Method for full hydrochloric acid pickling of iron-nickel alloy

Abstract

An iron-nickel alloy full hydrochloric acid pickling method belongs to the technical field of iron-nickel alloy surface oxide layer treatment, and specifically comprises the following steps: preparing hydrochloric acid solutions with different concentrations by using industrial hydrochloric acid, and adding a pickling accelerator into the hydrochloric acid solutions to obtain pickling solutions with different concentrations; the method comprises the steps that the iron-nickel alloy is completely immersed in an acid pickling solution, step-by-step acid pickling operation is conducted, the acid pickling condition of each stage is controlled in the acid pickling process, it is guaranteed that the acid pickling condition of each stage is improved, and the acid-pickled iron-nickel alloy is obtained; and the iron-nickel alloy obtained after acid pickling is subjected to scrubbing, water washing and air sweeping, and the iron-nickel alloy with the surface oxide layer completely removed is obtained. According to the method, a step-by-step hydrochloric acid pickling process system is adopted, the oxidation-reduction potential of a hydrochloric acid system, the hydrochloric acid concentration, the temperature and the pickling time are controlled, the iron-nickel alloy surface oxide layer structure is damaged step by step, and therefore the purpose of effectively removing the iron-nickel alloy surface oxide layer is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of surface oxide layer treatment of iron-nickel alloys, and particularly relates to a method for pickling iron-nickel alloys with full hydrochloric acid. Background Art

[0002] As a special-purpose stainless steel material, iron-nickel alloy is widely used in the manufacture of electromagnetic components, sensors, aerospace equipment parts, and precision parts, etc. due to its characteristics of high magnetic permeability and low magnetic conductivity loss.

[0003] The market demand for iron-nickel alloy is large and the development prospect is very broad. Therefore, strengthening the development and utilization of iron-nickel alloy is of great significance for giving full play to China's resource advantages, driving industrial progress, and promoting economic growth.

[0004] When producing iron-nickel alloy strip, the heat treatment process causes high-temperature oxidation on the surface of the iron-nickel alloy, and a dense oxide scale will be generated on its surface, which must be removed before rolling. The surface oxide scale of iron-nickel alloy has various oxidation structures, with different properties but all very dense and firmly combined with the matrix. Regarding the removal methods of the surface oxide layer of iron-nickel alloys (4J42K and 4J36), they are mainly mechanical polishing, and there are no patent and literature reports on the method of using pickling to remove the surface oxide layer of iron-nickel alloy strip.

[0005] At present, the methods for removing the surface oxide layer of iron-nickel alloy are as follows:

[0006] For example, CN118848792A discloses a stainless steel plate mirror polishing device and its process, providing a stainless steel plate mirror polishing device and its process with stable positioning and high polishing efficiency. However, it causes large wear and high loss on the stainless steel surface.

[0007] For example, CN216859193U discloses a grinding and polishing device for stainless steel processing, adding a conveyor belt, a mounting frame, an electric push rod, and an electric wheel, solving the problem of manual operation required when grinding materials, making the grinding and polishing of stainless steel safer. However, when using this grinding device, the production efficiency is low, the energy consumption is large, and the loss is also large.

[0008] In summary, regarding the removal methods of the surface oxide layer of iron-nickel alloys (4J42K and 4J36), they are mainly mechanical polishing, with low production efficiency, high energy consumption, and large loss. Summary of the Invention

[0009] Aiming at the deficiencies of the prior art, the method provided by the present invention solves the problem that the surface oxide layer of iron-nickel alloy is difficult to be removed by pickling process, not only has a low metal loss rate, but also has a low production cost, high production efficiency, and has important practical significance for the production of iron-nickel alloy strip in China.

