Pitting corrosion of Ni-Cr-Fe alloys at open circuit potential in chloride plus thiosulfate solutions

Alloys 600 and 800 exhibited stable pitting corrosion in 1 M NaCl plus thiosulfate deaerated solutions at the open circuit potential (OCP) and room temperature. Alloy 690 exhibited only metastable pitting. Stable pitting, stable passivity or metastable pitting were identified by observing features in the OCP vs. time transients and by analysis of electrochemical impedance spectroscopy measurements. Post-mortem scanning electron and optical microscopy observations confirmed these findings. Stable pits nucleated adjacent to Ti-rich inclusions, and reached diameters above 100 mu m after 50 h of OCP immersion. A prepassivation treatment inhibited this mechanism of pitting corrosion.

Automated summary

Alloys 600 and 800 showed stable pitting corrosion, while Alloy 690 exhibited only metastable pitting, in 1 M NaCl plus thiosulfate deaerated solutions at the open circuit potential and room temperature. Stable pits nucleated near Ti-rich inclusions and increased in diameter after a certain period of time. Prepassivation treatment can inhibit this mechanism of pitting corrosion.