Dissolution and passivation of iron in acetonitrile and acetonitrile–water mixtures

Abstract

The dissolution and passivation of iron in neutral acetonitrile–water mixtures have been studied under potentiodynamic and potentiostatic conditions. In “dry” acetonitrile (water content <0.02% or 200 ppm), passivation, due to an air-formed film, is maintained for a wide range of potentials up to 0.50 V, well above the corrosion potential E corr≈−0.4 V. Transpassive dissolution at higher potentials is normally controlled by interfacial reactions rather than diffusion in the solution. Addition of a relatively small amount of water (0.5% or ∼0.28 M) to acetonitrile initiates active dissolution on iron surfaces damaged by previous transpassive dissolution. At a level of 2% water (∼1.1 M) active dissolution is initiated on undamaged surfaces and proceeds relatively slowly under control by interfacial reactions. Transpassive dissolution at similar potentials meanwhile becomes diffusion-controlled. As water content is increased further (e.g., to 6% or ∼3.3 M) the electrochemistry of iron becomes progressively similar to that in aqueous solutions, with both active dissolution and passivation being enhanced. Passivation is promoted by addition of hydrogen peroxide as a passivator, but the dynamic balance between the two processes can, as expected, be shifted to dissolution by increasing solution acidity. The similarities and differences in acetonitrile, water and their mixtures are discussed in terms of the relative reactivity and concentration of the two solvents. The profound effects of water on the kinetics of iron dissolution and passivation are attributed to the dominant reactivity associated with acidic hydrogen. Based on the potentiostatic and potentiodynamic features, the mechanism of phase change during dissolution is described in terms of a model of a shifting oxide film formed through two-dimensional nucleation and growth.