How to improve the corrosion resistance of stainless steel thermos?
1. Material selection
1.1 Use high-quality stainless steel
The corrosion resistance of stainless steel thermos is closely related to the stainless steel material used. The common stainless steel materials on the market are 201, 304 and 316. Among them, 304 stainless steel is an internationally recognized food-grade stainless steel. It contains 18% chromium and 8% nickel. It has excellent corrosion resistance and good processing performance. It is a common choice for high-quality thermos liner. 316 stainless steel adds molybdenum to 304, which further improves its corrosion resistance and high-temperature strength. It can be used under more harsh conditions. It is a medical-grade stainless steel with better corrosion resistance than 304, but the cost is relatively high. In contrast, 201 stainless steel has weak acid and alkali resistance and is mostly used in the production of industrial steel pipes, decoration materials, etc. If it is used to make thermos, especially the liner, it is easy to rust after heating and use, which is harmful to health. Therefore, in order to improve the corrosion resistance of stainless steel thermos, 304 or 316 stainless steel should be used as the inner material.
1.2 Adding corrosion-resistant elements
In addition to choosing high-quality stainless steel materials, the corrosion resistance of stainless steel thermos can also be improved by adding specific corrosion-resistant elements. For example, adding molybdenum to stainless steel can significantly improve its corrosion resistance to chloride ions and effectively prevent pitting and crevice corrosion. Molybdenum can work synergistically with chromium to form a denser and more stable passivation film on the surface of stainless steel, enhancing its stability in a chlorine-containing environment. In addition, adding copper can also improve the corrosion resistance of stainless steel. Copper can promote the formation of a more uniform passivation film on the surface of stainless steel and improve its corrosion resistance in an acidic environment. By adding these corrosion-resistant elements, the corrosion resistance of the thermos can be effectively improved and the service life can be extended without changing the overall structure and function of the thermos.
2. Surface treatment process
2.1 Polishing treatment
Polishing treatment is one of the important links to improve the corrosion resistance of stainless steel thermos. Polishing can significantly improve the microstructure of the stainless steel surface, thereby improving its corrosion resistance.
Microstructure optimization: Polishing can remove tiny cracks, scratches and unevenness on the stainless steel surface, making the surface smoother and flatter. These tiny defects are often the starting point of corrosion. Polishing can reduce the source of corrosion, thereby reducing the probability of corrosion. Studies have shown that the surface roughness of polished stainless steel can be reduced to less than 0.1 microns, and the surface defect density is reduced by more than 80%.
Improved integrity of the passivation film: The polishing process helps to form a more complete and denser passivation film. The passivation film is a key factor in the corrosion resistance of stainless steel, and its integrity directly affects the corrosion resistance. Polishing can make the passivation film more evenly cover the stainless steel surface, reduce the defects and weak points of the film layer, and thus significantly improve its corrosion resistance. Experiments show that the corrosion resistance time of polished stainless steel in the salt spray test can be extended by more than 2 times.
Actual application effect: In actual use, the polished stainless steel thermos cup is not only more beautiful in appearance, but also significantly improved in corrosion resistance. For example, in daily use, when a polished thermos cup comes into contact with acidic beverages (such as orange juice, lemonade) or alkaline substances (such as soda water), the surface corrosion rate is significantly reduced, and the service life can be extended by more than 30%.
2.2 Coating treatment
Coating treatment is a method of enhancing the corrosion resistance of stainless steel by coating a protective film on the surface. A reasonable coating process can effectively improve the corrosion resistance and aesthetics of stainless steel thermos cups.
Primer selection: Primer is a key link in the coating process, and its role is to provide a good adhesion foundation for the topcoat. It is crucial to choose a primer designed specifically for stainless steel. For example, a primer containing epoxy resin has excellent mechanical properties and adhesion, and can form a strong bond with the stainless steel surface. Studies have shown that the coating system using epoxy resin primer can achieve an adhesion of more than 30N/cm², which is significantly better than ordinary primers.
Topcoat selection: The topcoat is the outermost layer of the coating layer, which is directly in contact with the external environment, and its performance directly affects the corrosion resistance and aesthetics of the thermos cup. It is recommended to use paints with high wear resistance and corrosion resistance, such as polyurethane or fluorocarbon paint. These paints can form a protective layer with high hardness after curing, which effectively reduces the scratching and peeling of the paint surface. Experiments show that the wear resistance of polyurethane topcoat is more than 5 times that of ordinary paints, and the corrosion resistance time of fluorocarbon paint can reach more than 1000 hours.
