Passivation and Depassivation of Alloy 2

Alloy 22 is a Ni–Cr–Mo alloy known for its exceptional corrosion resistance in harsh environments. This study aims to investigate the general corrosion behavior of Alloy 22 in both passive and active states while assessing the transition between these states in acidic chloride solutions at elevated temperatures (90 °C). Employing electrochemical techniques, including impedance spectroscopy and polarization tests, the research compared mill-annealed and thermally aged Alloy 22 specimens. Findings indicated that the depassivation pH was notably low at 1.7 under deaerated conditions and decreased further to 0.3 in aerated environments. The transition from passive to active states was marked by a dramatic increase in the corrosion rate by three orders of magnitude, alongside a significant rise in interfacial capacitance. Moreover, corrosion rates for mill-annealed and thermally aged samples were found to be comparable across all tested conditions. Intergranular attack was notably observed in the thermally aged Alloy 22 undergoing corrosion in the active state, attributed to the presence of precipitates and adjacent zones depleted of protective alloying elements.