Stainless steel corrosion
The metal surface exposed in corrosive environment undergoes electrochemical reaction or chemical reaction, and is uniformly corroded. The parts with weak corrosion resistance in the passive film of stainless steel surface form pitting reaction due to self-excited reaction, forming small holes, coupled with the proximity of chloride ions, forming a strong corrosive solution, accelerating the speed of corrosion reaction. There is also intergranular corrosion cracking inside the stainless steel, all of which damage the passivation film on the stainless steel watch surface. Therefore, the stainless steel surface must be cleaned and maintained regularly to maintain its gorgeous surface and prolong its service life. When cleaning the stainless steel surface, we must pay attention to the phenomenon of no surface scratching, avoid using bleaching ingredients and abradant washing liquid, steel wire ball, abrasive tool, etc. In order to remove washing liquid, wash the surface with clean water after washing. It can be used in telescopic mast.
Stainless steel is a thin, firm and fine stable chromium-rich oxide film (protective film) formed on its surface. Prevent oxygen atoms from continuing to penetrate and continue to oxidize, thus obtaining corrosion resistance. Once there is some reason, this kind of film is constantly damaged, oxygen atoms in air or liquid will constantly separate out, forming loose iron oxide, and the metal surface will be constantly rusted.
Due to cost considerations, many domestic manufacturers have reduced chromium and nickel in stainless steel and increased the content of manganese. Experts believe that the reason why stainless steel can be Stainless is that chromium and nickel exist. Reducing the content of these two components will reduce the rust resistance.
Matters needing attention in stainless steel machining process:
Processing Area: SST part of the processing area should be relatively fixed. The platform of the processing zone should take protective measures, such as laying rubber pad. SST part the surface protection layer of SST part should be avoided during processing.
Chloride ion exists in the use environment
Chloride ions exist widely, such as salt/sweat/sea water/Sea Breeze/soil, etc. Stainless steel corrodes rapidly in the environment where chloride ions exist, even exceeding ordinary low carbon steel. Therefore, there are requirements for the use environment of stainless steel, and it needs to be wiped frequently to remove dust and keep it clean and dry. (In this way, he can be given a "improper use".) There is an example in the United States: an enterprise uses an oak container to hold a solution containing chlorine ions. The container has been used for nearly 100 years and is planned to be replaced in the 1990 s. Because the oak material is not modern enough, 16 days after the replacement of stainless steel, the container leaks due to corrosion.
No solid solution treatment
Alloy elements do not dissolve into the matrix, resulting in low alloy content and poor corrosion resistance of the matrix structure.
Natural intergranular corrosion
This material which does not contain titanium and niobium has the tendency of intergranular corrosion. Adding titanium and niobium, and then adding it with stable treatment can reduce intergranular corrosion. A kind of high alloy steel that can resist corrosion in air or chemical corrosive medium. Stainless steel has beautiful surface and good corrosion resistance, and does not need surface treatment such as plating, however, giving full play to the inherent surface properties of stainless steel, a kind of steel used in many aspects is usually called stainless steel. Representative properties are 13 chromium steel, 18-8 chromium nickel steel and other high alloy steel. From the perspective of metallographic analysis, because stainless steel contains chromium, a very thin chromium film is formed on the surface. This film isolates the oxygen invading in the steel from corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, the Steel must contain more than 12% chromium. When welding is needed, the lower carbon content minimizes the carbide precipitation in the heat affected zone near the weld, while the precipitation of carbide may lead to intergranular corrosion of stainless steel in some environments.
Production is often carried out in dust sites, and there are often many dust in the air, which constantly fall on the surface of the equipment. They can be removed with water or alkaline solution. However, the dirt with adhesion needs to be cleaned by high pressure water or steam.
Floating iron powder or embedded iron
On any surface, free iron will rust and cause corrosion of stainless steel. Therefore, it must be cleared. Floating powder can generally be removed with dust. Some have strong adhesion and must be processed according to the embedded iron. In addition to dust, there are many sources of surface iron, including ordinary carbon steel wire brushing and sand, iron castings or other abrasive used on ordinary carbon steel, low alloy steel or micro glass bead before for shot Blasing treatment, or grinding non-stainless steel products mentioned above near stainless steel parts and equipment. If no protective measures are taken for stainless steel during blanking or hoisting, the steel wire rope, spreader and iron on the working table can easily be embedded or stained. Ordering requirements and post-Fabrication Inspection can prevent and detect the existence of free iron. ASTM Standard A380 stipulates the rust test method for inspecting iron or steel particles on the surface of stainless steel. This test method should be used when it is required that iron absolutely cannot exist. If the result is satisfactory, wash the surface with clean pure water or nitric acid until the dark blue completely disappears. As Standard A380 points out, if the rust test solution cannot be completely removed, it is not recommended to use this test method on the process surface of the equipment, that is, the direct contact surface used to produce human consumer goods. A relatively simple test method is to expose it in water for 12~24 hours to check whether there are rust spots. This test has poor sensitivity and time consuming. These are all test tests, not cleaning methods. If iron is found, it must be cleaned by the chemical and electrochemical methods described later.
