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

Abstract

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 µm after 50 h of OCP immersion. A prepassivation treatment inhibited this mechanism of pitting corrosion.