Effects of sigma phase precipitation on seawater pitting of duplex stainless steel

In this study the potentiodynamic polarization technique was employed to study the pitting corrosion characteristics of duplex stainless steel types SAF 2205 and 3RE60 duplex stainless steels in artificial seawater at various test temperatures (25, 50 and 80°C) and sigma phase precipitation (produced at pre-quenched heat treatment temperature of 845°C for different heating duration of 10, 60 and 300 minutes). The results showed that mill-annealed and sigma phase precipitated samples of type 2205 DSS were immune to pitting corrosion at ambient temperature, however, the alloy exhibited susceptibility to pitting attack at 50°C. The potential necessary to cause pitting corrosion decreased (became less noble) with increasing test temperature (to 80°C) and/or inappropriate heat treatment duration and become more effectual with long heat treatment exposure. In contrast, the mill-annealed 3RE60 exhibited seawater pitting attack at ambient temperature and the intensity of the pitting attack increased with increasing test temperature or sigma phase precipitation volume. The results clearly indicated that the presence of nitrogen and relatively higher chromium content in the 2205 alloy is an advantage for seawater pitting corrosion resistance.