A method for treating a stainless steel workpiece to resist high temperature discoloration and corrosion

Abstract

This invention discloses a method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion, relating to the field of stainless steel surface treatment technology. The method involves first subjecting the stainless steel workpiece to an oxidation baking treatment to form an oxide film layer on its surface. Then, an acidic solution is used to remove the oxide film layer, further separating the iron and chromium elements on the stainless steel surface. Finally, Fenton's reagent is used for passivation, resulting in a chromium-rich or chromium-nickel-rich system on the surface of the stainless steel workpiece. This improves both its high-temperature resistance and its resistance to chloride ion corrosion, effectively enhancing its resistance to high-temperature discoloration and chloride ion corrosion. This ensures that the stainless steel workpiece will not discolor under operating conditions of 350-400℃ and also exhibits resistance to salt corrosion.

Description

Technical Field

[0001] This invention relates to the field of stainless steel surface treatment technology, and in particular to a treatment method for stainless steel workpieces to resist high-temperature discoloration and corrosion. Background Technology

[0002] In the cookware industry, austenitic stainless steel is favored for its corrosion resistance, high strength, durability, and non-reactivity with food. Ferritic stainless steel, on the other hand, is magnetic and can be used as a contact material in induction cookers, also finding widespread use in the cookware industry. However, during cooking, stainless steel products begin to yellow when the temperature exceeds 200℃, and the higher the temperature, the more severe the yellowing. Since cooking temperatures exceeding 200℃ are common, yellowing indicates a defect and negatively impacts consumer reputation.

[0003] To address the issue of high-temperature discoloration in stainless steel, patent CN105543937B proposes an electrolytic passivation solution. This electrolytic passivation is essentially the common phosphoric acid-based electrochemical polishing of stainless steel. During electrochemical polishing, when stainless steel is subjected to severe corrosion, the corrosion rates of different alloying elements vary; iron corrodes faster, while chromium and nickel corrode much more slowly. The surface of the electrolytically polished stainless steel forms an iron-poor, chromium- and nickel-rich layer, thus providing a certain degree of resistance to high-temperature discoloration. This resistance is approximately 280-300℃. However, this method offers limited improvement in high-temperature discoloration resistance, requires altering the surface treatment of the stainless steel, and uses concentrated phosphoric acid, generating heavy metal ions such as chromium and nickel. This results in high costs and significant pollution, making its implementation subject to numerous limitations.

[0004] Patent CN 108359929B proposes a method of high-temperature baking oxidation followed by acid pickling to remove the oxide film, achieving an iron-poor, chromium-rich nickel layer. This method involves pre-oxidizing iron and then acid-washing to remove the oxide film. The baking temperature is very high, 420-500℃, resulting in high costs. Baking at this temperature does not completely separate the iron, and acid pickling damages the austenitic structure on the stainless steel surface, causing the actual discoloration temperature to be 60-100℃ lower than the baking temperature. In other words, the actual discoloration temperature of a 420-500℃ baking temperature is in the 350-400℃ range. Furthermore, the microscopic cavities created by the iron precipitation and the destruction of the austenitic and other metallographic structures significantly reduce the corrosion resistance of stainless steel, especially during neutral salt spray and boiling water tests, easily resulting in large rust spots and making it more sensitive to chloride ions than untreated stainless steel. Summary of the Invention

[0005] The technical problem to be solved by the present invention is how to improve the anti-discoloration performance of stainless steel materials at high temperatures and the chloride ion sensitivity after treatment, and to provide a treatment method for stainless steel workpieces to resist high-temperature discoloration.

[0006] To address the above problems, the present invention proposes the following technical solution:

[0007] This invention provides a method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion, the method comprising the following steps:

[0008] S1. The stainless steel workpiece is subjected to an oxidation baking treatment under oxygen conditions to form an oxide film on the surface. The baking treatment temperature is 350-400℃.

