Water is the most important resource for human beings. Due to the expansion of the population and the increase of water consumption, the freshwater resources are constantly decreasing, and the problem of water shortage in many parts of the world is becoming more and more serious. Two-thirds of the earth’s surface is covered by water, but 97% of the water reserves are seawater and brackish water. To use seawater, it must be desalinated. Therefore, seawater desalination came into being and developed rapidly.
The equipment materials used in the desalination process need to have corrosion-resistant characteristics. The selection and design principles of materials depend on the service environment of the materials. Stainless steel is an ideal material due to its corrosion resistance and durability, and is used in engineering for various desalination methods.
Stainless steel used in seawater desalination projects requires good corrosion resistance in various complex environments. Austenitic stainless steels 316L and 317 are the most important materials for the manufacture of desalination equipment. 316L can withstand corrosion in marine environments, but long-term immersion in seawater will cause pitting and crevice corrosion.
Pitting corrosion of stainless steels used in desalination plants that process hot concentrated seawater is notable in chloride-containing environments. Although the lack of oxygen in hot seawater reduces the tendency for pitting and 316L stainless steel is the traditional material for evaporators, most modern multi-stage flash desalination plants (MSFs) do not use 316L stainless steel, and most desalination equipment manufacturers begin to use 316L stainless steel. Duplex stainless steel is used as an alternative material.
The reason why duplex stainless steel has gradually become an important class of stainless steel for seawater desalination is that it has excellent corrosion resistance, especially stress corrosion cracking resistance. Stress corrosion cracking is a kind of corrosion method that causes the material to suddenly fail in the presence of tensile stress at a certain temperature and in a corrosive environment. The strength of duplex stainless steel is twice that of austenitic stainless steel, especially its excellent mechanical properties can reduce material cost. At present, 2205 duplex stainless steel has been used for the shell of the evaporator of the 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 environments, and economical stainless steel is used in slightly weaker corrosive environments. Economical duplex stainless steel is an economical duplex stainless steel, which has lower nickel and molybdenum content than 2205 stainless steel, and its corrosion resistance is similar to that of 316L. Due to its high mechanical properties, the economical duplex stainless steel can reduce the weight of the equipment by 50% compared with authentic stainless steel, thereby saving 35 to 40% of the cost. The “full duplex” concept has been applied to desalination plants with multiple effect distillation (MED).