When stainless steel is applied to water media, the salt content in the water medium needs to be taken into consideration. According to the different salt content, it can be divided into high-purity water, fresh water, brackish water and sea water. Among them, there is almost no salt in high-purity water; the salt content in fresh water is less than 0.05%; the salt content in brackish water is more than 0.05%, but lower than that of seawater; the salt content in seawater is 3%-3.5% about.

For stainless steel materials in water media, the most common corrosion damage is local corrosion, especially stress corrosion, pitting corrosion and crevice corrosion.

Among them, high-purity water is mainly used in the nuclear industry, especially in boiling water and pressurized water nuclear reactors that use water as cooling and working medium. Because chloride ions and oxygen are very low in high-purity water, stainless steel has an extremely low corrosion rate in this environment. Therefore, in addition to a small amount of materials such as nuclear fuel cladding, stainless steel is used in large quantities in water-cooled nuclear reactors.

Even if fresh water contains a small amount of chloride ions, saturated oxygen, and even other impurities, the general corrosion of stainless steel is usually very weak. Therefore, various types of stainless steel can be used in freshwater environments.

Fresh water is used not only as a medium but also as a raw material in industry, so its usage is very high. Among equipment and components in contact with industrial water, the most commonly used stainless steels are Cr13 type, Cr17 type and 18-8 type. The working environment of various heat exchange equipment, such as heat exchangers, evaporators, coolers, condensers, heat exchangers, etc., is the most demanding under industrial water conditions. The most commonly used steel is 18-8 type steel, such as OCr18Ni11Ti, OCr19Ni9, OOCr19Ni11, and sometimes OCr17Ni12Mo2 and OOCr17Ni14Mo2.

Daily used medical equipment, kitchen equipment and utensils, and household appliances are not only in contact with the atmosphere, but are also often corroded by fresh water. Stainless steel is used under these conditions because it is beautiful, durable, hygienic, and easy to clean, so it is widely used.

In the construction of hydropower stations, erosion of river water containing mud and sand is inevitable. Ultra-low carbon martensitic stainless steels, such as OOCr13Ni4Mo, OOCr13Ni5Mo, etc., have good applications.

In seawater, for general corrosion, the corrosion rate of stainless steel is not high. The main forms of damage to stainless steel by seawater are pitting corrosion, crevice corrosion and stress corrosion, and are often affected by the oxygen content and chloride ion concentration in seawater, and the relationship between seawater and materials. The relative flow rate, seawater pollution, temperature, marine life and other factors are directly affected. Usually, below room temperature, stainless steels such as OCr17Ni12Mo2 and OCr19Ni13Mo3 containing 2%-4% molybdenum can be used; for seawater below 40-50°C, austenitic stainless steel with high chromium and molybdenum is usually used.