Sensitization Phenomena on Aged SAF 2205 Duplex Stainless Steel and Their Control Using the Electrochemical Potentiokinetic Reactivation Test

Abstract

Microstructural changes and resulting properties were studied for SAF 2205† (UNS S31803) austeno-ferritic stainless steel (SS) aged between 700°C and 900°C for up to 2 weeks and then water-quenched. Quantitative metallography coupled with x-ray diffraction techniques were adopted to follow ferrite (α) transformation with subsequent formation of secondary austenite (γ2) and sigma (σ) phase. The kinetic model of α transformation was interpreted in the form of an Avrami-type expression. The electrochemical potentiokinetic reactivation (EPR) test was used to evaluate the degree of sensitization of the aged specimens. Results were compared with results from the corrosion test in boiling nitric acid (HNO3). Influences of the transformation of ferrite into austenite, sigma phase, and of other microstructural variations such as as chromium nitride (Cr2N) precipitation on stability of the passive film were shown. The susceptibility to inter-granular corrosion phenomena was caused by chromium depletion caused by sigma phase precipitation, while chromium nitrides appeared less harmful. Results were expressed as an isocharge line diagram that allowed concise identification of sensitization and desensitization ranges.