With the continuous development of the national economic construction, the seamless stainless steel pipes in the use of hygiene (food grade) in various industrial industries have become wider and wider. By analyzing the corrosion resistance capacity of the stainless surface passivation layer, and combined with the process and equipment application of the company's production of small sanitary -level seamless stainless steel pipes, high -quality steel pipes are produced to meet the needs of industrial fluid pipes, fill in domestic gaps, replace replacement import.
Hygiene (food grade) seamless stainless steel pipes are widely used in many industries such as pharmaceutical, food, beer, drinking water, biological engineering, chemical engineering, air purification, aviation, and nuclear industry. import. Here is the process and quality and quality of the production of sanitary (food grade) seamless stainless steel pipes. This management belongs to the most advanced and precise level in China and is widely used and exported abroad.
First. Surface analysis of stainless steel
Russia's Electric Energy spectrum (AES) method and X -ray optical energy spectrum (SPS) method can be used for stainless steel surface analysis, thereby determining the surface and corrosion resistance of the inside and outside of the stainless steel pipe. The analysis diameter of the AES method is very small and can be less than 20nm. Its original function is to identify as elements. The analysis of the XPS method is about 10 μm, which is mainly used to determine the chemical state of near the surface element.
The results of the scanning of mechanical polishing with AES and XPS detectors exposed to the atmosphere of the atmosphere showed that the most typical depth of the total depth of the surface analysis of stainless steel diamonds is 15nm, and it provides components, thickness and it about the passivation layer, and it Corrosion resistance, etc.
According to the definition, Austeen stainless steel contains high chromium and nickel, and some molybdenum (such as 316L00CR17NI14MO2), titanium, etc. generally contain 10.5%chromium and have better corrosion resistance. Corrosion resistance is because the chromium-rich passivation layer has the result of protective performance. The passivation layer is usually 3-5nm thick, or it is equivalent to 15 layers of atomic thickness. The passivation layer is formed during the process of chromium and iron oxidized-restore the reaction. If the passivation layer is damaged, a new passivation layer and/with electrochemical corrosion will occur quickly. Eclipse and crystal corrosion. Corrosion -resistant capabilities of the passivation layer are related to the chemical composition content contained in stainless steel. For example, high chromium, nickel and molybdenum, etc. can improve the passivation layer combined with energy potential, and strengthen the corrosion resistance of the passivation layer; Use fluid media.
Second, the surface corrosion of stainless steel pipes
1. The passivation layer of the stainless steel surface in the CI-medium is easily destroyed, because the CI-oxidation potential energy is large. If the passivation layer printing layer will continue to be corroded on the metal. In many cases, the abducting layer is only destroyed on the local area of the metal surface. The role of corrosion is to form a small hole or concave pits. Small pit -shaped corrosion that produces irregularly distributed on the surface of the material is called point erosion. The erosion rate increases with increased temperature, and increases with the concentration. The solution is to use ultra -low or low -carbon stainless steel (such as 316L 304L)
2. Austenitic Stainless King Kong is easily damaged when the stainless steel surface layer of stainless steel surface is made during manufacturing and welding. When the heating temperature and heating speed during welding and welding, when the stainless steel-sensitive temperature area (about 425-815 ° C), the oversaturated carbon in the material will be preempt first on the grain border, and combined with chromium to form chromium carbonic CR23C6. At this time, carbon carbon is carbon. The diffusion speed in Ao's body is larger than the diffusion speed of chromium. Chromium is not as good as chromium that the crystal world loses chromium carbide. As a result, the chromium content of the crystal boundary is continuously reduced with the continuous precipitation of chromium carbon. Chromium regions weaken the electric pad, and the corrosion resistance capacity of the passivation layer decreases. When the corrosive medium is contacted with CI-such as CI-, micro-battery corrosion is caused. Although the corrosion is only on the surface of the grain, it quickly deepen the interior corrosion. Especially stainless steel pipes are more obvious in the welding area.
