Dissolution and passivation of iron in acetonitrile and acetonitrile-watermixtures

The dissolution and passivation of iron in neutral acetonitrile-water mixtu res have been studied under potentiodynamic and potentiostatic conditions. In "dry" acetonitrile (water content <0.02% or 200 ppm), passivation, due t o an air-formed film, is maintained for a wide range of potentials up to 0. 50 V, well above the corrosion potential E-corr approximate to -0.4 V. Tran spassive dissolution at higher potentials is normally controlled by interfa cial reactions rather than diffusion in the solution. Addition of a relativ ely small amount of water (0.5% or similar to 0.28 M) to acetonitrile initi ates active dissolution on iron surfaces damaged by previous transpassive d issolution. At a level of 2% water (similar to 1.1 M) active dissolution is initiated on undamaged surfaces and proceeds relatively slowly under contr ol by interfacial reactions. Transpassive dissolution at similar potentials meanwhile becomes diffusion-controlled As water content is increased furth er (e.g., to 6% or similar to 3.3 M) the electrochemistry of iron becomes p rogressively similar to that in aqueous solutions, with both active dissolu tion 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 soluti on acidity. The similarities and differences in acetonitrile, water and the ir mixtures are discussed in terms of the relative reactivity and concentra tion 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 potentiody namic features, the mechanism of phase change during dissolution is describ ed in terms of a model of a shifting oxide film formed through two-dimensio nal nucleation and growth. (C) 1999 Elsevier Science Ltd. All rights reserv ed.