Passive Films on Iron: The Mechanism of Breakdown in Chloride Containing Solutions

Abstract

An examination has been made of the mechanism of breakdown of passive films on iron in borate buffer solution false(normalpH=8.4false) caused by chloride ions. Various electrochemical kinetic criteria were measured. XPS, SIMS, and ISS studies were made of the systems used in the electrochemical work. The rate of breakdown was found to be proportional to cnormalCl− and cH+ and exponentially dependent on the electrode breakdown potential and field drop in the oxide film. XPS data showed that when chloride ions caused breakdown, the Onormaltotal/normalFe and H2O/normalFe ratios changed from 2 to 1.5 and 0.5 to 0.1, respectively. SIMS data revealed that heating passive films up to 200°C drove out water from the films and that chloride ions penetrated the whole film thickness on breakdown. ISS data indicated that on changing from a passive to a depassivated film, the O/normalFe ratio changed from 2.07 to 1.5. Discussion of the electrochemical kinetic data shows that it is inconsistent with adsorption‐displacement models, pore models, and chemico‐mechanical models, but is not inconsistent with ion‐exchange processes, point‐defect models, and hydrated polymeric oxide models. Confrontation of the spectroscopic data with the expectations of the latter three models shows some points of agreement with all these models, but the data taken together is most consistent with the hydrated polymeric oxide model.

Analysis of the composition of the passive film on iron under pitting conditions in 0.05 M NaOH/NaCl using Raman microscopy in situ with anodic polarisation and MCR‐ALS

“…The presence of water in the passive film was found to result in an increase in the Fe–O distance, suggesting that the H atoms introduced can lead to increased structural flexibility by forming M–OH bonds in addition to M–O bonds, which would promote a more glass‐like structure for the film . The amorphous nature of the film afforded by the bound water prevents Fe 2+ from diffusing from the metal base to hydration sites at the oxide/solution interface …”

“…The major difference in the mechanisms of the stable pitting proposed by the various pitting theories concerns whether the aggressive anion is incorporated into the passive film or not. [4] Different models have been proposed [4] for the mechanism of pit formation in passive films on iron under the influence of chloride ions, the leading ones being: (1) the ion exchange model, in which Cl À ions enter the film via cation vacancies or by ion exchange of Cl À ions for O 2À ions; (2) the point defect model, in which cation diffusion occurs from the metal/film to the film/solution interface, resulting in formation of metal vacancies at the metal/film interface; when this occurs at a sufficiently high rate, voids form which grow and lead to local collapse of the passive film, resulting in faster dissolution than the rest of the film and growth of pits; (3) the adsorption-displacement models, in which breakdown results from dissolution of iron chloride complexes formed from simultaneous adsorption of Cl À ions and displacement of oxygen from the monolayer said to constitute the passive layer; (4) the chemico-mechanical models, in which Cl À ions adsorbed on the passive film reduce the interfacial tension at the film/solution interface, resulting in the formation of cracks or flaws from electrostriction pressure effects arising from repulsive forces between the adsorbed Cl À ions; (5) the pore models, in which Cl À ions pass through a porous passive film and form complexes with iron at the metal/film interface, which then diffuse outward through pores to the film/solution interface, resulting in pits at the bottom of the pores; and (6) the hydrated polymeric oxide model, in which Cl À ions are adsorbed onto the layer and displace the bound water molecules, then form chloridecontaining iron complexes that diffuse into the bulk solution to form soluble complexes.…”

The Effect of Chloride Ions on the Ambient Electrochemistry of Pyrite Oxidation in Acid Media

“…71,72 The chloride ion is a relatively strong Lewis base and so displaces hydroxyl ions or water molecules, forming chloride-containing iron complexes, which then dissolve into solution. The adsorption of chloride ions onto the surface also appears to inhibit pyrite oxidation, but not that of thiosulfate ions.…”

Corrosion Performance of Carbon Steel in Simulated Pore Solution in the Presence of Micelles

“…11͒ indicate that the micelles can also alter the chemical compositions of the product layers: the product layers in the micelles-containing specimens CN and CNC present mainly protective ␣-Fe 2 O 3 and/or Fe 3 O 4 , leading to an impeded Cl − penetration and consequently corrosion delay; in contrast, the product layers in the micelles-free specimen C and CC were more hydrated, exhibiting mainly FeOOH, FeO, and FeCO 3 , which are prone to chloride attack. 87,88 Correlation product layers composition/EIS parameters.-The XPS results ͑supported by ESEM and EDX͒ indicate that different types of iron oxide/hydroxide layers were formed on the steel surface after treatment in the test solutions with and without the presence of micelles. Further, a Ca-rich outer layer was formed ͑Figs.…”