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Corrosion resistance includes rust resistance and resistance to corrosive media such as acid, alkali, and salt, as well as resistance to oxidation, vulcanization, chlorination, and fluorination at high temperatures. Since the selection of different stainless steels is mainly to solve various corrosion problems encountered in practical projects, the corrosion resistance of stainless steels in corrosive environments is the first consideration for material selection personnel. Corrosion is the damage caused by chemical or electrochemical action between the metal and the medium, and corrosion resistance refers to the ability of stainless steel to resist the corrosion and damage of the medium. Therefore, when the corrosion resistance is involved in the selection of materials, the following points should be paid attention to:
1. The standard of corrosion resistance is artificially determined. It is necessary to recognize it, use it, and not be bound by it. The specific standard of corrosion resistance should be determined according to the specific use requirements.
At present, the corrosion resistance of stainless steel mostly adopts the 10-level standard, which level is selected as the corrosion resistance requirement, and the characteristics of equipment and parts (thickness, size) should be considered. Length of service life, product quality (such as impurities, color, purity) and other requirements, generally speaking, for equipment, instruments and components that require a smooth mirror surface or precise dimensions during use, you can choose grades 1 to 3 standards; closely match the requirements. , For equipment that does not leak for a long time or requires a limited length of use, the components should be selected from grades 2 to 5. For equipment that does not require high maintenance, easy maintenance or long service life, the components can be selected from grades 4 to 7. Except for special exceptions, stainless steel is used. Under the condition that the annual corrosion rate exceeds 1mm, it is generally not used. It should be pointed out that the 10-level standard is not applicable when local corrosion occurs.
2. Corrosion resistance is relative and conditional. It is often said that stainless steel is rust-resistant. Corrosion resistance refers to relative to rust and non-corrosion resistance, it refers to under certain conditions (medium, concentration, temperature, impurities, pressure, flow rate, etc. for a certain period of time). Up to now, there is no stainless steel with rust resistance and corrosion resistance in any corrosive environment, so the material selection personnel must choose according to the specific use conditions. The characteristics of stainless steel are used correctly, that is, reasonable selection of materials and correct use can achieve the purpose of rust or corrosion resistance.
3. The selection of stainless steel should not only consider its general corrosion resistance, but also its local corrosion resistance. In some aqueous media and chemical media, the latter needs to be paid more attention. This is because the selection personnel generally pay more attention to stainless steel The general corrosion resistance of stainless steel, and under the conditions of use, their sensitivity to local corrosion, such as stress corrosion pitting corrosion, is less considered; local corrosion of stainless steel mostly occurs in corrosive environments with good general corrosion resistance. , Local corrosion often leads to the sudden destruction of stainless steel equipment and components, which is far more harmful than general corrosion.
4. When applying the corrosion resistance data of stainless steel in various manuals, it should be noted that many of the data are only the test results in some experiments, and there are often large discrepancies with the actual medium environment. Corrosion data, generally, the corrosion test of the actual medium or the coupon test under field conditions should be carried out in the laboratory, and the simulation device test should be carried out if necessary.
Under some conditions of use, this situation will also be encountered. When the working medium or the industrial products produced, even if there is a trace amount of certain or certain metal ions in the stainless steel, it will affect the chemical process and industrial industry. The quality of the product (including gloss, color, purity, etc.). This situation is most common in industries such as nuclear fuel, pharmaceuticals and pigments. At this time, stainless steel without a certain element is often selected or the corrosion resistance grade of the selected stainless steel is appropriately increased, so as to reduce the metal ions to the allowable limit.
5. For stainless steel equipment, if the components fail due to corrosion, the cause of corrosion damage should be analyzed, and measures should be taken after identifying the cause, instead of throwing it away.
