On merchant ships, the major types of stainless steels used are austenitic, ferritic, martensitic, and duplex. For use in a seawater environment, Grade 316/316L and duplex stainless steels are recommended due to their superior resistance to chloride-induced corrosion.

Major Types of Stainless Steels Used on Ships 🚢

Austenitic Stainless Steel

These are the most common types of stainless steels used on ships, particularly Grades 304 and 316. They contain high levels of chromium and nickel, providing excellent corrosion resistance and ductility. They're non-magnetic and are widely used for deck fittings, tanks, and pipelines. The 316 grade is particularly notable as it contains molybdenum, which significantly boosts its resistance to chloride and pitting corrosion.

Ferritic Stainless Steel

Ferritic stainless steels have moderate corrosion resistance and are magnetic. They have a lower nickel content compared to austenitic types. They are typically used for less critical applications like interior housings and structural supports where corrosion risk is lower.

Martensitic Stainless Steel

These steels are known for their high hardness and moderate corrosion resistance. They are utilized for components that require high wear resistance, such as shafts, cutlery, and some pump parts.

Duplex Stainless Steel

Duplex steels have a dual-phase microstructure, combining properties of both austenitic and ferritic steels. This unique composition gives them high strength and very good resistance to both general corrosion and stress corrosion cracking. They are used in demanding marine and offshore applications, particularly for piping and pressure vessels.

Recommended Grades for a Seawater Environment 🌊

For a seawater environment, the following grades are recommended:

• 316/316L Austenitic Stainless Steel: Often called "marine grade," this is the most recommended type for general marine and saltwater environments. Its molybdenum content provides excellent resistance to salt-induced pitting and crevice corrosion. It is used for deck components, submerged pipelines, seawater pumps, and hull fittings.
• Duplex and Super Duplex Stainless Steels: Grades such as UNS S32750 (2507), UNS S32760, and S31803 offer even higher resistance to localized corrosion and stress corrosion cracking. They are ideal for harsh seawater and splash zone applications, as well as for critical submerged structural parts.

Why These Grades are Recommended 🧐

• Grade 316/316L: The key reason for recommending this grade is the addition of 2-2.5% molybdenum. This element significantly increases its pitting resistance in chloride-rich environments like seawater. It also offers excellent durability, weldability, and formability, making it suitable for a variety of marine services.
• Duplex Types: These steels are recommended due to their superior mechanical properties and exceptional resistance to seawater corrosion, especially in areas with high turbulence, high salinity, or long-term submersion. Their resistance to stress corrosion cracking is particularly valuable in these demanding conditions.

Note: While Grade 304 may be used in marine applications with only brief exposure to saltwater, it is not recommended for permanent immersion or highly saline zones due to its lower pitting resistance.