[0009] S2. The stainless steel workpiece that has been baked is chemically treated with an acid solution to remove the oxide film on the surface and then subjected to selective weak corrosion.

[0010] S3. Passivate stainless steel workpieces with Fenton solution to obtain stainless steel workpieces resistant to high temperature discoloration.

[0011] The acid solution is a mixture of sulfuric acid solution, fluoride ion solution and 5-sulfosalicylic acid solution.

[0012] It should be noted that in order to improve the corrosion efficiency of the surface oxide film, a strong acid solution should be used for corrosion, with sulfuric acid solution being the most suitable choice: hydrochloric acid contains chloride ions and is too corrosive to stainless steel; nitric acid is an oxidizing acid and is not suitable for removing oxides from the surface of stainless steel.

[0013] In this invention, the fluoride ion provider for the acid solution includes at least one selected from sodium fluoride, potassium fluoride, ammonium fluoride, sodium hydrogen fluoride, potassium hydrogen fluoride, ammonium hydrogen fluoride, and hydrofluoric acid. In the embodiments of this invention, sodium fluoride is selected as the fluoride ion provider for the fluoride-containing solution.

[0014] High-temperature discoloration of stainless steel can essentially be understood as the result of the oxidation of iron elements on the stainless steel surface at high temperatures. The more iron elements on the stainless steel surface, the more obvious the high-temperature discoloration. Therefore, any method that reduces the iron elements on the stainless steel surface is effective in combating high-temperature discoloration. In the processing method of this invention, during the high-temperature baking of the stainless steel workpiece, the iron elements in the stainless steel alloy separate from the chromium and nickel elements due to oxidation.

[0015] This invention generates an iron-rich and chromium- and nickel-poor oxide film system on stainless steel workpieces after oxidation baking. Then, chemical etching with an acid solution effectively removes the surface oxides and further selectively corrodes the iron element, thereby forming a chromium- or chromium- and nickel-rich system on the surface of the stainless steel workpiece. Finally, passivation treatment with Fenton's reagent forms a continuous chromium- or chromium- and nickel-rich system on the surface of the stainless steel workpiece, greatly improving its high-temperature resistance and resistance to chloride ion corrosion.

[0016] Preferably, the baking temperature is 360℃~400℃, and more preferably, it is 380℃-390℃. Baking within this temperature range facilitates the formation of an iron-containing oxide film on the surface of the stainless steel workpiece, and the thickness of this oxide film is suitable. It should be noted that this oxide film is a system rich in iron and low in chromium and nickel, formed by the oxidation of iron to iron oxide. After acid treatment, it can be effectively removed, resulting in a chromium-rich or chromium-nickel-rich surface on the stainless steel workpiece. Further passivation treatment with Fenton's reagent can restore and improve the stainless steel workpiece's resistance to chloride ion corrosion.

[0017] Furthermore, the acid solution is a mixture of sulfuric acid solution, fluoride ion solution, and 5-sulfosalicylic acid solution.

[0018] The aforementioned acid solution contains hydrogen ions and fluoride ions. The hydrogen ions provided by sulfuric acid dissolve the oxide film on the stainless steel surface and further corrode the Fe element; fluoride ions destroy the passivation film of stainless steel, assisting the hydrogen ions in further corrosion; 5-sulfosalicylic acid, under acidic conditions, can chelate iron ions, making the corrosion more targeted at iron. This invention uses a specially formulated acid solution that reacts with iron oxide and further selectively corrodes the Fe element, achieving the effect of removing oxides from the oxide film, restoring the original color of the stainless steel workpiece, and ensuring that the stainless steel surface is essentially undamaged. Simultaneously, the acid solution further weakly corrodes the stainless steel, enabling the surface of the workpiece to achieve a chromium-rich or chromium-nickel-rich system, resulting in better high-temperature resistance to discoloration. The basic principle of using an acidic solution for chemical etching treatment of the surface is as follows:

[0019] Fe2O3+6H + →2Fe 3+ +3H2O (removes the oxide film);

[0020] Fe-Cr-Ni+2H + →Cr-Ni+Fe 2+ +H2 (austenite, e.g., 304);

[0021] Fe-Cr+2H + →Cr+Fe 2+ +H2 (ferrite, e.g., 430).

