The determination of detrimental intermetallic phases for duplex and superduplex steels is critical to ensure the optimal performance of these materials in corrosive environments. Using ASTM A923 standards, specific tests can be performed to identify the presence of these phases.

• Method A: Metallographic evaluation
  Method A involves metallographic evaluation after electrolytic attack with sodium hydroxide. This procedure allows the microstructure to be classified, highlighting the sigma phase at the edge between ferrite and austenite.
• Method B: Impact Test
  To determine whether the presence of the sigma phase impairs the mechanical properties of the material, a impact test can be performed. The energy absorbed varies according to the percentage of sigma phase present, allowing one to check whether the limits imposed by the standards are met.
• Method C: Corrosion test
  Another method for evaluating the presence of intermetallic phases is the ferric chloride corrosion test, similar to that described in ASTM G48 Method A. This test, conducted according to the type of alloy analyzed, measures the corrosion rate expressed in milligrams per square decimeter per day (mdd).

Description of the phenomenon

Duplex and superduplex stainless steels are particularly resistant in corrosive environments containing chlorides due to their balanced microstructure of about 50 percent austenite and ferrite. This composition provides superior resistance to localized corrosion and remarkable insensitivity to intercrystalline corrosion, while maintaining the mechanical properties typical of austenitic alloys. Because of these characteristics, duplex steels are widely used in the chemical, petrochemical, and off-shore industries.

However, the presence of intermetallic phases can compromise this strength. These phases are generated when the material is subjected to temperatures between 300 and 1000°C. In particular, the sigma phase, which forms on ferrite at temperatures between 600 and 1000°C, can significantly reduce the corrosion resistance and resilience of the material. Therefore, it is essential to determine the presence of this phase to ensure that duplex or superduplex steel is produced properly and can operate safely under the expected service conditions.