Optimization of coating process: Optimization of coating process is crucial to improving the quality of paint layer. During the painting process, the temperature and humidity of the environment have a significant impact on the quality of the paint layer. The adhesion and curing effect of the paint layer are best when the temperature is 18-25℃ and the humidity is 40%-60%. In addition, the quality of painting technology is directly related to the uniformity and smoothness of the paint surface. Using a high-quality spray gun and maintaining a certain spraying distance and angle can greatly reduce the occurrence of sagging and unevenness on the paint surface. Proper flash-drying time between layers during spraying is also crucial. Each layer of paint should have enough time to dry to improve the integrity of the paint layer.
Actual application effect: After optimized coating treatment, the corrosion resistance of stainless steel thermos is significantly improved. For example, in daily use, when the thermos is exposed to acidic or alkaline substances, the paint layer is not easy to fall off, and the corrosion resistance time can be extended by more than 50%. At the same time, coating treatment can also improve the aesthetics of the thermos, making its color brighter, the surface smoother, and having a better visual effect.
3. Manufacturing process improvement
3.1 Welding process optimization
Welding process is a key link in the manufacturing process of stainless steel thermos cups, and its quality directly affects the sealing and corrosion resistance of the thermos cups.
Choice of welding process: Although the traditional argon arc welding has a low cost, the heat affected zone is large, the weld is obvious and the bulge is high, and subsequent polishing treatment is required, which not only increases the production cost, but also may introduce new corrosion risks during the processing. In contrast, laser welding has the advantages of small heat affected zone, small welding deformation, and smooth weld, which can effectively reduce the subsequent processing steps and improve production efficiency and product quality. For example, the corrosion resistance time of the thermos cup welded by laser welding in the salt spray test is more than 30% longer than that of the thermos cup welded by argon arc welding.
Optimization of welding parameters: In the laser welding process, parameters such as welding power, welding speed, and defocus have an important influence on the welding quality. By precisely controlling these parameters, it is possible to ensure that the penetration depth and width of the weld are uniform and consistent, and reduce welding defects. Studies have shown that when the laser power is 1000W, the welding speed is 5m/min, and the defocus is 0.5mm, the welding quality is optimal and the corrosion resistance of the weld is significantly improved.
Welding protection measures: During the welding process, the use of appropriate shielding gas can effectively prevent the oxidation of the weld and improve the corrosion resistance of the weld. Using pure argon or a mixture of argon and helium as a shielding gas can form a good protective atmosphere and reduce the thickness of the oxide layer on the weld surface. Experiments show that the corrosion resistance time of welds protected by argon in an acidic environment is 50% longer than that of unprotected welds.
3.2 Heat treatment process
The heat treatment process can significantly improve the microstructure and properties of stainless steel, thereby improving the corrosion resistance of stainless steel thermos cups.
Solution treatment: Solution treatment is to heat stainless steel to a high temperature and then cool it rapidly, so that the alloy elements are evenly distributed in the matrix to form a uniform solid solution. This process can improve the corrosion resistance and toughness of stainless steel. For example, after solution treatment at 1050℃, the corrosion resistance of 304 stainless steel increased by 20% and the yield strength increased by 15%. The surface corrosion rate of stainless steel thermos cups after solution treatment is significantly reduced in daily use, especially when in contact with acidic or alkaline substances.
Stabilization treatment: For stainless steel containing stabilizing elements such as titanium or niobium, stabilization treatment can prevent intergranular corrosion during welding or high-temperature processing. By stabilizing treatment in the temperature range of 850℃-900℃, titanium or niobium can be combined with carbon to form stable carbides, thereby reducing the precipitation of chromium carbides and improving corrosion resistance. For example, the corrosion resistance time of 316 stainless steel thermos cups that have been stabilized is 40% longer in high temperature environments than that of untreated thermos cups.
Aging treatment: For some stainless steel thermos cups that need to improve hardness and strength, aging treatment can be used. By heating at a lower temperature for a long time, the alloy elements precipitate fine precipitation phases in the matrix, thereby improving the strength and hardness of the material. For example, the hardness of the 304 stainless steel thermos cup treated at 450℃ has increased by 30%, and its wear resistance has been significantly enhanced. At the same time, the corrosion resistance of the thermos cup after aging treatment has also been improved to a certain extent, especially in the composite environment of contact wear and corrosion.
4. Quality control and testing
4.1 Material testing
Material testing is the basic link to ensure the corrosion resistance of stainless steel thermos cups. By strictly testing the quality of raw materials, the corrosion resistance caused by material problems can be effectively avoided.