[0022] Furthermore, in the acidic solution, the concentration of sulfuric acid solution is 50-200 g / L, the concentration of fluoride ions is 5-10 g / L, and the concentration of 5-sulfosalicylic acid solution is 10-30 g / L.

[0023] Furthermore, the Fenton solution is composed of hydrogen peroxide with a concentration of 30-100 g / L and ferrous sulfate solution with a concentration of 10-50 g / L.

[0024] In the processing method of this invention, the workpiece after acid solution chemical treatment needs to be further passivated with Fenton's reagent. The Fenton's reagent of this invention is a mixed solution of hydrogen peroxide (H2O2) and ferrous ions, possessing strong oxidizing properties. It contains a large number of hydroxyl radicals and is a highly oxidizing substance, which can force the discontinuous passivated surface of stainless steel, which has been selectively separated by iron and chromium elements, into the anodic passivation zone, thereby restoring the passivation film on the stainless steel surface to its resistance to chloride ion corrosion.

[0025] Furthermore, the pH value of the Fenton solution is 2.0-3.0. This invention controls the pH value of the Fenton reagent by adding sulfuric acid. Within this pH range, the Fenton reagent can generate a large number of hydroxyl radicals. The basic principle of using Fenton reagent for surface passivation is as follows:

[0026] Fe 2+ +H₂O₂→Fe 3+ +(OH) - +OH·.

[0027] Furthermore, the ORP value of the Fenton solution is >1000mV.

[0028] Furthermore, the baking time in step S1 is 5 minutes or more, preferably 5-10 minutes.

[0029] Furthermore, the temperature of the chemical treatment in step S2 is 30-50℃, and the time is 2-5 minutes.

[0030] Furthermore, the passivation process in step S3 takes more than 30 seconds, preferably 30-180 seconds.

[0031] Furthermore, the stainless steel workpiece is made of 304 stainless steel, 340L stainless steel, 430 stainless steel or 316 stainless steel.

[0032] In the above-mentioned anti-discoloration treatment method for stainless steel workpieces, preferably, the stainless steel workpiece is made of ferritic stainless steel or austenitic stainless steel.

[0033] In the above-mentioned treatment method for stainless steel workpieces to resist discoloration, the stainless steel workpieces can be stainless steel sheets or various products made of stainless steel materials. There are no specific limitations on the specific form or product of the workpiece. As long as it is to improve the high-temperature discoloration resistance and corrosion resistance of stainless steel materials, this scope can be applied.

[0034] As a further preferred option, the stainless steel workpiece is selected from stainless steel cookware.

[0035] Compared with the prior art, the technical effects achieved by the present invention include:

[0036] The present invention provides a method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion. First, the stainless steel workpiece undergoes an oxidation baking treatment to form an oxide film on its surface. Then, an acidic solution is used to remove the oxide film, further separating the iron and chromium elements on the stainless steel surface. Finally, Fenton's reagent is used for passivation, resulting in a chromium-rich or chromium-nickel-rich system on the surface of the stainless steel workpiece. This improves both its high-temperature resistance and its resistance to chloride ion corrosion, effectively enhancing its resistance to high-temperature discoloration and chloride ion corrosion. This ensures that the stainless steel workpiece will not discolor under operating conditions of 350-400℃ and also exhibits resistance to salt corrosion. Detailed Implementation

[0037] The technical solutions in the embodiments will be clearly and completely described below. Obviously, the embodiments described below are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0038] It should be understood that, when used in this specification and the appended claims, the terms "comprising" and "including" indicate the presence of the described features, integrals, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integrals, steps, operations, elements, components and / or collections thereof.

