Anodic Dissolution of Armco Iron in a Mixed Kinetics Mode: Effect of Macroscopic Heterogeneity

Abstract

Dependence of the current density of dissolution of an RDE made of Armco iron on the solution agitation rate is studied. Experiments are performed in weakly acid nitrate solutions of pH 1 containing 1–3 M of nitrate ions, at different charges passed, in a prepassive region. On the basis of these dependences, the effect the macroscopic heterogeneity (accumulation of passive phase of iron carbide Fe₃C during dissolution) exerts on the dissolution is analyzed. The anomalous behavior of the dissolution rate (which increases with decreasing rotation rate ω of RDE in a certain ω interval) is due to transition from a mixed diffusion–kinetic control of dissolution rate at a macroscopically heterogeneous electrode to a diffusion control, when the salt film that forms during dissolution screens the heterogeneity (passive areas). Heterogeneity parameters under mixed control may quantitatively be analyzed at high ω, if at low ω one observes a diffusion dissolution connected with a complete surface coverage by the formed salt film.