Pitting Corrosion of Steam Turbine Blading Steels: The Influence of Chromium Content, Temperature, and Chloride Ion Concentration

Two industry-standard steam turbine blading steels, FV566 containing 11% Cr and FV520B containing 13.5% Cr by weight, are characterized with respect to chloride-induced pitting corrosion using potentiodynamic polarization measurements and the scanning vibrating electrode technique (SVET). In deaerated aqueous sodium chloride (NaCl), at near-neutral pH, both alloys exhibit a dependence of pitting potential (E_b) on chloride concentration ([Cl^–]) such that E_b = A + B log 10[Cl^–]. The quantity B is temperature (T) independent and A decreases linearly with increasing T. Similar B values of –139 ± 8 mV/decade and –141 ± 9 mV/decade, and similar dA/dT slopes of –3.9 ± 0.2 mV/K and –3.8 ± 0.4 mV/K, are found for 11Cr and 13.5Cr, respectively. However, A for 13.5Cr remains ca. 0.25 V higher than for 11Cr throughout. At 20°C and under nonpolarized, aerated conditions, 13.5Cr shows no evidence of pitting, even when [Cl^–] is 5 mol/dm³. Conversely, stable pitting commences on 11Cr within 1 h of immersion at [Cl^–] ≥ 1 mol/dm³. Each pit emits a steadily increasing anodic current over 24 h, and pit number density increases from 1 cm^–2 to 4 cm^–2 as [Cl^–] is increased from 1 mol/dm³ to 5 mol/dm³. However, the total (sum) anodic current-time characteristic is independent of both [Cl^–] and pit density. The anodic currents detected imply that pit radii of ca. 100 μm may be attained within 12 h of immersion.