[0010] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0011] The present invention provides a method for pickling iron-nickel alloy with full hydrochloric acid, which specifically includes the following steps:

[0012] S1. Prepare pickling solution: Use industrial hydrochloric acid to prepare hydrochloric acid solutions with different concentrations, and add pickling accelerators thereto to obtain pickling solutions with different concentrations;

[0013] S2. Step-by-step pickling process: Immerse the iron-nickel alloy completely in the pickling solution and perform step-by-step pickling operations. During the pickling process, control the pickling conditions at each stage to ensure that the pickling conditions at each stage are improved, and obtain the pickled iron-nickel alloy;

[0014] The pickling conditions are selected from one or more of pickling temperature, pickling time, hydrochloric acid concentration in the pickling solution, and redox potential of the pickling solution;

[0015] S3. The pickled iron-nickel alloy is brushed, washed with water, and air-swept to obtain an iron-nickel alloy with the surface oxide layer completely removed.

[0016] Wherein:

[0017] In step S1, the mass concentration of the hydrochloric acid solution is 10% - 25%.

[0018] In step S1, the pickling accelerator is selected from one of benzylidene acetone, nonylphenol polyoxyethylene ether, and triethanolamine, and the addition amount is 0.5% - 2% of the mass of the hydrochloric acid solution.

[0019] In step S2, the iron-nickel alloy is 4J36 sheet or 4J42K sheet after removing oil, and the thickness of the sheet is 3mm - 5mm.

[0020] In step S2, the number of pickling stages is 6, the pickling temperature is 50°C - 80°C, and the pickling time is 20s - 100s.

[0021] In step S2, hydrogen peroxide is used to regulate the redox potential of the pickling solution, and the range of the redox potential of the pickling solution is 390mV - 520mV.

[0022] In step S2, after each stage of pickling, it is cleaned with alcohol and dried after cleaning.

[0023] In step S3, the total weight loss rate of the iron-nickel alloy is 2.5% - 7%.

[0024] Compared with the prior art, the beneficial effects of the present application are as follows:

[0025] 1. By virtue of the characteristics of small atomic radius of chloride ions, strong ability to penetrate oxides, and easy occurrence of electrochemical reactions with corrosion-resistant metals, etc., the present invention utilizes the activation-passivation electrochemical phenomenon that is easily formed between the surface of the iron-nickel alloy matrix and chloride ions, and realizes the stripping of the oxide layer on the surface of the iron-nickel alloy through hydrochloric acid pickling, thereby achieving the effect of effectively removing the oxide layer on the surface of the iron-nickel alloy.

[0026] 2. The present invention adopts a step-by-step hydrochloric acid pickling process system, and by controlling the redox potential, hydrochloric acid concentration, temperature, and pickling time of the hydrochloric acid system, the oxide layer structure on the surface of the iron-nickel alloy is gradually destroyed to achieve the purpose of effectively removing the oxide layer on the surface of the iron-nickel alloy.

[0027] 3. The present invention solves the problem that the oxide layer on the surface of the iron-nickel alloy cannot be removed by pickling and can only be removed by mechanical polishing methods, and has the characteristics of low metal loss rate, low energy consumption, high efficiency, and low production cost. Specific Embodiments

[0028] In order to make the technical solutions, creative purposes, and advantages achieved by the present invention clearer and more distinct, the present invention will be further described below in conjunction with specific embodiments.

[0029] Example 1

[0030] A method for full hydrochloric acid pickling of iron-nickel alloy specifically includes the following steps:

[0031] S1. Prepare hydrochloric acid pickling solution: Use industrial hydrochloric acid to prepare hydrochloric acid solutions with mass fractions of 10%, 15%, 20%, and 25%, and add 0.5% of benzylideneacetone based on the mass of the hydrochloric acid solution to obtain pickling solutions.

