Method for cleaning medium-high chromium stainless steel welding seam

Abstract

The invention provides a rapid cleaning method for a stainless steel welding seam, which adopts a method of carrying out constant-current anodic polarization on a stainless steel welding part in a sodium sulfate solution, and can rapidly and completely remove surface oxidation color after stainless steel welding. The method is simple and easy to operate, the used solution is safe and pollution-free, and the corrosion resistance of the surface of the treated weld joint is remarkably improved.

Description

Technical Field

[0001] The invention relates to the technical field of stainless steel, and in particular to a method for cleaning a medium- and high-chromium stainless steel weld. Background Art

[0002] Medium-high chromium stainless steel generally refers to stainless steel with a chromium content greater than 16%. This type of stainless steel usually has excellent corrosion resistance and is therefore widely used in industries such as aerospace, marine engineering, energy industry, transportation, chemical industry, papermaking, textile and food processing. In the actual use of medium-high chromium stainless steel, welding is the most important means of connection. However, due to the high temperature of welding, the metal surface of the weld and its heat-affected zone (HAZ) may oxidize to form an oxidized color. The appearance of this oxidized color will destroy the passivation film on the surface of the stainless steel, making the stainless steel surface more susceptible to corrosion and causing serious damage. Therefore, after welding, the stainless steel parts usually need to remove the surface oxide scale, and undergo pickling and passivation treatment to rebuild the passivation film and restore its corrosion resistance.

[0003] There are many methods to remove the oxidation color of stainless steel welds, mainly including mechanical removal methods and chemical removal methods. The mechanical removal method uses metal brushes, sandpaper, sanding belts or grinding tools to remove the oxidation color. This is a physical method that peels off the oxide layer through friction. The chemical removal method uses chemical solutions, such as pickling solutions (sulfuric acid, hydrochloric acid, nitric acid and hydrofluoric acid, etc.) or pickling passivation pastes, to dissolve or convert the oxidation color and make it fall off the stainless steel surface. However, when the mechanical grinding method is used to remove the oxidation color on the surface of stainless steel, grinding marks are very likely to appear, resulting in uneven stainless steel surface and rough surface grinding marks, affecting the overall appearance and corrosion resistance of the product; the splashes during the mechanical grinding process may further contaminate the stainless steel base material and affect its corrosion resistance; in addition, for welds of complex shapes, mechanical grinding is difficult to completely remove the oxidation color of stainless steel welds in dead corners, thus leaving potential corrosion problems. The method of chemically removing the oxidation color of stainless steel welds requires strict control of acid concentration, temperature and treatment time to prevent unclean or excessive pickling, which damages the stainless steel surface; during the acid treatment operation, the acid should be avoided from splashing onto the skin or clothing to avoid burns or damage; the pickling site should have good exhaust equipment to reduce the corrosion and harm of acid-containing gases to equipment and human body; after pickling, it must be thoroughly washed with water, and no residual acid is allowed to remain on the surface of the workpiece to avoid corrosion; the pickling liquid needs to be properly collected and treated to avoid waste acid from polluting the land and water sources. Pickling passivation paste is a chemical preparation used to treat stainless steel welds. Its essence is a colloid containing various corrosive acids. Pickling passivation paste is usually used by manual application, which is simple to operate and suitable for on-site construction. It is suitable for low-Cr stainless steel varieties and can effectively remove weld spots, oxide scales, etc. However, pickling passivation paste is easy to remain, the operating environment is poor, the labor intensity is high, necessary acid prevention measures need to be taken, and the effect on the treatment of welds in pipelines is limited.

[0004] The welding oxidation color of medium and high chromium stainless steel is more difficult to remove than that of low chromium stainless steel. Mechanical grinding cannot completely remove the oxide scale at complex shaped welds; chemical removal requires the use of high-concentration acid, which is difficult to carry out in actual operation due to safety and environmental issues.

[0005] Therefore, it is urgent to establish a safe, pollution-free, easy-to-operate method that can completely remove the surface oxidation color of medium and high chromium stainless steel, so as to properly treat the surface of medium and high chromium stainless steel welds and ensure the corrosion resistance of medium and high chromium stainless steel after welding.

