Effects of Nb on Stress Corrosion Cracking of Various Heat-Affected Zone Microstructures of E690 Steel under Cathodic Potential

This research examines how Nb affects the stress corrosion cracking (SCC) behavior of various heat-affected zone (HAZ) microstructures of E690 steel in simulated saltwater under cathodic protection. The results showed that the Nb is in the form of solid solution in coarse-grain HAZ (CGHAZ). In fine-grain HAZ and intercritical HAZ (ICHAZ), the nanosized NbC can precipitate. The anodic dissolution (AD) mechanism governs the SCC behavior of HAZ at open-circuit potential (OCP), and the addition of Nb lowers the SCC susceptibility. Nb in CGHAZ solubilized in the matrix can decrease the corrosion rate by reducing the corrosion current density. In FGHAZ, intercritical HAZ (ICHAZ), the formation of NbC precipitates can restrain the growth of MA islands by optimizing the microstructures. Under cathodic protection, the SCC behavior of steels is controlled by HE. Adding Nb in different microstructures acts as different roles to reduce the SCC susceptibility. In CGHAZ, Nb exists as atoms in solid solution, which can avoid recrystallization and refine the microstructures. In FGHAZ and ICHAZ, nanosized NbC precipitates play a significant role in avoiding the aggregation of hydrogen.