[0032] S2. Step-by-step pickling process: Use 4J36 iron-nickel alloy plates after removing oil stains as samples, with a plate thickness of 3 mm. Immerse the plates completely in the hydrochloric acid pickling solution for step-by-step pickling operations. The specific steps are as follows:

[0033] First-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 10%, adjust the redox potential of the pickling solution to 390 mV, control the temperature at 50 °C, immerse the iron-nickel alloy plate completely in the hydrochloric acid pickling solution, pickle for 20 s, clean with alcohol and then dry, and proceed to the second-stage pickling;

[0034] Second-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 10%, adjust the redox potential of the pickling solution to 390 mV, control the temperature at 60 °C, immerse the iron-nickel alloy plate after the first-stage pickling completely in the hydrochloric acid pickling solution, pickle for 30 s, clean with alcohol and then dry, and proceed to the third-stage pickling;

[0035] Tertiary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 10%, adjust the redox potential of the pickling solution to 420 mV, control the temperature at 60 °C, completely immerse the iron-nickel alloy sheet after secondary pickling in the hydrochloric acid pickling solution, pickle for 40 s, clean with alcohol and then dry, and proceed to quaternary pickling;

[0036] Quaternary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%, adjust the redox potential of the pickling solution to 460 mV, control the temperature at 70 °C, completely immerse the iron-nickel alloy sheet after tertiary pickling in the hydrochloric acid pickling solution, pickle for 50 s, clean with alcohol and then dry, and proceed to quinary pickling;

[0037] Quinary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the redox potential of the pickling solution to 500 mV, control the temperature at 70 °C, completely immerse the iron-nickel alloy sheet after quaternary pickling in the hydrochloric acid pickling solution, pickle for 50 s, clean with alcohol and then dry, and proceed to senary pickling;

[0038] Senary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the redox potential of the pickling solution to 520 mV, control the temperature at 80 °C, completely immerse the iron-nickel alloy sheet after quinary pickling in the hydrochloric acid pickling solution, pickle for 50 s, and the pickled iron-nickel alloy;

[0039] S3. The pickled iron-nickel alloy is scrubbed, washed with water and air-swept to obtain an iron-nickel alloy with the surface oxide layer completely removed, and the total weight loss rate of the iron-nickel alloy is 2.5%.

[0040] Example 2

[0041] A method for full hydrochloric acid pickling of iron-nickel alloy specifically includes the following steps:

[0042] S1. Prepare hydrochloric acid pickling solution: Use industrial hydrochloric acid to prepare hydrochloric acid solutions with mass fractions of 10%, 15%, 20%, and 25%, and add 0.5% of benzalacetone based on the mass of the hydrochloric acid solution to obtain pickling solutions.

[0043] S2. Step-by-step pickling process: Use the 4J36 iron-nickel alloy sheet after degreasing as a sample, with a sheet thickness of 3 mm. Completely immerse the sheet in the hydrochloric acid pickling solution and perform step-by-step pickling operations. The specific steps are as follows:

[0044] Primary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 10%, adjust the redox potential of the pickling solution to 390 mV, control the temperature at 50 °C, completely immerse the iron-nickel alloy sheet in the hydrochloric acid pickling solution, pickle for 40 s, clean with alcohol and then dry, and proceed to secondary pickling;

[0045] Secondary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%, adjust the oxidation-reduction potential of the pickling solution to 390 mV, control the temperature at 60 °C, completely immerse the iron-nickel alloy sheet after primary pickling in the hydrochloric acid pickling solution, pickle for 50 s, clean with alcohol and then dry, and proceed to tertiary pickling;

[0046] Tertiary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%, adjust the oxidation-reduction potential of the pickling solution to 440 mV, control the temperature at 60 °C, completely immerse the iron-nickel alloy sheet after secondary pickling in the hydrochloric acid pickling solution, pickle for 60 s, clean with alcohol and then dry, and proceed to quaternary pickling;

[0047] Quaternary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the oxidation-reduction potential of the pickling solution to 460 mV, control the temperature at 70 °C, completely immerse the iron-nickel alloy sheet after tertiary pickling in the hydrochloric acid pickling solution, pickle for 50 s, clean with alcohol and then dry, and proceed to quinary pickling;