[0006] In order to solve the above defects and shortcomings in the prior art, the present invention provides a method for quickly cleaning the weld of medium-high chromium stainless steel, which can completely remove the surface oxidation color of medium-high chromium stainless steel after welding, is simple and easy to operate, is suitable for welds of various complex shapes, and the solution used is safe and pollution-free. The corrosion resistance of the weld surface after treatment is significantly improved. Summary of the invention

[0007] The purpose of the present invention is to provide a method for cleaning the weld of medium- and high-chromium stainless steel in view of the above problems.

[0008] The purpose of the present invention is achieved as follows: 1. A method for cleaning medium-high chromium stainless steel welds, comprising the following steps: Step 1: After the medium-high chromium stainless steel weld is naturally cooled, the heavy oil stains on the surface are removed, the area requiring electrolytic treatment is measured and calculated, and the anode current required to be applied is calculated. Step 2: Prepare weld treatment solution: The weld treatment solution is an aqueous solution of sodium sulfate, and the mass fraction of sodium sulfate is selected to be 5%-20%; Step 3: The stainless steel weld is connected to the positive electrode of the constant current meter as the anode, and another piece of conductive metal is connected to the negative electrode of the constant current meter as the cathode. The cathode metal can be the same stainless steel material as the welding material, or lead or copper. The output current of the constant current meter is adjusted to the anode current value calculated in step 1; Step 4: Place the stainless steel weld sample and the cathode together in the sodium sulfate treatment solution, turn on the power supply to start the electrolytic treatment; Step 5: After electrolysis for 1-5 minutes, take out the stainless steel weld sample to observe whether the oxide scale on its surface is removed cleanly; if not, put it into the electrolyte again and continue the electrolytic treatment for 1-5 minutes; Step 6: After the oxide scale on the surface of the stainless steel is removed cleanly, take out the stainless steel weld sample and wash it with clean water, and dry it naturally or heat it to dry.

[0009] The area to be electrolytically treated and the anode current to be applied in step 1 are calculated as follows: the area to be electrolytically treated refers to the sample area to be immersed in the treatment solution, which must include the complete weld oxidation discoloration area and the necessary surrounding stainless steel base material, and the area measurement is accurate to 0.1 square centimeters; the anode current to be applied refers to the current applied to the weld sample by the constant current instrument, and the applied current value is equal to the product of the anode area and the anode current density, and the anode current density is 50 mA / cm 2 -1000mA / cm 2 .

[0010] The beneficial effects of the present invention are as follows: the present invention provides a method for rapid cleaning of medium- and high-chromium stainless steel welds. The method of constant current anodic polarization of stainless steel welds in a sodium sulfate solution can quickly and completely remove the surface oxidation color of stainless steel after welding. The method is simple and easy to operate, the solution used is safe and pollution-free, and the corrosion resistance of the weld surface after treatment is significantly improved. DETAILED DESCRIPTION

[0011] The technical solution adopted by the present invention to solve the above problems is: design a method for rapid cleaning of medium-high chromium stainless steel welds. The method takes the oxide scale after welding of medium-high chromium stainless steel as the treatment object, uses sodium sulfate solution as the treatment solution, and uses a constant current meter to apply a stable anodic current to the oxide scale of the medium-high chromium stainless steel, so as to quickly remove the oxide scale on its surface in a short time and improve the corrosion resistance of the weld surface. The method is simple and easy to operate, the solution used is safe and pollution-free, and the corrosion resistance of the weld surface after treatment is significantly improved.

[0012] Specifically, the following steps are included: Step 1: After the medium and high chromium stainless steel is welded and cooled naturally, the heavy oil stains on the surface are removed, the area that needs electrolytic treatment is measured and calculated, and the anode current that needs to be applied is calculated. The area that needs electrolytic treatment refers to the area of ​​the sample to be immersed in the treatment liquid. This area must include the complete weld oxidation discoloration area and the necessary surrounding stainless steel base material. The area must be measured with an accuracy of 0.1 square centimeters. The anode current that needs to be applied refers to the current applied to the weld sample by the constant current meter. The applied current value is equal to the product of the anode area and the anode current density. The commonly used anode current density is 50mA / cm 2 -1000mA / cm 2 .

