Passive and Electrochemical Response of Alloy 22 in Alkaline Solutions

Among Ni-base alloys, Alloy 22 is one the most important alloys because of its corrosion response under both oxidizing and reducing solutions. In this paper, passive and electrochemical response of Alloy 22 was investigated in alkaline solutions with focus on the variation of passive film stability with KOH concentrations. The resistance, capacitance and thickness of the passive film were measured by electrochemical impedance spectroscopy (EIS). Moreover, the semiconducting properties were evaluated from Mott–Schottky (M–S) tests. M–S plots depicted that this Ni-base alloy in KOH solutions behave as p-type semiconductor. Also, these tests indicated that defect (acceptor) density is in the range 1022 cm-3 and increased with solution concentration. In conclusion, all electrochemical results revealed that dilute KOH solutions offer better conditions of the passive film formation on this Ni-base alloy than concentrated KOH solutions. Finally, scanning electron microscope micrographs revealed that Alloy 22 is well passivated even in concentrated KOH solutions.