Duplex and super duplex stainless steel in marine service

Duplex stainless steels hold roughly equal fractions of austenite and ferrite, which gives them about twice the yield strength of a standard austenitic grade and markedly better resistance to chloride pitting and stress corrosion cracking. In marine service they appear in seawater piping, pump and valve bodies, cargo tanks on chemical carriers, and in the wetted parts of an exhaust gas cleaning system , where low pH meets chloride.

Selection is conventionally indexed by the pitting resistance equivalent number, PREN, calculated as the chromium content plus 3.3 times molybdenum plus 16 times nitrogen. Grade 2205, EN 1.4462, is the standard duplex; the super duplex grades such as 2507, EN 1.4410, sit at PREN 40 and above. For tungsten-bearing grades such as UNS S32760 the tungsten-inclusive PREW index is used instead, which is a distinction often lost in specification documents.

The full article will cover the grade families and their designations, phase balance and the intermetallic precipitation window, welding procedure and heat input control, the alternatives of glass reinforced plastic and coated carbon steel, and the cathodic protection interaction.