Effect of cold rolling on pitting resistance in duplex stainless steels

Duplex Stainless Steels (DSS) are biphasic austeno-ferritic steels in which the best combination of mechanical and corrosion-resistance properties is achieved for almost equal volume fractions of the phases. These steels are classified according to their pitting corrosion resistance, assessed by the PREN index (Pitting Resistance Equivalent Number) which, although qualitatively, is widely employed as comparison. The present work is aimed to study the pitting resistance of four DSS grades (SAF 2101, 2304, 2205 and 2507) in the as-received conditions and after cold rolling. at various thickness reductions (from 15% to 85%), to highlight the effects of cold working on the corrosion behaviour. The materials were potentiodynamically tested in artificial seawater (pH 7) and the corresponding Critical Pitting Temperatures (CPT) were determined. Cold deformation mainly affected the Lean DSS grades, whereas the high-alloyed DSS were stable, even after heavy deformations. These differences can be attributed not only to composition but also to the onset of diffusionless phase transformations induced by cold working, which make the materials more prone to corrosive attacks.