In order to prevent the accumulation of process lubricants or products and/or dirt, mechanical cleaning of scratches and other rough surfaces must be carried out. Generally, special polishing machines for stainless steel are used to remove them. If stainless steel is heated to a certain high temperature in the air during welding or grinding, chromium oxide heat tempering color will appear on both sides of the weld, the lower surface and the bottom of the weld. The thermal tempering color is thinner than the oxidation protective film, and it is obviously visible. The color depends on the thickness and can be seen in Rainbow, blue, purple to light yellow and brown. The thicker oxide is generally black. It is caused by staying at high temperature or at a higher altitude for a long time. When any such oxide layer appears, the chromium content on the metal surface will decrease, resulting in a decrease in the corrosion resistance of these areas. In this case, not only the heat tempering color and other oxide layers should be eliminated, but also the chromium-poor metal layers under them should be cleaned.
Rust on stainless steel products or equipment is sometimes seen before or during the manufacturing process, which indicates that the surface is seriously polluted. The rust must be removed before the equipment is put into use, and the thoroughly cleaned surface should be inspected by iron test and/or water test.
Rough grinding and machining
Grinding and machining will cause surface roughness, leaving defects such as grooves, overlap and burrs. Each defect may also damage the metal surface to a certain depth, so that the damaged metal surface cannot be cleaned up by pickling, Electropolishing or shot Blasing (such as dry sand blasting and micro glass bead abrasive). The rough surface can become the birthplace of corrosion and deposition products. Before re-welding, cleaning the weld defects or removing the excess weld reinforcement height cannot be ground by coarse grinding. In the latter case, fine abrasive should be used for grinding.
Welding arc striking patch stain
When welders arc on the metal surface, they will cause surface roughness defects. The protective film is damaged, leaving potential corrosion sources. Welders should arc on the welded track or on the side of the weld joint. Then fuse the arc striking trace into the weld.
Welding spatter has a great relationship with welding process. For example, GTAM (gas shielded tungsten arc welding) or TIG (inert gas shielded tungsten arc welding) does not splash. However, when GMAW (gas shielded metal arc welding) and FCAW (arc welding with solder cores) are used, a large amount of spatter will be caused if the welding parameters are not used properly. In this case, you must adjust the parameters. If you want to solve the problem of welding spatter, antispattering agent should be coated on each side of the joint before welding, which can eliminate the adhesion of spatter. After welding, this antispattering agent and various splashes can be easily removed without damaging the surface or causing slight damage.
The welding process using solder includes manual welding, solder core arc welding and submerged arc welding. These welding processes will leave fine solder particles on the surface, and common cleaning methods cannot remove them. This particle will be the corrosion source of crevice corrosion, and the residual solder must be removed by mechanical cleaning.
Welding defects such as undercut, incomplete penetration, dense pores and cracks not only reduce the firmness of the joint, but also become the corrosion source of crevice corrosion. When cleaning operations are performed to improve this result, they will also entrain solid particles. These defects can be repaired by re-welding or re-welding after grinding.
Oil and grease
Organic substances such as oil, grease and even fingerprints will become the corrosion source of local corrosion. Because these substances can act as a barrier, they will affect the chemical and electrochemical cleaning effect, so they must be thoroughly cleaned up. ASTM A380 has a simple water cut-off (WATERBREAK) test to detect organic pollutants. During the test, water is poured from the top of the vertical surface, and the water will be separated along the surrounding of organic substances during the downward flow. Flux and/or acidic chemical cleaning agent can remove oil stains and grease.
When tearing off the tape and protection cloth, a part of the adhesive always remains on the stainless steel surface. If the adhesive agent is not hard, it can be removed with organic flux. However, when exposed to light or air, the adhesive agent hardens to form a corrosion source of crevice corrosion. Then it needs to be mechanically cleaned with fine abrasive.
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The effects of these pollutants are similar to those of oil and grease. It is recommended to use a clean brush and clean water or alkaline cleaning agent for washing, or use high-pressure water or steam to wash. Stainless steel with ferrite structure in use. The chromium content ranges from 11% to 30%, with a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo, Ti, Nb and other elements. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, etc, it is mostly used to manufacture components that are resistant to atmospheric, water vapor, water and oxidative acid corrosion. This kind of steel has the disadvantages of Poor plasticity, obvious reduction of plasticity and corrosion resistance after welding, which limits its application. The application of seconclary refining technology (AOD or VOD) can greatly reduce interstitial elements such as carbon and nitrogen, thus making this kind of steel widely used.