Application of Stainless Steel in Seawater Desalination Project
Water is the most important resource for mankind. Due to the expansion of population and the increase in water consumption, freshwater resources have been continuously reduced, and the problem of water shortage in many areas of the world has become increasingly prominent. 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 has emerged and is developing rapidly.
The equipment and materials used in the desalination process need to have corrosion resistance. The selection and design principles of the materials depend on the service environment of the materials. Stainless steel has become an ideal material due to its corrosion resistance and durability and is used in various desalination projects.
The stainless steel used in seawater desalination projects requires good corrosion resistance in various complex environments. Austenitic stainless steel 316L and 317 are the most important materials for the manufacture of seawater desalination equipment. 316L can withstand corrosion in the marine environment, but long-term immersion in seawater will cause pitting corrosion and crevice corrosion.
The pitting corrosion of stainless steel used in desalination equipment for processing hot concentrated seawater in an environment containing chloride ions is very noteworthy. Although the lack of oxygen in hot seawater reduces the tendency of pitting corrosion, and 316L stainless steel is the traditional material for evaporators, most modern multi-stage flash desalination plants (MSF) do not use 316L stainless steel, and most desalination equipment manufacturers start to Use duplex stainless steel as an alternative material.
The reason why duplex stainless steel has gradually become an important type of stainless steel for seawater desalination is due to its excellent corrosion resistance, especially stress corrosion cracking resistance. Stress corrosion cracking is a corrosion method that causes the material to suddenly fail under the condition of a certain temperature, a material in a corrosive environment, and the presence of tensile stress. The strength of duplex stainless steel is twice that of austenitic stainless steel, especially its excellent mechanical properties can reduce material costs. At present, 2205 duplex stainless steel has been used in the shell of the MSF desalination unit evaporator.
In recent years, the seawater desalination equipment supplier has also introduced the “full-duplex” concept, that is, 2205 duplex stainless steel is used in harsh environments, and economical stainless steel is used in weaker corrosive environments. The economical duplex stainless steel is a kind of economical duplex stainless steel. Compared with 2205 stainless steel, it has lower nickel and molybdenum content, and its corrosion resistance is similar to that of 316L. Due to its high mechanical properties, compared with austenitic stainless steel, the economical duplex stainless steel can reduce the weight of the equipment by 50%, thereby saving 35-40% of the cost. The “full two-phase” concept has been applied to multiple-effect distillation (MED) desalination plants.