Passivation and Depassivation of Alloy 22 in Acidic Chloride Solutions

Abstract

Alloy 22 (N06022) is a Ni–Cr–Mo alloy that offers an outstanding corrosion resistance in a wide variety of highly corrosive environments. In the present work, the general corrosion of alloy 22 in the passive and active states and the transition from passive and active states were studied in acidic chloride solutions at . Electrochemical studies, including electrochemical impedance spectroscopy and polarization tests, were performed using mill-annealed and thermally aged (10 h at ) alloy 22 specimens. The depassivation pH was 1.7 in deaerated conditions and in aerated conditions. The transition from passive to active states was characterized by a 3 orders of magnitude increase in the corrosion rate (CR) and a significant increase in the interfacial capacity. The CRs obtained via electrochemical tests for mill-annealed (MA) and thermally aged alloy 22 were comparable in all the tested conditions used in the present work. Intergranular attack was observed in thermally aged alloy 22 corroding in the active state due to the presence of precipitates and adjacent depleted zones of the protective alloying elements.