EVIDENCE FOR A CORROSION SLOT/TUNNELING MECHANISM FOR LEAD-INDUCED TRANSGRANULAR SCC OF ALLOY 690 IN PbO + 10 NaOH SOLUTION

摘要

Detailed microstructural characterization of stress corrosion cracking produced in Alloy 690 tested in PbO+10% NaOH has been performed. Analytical Electron Microscopy and the Focused Ion Beam (FIB) technique have been successfully applied to gain a different perspective on the nature of transgranular SCC. A unique series of FIB secondary electron images have been obtained from the tip of a transgranular stress corrosion crack produced in PbO + 10% NaOH solution, providing evidence of a series of aligned corrosion "tunnels" or "slots". The presence of corrosion slots/tunnels suggests that the transgranular PbSCC occurring in Alloy 690 is progressing via a slip step dissolution mechanism along select crystallographic planes. Alloy 690 has a lower stacking fault energy than Alloy 600, and deforms readily via planar slip. These factors favor a corrosion "tunneling"/slot formation SCC mechanism. This can potentially explain why Alloy 690 can be more susceptible to PbSCC in this Pb-caustic solution than Alloy 600.