The materials of shell and tube heat exchangers are mainly carbon steel, stainless steel and copper. Among them, when carbon steel tube sheets are used as coolers, the welds between the tube sheet and the tube are often corroded and leaked. The cooling water system pollutes the environment and causes waste of materials.
In the manufacture of shell-and-tube heat exchangers, the welding of tube plates and tubes is generally performed by manual arc welding. There are different degrees of defects in the shape of the weld, such as depressions, pores, slag inclusions, etc., and the stress distribution of the weld is uneven. In use, the tube sheet part is generally in contact with industrial cooling water, and impurities, salts, gases, and microorganisms in industrial cooling water will cause corrosion to the tube sheet and the weld. This is what we often call electrochemical corrosion. Studies have shown that whether industrial water is freshwater or seawater, there will be various ions and dissolved oxygen. The changes in the concentration of chloride ions and oxygen play an important role in the shape of metal corrosion. In addition, the complexity of the metal structure also affects the corrosion morphology. Therefore, the corrosion of the weld between the tube sheet and the tube is mainly pitting and crevice corrosion. Viewed from the outside, there will be many corrosion products and deposits on the surface of the tube sheet, with pits of various sizes distributed. When seawater is used as a medium, galvanic corrosion will also occur. Chemical corrosion is the corrosion of the medium, and the heat exchanger tube sheet will be corroded by the chemical medium when it contacts various chemical mediums. In addition, there will be some bimetal corrosion between the heat exchanger tube sheet and the heat exchanger tube.
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 hydrochloric acid, the larger the concentration, the more severe the corrosion. Carbon steel and stainless steel have the most severe corrosion in sulfuric acid with a concentration of about 50%, and when the concentration increases to more than 60%, the corrosion decreases sharply;
(2) Impurities: Harmful impurities include chloride ions, sulfur ions, cyanide ions, ammonia ions, etc. These impurities can cause severe corrosion in some cases
(3) Temperature: Corrosion is a chemical reaction. For every 10 ° C increase in temperature, the corrosion rate increases about 1 to 3 times, but there are exceptions;
(4) Ph value: Generally, the smaller the ph value, the greater the corrosion of the metal;
(5) Flow rate: In most cases, the greater the flow rate, the greater the corrosion.