An electrochemical impedance spectroscopic study of the passive state on Alloy-22

The application of electrochemical impedance spectroscopy (EIS) for interrogating the passive states on metals and alloys is outlined, with particular reference to Alloy-22 (nominally 22Cr 13Mo 3.6Fe 2.8W 1.0Co 0.1V balNi, w/o) in saturated NaCl (6.2 M) + 0.001 M HCl brine (pH 3) at 80 °C. The passive film is found to be an n-type semi conductor due to the predominance of oxygen vacancies and/or metal interstitials in the barrier layer. At higher potentials, starting just prior to the transition into the transpassive state, the barrier layer becomes p-type in character, due to the oxidative ejection of cations leading to the generation of an excess of cation vacancies. The steady-state barrier layer thickness and passive current as a function of voltage, as derived from the point defect model (PDM), are found to be consistent with the electronic character in both the passive and transpassive states. EIS data for the passive state are interpreted in terms of the PDM by optimizing the model on the data using commercial optimization program. Importantly, the impedance data across the entire passive range can be accounted for by a single set of parameters (standard rate constants, transfer coefficients, etc.). Finally, in a sensitivity analysis, the impedance of the passive state is calculated as a function of the standard rate constants for the defect generation and annihilation reactions at the interfaces and it is found that the rate of barrier layer dissolution has a major impact on the interphasial impedance and hence corrosion resistance.