Application of stainless steel in seawater desalination project
Water is the most important resource of human beings. Due to the expansion of population and the increase of water consumption, fresh water resources are decreasing, and the shortage of water resources in many parts of the world is becoming increasingly prominent. Two thirds of the earth's surface is covered by water, but 97% of the water reserves are sea water and brackish water. To use sea water, it must be desalinated. Therefore, sea water desalination came into being and developed rapidly.
The equipment and materials used in desalination process should have the corrosion resistance characteristics. The selection and design principles of the materials depend on the service environment of the materials. Stainless steel has become an ideal material because of its corrosion resistance and durability, and is used in various desalination methods.
The stainless steel used in seawater desalination project requires good corrosion resistance in various complex environments. Austenitic stainless steel 316L and 317 are the most important materials for seawater desalination equipment. 316L can withstand the corrosion of marine environment, but pitting corrosion and crevice corrosion will appear when it is immersed in seawater for a long time.
The pitting corrosion of stainless steel used in seawater desalination equipment for treating heat concentrated seawater in chloride environment is very noteworthy. Although hot seawater anoxia reduces the tendency of pitting corrosion, and 316L stainless steel is the traditional material of evaporator, most modern multi-stage flash desalination plants (MSF) do not use 316L stainless steel, and most desalination equipment manufacturers begin to use duplex stainless steel as an alternative material.
The reason why duplex stainless steel has become an important material for seawater desalination is that it has excellent corrosion resistance, especially stress corrosion cracking resistance. Stress corrosion cracking (SCC) is a kind of corrosion mode that causes sudden failure of materials under certain temperature, corrosive environment and tensile stress. The strength of duplex stainless steel is twice that of austenitic stainless steel, especially its excellent mechanical properties can reduce the material cost. At present, 2205 duplex stainless steel has been used in the shell of evaporator of MSF desalination unit.
In recent years, seawater desalination equipment suppliers have also introduced the concept of "full duplex", that is, 2205 duplex stainless steel is used in harsh environment, and economical stainless steel is used in slightly weak corrosive environment. The economical duplex stainless steel is a kind of economical duplex stainless steel. It has lower nickel and molybdenum content than 2205 stainless steel, and its corrosion resistance is similar to 316L. Because of its high mechanical properties, the self weight of the equipment can be reduced by 50% compared with austenitic stainless steel, thus saving 35-40% of the cost“ The concept of "full two-phase" has been applied in the desalination plant of multi effect distillation (MED).