SPECIFICITY OF THE PASSIVE STATE OF IRON IN SLIGHTLY ALKALINE AQUEOUS MEDIA WITH DIFFERENT ANIONIC COMPOSITION

Abstract

A comparative analysis of the effect of anionic composition and hydrodynamic conditions on the passivation of iron in borate buffer (pH 8.4) and hydrocarbonate (pH 8.3 с(NaHCO₃) = 0.075 М) aqueous solutions was carried out. Experiments were performed on a stationary electrode (iron / borate buffer) and on a rotating disk electrode at ω = 350 rpm (iron/hydrocarbonate electrolyte). In both cases, a classical electrolytic three-electrode cell with free air access was used at a temperature of 20 ± 2 °C. To obtain reliable results, a complex of physicochemical methods was used: electrochemical methods with varying the scan speed of the potential, optical and scanning electron microscopy, X-ray microanalysis. In the media studied, the nature of the anions is very active, despite the fact that the passive state of iron is associated with the formation of oxide-hydroxide films on its surface. It is found that the formation of films occurs with diffusion limitations, and is complicated by adsorption on the surface intermediates, presumably borates or carbonates of iron. In addition boron-containing complexes are in the layer of the oxide-hydroxide (in the system iron / borate buffer), and hydrocarbonate complexes are present on films surface (in the system iron / hydrocarbonate electrolyte). It is established that the change in hydrodynamic conditions does not have such a significant effect on the processes of iron passivation in the studied media. Schemes for the formation of a passive layer on iron in borate and hydrocarbonate aerated solutions are presented, based on generalized data on the composition of passive films in the media studied. The proposed schemes include the formation and transformation of oxide-hydroxide and salt films on the metal surface.