Materials used in environments that are highly corrosive or contain high levels of salinity, such as seawater, must meet certain chemistry levels to ensure proper corrosion protection. Historically, 300 series stainless steels such as stainless type 304 and 316 have been used to combat corrosion, but studies have shown these materials can lack the grain structure to withstand the aggressive nature of seawater. Therefore, alternate types of stainless steels have been developed for seawater applications. Materials such as super-austenitic and duplex stainless steels have maximum strength and corrosive resistant capabilities and have been used extensively in seawater and brackish water applications.
Roscoe Moss Company manufactures its well casings and screens used in seawater applications from super-austenitic steels to maximize strength and corrosion resistance. Super austenitic steels are characterized as having low carbon, high purity and nitrogen, along with high levels of nickel and molybdenum (6% minimum) to improve resistance to chloride stress corrosion cracking and chloride induced pitting. The combination of corrosion resistance, weldability, formability, and strength make super-austenitic stainless steels a preferred material for seawater applications, as well as providing excellent resistance to pitting and corrosion in sodium chloride solutions and environments.
Roscoe Moss Company also manufactures well casings and screens
in duplex stainless steels. These materials
are referred to as "duplex" because they are
a balance of ferritic and austenitic stainless
steel. Applications for duplex stainless steels
are more commonly found in brackish water environments.
Despite having lower nickel and molybdenum contents
than their austenitic counterparts of similar
corrosion resistance(such as SST-316L), duplex stainless steels
have increased yield strength allowing for the
use of lighter material, thereby creating a
more cost effective alternative.
The table below highlights the approximate percentage of key chemical elements in certain stainless steels used in seawater and brackish water applications.