[0013] Step 2: Prepare the weld treatment solution: The weld treatment solution is an aqueous solution of sodium sulfate, and the mass fraction of sodium sulfate is selected to be 5%-20%; Step 3: Connect the stainless steel weld as the anode to the positive pole of the galvanometer, and take another piece of conductive metal as the cathode to the negative pole of the galvanometer. The cathode metal can be the same stainless steel material as the welding material, or lead or copper. Adjust the output current of the galvanometer to the anode current value calculated in step 1; Step 4: Place the stainless steel weld sample and the cathode together in the sodium sulfate treatment solution, and turn on the power to start the electrolytic treatment; Step 5: After electrolysis for 1-5 minutes, take out the stainless steel weld sample and observe whether the surface oxide scale is completely removed; if not, put it into the electrolyte again and continue electrolysis for 1-5 minutes; Step 6: After the oxide scale on the stainless steel surface is removed, take out the stainless steel weld sample and wash it with clean water, and dry it naturally or heat it to dry.

[0014] Among them, in the step one, the heavy oil and other pollution on the surface of the sample can be cleaned with appropriate solvents (such as ethanol, acetone), and then rinsed with clean water. The area that needs electrolytic treatment refers to the area of ​​the anode sample to be immersed in the treatment solution. This area must include the complete weld oxidation discoloration area and the necessary surrounding stainless steel base material. The area is measured with an accuracy of 0.1 square centimeters. The anode current that needs to be applied refers to the current applied to the weld sample by the constant current meter. The applied current value is equal to the product of the anode area and the anode current density. The commonly used anode current density is 50mA / cm 2 -1000mA / cm 2 The greater the anodic current density, the faster the oxide scale on the sample surface is removed and the shorter the required processing time.

[0015] In step 2, the weld treatment solution refers to an aqueous solution prepared with deionized water and analytical grade sodium sulfate, and the content of harmful ions such as chloride ions should be minimized in the solution. The commonly used mass fraction is 5%-20%. This is because when the mass fraction of sodium sulfate is lower than 5%, the conductivity of the solution is poor, which places higher requirements on the power of the potentiostat; when the mass fraction of sodium sulfate is higher than 20%, the sodium sulfate in the solution is easily oversaturated and precipitated, wasting reagents and increasing treatment costs.

[0016] In step 3, the metal used as the cathode can be stainless steel, the same as the welding material, or lead or copper, but carbon steel and other materials that are prone to corrosion cannot be used to avoid new contamination of the stainless steel and affect its corrosion resistance. The shape of the cathode depends on the characteristics of the welded part to be processed, and can be a plate, a wire rod or other special-shaped parts.

[0017] In step 4, the weld to be treated should be immersed in the treatment solution together with the cathode, so that a solution loop can be formed between the anode and cathode of the stainless steel weld sample. However, the cathode and cathode cannot be in direct contact, and a necessary distance must be maintained between them to prevent short circuit and burnout of the constant current instrument.

[0018] In step 5, the treatment time of the weld surface oxide scale depends on the anode current density. The greater the anode current density, the shorter the treatment time required, and vice versa. The treatment time of the weld surface oxide scale cannot be too long to prevent excessive dissolution of the material in the weld area. Interruption of the electrolytic treatment of the sample will not affect the surface quality of the sample. Therefore, the single electrolysis time can be appropriately shortened and the number of observations can be increased.

[0019] In step 6, after the stainless steel weld scale is completely removed, the weld surface should be thoroughly cleaned with clean water to avoid the introduction of corrosive ions such as chloride ions. The stainless steel surface should be kept as dry and pollution-free as possible.

[0020] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with embodiments. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the protection scope of the present invention. Example 1 Cleaning of 443 stainless steel (TTS443M) autogenous welding seam

[0021] Step 1: After the 443 stainless steel is self-melted and welded, wait for it to cool naturally. Use alcohol ultrasonic cleaning to remove heavy oil and other contaminants on the surface, and measure and calculate the area that needs electrolytic treatment. In this example, the surface area of ​​the sample is about 9.5 cm 2 , the anode electrolysis current density was set to 200 mA / cm 2 , 400 mA / cm 2 and 600 mA / cm 2 The current values ​​that the constant current instrument should output are calculated to be 1.9A, 3.8A, and 5.7A respectively.