Composition testing: Use a spectrometer to accurately test the chemical composition of stainless steel raw materials to ensure that the content of key elements such as chromium, nickel, and molybdenum meets the standards. For example, the chromium content of 304 stainless steel should reach about 18%, and the nickel content should be about 8%. Materials that fail the composition test are strictly prohibited from being used in production.
Surface quality testing: Use ultrasonic flaw detectors and surface roughness meters to test the surface of stainless steel raw materials. Ultrasonic flaw detectors can detect tiny cracks and defects inside raw materials, and surface roughness meters can measure the flatness of the surface. The surface roughness should be controlled below 0.5 microns to reduce the source of corrosion.
Corrosion resistance test: The corrosion resistance of the raw materials is evaluated through salt spray test. The stainless steel sample is placed in the salt spray test chamber, and after 96 hours of testing, its surface corrosion is observed. The corrosion rate of 304 stainless steel should be less than 0.1mm/year, and the corrosion rate of 316 stainless steel should be less than 0.05mm/year. Only materials that meet the standards can be used in the production of thermos cups.
4.2 Finished product inspection
Finished product inspection is the last line of defense to ensure that the corrosion resistance of stainless steel thermos cups meets the design requirements. Through a series of strict testing methods, the corrosion resistance of the thermos cup can be fully evaluated.
Appearance inspection: Check whether there are scratches, dents, paint peeling and other defects on the surface of the thermos cup. Use high-precision visual inspection equipment to scan the surface of the thermos cup 360 degrees to ensure that the number of surface defects does not exceed 3 and the area of each defect is less than 1 square millimeter.
Sealing test: The sealing of the thermos cup is tested by air pressure detection. Inject a certain pressure of gas into the thermos cup, then immerse it in water to observe whether there are bubbles escaping. Under a pressure of 0.1MPa, the thermos cup should remain sealed without bubbles escaping. The thermos cup that fails the sealing test will be scrapped directly.
Corrosion resistance test: The finished thermos cup is subjected to salt spray test and chemical immersion test. In the salt spray test, the thermos cup should be tested for 240 hours, and the surface corrosion area should not exceed 10%. In the chemical immersion test, the thermos cup is immersed in 3% hydrochloric acid solution and 5% sodium hydroxide solution respectively. After immersion for 48 hours, its surface corrosion is observed. The corrosion rate of 304 stainless steel thermos cup in hydrochloric acid solution should be less than 0.2mm/year, and the corrosion rate in sodium hydroxide solution should be less than 0.15mm/year; the corrosion rate of 316 stainless steel thermos cup in hydrochloric acid solution should be less than 0.1mm/year, and the corrosion rate in sodium hydroxide solution should be less than 0.1mm/year. Only thermos cups that pass these tests can enter the market for sale.
5. Summary
Through the multi-faceted research on the improvement of the corrosion resistance of stainless steel thermos, the following key points can be summarized:
The importance of material selection: 304 or 316 stainless steel is preferred as the material of the thermos liner because of its excellent corrosion resistance. At the same time, by adding corrosion-resistant elements such as molybdenum and copper, the corrosion resistance of stainless steel can be further improved.
Optimization of surface treatment process:
Polishing: It can significantly improve the microstructure of the stainless steel surface, reduce surface roughness, reduce corrosion sources, and improve the integrity of the passivation film, significantly improving corrosion resistance.
Coating treatment: Selecting appropriate primer and topcoat, and optimizing the coating process can effectively enhance the corrosion resistance and aesthetics of the thermos.
The role of manufacturing process improvement:
Welding process optimization: Using advanced welding technologies such as laser welding, and optimizing welding parameters and protection measures, can reduce welding defects and improve the corrosion resistance of welds.
Heat treatment process: Through solid solution treatment, stabilization treatment and aging treatment, the microstructure and performance of stainless steel are improved, thereby improving the corrosion resistance and strength of the thermos.
Guarantee of quality control and testing:
Material testing: Strictly test the composition, surface quality and corrosion resistance of raw materials to ensure that the raw materials meet the standards and avoid the decline of corrosion resistance due to material problems.
Finished product testing: Carry out comprehensive appearance, sealing and corrosion resistance testing on the finished thermos cup. Only products that pass strict testing can enter the market for sale, thus ensuring that consumers can use thermos cups with excellent corrosion resistance.
In summary, improving the corrosion resistance of stainless steel thermos cups requires starting from material selection, surface treatment process, manufacturing process improvement, quality control and testing, etc. By comprehensively using these methods, the corrosion resistance of stainless steel thermos cups can be significantly improved, their service life can be extended, and consumers' demand for high-quality thermos cups can be met.