[0039] It should also be understood that the terminology used in this specification of embodiments of the invention is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of the invention. As used in this specification of embodiments of the invention and the appended claims, the singular forms “a,” “an,” and “the” are intended to include the plural forms unless the context clearly indicates otherwise.

[0040] The stainless steel workpiece resistant to high-temperature discoloration and corrosion treatment provided by this invention can be formed by first machining or stamping stainless steel material to create the corresponding stainless steel workpiece. This can be done through methods such as deep drawing, spinning, or extrusion. The specific product of the stainless steel workpiece is not specifically limited; it can be stainless steel sheet, stainless steel oven, or stainless steel cookware such as stainless steel pots or spoons. The material used for the stainless steel workpiece can be various types of stainless steel, such as 304 stainless steel, 340L stainless steel, 430 stainless steel, 316 stainless steel, etc., without specific limitations.

[0041] Furthermore, the process also includes performing certain mechanical pretreatments on the stainless steel workpieces that have been machined and formed, such pretreatments including mechanical polishing, wire drawing, sanding, tumbling, grinding, etc. It may also include conventional treatments such as degreasing. The corresponding stainless steel workpieces in the following embodiments and comparative examples can all be processed and formed using the above-described pretreatment methods.

[0042] The present invention provides a method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion, the method comprising the following steps:

[0043] S1. The stainless steel workpiece is subjected to an oxidation baking treatment under oxygen conditions to form an oxide film on the surface. The baking treatment temperature is 350-400℃.

[0044] The shaped stainless steel workpiece is placed in an oven or drying tunnel and subjected to oxidation baking under aerobic conditions for a certain period of time. The resulting surface oxide film is an iron-rich and chromium- and nickel-poor oxide system. This oxide can be effectively removed during subsequent acid treatment, thereby forming a chromium- and nickel-rich or chromium-rich system on the surface of the stainless steel workpiece.

[0045] The oxidation baking temperature in this embodiment of the invention is between 350-400°C. The baking time is to reach the set temperature and maintain it for more than 5 minutes, usually 5-10 minutes.

[0046] S2. The stainless steel workpiece that has been baked is chemically treated with an acidic solution to remove the oxide film on the surface and then subjected to selective weak corrosion.

[0047] The stainless steel workpieces that have undergone the above baking treatment are then subjected to chemical etching in an acidic solution, which effectively removes oxides from the surface oxide film and further selectively etches away iron from the cavities.

[0048] The acid solution formulation used in the embodiments of the present invention is as follows:

[0049] Sulfuric acid (provides H+) + 50-200g / L;

[0050] Sodium fluoride (provides F) - 5-10g / L;

[0051] 5-Sulfosalicylic acid 10-30 g / L.

[0052] During acid treatment, appropriate heating can accelerate the reaction rate. In this embodiment of the invention, the temperature range for acid chemical treatment is 20-50°C, and the reaction time is 2-5 minutes. After this step, the stainless steel workpiece can restore its original color, and a chromium-nickel or chromium-rich system is formed on the surface, further improving its resistance to high-temperature discoloration.

[0053] It should be noted that the concentration of the acid solution is related to the treatment time and temperature. The higher the concentration, the stronger the corrosiveness of the solution, the higher the risk, the more stringent the operational requirements, and the easier it is to cause over-corrosion. Therefore, in actual production, the concentration of the acid solution should not be too high.

[0054] In other embodiments, potassium fluoride may also be used to provide fluoride ions.

[0055] S3. Passivate the stainless steel workpiece with Fenton solution to obtain a stainless steel workpiece resistant to high temperature discoloration.

[0056] In this embodiment, after acidification, the workpiece is passivated with Fenton's reagent solution. The high potential of the hydroxyl radicals in Fenton's reagent re-passivates the damaged austenite and ferrite structures in the workpiece, resulting in excellent resistance to chloride ion corrosion. The passivation treatment using the Fenton's reagent solution can be performed at room temperature, with a reaction time of at least 30 seconds, preferably 30-180 seconds.

