Detection of sigma phase by electrochemical methods in a 22%Cr DSS

Duplex stainless steels (DSS) may undergo various structural transformations as a function of temperature, leading to formation of secondary phases when exposed at temperatures in the range 600-1000 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 the mechanical properties and corrosion resistance of DSS alloys as a consequence of depletion of Cr and Mo in the boundary zones of ferrite grains. As a result of sigma precipitation, DSS become susceptible to localized corrosion via a mechanism similar to sensitization in austenitic stainless steels. Electrochemical methods, reported to successfully detect sensitization in austenitic SS have been applied with the aim to define reliable test conditions with appropriate sensitivity also for DSS. A modified Electrochemical Potentiokinetic Reactivation (EPR) method was developed for investigating 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 (OM) examinations and XRD analysis.