Kinetics of chloride ion adsorption on stainless alloys by in situ contact electric resistance technique

As the primary reason for pitting of stainless alloys, chloride adsorption is not adequately studied, e.g. kinetic investigations of chloride adsorption are actually absent. We discuss and partly reconsider some well-known facts (e.g. halides order: Cl− > Br− > I−), disputed points (chloride penetration in passive film), and still unknown aspects of chloride adsorption. For the first time, we report kinetic studies of chloride adsorption on stainless alloys by in situ contact electric resistance technique. The peak-like character of kinetic curves has been found for all studied stainless alloys, but not for pure iron and nickel. This has been considered as a sequence of the substantial charge transfer during chloride adsorption. Opposite to typical d metals, stainless materials are alloys of early and late transition metals having unfilled d-bands with increased number of d-electron vacancies. Such electronic structure is favorable for adsorption of electron donating adsorbates like halide ions. Experimental data of this work are more compatible with possibility of chloride penetration into the passive films on stainless alloys that also might involve a transformation of primary oxy-hydroxide films into oxy-chloride films.