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

Year: 2026
Type: article
Abstract: 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 approximately 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 (scanning electron microscope, focused ion beam-transmission electron microscopy, and scanning transmission electron microscopy with energy-dispersive x-ray) revealed similar crack-tip oxide compositions across all environments, with Pb incorporated in a (Cr,Fe)_3O_4 spinel phase at approximately 1 wt% to 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.
Source: CORROSION
DOI: 10.5006/4887
Language: English
Open Access status: green
OpenAlex ID: W7147217186
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