[0057] The Fenton reagent formulation described in this embodiment is as follows:

[0058] Hydrogen peroxide 30-100g / L;

[0059] Ferrous sulfate 10-50 g / L.

[0060] pH value is 2.0-3.0, ORP value is >1000mV.

[0061] Finally, the passivated stainless steel workpieces undergo further cleaning and drying to obtain products ready for packaging and shipment.

[0062] To further describe the technical solution of the present invention, the following specific implementation description uses stainless steel cookware as the selected stainless steel workpiece. The main structure of the cookware selected here is made of 304 stainless steel, the composite bottom is made of 430 stainless steel, the inner surface is sanded, the outer surface is mirror polished, and the composite bottom is sanded.

[0063] Example 1

[0064] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 350℃.

[0065] The baked stainless steel workpiece is immersed in an acid solution at 40°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion.

[0066] After the workpiece has been treated with acid solution, soak it in Fenton's reagent solution for 1 minute, then rinse it with deionized water and dry it.

[0067] The acid solution used in this embodiment is a mixed solution of 100 g / L sulfuric acid, 8 g / L sodium fluoride, and 20 g / L 5-sulfosalicylic acid.

[0068] The Fenton reagent used in this embodiment is a mixed solution of 50 g / L hydrogen peroxide and 30 g / L ferrous sulfate, with a pH of 2.0 and an ORP value > 1000 mV.

[0069] Example 2

[0070] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 380℃.

[0071] The baked stainless steel workpiece is immersed in an acid solution at 40°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion.

[0072] After the workpiece has been treated with acid solution, soak it in Fenton's reagent solution for 1 minute, then rinse it with deionized water and dry it.

[0073] The acid solution used in this embodiment is a mixed solution of 100 g / L sulfuric acid, 8 g / L sodium fluoride, and 20 g / L 5-sulfosalicylic acid.

[0074] The Fenton reagent used in this embodiment is a mixed solution of 50 g / L hydrogen peroxide and 30 g / L ferrous sulfate, with a pH of 2.0 and an ORP value > 1000 mV.

[0075] Example 3

[0076] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 400℃.

[0077] The baked stainless steel workpiece is immersed in an acid solution at 40°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion.

[0078] After the workpiece has been treated with acid solution, soak it in Fenton's reagent solution for 1 minute, then rinse it with deionized water and dry it.

[0079] The acid solution used in this embodiment is a mixed solution of 100 g / L sulfuric acid, 8 g / L sodium fluoride, and 20 g / L 5-sulfosalicylic acid.

[0080] The Fenton reagent used in this embodiment is a mixed solution of 50 g / L hydrogen peroxide and 30 g / L ferrous sulfate, with a pH of 2.0 and an ORP value > 1000 mV.

[0081] Example 4

[0082] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 380℃.

[0083] The baked stainless steel workpiece is immersed in an acid solution at 30°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion.

[0084] After the workpiece has been treated with acid solution, soak it in Fenton's reagent solution for 1 minute, then rinse it with deionized water and dry it.

[0085] The acid solution used in this embodiment is a mixed solution of 100 g / L sulfuric acid, 8 g / L sodium fluoride, and 20 g / L 5-sulfosalicylic acid.

[0086] The Fenton reagent used in this embodiment is a mixed solution of 50 g / L hydrogen peroxide and 30 g / L ferrous sulfate, with a pH of 2.0 and an ORP value > 1000 mV.

[0087] Example 5

[0088] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 380℃.

[0089] The baked stainless steel workpiece is immersed in an acid solution at 50°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion.

[0090] After the workpiece has been treated with acid solution, soak it in Fenton's reagent solution for 1 minute, then rinse it with deionized water and dry it.

[0091] The acid solution used in this embodiment is a mixed solution of 100 g / L sulfuric acid, 8 g / L potassium fluoride, and 20 g / L 5-sulfosalicylic acid.

