Passivity Breakdown of Ni-Cr Alloys: From Anions Interactions to Stable Pits Growth

Abstract

The susceptibility of Ni-Cr alloys to pitting corrosion is known to be closely related to the Cr content in the alloy. However, the role of Cr in the different stages of the pitting process is not totally understood.To determine the role Cr in the different stages of the pitting process, the passive films formed on different nickel-base alloys (with 16, 20, 24 and 28 wt% Cr) in a sulfate solution were characterized using electrochemical techniques and X-ray photoelectron spectroscopy. Electrochemical impedance spectroscopy (EIS) analyses are discussed in terms of the migration of point defects. Metastable and stable pitting was then studied using polarization curves in a 0.1 M NaCl solution. The alloys studied exhibit a wide range of susceptibility. The 16 wt% Cr alloy was the most susceptible to metastable and stable pitting while the 28 wt% Cr alloy was almost unaffected by both phenomena. By studying the chemical composition of the passive film, it was shown that nickel incorporated into the passive film was responsible for deleterious interactions between the surface and chloride ions leading to metastable pit events. Migration resistance is a good indicator of the passivity breakdown process based on the EIS results. Low migration resistance values imply the presence of a vacancy condensation process while high values imply a thinning process. As a result, a step by step mechanism is proposed showing chloride ion interaction with passive films at stable pit initiation.