Air-formed Oxide Film Research Articles

Significance of the Redox Potential in the Inhibition of the Corrosion of Iron by Non-Oxidising Inhibitors in the pH Range 5–13

Abstract Previous work has indicated that during the inhibition of the corrosion of iron, in the pH range 5–13, the air-formed oxide film was stabilised either by the formation of material of similar composition, or by local deposits of insoluble ferric compounds.The effect of three corrosive and six inhibitive ions on the redox potential of the Fe3+/Fe2+ system has been examined and it is concluded that an essential condition for inhibition is that the redox potential must be sufficiently low for ferrous ions to be oxidised rapidly by the oxygen in the solution.

Significance of the Redox Potential in the Inhibition of the Corrosion of Iron by Non-Oxidising Inhibitors in the pH Range 5–13

Previous work has indicated that during the inhibition of the corrosion of iron, in the pH range 5–13, the air-formed oxide film was stabilised either by the formation of material of similar composition, or by local deposits of insoluble ferric compounds.The effect of three corrosive and six inhibitive ions on the redox potential of the Fe3+/Fe2+ system has been examined and it is concluded that an essential condition for inhibition is that the redox potential must be sufficiently low for ferrous ions to be oxidised rapidly by the oxygen in the solution.

The electrochemical behaviour of 18Cr-8Ni stainless steel in hot concentrated aqueous magnesium chloride solution

The electrochemical behaviour of 18-8 stainless steel in concentrated aqueous magnesium chloride solution (bp 154°C) at 140°C has been examined. “Active” anodic dissolution of material carrying the original air-formed oxide film begins below −250 mV(she). As the potential is raised, passivation slowly sets in, possibly through the selective dissolution of iron and chromium. Active dissolution begins again at −170 mV(she) and increases up to ca. −130 mV, when passivation by oxide-film formation begins; this is substantially complete between −120 and −110 mV(she). Breakdown of the “oxide” passivity by chloride ion begins at −110 mV(she) and major pitting with large anodic c.d. is observed at ca. −80 mV(she) and above. These values, obtained by slow potentiodynamic scan (2·5 mV/min) in the positive direction, are confirmed by potentiostatic current/time observations. The very large variation of anodic dissolution rate over a very small potential range is noted as being significant in the mechanism of stress-corrosion crack propagation in these alloys exposed to chloride-containing environments.

Nature of the Oxide Film Present on Iron after Inhibition in 0·05 M Chromate Solutions

Chemical and microprobe analyses have been used to study the composition of the oxide films formed on iron by 0·05 M potassium chromate, pH 4–8.Chemical analyses have shown that the air-formed oxide film was thinned and evenly reinforced with a normal iron chromium spinel having a composition in the range Fe2+ (Fe3+0·5 Cr3+1·5)O4 — Fe3+ Cr3+ O3 Microprobe analyses indicated enrichment of chromium at scratch lines, but the effect was small compared with the overall thickening of the films.

Inhibition of the Corrosion of Iron by Benzoate and Acetate Ions

AbstractThe uptake of benzoate and acetate ions from aqueous solutions into the air-formed oxide film on mild steel has been studied by means of radioactive tracers. The extent of uptake is found to depend on solution pH in a regular manner, consistent with the concept of repair of defects in the air-formed oxide by insoluble material, the composition of which depends on the level of anodic acidity. Direct evidence for the formation of discrete, microscopic regions of intense uptake of the inhibitive anions is provided by high resolution autoradiography.

Mechanism of Inhibition of Corrosion of Mild Steel in Aqueous Solutions of Sodium Azelate: I. Effect of solutions of sodium azelate on the air-formed oxide film on mild steel

AbstractChanges in the thickness and ferrous ion permeability of the air-formed oxide film on mild steel, immersed in solutions of sodium azelate, have been studied as functions of immersion time and solution pH. In neutral solutions, characteristic thickening of the air-formed oxide film takes place, whilst at lower pH values slight, but significant, film breakdown occurs during the first twenty-four hours of immersion with no detectable change thereafter. The role of azelate ions in giving rise to these different effects will be considered in Part II.

