Evaluating the Use of Phosphate to Mitigate Caustic-PbSCC of Alloy 690TT

Phosphate additions to Pb caustic water chemistry were investigated as a potential mitigation strategy for Lead Assisted Stress Corrosion Cracking (PbSCC) of Alloy 690TT at ~310 °C. Autoclave tests were performed for 500 h using C ring specimens strained in the plastic regime and exposed to four caustic environments (pH_high T ~10) containing different PbO and phosphate concentrations. Two surface conditions were examined: as received and OPS polished. Results showed that 1,700 ppm phosphate provided only a minor benefit in reducing PbSCC severity, while 3,000 ppm phosphate mitigated cracking for as received surfaces but promoted localized attack on polished surfaces. Advanced microstructural characterization (SEM, FIB TEM, STEM EDX) revealed similar crack tip oxide compositions across all environments, with Pb incorporated in a (Cr,Fe)_3O_4 spinel phase at ~1–3 wt.%. These findings show that phosphate reduces PbSCC kinetics by partially sequestering Pb from solution rather than altering crack tip chemistry. Although mitigation was incomplete, the results provide insight into inhibitor performance under concentrated crevice-like conditions and highlight the complexity of controlling PbSCC. These findings inform inhibitor selection and water chemistry strategies aimed at improving long-term PWR steam generator integrity.