Volume 6, Issue 3 (March 2009)
Role of Thiosulfate on Chloride Breakdown Potential of Heat Treated Ni-18Cr-6Fe Alloys
The breakdown potentials of mill-annealed Ni-18Cr-6Fe alloys were measured in wide range of chloride concentrations (0.001–1 M NaCl) and various concentrations of thiosulfate ranging from 0–0.25 M (Na2S2O3) at 80°C using the potentiodynamic technique. These potentials were compared with heat treated alloys for one or five hours at 900°C (and quenched). This work aimed to study the effect of sensitization on the pitting characteristics of this alloy in chloride-thiosulfate solutions. The anodic polarization curves revealed that sensitized alloys became more susceptible to chloride and thiosulfate pitting attack, or both. Addition of thiosulfate was potentially detrimental to chloride pitting resistance and the aggressiveness of thiosulfate species was found dependent on the ratio of chloride to thiosulfate concentration as the overall ionic strength is varied. A thiosulfate to chloride ratio of 1:40 (S2O2–3 / Cl–=0.025) was found be the most aggressive, regardless of ionic strength or heat treatment duration.