Adsorption and Absorption of Chloride Ions on Passive Iron Systems

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 .