Stress Corrosion Cracking of Steam Generator Tubes in Alloy 690TT: Effect of Sulfur and Lead on the Damage Mechanisms

Outer diameter stress corrosion cracking (ODSCC) is an active and significant degradation mode that affects steam generator (SG) tubes in pressurized water reactors. This degradation mode is mostly due to the local and continuous accumulation of deleterious pollutants in confined areas, known as heat transfer crevices, in steam generators. Understanding the role of these pollutants, in particular lead and sulfur, in the degradation of SG tubes is a key gap in the safe long-term operation of nuclear power plants (NPP). This study focuses on Alloy 690 (Ni-30Cr-11Fe) thermally treated (TT) SG tubes, mainly used in French NPP. Reverse U-bend specimens made of Alloy 690TT were exposed for 7,000 h at a temperature of 320°C to simulate crevice environments. These environments included the presence of sludge, liquid, and wet steam phases, polluted with S only or S combined with Pb. Postexposure characterization of specimens extracted from the steam phase, in the presence of S and Pb, revealed intergranular stress corrosion cracking. At the nanoscale, traces of S and Pb were detected along the cracks, especially at the crack tip and at the oxide/metal interface.