[0048] Quinary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the oxidation-reduction potential of the pickling solution to 500 mV, control the temperature at 70 °C, completely immerse the iron-nickel alloy sheet after quaternary pickling in the hydrochloric acid pickling solution, pickle for 60 s, clean with alcohol and then dry, and proceed to senary pickling;

[0049] Senary pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 25%, adjust the oxidation-reduction potential of the pickling solution to 520 mV, control the temperature at 80 °C, completely immerse the iron-nickel alloy sheet after quinary pickling in the hydrochloric acid pickling solution, pickle for 50 s, and obtain the pickled iron-nickel alloy;

[0050] S3. The pickled iron-nickel alloy is scrubbed, washed with water and air-swept to obtain an iron-nickel alloy with the surface oxide layer completely removed, and the total weight loss rate of the iron-nickel alloy is 3%.

[0051] Example 3

[0052] A method for full hydrochloric acid pickling of iron-nickel alloy specifically includes the following steps:

[0053] S1. Prepare hydrochloric acid pickling solution: Use industrial hydrochloric acid to prepare hydrochloric acid solutions with mass fractions of 10%, 15%, 20%, and 25%, and add 1% of benzalacetone based on the mass of the hydrochloric acid solution to obtain pickling solutions.

[0054] S2. Step-by-step pickling process: Use the 4J42K iron-nickel alloy sheet after degreasing as a sample. The thickness of the sheet is 3 mm. Immerse the sheet completely in the hydrochloric acid pickling solution and perform step-by-step pickling operations. The specific steps are as follows:

[0055] First-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 10%. Adjust the redox potential of the pickling solution to 390 mV, control the temperature at 50 °C, immerse the iron-nickel alloy sheet completely in the hydrochloric acid pickling solution, pickle for 50 s, clean with alcohol and then dry, and proceed to the second-stage pickling;

[0056] Second-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%. Adjust the redox potential of the pickling solution to 390 mV, control the temperature at 60 °C, immerse the iron-nickel alloy sheet after the first-stage pickling completely in the hydrochloric acid pickling solution, pickle for 50 s, clean with alcohol and then dry, and proceed to the third-stage pickling;

[0057] Third-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%. Adjust the redox potential of the pickling solution to 440 mV, control the temperature at 60 °C, immerse the iron-nickel alloy sheet after the second-stage pickling completely in the hydrochloric acid pickling solution, pickle for 60 s, clean with alcohol and then dry, and proceed to the fourth-stage pickling;

[0058] Fourth-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%. Adjust the redox potential of the pickling solution to 460 mV, control the temperature at 70 °C, immerse the iron-nickel alloy sheet after the third-stage pickling completely in the hydrochloric acid pickling solution, pickle for 60 s, clean with alcohol and then dry, and proceed to the fifth-stage pickling;

[0059] Fifth-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%. Adjust the redox potential of the pickling solution to 500 mV, control the temperature at 70 °C, immerse the iron-nickel alloy sheet after the fourth-stage pickling completely in the hydrochloric acid pickling solution, pickle for 60 s, clean with alcohol and then dry, and proceed to the sixth-stage pickling;

[0060] Sixth-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 25%. Adjust the redox potential of the pickling solution to 520 mV, control the temperature at 80 °C, immerse the iron-nickel alloy sheet after the fifth-stage pickling completely in the hydrochloric acid pickling solution, pickle for 60 s, and the pickled iron-nickel alloy;

[0061] S3. The pickled iron-nickel alloy is brushed, washed with water and air-swept to obtain an iron-nickel alloy with the surface oxide layer completely removed. The total weight loss rate of the iron-nickel alloy is 4.4%.

[0062] Example 4

[0063] A method for full hydrochloric acid pickling of iron-nickel alloy, specifically including the following steps:

[0064] S1. Prepare hydrochloric acid pickling solution: Use industrial hydrochloric acid to prepare hydrochloric acid solutions with mass fractions of 10%, 15%, 20%, and 25%, and add 1.5% of benzylideneacetone based on the mass of the hydrochloric acid solution to obtain pickling solutions.

