Influence of Sigma Phase Formation on Pitting Corrosion of an Aged UNS S31803 Duplex Stainless Steel

The main purpose of this contribution was the study of the relation between sigma formation and pitting corrosion resistance of a UNS S31803 duplex stainless steel aged at 850°C. Solution-treated samples were isothermally aged at 850°C for up to 360 h. The corrosion resistance of aged UNS S31803 steel was analyzed through cyclic potentiodynamic polarization tests in 0.6 M sodium chloride (NaCl) solution. The main mechanism of sigma phase formation was nucleation and growth from ferrite, in agreement to the Johnson-Mehl-Avrami-Kolmogorov kinetics model. This model suggests a change on the mechanisms of sigma phase formation from nucleation and growth from ferrite to the growth from austenite. Pitting morphology was related to the material microstructure. The pitting growth mechanism occurred by selective corrosion of the chromium-depleted areas of the specimens. Both pitting and repassivation potentials decreased with an increase of sigma volume fraction. Recovery of the corrosion resistance was observed in the early stages of sigma formation, probably related to chromium redistribution to depleted areas.