[0022] Step 2: Prepare a weld treatment solution, which is an aqueous solution of sodium sulfate, and the mass fraction of sodium sulfate is selected to be 10%.

[0023] Step 3: Connect the 443 stainless steel weld sample as the anode to the positive pole of the constant current instrument, and connect another 443 stainless steel plate as the cathode to the negative pole of the constant current instrument. Adjust the output current of the constant current instrument.

[0024] Step 4: Place the 443 stainless steel weld sample and the cathode plate together in a 10% sodium sulfate treatment solution, turn on the power supply and output a constant current.

[0025] Step 5: After electrolysis for 1-5 minutes, take out the stainless steel weld sample and observe whether the surface oxide scale is completely removed. In this embodiment, the anode current density is 200 mA / cm 2 , 400 mA / cm 2 and 600 mA / cm 2 When the electrolysis time was 5 min, 2 min and 1 min respectively, the oxide scale at the weld on the surface of the sample was completely removed, and its surface color was consistent with the color of the stainless steel matrix.

[0026] Step 6: After the oxide scale on the stainless steel surface is removed, take out the sample and wash it with clean water, and then dry it naturally or heat it to dry.

[0027] The pitting corrosion resistance of the samples before and after the 443 weld cleaning was compared, and the pitting corrosion immersion test in the national standard pitting corrosion test method (GB T 17897-2016) was carried out. After 24 hours of pitting corrosion immersion, a large number of pitting pits appeared on the surface of the original sample of the autogenous welding base material and the weld; while after the weld cleaning treatment, the base material of the sample had almost no pitting corrosion, and only a small amount of pitting pits appeared on the weld. The pitting corrosion rate of the original sample of the autogenous welding was measured to be 9.5g / (m 2 *h), while the pitting corrosion rates of the samples after weld cleaning were 7.1 g / (m 2 *h)、6.9 g / (m 2 *h) and 6.5 g / (m 2 *h). The pitting corrosion resistance of 443 welds has been significantly improved. Example 2 Cleaning of 444 stainless steel (00Cr18Mo2) filler wire welding seam

[0028] Step 1: After welding the 444 stainless steel wire, wait for it to cool naturally. Use alcohol ultrasonic cleaning to remove heavy oil and other contaminants on the surface, and measure and calculate the area that needs electrolytic treatment. In this example, the surface area of ​​the sample is about 10.6 cm 2 , the anode electrolysis current density was set to 200 mA / cm 2 , 400 mA / cm 2 and 600 mA / cm 2 The current values ​​that the constant current instrument should output are calculated to be 2.1A, 4.2A, and 6.4A respectively.

[0029] Step 2: Prepare a weld treatment solution, which is an aqueous solution of sodium sulfate with a mass fraction of 15%.

[0030] Step 3: Connect the 444 stainless steel weld sample as the anode to the positive pole of the constant current instrument, and connect another 444 stainless steel plate as the cathode to the negative pole of the constant current instrument. Adjust the output current of the constant current instrument.

[0031] Step 4: Place the 444 stainless steel weld sample and the cathode plate together in a 15% sodium sulfate treatment solution, turn on the power supply and output a constant current.

[0032] Step 5: After electrolysis for a period of time, take out the stainless steel weld sample and observe whether the surface oxide scale is completely removed. In this embodiment, the anode current density is 200 mA / cm 2 , 400 mA / cm 2 and 600 mA / cm 2 When the electrolysis time was 5min, 5min and 3min respectively, the oxide scale on the weld of the sample surface was completely removed. The surface color of the sample after electrolytic cleaning was consistent with the color of the stainless steel substrate.

[0033] Step 6: After the oxide scale on the stainless steel surface is removed, take out the sample and wash it with clean water, and then dry it naturally or heat it to dry.

