Temperature In the design process of petrochemical plants, the selection of equipment materials often needs to consider various factors, of which corrosion is one of the factors to be considered, especially considering the long-term continuous operation of the device to ensure internal leakage of the equipment and select the appropriate resistance. Corrosion of equipment materials is more important.
The author analyzes the possible corrosiveness of the nickel brazed plate heat exchanger and discusses the influence of the circulating cooling water chloride ion content on the material selection of the nickel brazed plate heat exchanger with circulating water as the cooling medium. We know that nickel brazed plate heat exchangers are widely used due to their high heat transfer efficiency, compact structure, easy disassembly, small footprint, and wide application range. The plate heat exchanger is composed of two side pressure plates, multiple inner plates, cold and hot medium inlets and outlets, and tightening screws. For nickel brazed plate heat exchangers used for non-corrosion of the cold medium, the material of the side pressure plates and the inlet and outlet ports at both ends is carbon steel, and the inner plate is usually made of 0.5 0.8 mln thick stainless steel or alloy plate pressing. Since the chloride ions in the water will produce different degrees of corrosion on stainless steel and alloy steel, so for nickel brazed plate heat exchangers that are not corroded by the cold medium, the choice of inner plate material depends on the content of chloride ions in the circulating cooling water. Of course, the temperature is also the main factor that determines the degree of chloride ion corrosion of the inner plate.
1. Corrosion analysis
There are many types of corrosion, and the forms of metal corrosion can be divided into uniform corrosion and local corrosion. The former occurs more uniformly on the entire surface of the metal, and the latter occurs only locally. Typical localized corrosion are: intergranular corrosion, pitting corrosion, crevice corrosion, stress corrosion, galvanic corrosion, erosion corrosion, corrosion fatigue, delamination corrosion. The main damage and corrosion of nickel brazed plate heat exchanger by circulating cooling water in the presence of chloride ions are pitting corrosion, stress corrosion and crevice corrosion. Pitting corrosion, also known as hole corrosion, is a highly localized corrosion pattern that corrodes into pits on the metal surface, further forming deep holes to penetrate the metal plate. In the plate heat exchanger, the surface of the inner plate is generally covered with a protective passivation film, and the corrosion is slight, but due to defects on the plate surface (such as: scratches, collision points, non-metallic inclusions, etc.), the micro-break The exposed metal becomes the anode of the battery, the membrane around the enlarged area becomes the cathode, and the anode current is highly concentrated, which causes the corrosion to rapidly develop inwards, and then produces localized serious corrosion points. Stress corrosion is when the metal plate has tensile stress and the corrosion causes the metal plate to crack, also known as stress corrosion cracking. For the plate heat exchanger, stress will be generated when the inner plate is pressed. Therefore, if the inner plate is made of austenitic stainless steel, the environment with chloride ions will produce typical stress corrosion. Crevice corrosion is a special form of pitting corrosion, which occurs in the crevices, and the damage form is trench-like, which can penetrate in severe cases. For plate heat exchangers, the inner plate is surrounded by sealing pads compressed by the plates. Between the two plates, one side has a sealing groove and the other side is generally flat. After the compression, the gasket and the plate There will be gaps. In addition, the plates are alternately arranged and clamped, so that the corrugated tips of adjacent plates cross each other to form a large number of contact points, and gaps will also be formed around the contacts. The gap is a hypoxic zone, which is also in an occlusive state. The pH value in the gap decreases and the chloride ion concentration increases, thereby accelerating corrosion. Therefore, crevice corrosion is also a typical corrosion of austenitic stainless steel plate heat exchangers.
2. The effect of temperature on corrosion
Corrosion is a chemical reaction. The rate of corrosion increases approximately 1-3 times for each temperature increase of 10 ° C. Generally, the corrosion rate always accelerates with increasing temperature, the temperature increases, the diffusion rate increases, and the electrolyte resistance decreases, which accelerates the reaction of the corrosion battery. Of course, there are exceptions. When the temperature rise can reduce the effects of other factors, corrosion may be reduced; but this exception is not applicable to nickel brazed plate heat exchangers.
3. Source and content of chloride ion in circulating water
The chloride ion in the circulating water comes from the chloride ion in the make-up water of the circulating cooling water and the treatment and addition of chlorine in the supplementary water, and the treatment and addition of chlorine in the circulating cooling water. The amount of chloride ion mainly depends on the concentration of circulating water. For the treatment of supplementary water treatment and the addition of chlorine from the circulating cooling water treatment, the chloride ion is not the main one. According to the national standard "Design Specification for Industrial Circulating Cooling Water Treatment)) GBS0050-95, the freeness of the supplementary water and chlorine treatment is free. The amount of chlorine needs to be controlled at 0.1-0.2 mg / L, and the residual chlorine amount of the chlorine treatment of the open circulating cooling water is controlled at 0.5-1.0 mg / Ll1 J. The main thing is to replenish the chloride ions in the water, so the source of the supplemental water is different, so that the content of chloride ions in the circulating cooling water is different.
(1) For the open circulating cooling water system, the water will continuously evaporate during the cooling of the circulating water tower. The evaporated water is equivalent to distilled water. The impurities, salts, and chloride ions contained in the original part of the water are left in the circulation In water, the concentration of salt and chloride ions increases. According to GB 50050-95. The design concentration of open circulating cooling water should not be less than 3. Ol2], so the concentration multiple is often greater than 3 during design, usually 4 or even 5. Then, the chloride ion content in the circulating cooling water will increase exponentially with the chloride ion content in the supplemental water as the concentration factor increases.
(2) According to the national standard "Water Quality Standard for Drinking Water Sources" CJ3O2O-93 and "Environmental Quality Standards for Surface Water" GB 3838-2OO2, the water quality standard for chlorine content (calculated as chloride ion) is 250 mg / L This is the standard, and then consider the requirement that the design concentration of circulating cooling water should not be less than 3.0, then it means that the water content should be replenished according to the national standard, and the minimum chloride ion content of the circulating cooling water will reach 750 mg / L. In fact, no matter whether it is surface water, groundwater, or tap water, the supplemental water source does not have much chloride ion content of 250 mg / L, and the circulating cooling water does not reach 750 mg / L. According to the provisions of GB 50050-95, for carbon steel heat exchange equipment, the circulating cooling water chloride ion content ≤ 1 000mg / L, for stainless steel heat exchange equipment, circulating cooling water chloride ion content ≤ 300 mg / LE. If the specific gravity of water is considered as approximately 1 000 g / L, 300 mg / L is equivalent to 3X10I4 (300 ppm) [wt, mass fraction]. Therefore, the maximum chloride ion content of circulating cooling water can be considered as 3 X 10 ~ (3OO ppm.).
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