When selecting stainless steel in a corrosive environment, in addition to a detailed understanding of the specific conditions for the use of stainless steel, the main factors to be considered are: corrosion resistance, strength, toughness and physical properties of stainless steel, processing, Formability, resources, price and difficulty.
Corrosion resistance
Corrosion resistance includes stainless steel and acid, alkali, salt and other corrosive media properties as well as high temperature resistance to oxidation, sulfurization, chlorination, fluorination, etc.. Because of the choice of different stainless iron is mainly to solve the actual project The various corrosion problems, for this reason, the corrosion resistance of stainless steel in the corrosive environment is the first consideration for the material selection. Corrosion is the damage caused by chemical or electrochemical interaction between the metal and the medium, and the corrosion resistance Refers to the ability of stainless steel to resist corrosion damage of the medium, so when the corrosion resistance is involved in the selection of materials, the following points should be noted.
1. The standard of corrosion resistance is determined by human beings. It is necessary to recognize it, use it, and not be bound by it. It is necessary to determine the specific standard of corrosion resistance according to the specific use requirements.
At present, the corrosion resistance of stainless steel is mostly based on the 10-level standard. Which one is selected as the corrosion-resistant requirement, the equipment, the characteristics of the part (thin thickness, size) should be considered. The service life is long, the product quality (such as impurities, Requirements for color, purity, etc. Generally speaking, for equipment instruments and components that require a smooth mirror or a precise size during use, the standard of Class 1~3 can be selected; the requirements are closely matched, and the long-term leakage or required length limit is required. Equipment, parts selected 2~5, for equipment that is not easy to repair or require a long life, the parts can be used 4~7, except for special exceptions, the annual corrosion rate of stainless steel under the conditions of use is more than 1mm Generally, it is not used. It should be pointed out that the 10-level standard is not applicable for local corrosion.
2. Corrosion resistance is relative, conditional, often said stainless steel rust, corrosion resistance refers to relative to rust and non-corrosion, refers to under certain conditions (medium, Concentration, temperature, impurities, pressure, flow rate, etc.). Up to now, there is no stainless steel with corrosion resistance and corrosion resistance in any corrosive environment, so the members of the material must be applied according to the specific conditions of use. After the selection of the stainless steel grade, the use department should also correctly use the characteristics of the selected stainless steel, that is, the rational selection and correct use can achieve the purpose of stainless or corrosion resistance.
3. The choice of stainless steel should consider its resistance to general corrosion and its resistance to local corrosion. In some aqueous media and chemical media, the latter needs more attention. This is because the selection of materials is generally more Pay attention to the general corrosion resistance of stainless iron, and under the conditions of use, they are less sensitive to local corrosion, such as stress corrosion pitting, etc.; local corrosion of stainless steel is mostly resistant to general corrosion. Occurred in a good corrosive environment, local corrosion often leads to stainless steel equipment, sudden destruction of components, and its harmfulness is far greater 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 experimental results in some experiments, and often have a large difference with the actual media environment, in order to obtain closer to the actual use conditions. Corrosion resistance data, generally should be carried out in the laboratory in the actual medium corrosion test or on-site conditions of the coupon test, if necessary, the simulation device test.
Under some conditions of use, this is also the case. In the working medium or industrial products produced, even if it contains a trace amount of metal ions in some or some of the stainless steel, it will affect the chemical process. The quality of industrial products (including gloss, color, purity, etc.). This situation is most common in industries such as nuclear fuel pharmaceuticals and pigments. In this case, it is often preferred to use stainless steel that does not contain an element or to properly increase the selected stainless steel. Corrosion resistance grade to reduce metal ions to the allowable limit.
5. Stainless steel equipment, if the parts fail due to corrosion, the analysis of the cause of corrosion damage should be carried out, and the measures should be taken after the cause is identified, and should not be thrown away.
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