3. Stress corrosion cracks: It is the common effect of static stress and corrosion that causes cracks and metal crisps. The environment of stress corrosion cracks is usually quite complicated. Not only is stress stress, but this stress is combined with remnant stress produced in metals due to production, welding, or heat treatment.
Third, the surface treatment and corrosion resistance of stainless steel tubes
The inside and outside surface of the stainless steel pipe (especially like electrolyte polishing and mechanical polishing) has a good passivation layer, and the corrosion resistance is strong. The surface of the inside and outside is high, and the media adhesion is rarely conducive to corrosion resistance. The high surface of the tube is high, and the less liquid medium is stranded, which is conducive to rinse, especially in the pharmaceutical industry.
1. Electrolytic polishing in the tube (electrochemical polishing): Electrolytic polishing fluid is phosphate, sulfate, chromicide, gum, potassium chromate, etc. The surface of the stainless steel pipe is on the anode, and the polishing fluid flows inwardly with a low voltage and large current to perform electrolytic polishing treatment. At this time, two contradictory processes are performed at the same time, that is, the production and dissolution of the metal surface passivation layer (thickness of the mucous membrane). Due to the different conditions of the surface of the micro -micro -surface and the concave part of the film, the conditions for entering passivation are different, and the anode is soluble. Due to the different conditions of the micro -raised part and the concave part of the film, the condition of the film into the film is different, and due to the dissolved of the anode, the concentration of metal salt in the anode area continues to increase, forming a high -resistant thick mucosa on the surface. The thickness of the membrane at the unevenness causes the anode surface current density, and the cutting-end discharge is fast. In a short period of time, the purpose of flattening the protruding micro-part can achieve a high smoothness RA ≤ 0.2-0.4 μm. Under this action, the inside of the tube increases chromium content and increases the corrosion resistance of the metal surface passivation layer.
How to master the polishing should be related to the formula of electrolyte, concentration, temperature, power -powered time, current density, electrode condition, and tube surface treatment. Instead of technical mastering, it will destroy the smoothness of the tube surface. Excessive electrolysis will occur more and more bump surfaces. Even the tube is scrapped. The real quality requires a certain amount of technology, and the cost cost is higher.
2. Mechanical polishing in the tube: there are rotation and straight line polishing. Here is a rotating mechanical polishing as an example: mechanical polishing equipment is relatively simple, power and polishing discs, high -end polishing equipment are simpler, power and polishing disk, and high -level polishing wax. The cloth plate and cloth plate made of level-grade fine sand granules are polished multiple times on the surface and outside of the tube on the surface and outside of the tube. The smoothness can reach RA ≤ 0.2-0.4 μm.
Compared with electrolytic polishing, mechanical polishing has simple equipment, low technical content, and easy to master. The cost cost is also low. It will not damage the tube and cause scrap, so it is widely used. However, the surface printing layer's corrosion resistance is much better.
Fourth, sanitary seamless stainless steel pipe production process
Steelmaking-Rolling Round Steel-Passing-Cold Draw-Cold Rolling-Light Enemy-the inner surface polishing-outer surface polishing-testing and acceptance-packaging into the warehouse.
Fifth, the purchase of hygiene seamless stainless steel pipes
Stainless steel contains less than 50%iron and 10.5%chromium, and also adds nickel, titanium, molybdenum, etc. Depending on the different school composition, the gold phase tissue inside the metal is also different. Body shape, Austrian body, double phase, precipitated hardening stainless steel, etc. Hygienic seamless stainless steel pipes also have different materials.
Generally, it is commonly used in industries to use Olympic stainless steel; 0CR19NI9 (USU304), 00CR19NI11 (USU 304L), 0CR17NI12MO2 (USU 316), 00CR17NI14MO2 (USU 316L), and 1CR18NI9TI (SUS 32111 ) Wait. It is necessary to meet the standards of GB/T14976-94 "Seamless Steel Steel Pipe for fluid transportation", as well as the requirements for the surface smoothness of the inside and outside of the steel pipe.