Detection of Sigma Phase in 22% Cr Duplex Stainless Steel by Electrochemical Methods

Duplex stainless steels (DSS) may undergo various structural transformations as a function of temperature, leading to formation of secondary phases when exposed to temperatures in the range 600-1000 degrees C for prolonged periods of time. Among these phases, apart from carbides and nitrides, sigma is the most prominent intermetallic compound, since it is known to adversely affect mechanical properties and corrosion resistance of DSS as a consequence of Cr and Mo depletion in the boundary zones of ferrite grains. As a result of sigma precipitation, DSS become susceptible to localized corrosion via a mechanism resembling sensitization in austenitic stainless steels. Electrochemical methods, known to successfully detect sensitization in austenitic SS, have been used to identify reliable test conditions with appropriate sensitivity also for DSS. A modified electrochemical potentiokinetic reactivation method was developed to investigate 22% Cr DSS sensitivity under different thermal cycles. Modifications were designed by closely analysing DSS anodic behaviour changes with respect to the operating testing conditions. All findings were confirmed by SEM, optical microscopy and XRD.