Electrochemical characterisation and modelling of passive films on Ni- and Fe-based alloys: Dissertation

The behaviour of oxide films formed on Ni- and Fe-based alloys in different environments has been studied electrochemically. The aim was to study the effect of pH, temperature and Cr content of the alloy on the protectiveness of the oxide film using a wide combination of electrochemical techniques: conventional linear sweep cyclic voltammetry, rotating ring-disc voltammetry, electrochemical impedance spectroscopy (EIS), contact electric resistance technique (CER) and contact electric impedance technique (CEI). The other goal was to develop modelling tools to describe and predict the oxide film behaviour in different conditions. Increasing pH has been found to decrease oxidation rates in both the passive and transpassive regions. Increasing pH as well as increasing temperature shifts the passive region in the negative direction on the potential scale. A higher amount of Cr in the alloy leads to a more passive oxide film on the metal surface both at low and high temperatures. On the other hand, transpassive dissolution takes place at lower potentials and its rate increases with higher Cr content of the alloy. The potential region of transpassive oxidation and secondary passivation increases and the effect of Cr on the electrochemical behaviour especially on Ni-Cr alloys decreases at high temperatures. The behaviour of oxide films in the passive state in different environments was simulated using the Mixed Conduction Model (MCM). Using this model the diffusion coefficients of current carriers and reaction rate constants at room temperature and profiles of resistances against ionic transport in the oxide films at 200°C were estimated. Also the transpassive dissolution of Ni-Cr alloys at room temperature was studied and a kinetic model proposed to determine quantitatively the reaction rates in the transpassive region. The model describes the oxidation and dissolution reactions of metal cations at the film/solution interface. The proposed model can be used to estimate steady-state current densities as well as dependencies of surface fractions of dissolving species on potential.