[0092] The Fenton reagent used in this embodiment is a mixed solution of 50 g / L hydrogen peroxide and 30 g / L ferrous sulfate, with a pH of 2.0 and an ORP value > 1000 mV.

[0093] Example 6

[0094] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 380℃.

[0095] The baked stainless steel workpiece is immersed in an acid solution at 60°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion.

[0096] After the workpiece has been treated with acid solution, soak it in Fenton's reagent solution for 1 minute, then rinse it with deionized water and dry it.

[0097] The acid solution used in this embodiment is a mixed solution of 100 g / L sulfuric acid, 8 g / L sodium fluoride, and 20 g / L 5-sulfosalicylic acid.

[0098] The Fenton reagent used in this embodiment is a mixed solution of 50 g / L hydrogen peroxide and 30 g / L ferrous sulfate, with a pH of 2.0 and an ORP value > 1000 mV.

[0099] The workpiece obtained in this embodiment is white, indicating a quality problem due to over-corrosion.

[0100] Comparative Example 1

[0101] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 380℃.

[0102] The stainless steel workpiece that has undergone baking is immersed in an acid solution at 40°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion. Then it is rinsed with deionized water and dried.

[0103] The acid solution used in this embodiment is a mixed solution of 100 g / L sulfuric acid, 8 g / L sodium fluoride, and 20 g / L 5-sulfosalicylic acid.

[0104] Comparative Example 2

[0105] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 380℃.

[0106] The baked stainless steel workpiece is immersed in an acid solution at 40°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion.

[0107] After the workpiece has been treated with acid solution, soak it in Fenton's reagent solution for 1 minute, then rinse it with deionized water and dry it.

[0108] The acid solution used in this comparative example was a 10 wt% sulfuric acid solution.

[0109] The Fenton reagent used in this comparative example was a mixed solution of 50 g / L hydrogen peroxide and 30 g / L ferrous sulfate, with a pH of 2.0 and an ORP value > 1000 mV.

[0110] Comparative Example 3

[0111] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 380℃.

[0112] The stainless steel workpiece that has undergone baking is immersed in an acid solution at 40°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion. Then it is rinsed with deionized water and dried.

[0113] The acid solution used in this comparative example was a 10 wt% sulfuric acid solution.

[0114] Comparative Example 4

[0115] Stainless steel cookware is subjected to an oxidative baking treatment under aerobic conditions to form an oxide film on the surface. The baking temperature is 450℃.

[0116] The stainless steel workpiece that has undergone baking is immersed in an acid solution at 40°C for 3 minutes to chemically remove the oxide film on the surface and perform selective weak corrosion. Then it is rinsed with deionized water and dried.

[0117] The acid solution used in this comparative example was a 10 wt% sulfuric acid solution.

[0118] Comparative Example 5

[0119] Stainless steel cookware was placed in an electrolytic polishing solution composed of 60% phosphoric acid, 10% sulfuric acid, and 10% chromic anhydride by mass, at a voltage of 15V and an A / dm³. 2 Electrolyze at a current density for 5 minutes, then rinse with deionized water and dry. The stainless steel cookware in this comparative example was not baked.

[0120] Comparative Example 6

[0121] Stainless steel cookware undergoes no baking or chemical treatment after mechanical pretreatment.

[0122] The corresponding stainless steel cookware products obtained from the above embodiments and comparative examples were randomly selected for the highest discoloration temperature test and the neutral salt spray test. The results are shown in Table 1 below.

[0123] The specific method for testing the high-temperature discoloration resistance temperature is as follows: Adjust the oven temperature to the temperature to be tested, record the temperature value, then place the workpiece in the oven and bake for 1 hour. Take out the average b-value recorded in the Lab value of the colorimeter and compare it with the average b-value recorded before placing it in the oven. The difference in b-values ​​before and after is Δb; Δb < 0.2 indicates no change. The highest temperature value with no change is the high-temperature discoloration resistance temperature. In the colorimeter, L represents the brightness difference, a represents the red-green difference, and b represents the yellow-blue difference. b- indicates blue discoloration, and b+ indicates yellow discoloration. For stainless steel, after exceeding the discoloration point, the higher the temperature, the larger the Δb value. After reaching the discoloration point, for every 10℃ increase in temperature, the b-value increases by 0.4-0.5.

