Adsorption and Absorption of Chloride Ions on Passive Iron Systems

Abstract

The adsorption and absorption of chloride on iron in borate buffer solution have been studied using a radiotracer technique. The two processes are clearly distinguishable by their different time constants, ∼ 1 min for adsorption and ∼ 1h for absorption. The adsorpton of chloride on bare iron and passive layer surfaces follows the Tempkin type of isotherm. The free energy of adsorption of chloride on passive layer is significantly higher than that on bare iron. Partial charge transfer between adsorbed chloride ion and Fe2+ and/or Fe3+ on the passive surface has been proposed to account for this difference. The “saturation” coverage of chloride on passive layer varies with concentration and attains a value of 0.8 at higher concentrations. The absorption of chloride into the passive layer starts at about −0.1 V/NHE, followed by a linear increase with the potential. The steady‐state value of absorption of chloride at the breakdown potential varies with concentration of chloride in the solution and breakdown potential. The maximum value of absorption before breakdown occurs is θ=2.5 .

Chloride in cement

“…It can be concluded that the model is conceptually correct but needs scaling: it attempts to embrace processes occurring across a range of scales from atomistic to macroscopic and it is difficult to see how these mechanisms relate to each other and how they impact on the observed property changes. Jovancicevic et al (1986) conclude that desorption and absorption of chloride on iron are two distinct processes. Adsorption, being potential and concentration dependent, is stronger in the passive layer than on bare iron, which is explained by a charge transfer between chloride (Cl À ) and iron (Fe 2+ and/or Fe 3+ ).…”

“…Jovancicevic et al (1986) classified the existing models for describing the action of chloride on passive layer breakdown into three stages in terms of the mechanisms involved…”

“…This model explains the dependence of film breakdown kinetics on chloride concentration, but does not take into account the film thicknessoften much more than a monolayer -and its possible role on the breakdown kinetics (Jovancicevic et al, 1986).…”

“…Figure 11 shows the linear dependence of absorption on concentration. This relation is attributed to 'mutual repulsion of the anions within the film, with an increase of the saturation value as the potential grows more positive, corresponding to the increasing film thickness' (Jovancicevic et al, 1986).…”

“…Figure 11. Concentration dependence of absorption of chloride (mol/cm 2 ) for different potentials (Jovancicevic et al, 1986).ˆis the surface concentration of adsorbed and/or absorbed species. The parameter Ł on the right-hand vertical scale is the surface coverage, defined as the ratio betweenˆand the maximum surface concentration of adsorbed species (ˆm ax ).…”