[0065] S2. Step-by-step pickling process: Use 4J36 iron-nickel alloy plates after removing oil stains as samples. The thickness of the plates is 3 mm. Immerse the plates completely in the hydrochloric acid pickling solution and perform step-by-step pickling operations. The specific steps are as follows:

[0066] First-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 10%, adjust the oxidation-reduction potential of the pickling solution to 390 mV, control the temperature at 50 °C, immerse the iron-nickel alloy plates completely in the hydrochloric acid pickling solution, pickle for 70 s, clean with alcohol and then dry, and perform second-stage pickling;

[0067] Second-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%, adjust the oxidation-reduction potential of the pickling solution to 390 mV, control the temperature at 60 °C, immerse the iron-nickel alloy plates after the first-stage pickling completely in the hydrochloric acid pickling solution, pickle for 70 s, clean with alcohol and then dry, and perform third-stage pickling;

[0068] Third-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the oxidation-reduction potential of the pickling solution to 440 mV, control the temperature at 60 °C, immerse the iron-nickel alloy plates after the second-stage pickling completely in the hydrochloric acid pickling solution, pickle for 80 s, clean with alcohol and then dry, and perform fourth-stage pickling;

[0069] Fourth-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the oxidation-reduction potential of the pickling solution to 480 mV, control the temperature at 70 °C, immerse the iron-nickel alloy plates after the third-stage pickling completely in the hydrochloric acid pickling solution, pickle for 60 s, clean with alcohol and then dry, and perform fifth-stage pickling;

[0070] Fifth-stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the oxidation-reduction potential of the pickling solution to 500 mV, control the temperature at 70 °C, immerse the iron-nickel alloy plates after the fourth-stage pickling completely in the hydrochloric acid pickling solution, pickle for 80 s, clean with alcohol and then dry, and perform sixth-stage pickling;

[0071] Sixth - stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 25%, adjust the oxidation - reduction potential of the pickling solution to 520 mV, control the temperature at 80 °C, completely immerse the iron - nickel alloy sheet after fifth - stage pickling in the hydrochloric acid pickling solution, pickle for 80 s, and obtain the pickled iron - nickel alloy;

[0072] S3. The pickled iron - nickel alloy is scrubbed, washed with water and air - swept to obtain an iron - nickel alloy with the surface oxide layer completely removed, and the total weight loss rate of the iron - nickel alloy is 2.5%.

[0073] Example 5

[0074] A method for full - hydrochloric - acid pickling of iron - nickel alloy, specifically including the following steps:

[0075] S1. Prepare hydrochloric acid pickling solution: Use industrial hydrochloric acid to prepare hydrochloric acid solutions with mass fractions of 10%, 15%, 20%, and 25%, and add 2% of benzylideneacetone based on the mass of the hydrochloric acid solution to obtain pickling solutions.

[0076] S2. Step - by - step pickling process: Use the 4J42K iron - nickel alloy sheet after degreasing as a sample, with the thickness of the sheet being 3 mm. Completely immerse the sheet in the hydrochloric acid pickling solution and perform step - by - step pickling operations. The specific steps are as follows:

[0077] First - stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 10%, adjust the oxidation - reduction potential of the pickling solution to 390 mV, control the temperature at 60 °C, completely immerse the iron - nickel alloy sheet in the hydrochloric acid pickling solution, pickle for 70 s, clean with alcohol and then dry, and perform second - stage pickling;

[0078] Second - stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%, adjust the oxidation - reduction potential of the pickling solution to 420 mV, control the temperature at 70 °C, completely immerse the iron - nickel alloy sheet after first - stage pickling in the hydrochloric acid pickling solution, pickle for 70 s, clean with alcohol and then dry, and perform third - stage pickling;

[0079] Third - stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 15%, adjust the oxidation - reduction potential of the pickling solution to 440 mV, control the temperature at 70 °C, completely immerse the iron - nickel alloy sheet after second - stage pickling in the hydrochloric acid pickling solution, pickle for 80 s, clean with alcohol and then dry, and perform fourth - stage pickling;

