Corrosion analysis of shell and tube heat exchangers

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The materials used in shell-and-tube heat exchangers are generally carbon steel, stainless steel, and copper. In the case of carbon steel tube sheets used as coolers, corrosion and leakage often occur at the welds between the tube sheet and the tubes; these leaks cause contamination of the cooling water system, leading to environmental damage as well as waste of materials. 　　During the fabrication of shell-and-tube heat exchangers, the welding of the tube sheet to the tubes is generally carried out using manual arc welding. The welds suffer from various defects such as depressions, pores, and slag inclusions, and the stress distribution within the welds is also uneven. During use, the tube sheet area is generally in contact with industrial cooling water, and the impurities, salts, gases, and microorganisms present in this water can cause corrosion of the tube sheet and welds – this is what is commonly referred to as electrochemical corrosion. Studies have shown that industrial water, whether freshwater or seawater, contains various ions and dissolved oxygen; changes in the concentrations of chloride ions and oxygen play an important role in determining the pattern of metal corrosion. Furthermore, the complexity of the metal structure also affects the corrosion pattern. Therefore, the corrosion of the tube sheet and tube sheet-to-tube welds is mainly pitting and crevice corrosion. Visually, the tube sheet surface is covered with numerous corrosion products and deposits, along with pits of varying sizes. When seawater is used as the medium, galvanic corrosion also occurs. Chemical corrosion refers to the corrosion caused by chemicals; heat exchanger tube sheets that come into contact with various chemical substances are subject to corrosion by those chemicals. Additionally, a certain degree of bimetallic corrosion can also occur between the heat exchanger tube sheet and the heat exchange tubes. 　　In summary, the main factors affecting the corrosion of shell-and-tube heat exchangers are: (1) Medium composition and concentration: The effect of concentration varies; for example, in saline acids, generally the higher the concentration, the more severe the corrosion. Carbon steel and stainless steel suffer the most severe corrosion in sulfuric acid with a concentration of around 50%; however, when the concentration increases to over 60%, corrosion decreases sharply ； 　　(2) Impurities: Harmful impurities include chloride ions, sulfide ions, cyanide ions, ammonia ions, etc.; these impurities can cause severe corrosion under certain conditions ； 　　(3) Temperature: Corrosion is a chemical reaction; for every 10°C increase in temperature, the corrosion rate increases by about 1 to 3 times, though there are exceptions ； 　　(4) pH value: Generally, the lower the pH value, the greater the corrosion of the metal ； 　　(5) Flow rate: In most cases, the higher the flow rate, the greater the corrosion.