[0034] The original sample of the wire-filled welding and the sample of the weld cleaned after the wire-filled welding were selected for comparison, and the pitting corrosion immersion test in the national standard pitting corrosion test method (GB T 17897-2016) was carried out. After 24 hours of pitting corrosion immersion, a large number of pitting pits appeared on the surface of the base material and weld of the original welding sample; while the base material of the sample after the weld cleaning treatment had almost no pitting corrosion, and only a small amount of pitting corrosion pits appeared at the junction of the weld and the base material. The pitting corrosion rate of the original welding sample was measured to be 15.5g / (m 2 *h), while the pitting corrosion rates of the samples after electrolytic cleaning were 9.1 g / (m 2 *h)、7.9 g / (m 2 *h) and 8.5 g / (m 2 *h). The pitting corrosion resistance of 444 welds has been significantly improved. Example 3 Cleaning of the weld inside the 2001 stainless steel (S32001) self-melting welded pipe

[0035] Step 1: After the 2001 stainless steel self-melting welding is made into a welded pipe, wait for it to cool naturally. The outer surface can be polished, brushed, etc. to remove the surface oxidation color of the weld, but the inner surface cannot be treated. Measure and calculate the area that needs electrolytic treatment. In this example, the diameter of the welded pipe is 80mm, the width of the weld oxidation color is about 12mm, and the pipe length is 4m. The surface area that needs to be treated is about 5000cm 2 , set the anode electrolysis current density to 500 mA / cm 2 . Calculate that the current value that the constant current instrument should output is 2500A.

[0036] Step 2: Prepare a weld treatment solution, which is an aqueous solution of sodium sulfate with a mass fraction of 12%.

[0037] Step 3: Connect the 2001 stainless steel welded pipe as the anode to the positive pole of the constant current instrument, and take another 2001 rod with a diameter of 5 cm as the cathode to the negative pole of the constant current instrument. Insert the 2001 rod into the 2001 welded pipe, and install semi-sealed plugs on both sides of the welded pipe to ensure that there is no short circuit between the positive and negative poles. Adjust the output current of the constant current instrument.

[0038] Step 4: Add 12% sodium sulfate treatment solution into the 2001 welded pipe, ensure that the weld and cathode rod are both in the sodium sulfate solution, turn on the power supply and output a constant current.

[0039] Step 5: Electrolyze for 3 minutes, remove the stainless steel cathode rod, and completely remove the oxide scale on the weld inside the welded pipe. The surface color of the inner weld after electrolytic cleaning is consistent with the color of the stainless steel matrix.

[0040] Step 6: Release the treatment solution in the stainless steel welded pipe, clean the inside of the welded pipe with clean water and dry it naturally.

[0041] The above is only a preferred embodiment of the present invention. It should be pointed out that a person skilled in the art can make several improvements and modifications without departing from the concept of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for cleaning medium and high chromium stainless steel welds, characterized in that: The following steps are involved: Step 1: After the medium- and high-chromium stainless steel is welded and cooled naturally, the heavy oil stains on the surface are removed, the area that needs electrolytic treatment is measured and calculated, and the anode current that needs to be applied is calculated; Step 2: Prepare the weld treatment solution: The weld treatment solution is an aqueous solution of sodium sulfate, and the mass fraction of sodium sulfate is selected to be 5%-20%; Step 3: Connect the stainless steel weld as the anode to the positive electrode of the constant current meter, and take another piece of conductive metal as the cathode to the negative electrode of the constant current meter. The cathode metal can be the same stainless steel material as the welding material, or lead or copper. Adjust the output current of the constant current meter to the anode current value calculated in step 1; Step 4: Place the stainless steel weld sample and the cathode together in the sodium sulfate treatment solution, and turn on the power to start the electrolytic treatment; Step 5: After electrolysis for 1-5 minutes, take out the stainless steel weld sample and observe whether the surface oxide scale is completely removed; if not, put it into the electrolyte again and continue electrolysis for 1-5 minutes; Step 6: After the oxide scale on the stainless steel surface is removed, take out the stainless steel weld sample and wash it with clean water, and dry it naturally or heat it to dry.

2. A method for cleaning a medium-high chromium stainless steel weld according to claim 1, characterized in that: The area to be electrolytically treated and the anode current to be applied in step 1 are calculated as follows: the area to be electrolytically treated refers to the sample area to be immersed in the treatment solution, which must include the complete weld oxidation discoloration area and the necessary surrounding stainless steel base material, and the area measurement is accurate to 0.1 square centimeters; the anode current to be applied refers to the current applied to the weld sample by the constant current instrument, and the applied current value is equal to the product of the anode area and the anode current density, and the anode current density is 50 mA / cm 2 -1000mA / cm 2 .