1 / 6.70 16.23 0.89 64.56 9.60 2.03

[0073] Based on the analysis of morphology images and chemical composition data, Comparative Example 1 underwent pickling treatment, while Example 1 underwent pickling treatment followed by passivation treatment. Comparing the chemical elements of Example 1 and Comparative Example 1, the chromium content did not change significantly, but the oxygen and iron content changed considerably. Furthermore, based on the morphology image analysis, blistering appeared on the surface of the stainless steel pipe in Comparative Example 1 after pickling treatment. Figure 3 The film formed after acid washing is loose, porous, and uneven. Figure 4 In Example 1, after pickling and passivation, an orderly film layer was formed on the stainless steel pipe at the original blistering site. Figure 1 Furthermore, the resulting film is dense, smooth, and free of loose or porous phenomena. This indicates that even after pickling, stainless steel pipes still exhibit blistering and uneven surfaces. Adding a passivation process after pickling can form an orderly passivation film on the original blistering surface, thereby isolating the stainless steel pipe from the corrosive medium and preventing localized corrosion that could lead to the precipitation of hexavalent chromium.

[0074] Examples 1 and Comparative Examples 1-2 were tested using an electrochemical workstation (CH-750E, Chenhua, China). The test solution medium was a 3.5% sodium chloride solution, and the temperature was room temperature. A three-electrode system was used, with a steel sheet layer as the working anode, a platinum electrode as the auxiliary cathode, and a saturated calomel electrode as the reference electrode. The working anode was immersed in the electrolyte solution, and the polarization curve was determined using the continuous potential scanning method. After the anode open-circuit potential stabilized, the polarization curve test began. The potential scan range was -1.5 to 0 V (Vs SCE), and the scan frequency was 10 mV / s. The test results are shown in [Figure number missing]. Figure 5 .

[0075] All three polarization curves show obvious passivation regions. When the potential shifts towards the cathode potential, a reduction reaction occurs at the cathode, and a hydrogen release reaction occurs on the stainless steel surface, precipitating H. + As the potential shifts towards the anodic potential, the current density increases accordingly, and anodic dissolution occurs on the stainless steel surface. The dissolution and destruction of the anodic oxide layer, as well as the dissolution of the substrate, lead to a gradual increase in the anodic current. This indicates that a high-resistance, corrosion-resistant passivation film is formed on the stainless steel surface, thereby isolating the stainless steel from the corrosive medium and preventing localized corrosion.

[0076] Then Figure 5 The curves in the table are fitted to obtain the Tafel curve fitting parameter table (see Table 3).

[0077] Table 3. Parameters for Tafel Curve Fitting

[0078]

[0079] Analysis of the data in Table 3 shows that the self-corrosion potentials of Example 1, Comparative Example 1, and Comparative Example 2 are -959.636 mV, -993.47 mV, and -1122.3 mV, respectively, and the corrosion current densities are 0.14165 mA / cm², respectively. 2 0.31909 mA / cm 2 and 1.0293 mA / cm 2 Compared with Comparative Example 2, Example 1 and Comparative Example 1 both showed a positive shift in self-corrosion potential and a significant decrease in corrosion current density, indicating that the corrosion resistance of stainless steel pipes is improved after pickling and passivation treatment or pickling treatment alone. In particular, Example 1 showed the largest positive shift in self-corrosion potential, the smallest corrosion current density, the lowest corrosion rate, and a corrosion inhibition rate as high as 86.24%, indicating that the stainless steel pipes treated with pickling and passivation can maximize the corrosion resistance of stainless steel pipes.

[0080] The corrosion resistance of stainless steel pipes in Example 1 and Comparative Examples 1-3 was determined by blue spot test and neutral salt spray test, and the results are shown in Table 4.

[0081] Table 4 Results of Blue Dot Test and Neutral Salt Spray Test

[0082] Blue Dot Test neutral salt spray test Example 1 Slight blue spots appeared in the weld area after 30 minutes. 956h Comparative Example 1 Slight blue spots appeared in the weld area after 15 minutes. 249h Comparative Example 2 Slight blue spots appeared in the weld area within 1 minute. 3h Comparative Example 3 Slight blue spots appeared in the weld area after 3 minutes. 138h

[0083] The corrosion resistance of the stainless steel pipe treated with the pickling solution of the present invention in Comparative Example 1 is obviously higher than that of the stainless steel pipe treated with the conventional pickling solution in Comparative Example 3, indicating that the pickling solution of the present invention can effectively improve the corrosion resistance of stainless steel pipes.

[0084] The technical solutions provided by the embodiments of the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the embodiments of the present invention. The descriptions of the embodiments above are only for helping to understand the principles of the embodiments of the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the embodiments of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes, characterized in that: Includes the following steps: (1) Pickling: Immerse the stainless steel pipe in the pickling solution, rinse with water and set aside. (2) Passivation: Immerse the pickled stainless steel pipe in the passivation solution, rinse with water and set aside. (3) Cleaning and testing: The passivated stainless steel pipe is neutralized with sodium carbonate solution, rinsed with water after neutralization, and then soaked in deionized water to clean it. The soaking solution is collected and the content of hexavalent chromium in the soaking solution is tested. The cleaned stainless steel pipe is air-dried naturally. The pickling solution comprises the following raw materials in parts by weight: 4-7 parts sulfuric acid, 15-22 parts nitric acid, 2-3 parts polyferric sulfate, 4-6 parts ammonium fluoride, 3-8 parts ammonium bifluoride, 1-2 parts acetic acid, and 50-75 parts water. The passivation solution comprises the following raw materials in parts by weight: 8-12 parts nitric acid, 1.2-2.5 parts ethylenediaminetetramethylene phosphoric acid, 0.3-0.6 parts formic acid, 0.3-0.6 parts acetic acid, 0.02-0.05 parts polyether, 1-2 parts sodium dihydrogen phosphate, and 80-90 parts water.

2. The method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes according to claim 1, characterized in that: The sodium carbonate solution has a mass concentration of 4.5-5.5%.

3. The method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes according to claim 1, characterized in that: In step (1), the soaking temperature for pickling is 15-30℃.

4. The method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes according to claim 1, characterized in that: In step (1), the soaking time for pickling is 10-20 minutes.

5. The method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes according to claim 1, characterized in that: In step (2), the immersion temperature for passivation is 15-30℃.

6. The method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes according to claim 1, characterized in that: In step (2), the passivation soaking time is 10-20 min.

7. The method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes according to claim 1, characterized in that: In step (3), the soaking temperature of the deionized water is 15-20℃, and the soaking time of the deionized water is 5-10 min.

8. The method for inhibiting the precipitation of hexavalent chromium in stainless steel pipes according to claim 1, characterized in that: In step (3), the method for detecting hexavalent chromium is diphenylcarbazide spectrophotometry.

Images