Effect of Sigma Phase on the Corrosion and Stress Corrosion of 2205 and 2507 Duplex Stainless Steels

Offshore operations worldwide have recently reported the presence of improperly heat treated 2205 and 2507 duplex stainless steel (SS) pipe fittings with varying levels of embrittling secondary phases (such as sigma s). Duplex SS plate and pipe samples were heat treated to generate levels of s phase ranging from 0 to 14%. Test coupons were then prepared from these sigmatized duplex SS materials for cyclic polarization, galvanostatic and potentiodynamic testing in simulated seawater solutions at 25°C and 80°C. These tests helped understand and characterize the reduction in pitting resistance of sigmatized duplex SS. To evaluate the environmentally assisted cracking (EAC) susceptibility of these materials, slow strain rate testing (SSRT) was carried out. The above tests were also replicated using samples prepared from duplex SS components obtained from offshore facilities. The results obtained suggest that pitting is more stable at higher s contents. However, the potential gap between open circuit potential (OCP) and pitting potential is insensitive to the increase in the s volume fraction. This is consistent for both duplex SS evaluated. On the other hand, a monotonical increase in potential gap between pitting potential obtained at the stable pitting current and that obtained from cyclic polarization was observed with increasing s content. This behavior is consistent with the occurrence of crevice-like corrosion at the intermetallic–ferrite interface. Evaluation of EAC susceptibility indicates that cracking does not occur at the test conditions. Using the hypothesis of crevice-like corrosion at near OCP conditions, a model for EAC was implemented which explained the relationship between the electrochemical results and those obtained from the SSRT testing.