[0080] Fourth - stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 20%, adjust the oxidation - reduction potential of the pickling solution to 480 mV, control the temperature at 80 °C, completely immerse the iron - nickel alloy sheet after third - stage pickling in the hydrochloric acid pickling solution, pickle for 80 s, clean with alcohol and then dry, and perform fifth - stage pickling;

[0081] Five - stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 25%, adjust the redox potential of the pickling solution to 500 mV, control the temperature at 80 °C, completely immerse the iron - nickel alloy sheet after four - stage pickling in the hydrochloric acid pickling solution, pickle for 90 s, clean with alcohol and then dry, and then perform six - stage pickling;

[0082] Six - stage pickling: Add hydrogen peroxide to the pickling solution with a hydrochloric acid mass fraction of 25%, adjust the redox potential of the pickling solution to 520 mV, control the temperature at 80 °C, completely immerse the iron - nickel alloy sheet after five - stage pickling in the hydrochloric acid pickling solution, pickle for 100 s, and the pickled iron - nickel alloy;

[0083] S3. The pickled iron - nickel alloy is scrubbed, washed with water and air - swept to obtain an iron - nickel alloy with the surface oxide layer completely removed, and the total weight loss rate of the iron - nickel alloy is 6.5%.

Claims

1. A method for full hydrochloric acid pickling of iron-nickel alloy, characterized in that, specifically includes the following steps: S1. Prepare pickling solution: Use industrial hydrochloric acid to prepare hydrochloric acid solutions with different concentrations, and add pickling accelerators thereto to obtain pickling solutions with different concentrations; S2. Step-by-step pickling process: Immerse the iron-nickel alloy completely in the pickling solution and perform step-by-step pickling operations. Control the pickling conditions at each stage during the pickling process to ensure that the pickling conditions at each stage are improved, and obtain the pickled iron-nickel alloy; The pickling conditions are selected from one or more of pickling temperature, pickling time, hydrochloric acid concentration in the pickling solution, and redox potential of the pickling solution; S3. The pickled iron-nickel alloy is brushed, washed with water, and air-swept to obtain an iron-nickel alloy with the surface oxide layer completely removed.

2. The method for full hydrochloric acid pickling of iron-nickel alloy according to claim 1, characterized in that, in the step S1, the mass concentration of the hydrochloric acid solution is 10% - 25%.

3. The method for full hydrochloric acid pickling of iron-nickel alloy according to claim 1, characterized in that, in the step S1, the pickling accelerator is selected from one of benzalacetone, nonylphenol polyoxyethylene ether, and triethanolamine, and the addition amount is 0.5% - 2% of the mass of the hydrochloric acid solution.

4. The method for full hydrochloric acid pickling of iron-nickel alloy according to claim 1, characterized in that, in the step S2, the iron-nickel alloy is 4J36 sheet or 4J42K sheet after removing oil stains, and the thickness of the sheet is 3mm - 5mm.

5. The method for full hydrochloric acid pickling of iron-nickel alloy according to claim 1, characterized in that, in the step S2, the number of pickling stages is 6, the pickling temperature is 50°C - 80°C, and the pickling time is 20s - 100s.

6. The method for full hydrochloric acid pickling of iron-nickel alloy according to claim 1, characterized in that, in the step S2, hydrogen peroxide is used to regulate the redox potential of the pickling solution, and the range of the redox potential of the pickling solution is 390mV - 520mV.

7. The method for full hydrochloric acid pickling of iron-nickel alloy according to claim 1, characterized in that, in the step S2, after each stage of pickling, it is cleaned with alcohol and dried after cleaning.

8. The method for full hydrochloric acid pickling of iron-nickel alloy according to claim 1, characterized in that, in the step S3, the total weight loss rate of the iron-nickel alloy is 2.5% - 7%.