What are the precautions for stainless steel corrosion?

1, the matters needing attention in the process of stainless steel machining Processing area: the processing area of stainless steel parts should be relatively fixed. Protective measures should be taken for the platform in the processing area, such as laying rubber pads. Damage to the surface protective layer of stainless steel parts should be avoided when processing stainless steel parts. 2. There are chloride ions in the use environment. Chloride ion exists widely, such as salt/sweat/seawater/sea breeze/soil and so on. In the presence of chloride ion, stainless steel corrodes rapidly, 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. (This will give him a "misuse". There is an example in the United States: an enterprise used an oak container to hold a solution containing chloride ions. The container has been used for nearly a hundred years, and it was planned to be replaced in the 1990s. Because the oak material was not modern enough, the container leaked due to corrosion 16 days after being replaced with stainless steel. 3, without solid solution treatment The alloy elements are not dissolved into the matrix, which leads to low alloy content and poor corrosion resistance of the matrix. 4. Natural intergranular corrosion This material without titanium and niobium has a tendency of intergranular corrosion. Adding titanium and niobium and stabilizing treatment can reduce intergranular corrosion. A kind of high alloy steel that can resist corrosion in air or chemical corrosive medium. Stainless steel is a kind of steel with beautiful surface and good corrosion resistance. It does not need surface treatment such as color plating, but exerts its inherent surface properties. It is used in many aspects, and is usually called stainless steel. Representative properties are 13 chromium steel, 18-8 chromium-nickel steel and other high alloy steels. From the metallographic point of view, because stainless steel contains chromium, a very thin chromium film is formed on the surface, which is isolated from oxygen invading the steel and plays a role in corrosion resistance. In order to maintain the inherent corrosion resistance of stainless steel, steel must contain more than 12% chromium. When welding is needed, the lower carbon content minimizes carbide precipitation in the heat affected zone near the weld, which may lead to intergranular corrosion of stainless steel in some environments. 5. Dust Production is often carried out in dusty places, and there is often a lot of dust in the air, which keeps falling on the surface of the equipment. They can be removed with water or alkaline solution. However, adhesive dirt needs high-pressure water or steam to clean it. 6, floating iron powder or embedded iron On any surface, free iron will rust and corrode stainless steel. Therefore, it must be removed. Float powder can generally be removed together with dust. Some of them have strong adhesion and must be treated as embedded iron. In addition to dust, there are many sources of surface iron, including cleaning with ordinary carbon steel wire brush, shot peening with sand, glass beads or other abrasives previously used on ordinary carbon steel, low alloy steel or iron castings, or grinding the aforementioned non-stainless steel products near stainless steel parts and equipment. If stainless steel is not protected in the process of discharging or hoisting, the iron on the wire rope, spreader and workbench is easy to embed or stain the surface. Ordering requirements and post-production inspection can prevent and find 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 there must be no iron. If the result is satisfactory, wash the surface with clean pure water or nitric acid until the dark blue disappears completely. As the standard A380 points out, if the rust test solution cannot be completely removed, it is not recommended to use this test method on the technical 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 to water for 12~24 hours to check whether there are rust spots. This test is not sensitive and time-consuming. These are testing tests, not cleaning methods. If iron is found, it must be cleaned by chemical and electrochemical methods described later. 7. Scratches In order to prevent the accumulation of process lubricants or products and/or dirt, it is necessary to mechanically clean scratches and other rough surfaces, which are usually removed by a special polishing machine for stainless steel. If stainless steel is heated to a certain high temperature in the air during welding or grinding, chromium oxide hot 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 oxide protective film and is obviously visible. The color depends on the thickness, and can be rainbow, blue, purple to light yellow and brown. Thicker oxides are generally black. It is caused by staying at high temperature or high altitude for a long time. When any kind of oxide layer appears, the chromium content on the metal surface will decrease, which will reduce the corrosion resistance of these areas. In this case, not only the thermal tempering color and other oxide layers should be eliminated, but also the chromium-poor metal layer below them should be cleaned. 8. Rust spots Rust is sometimes seen on stainless steel products or equipment before or during production, which indicates that the surface is seriously polluted. 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. 9. Rough grinding and machining Both grinding and machining will cause surface roughness, leaving defects such as grooves, overlaps and burrs. Each defect may also damage the metal surface to a certain depth, so that the damaged metal surface cannot be cleaned by pickling, electropolishing or shot blasting (such as dry sand blasting and glass beads for abrasives). Rough surface can be the birthplace of corrosion and deposition products, and it is not allowed to use rough grinding to clean up weld defects or remove excess weld reinforcement before re-welding. In the latter case, it should be ground with fine abrasive. 10, welding arc spot When the welder strikes the arc on the metal surface, it will cause the surface roughness defect. The protective film is damaged, leaving a potential corrosion source. The welder should strike the arc on the welded bead or on the side of the welded joint. Then the arc striking trace is melted into the weld. 11, welding spatter Welding spatter has a great relationship with welding technology. For example, GTAM (gas shielded tungsten arc welding) or TIG (inert gas shielded tungsten arc welding) has no splash. However, when GMAW (gas shielded metal arc welding) and FCAW (arc welding with flux core) are used, a lot of spatter will be caused if the welding parameters are not used properly. When this happens, the parameters must be adjusted. If the problem of welding spatter is to be solved, anti-splash agent should be coated on each side of the joint before welding, which can eliminate the adhesion of spatter. After welding, the anti-splash agent and various splashes can be easily cleaned without damaging the surface or causing slight damage. The welding processes using flux include manual welding, arc welding with flux core and submerged arc welding. These welding processes will leave fine flux particles on the surface, which cannot be removed by ordinary cleaning methods. This particle will be the corrosion source of crevice corrosion, and these residual fluxes must be removed by mechanical cleaning. 12, welding defects 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. To improve this result, they will also carry solid particles when cleaning operations are carried out. These defects can be repaired by re-welding or re-welding after grinding. 13, 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 barriers, they will affect the chemical and electrochemical cleaning effect, so they must be thoroughly cleaned up. ASTMA380 has a simple 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 periphery of the organic matter during the downward flow. Fluxes and/or acidic chemical cleaning agents can remove oil stains and grease. 14. Residual adhesive When the adhesive tape and protective paper are torn off, a part of the adhesive always remains on the stainless steel surface. If the adhesive is not hard, it can be removed with organic solvent. However, when exposed to light or air, the adhesive hardens and forms the corrosion source of crevice corrosion. Then mechanical cleaning with fine abrasive is needed. 15, paint pen print 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 for washing. Stainless steel with ferrite structure in use. The chromium content is 1 1%~30%, and it has a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of elements such as Mo, Ti, Nb, etc. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance and excellent stress corrosion resistance, and is mostly used to manufacture parts that are resistant to atmospheric, steam, water and oxidative acid corrosion. This kind of steel has some disadvantages, such as poor plasticity, obviously reduced plasticity and corrosion resistance after welding, which limits its application. The application of refining technology outside the furnace (AOD or VOD) can greatly reduce the interstitial elements such as carbon and nitrogen, so this kind of steel is widely used.