Water is the most important resource for human beings. Due to the expansion of population and the increase of water consumption, fresh water resources are constantly 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 percent of its water reserves are seawater and brackish water. Desalination has been developed rapidly since seawater has to be desalinated to make use of it.
The equipment materials used in the desalination process should have the characteristics of corrosion resistance. The selection and design principles of the materials depend on the service environment of the materials. Stainless steel is an ideal material because of its corrosion resistance and durability. It is used in various desalination projects.
Stainless steel used in desalination projects requires good corrosion resistance in a variety of complex environments. Austenitic stainless steel 316L and 317 are the most important materials for desalination equipment. The 316L can withstand corrosion in Marine environments, but long-term immersion in seawater can cause pitting and creepage corrosion.
Pitting corrosion of stainless steel in the presence of chloride ions is of great concern for desalination equipment used to process thermally concentrated seawater. Despite the fact that hot seawater is hypoxic to reduce pitting and 316L stainless steel is the traditional material for evaporators, most modern multistage flash desalination units (MSFS) do not use 316L stainless steel, and most desalination equipment manufacturers are beginning to use biphasic stainless steel as an alternative material.
Duplex stainless steel has gradually become an important class of stainless steel materials for seawater desalination, because it has excellent corrosion resistance, especially stress corrosion cracking resistance. Stress corrosion cracking is a corrosion mode that causes sudden failure of materials under certain temperature, corrosion 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 the evaporator of the MSF desalination plant.
In recent years, desalination equipment suppliers have also introduced the “full duplex” concept, that is, 2205 duplex stainless steel for harsh environment, saving stainless steel for slightly weaker corrosion environment. Economical duplex stainless steel is a kind of economical duplex stainless steel, which has lower nickel and molybdenum content than 2205 stainless steel, and the corrosion resistance is similar to 316L. Due to its high mechanical properties, the saving duplex stainless steel can reduce the dead weight of equipment by 50% compared with austenitic stainless steel, thus saving 35 ~ 40% of the cost. The “full biphasic” concept has been applied to multi-effect distillation (MED) desalination plants.