[0124] The neutral salt spray test was conducted for 24 hours according to the national standard GB / T10125-2021, "Artificial Atmosphere Corrosion Test: Salt Spray Test". (Note: After 24 hours, most stainless steel cookware would have failed the test.)

[0125] Table 1 Performance test results of the examples and comparative examples

[0126]

[0127] The results from Examples 1-3 show that under the same chemical treatment (acid treatment and Fenton's reagent treatment), the color change temperature and baking temperature are basically the same, and the higher the baking temperature, the higher the color change temperature. However, in Comparative Examples 2-4, after different chemical treatments using conventional acid solutions, the color change temperature was 60-80°C lower than the baking temperature.

[0128] In Examples 2, 4, and 5, the baking temperature is the same. The main difference is the acid treatment temperature. Under the same acid solution and the same treatment time, the acid treatment temperature is higher, and the final discoloration temperature point will also be higher. Therefore, the acid treatment temperature can be appropriately increased (40-50℃ is appropriate). However, excessively high acid treatment temperature (such as 60℃) will cause over-corrosion of the workpiece, resulting in whitening of the workpiece and quality problems.

[0129] Comparative Examples 2 and 2-4 show that acid treatment using an acid solution containing specific components of the present invention can increase the discoloration temperature by 50-80°C. In other words, the present invention can achieve a higher discoloration temperature at a lower baking temperature, resulting in greater energy savings. Comparative Examples 1-3 and 1-4 reveal that treatment with Fenton's reagent solution significantly repairs the reduced resistance to chloride ion corrosion caused by metallographic damage after baking and acidic solution immersion, and also shows improvement compared to ordinary stainless steel workpieces.

[0130] In the above embodiments, the descriptions of each embodiment have different focuses. For parts that are not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0131] The above description describes specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in the present invention, and these modifications or substitutions should all be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion, characterized in that, The processing method includes the following steps: S1. The stainless steel workpiece is subjected to an oxidation baking treatment under oxygen conditions to form an oxide film on the surface. The baking treatment temperature is 350-400℃. S2. The stainless steel workpiece that has been baked is chemically treated with an acid solution to remove the oxide film on the surface and then subjected to selective weak corrosion. S3. Passivate stainless steel workpieces with Fenton solution to obtain stainless steel workpieces resistant to high temperature discoloration. The acid solution is a mixture of sulfuric acid solution, fluoride ion solution, and 5-sulfosalicylic acid solution. The Fenton solution is composed of hydrogen peroxide with a concentration of 30-100 g / L and ferrous sulfate solution with a concentration of 10-50 g / L; the pH value of the Fenton solution is 2.0-3.

0.

2. The method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion as described in claim 1, characterized in that, In the acid solution, the concentration of sulfuric acid solution is 50-200 g / L, the concentration of fluoride ions is 5-10 g / L, and the concentration of 5-sulfosalicylic acid solution is 10-30 g / L.

3. The method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion as described in claim 1, characterized in that, The ORP value of the Fenton solution is >1000mV.

4. The method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion as described in claim 1, characterized in that, The baking time for step S1 is 5-10 minutes.

5. The method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion as described in claim 1, characterized in that, The chemical treatment in step S2 is carried out at a temperature of 30-50°C for 2-5 minutes.

6. The method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion as described in claim 1, characterized in that, The passivation process in step S3 takes 30-180 seconds.

7. The method for treating stainless steel workpieces to resist high-temperature discoloration and corrosion as described in claim 1, characterized in that, The stainless steel workpiece is made of 304 stainless steel, 340L stainless steel, 430 stainless steel or 316 stainless steel.

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