On the mechanism of crevice corrosion of stainless Cr steels

The mechanism of crevice corrosion of stainless Cr steels in neutral 3%NaCl solution has been studied in a cell constructed for this purpose. Immediately after the start of an experiment, both pH and potential of the steel surface increase as a result of alkali formation (O 2 reduction) during corrosion of the steel in the passive state and thickening of the air-formed oxide film. After the oxygen available within the crevice has been consumed, the cathode process is concentrated to the outer surface whereas on the crevice surface only metal dissolution takes place. By hydrolysis of metal ions (Fe 2+ and Cr 3+) the crevice solution is now acidified. After having passed a maximum value of about 10, the pH in the crevice is lowered to a steady state value of about 4. The crevice surface is first activated locally (pitting) and finally in its entire extent and e crev. and e out. are both lowered. At the anode (crevice) surface, the Flade potential is raised whereas it is lowered on the outer surface where the pH is raised due to the cathode reaction. The crevice must therefore be active and the outer surface passive even if their potential difference is quite small (150 mV for 13%Cr, 50 mV for 17%Cr).

An electrochemical investigation of fretting corrosion in stainless steel

Fretting the surface of a 18−Cr 8−Ni stainless steel electrode in 1% NaCl results in an increased corrosion current. In the cathodic region the increase is attributed to the stirring effect in the electrolyte. At low values of pH the greater rise in current is due to plastic deformation of the surface. In the region of the corrosion potential, the air-formed oxide film is disrupted by the fretting and does not reform. The major part of the current increase is attributed to plastic deformation of the metal surface resulting in the emergence of active slip bands.

Uptake of Phosphate Ions by Oxide Films on Mild Steel

phosphate ions were foud by radiotracer methods to be firmly bound by oxide films on mild steel after immersion in 0·1 M sodium phosphate over the pH range 7–13. Under inhibitive conditions, autoradiography showed.that the major part of the phosphate uptake was distributed uniformly over the surface, but localy increased uptake occurred to a variable extent at points and areas of the surface. At pH 8, approximately a monolayer of phosphate ions was taken up initially by the oxide. Thereafter the phosphate ion uptake increased linearly with the logarithm of time of immersion, this uptake being decreased by icreasing time of exposure to air before immersion. Increasing the pH decreased the amount of imtialy uptake of phosphate ions. The results are consistent with ion exchange of phosphate ions initially with the surfce layer of the air-formed oxide film and continuing during the thickening of the oxide film in the solution.

The corrosion of aluminium in solutions of sodium fluoride and sodium chloride

The corrosion of 99·997% aluminium by 0·1 N-sodium fluoride and chloride has been examined in the presence of air over the pH range 2—13. When a solution of either salt was saturated with aluminium oxide, corrosion was arrested.Under anaerobic conditions, 0·1 N-sodium fluoride attacked the air-formed oxide film and corrosion was rapid, in contrast no corrosion was observed in 0·1 N-sodium chloride solution.

The breakdown of the air-formed oxide film on iron upon immersion in solutions of pH 6–13

The breakdownof the air-formed oxide film on iron is greater upon immersion in corrosive than inhibitive solutions; it is retarded by dissolved oxygen and in some solutions marginally affected by the age, or thickness,of the film. The breakdown is not due to direct solution, as with aluminium, but to reductive dissolution, a two-stage process, in which ferric ions are first reduced to ferrous ions, which then pass into solution as hydrated cations. It follows that inhibition will occur if either process is impeded.

The oxidation of iron at room temperature

The thickness of the air-formed oxide film on pure iron has been measured by the electrometric method over the period of a year. The growth followed the inverse, rather than the direct, logarithmic relationship. The degradation products of hydrocarbon grease were found to affect appreciably the film thickness. We conclude that film growth is controlled by the movement of material and not by movement of electrons.

839. An electrometric method for measuring the thickness of the air-formed oxide film on pure iron and mild steel

The first page of this article is displayed as the abstract.

142. The thickness of air-formed oxide films on iron

142. The thickness of air-formed oxide films on iron

The Primary Oxide Film on Iron

The identity and structure of the invisible, air-formed oxide film on iron has never been determined although the practical importance of a knowledge of its properties in the study of corrosion has been demonstrated. Electron diffraction experiments are now reported which show this film to be structurally identical with Fe3O4. In addition, it is found that the oxide has a definite orientation relationship with the underlying iron, viz., the [√210] directions in the oxide crystals are parallel to the [111] directions in the iron. Freshly abraded surfaces of electrolytic and ingot iron were investigated. Evaporated films of electrolytic iron were prepared in the diffraction camera and pictures taken before and after exposure to air show that Fe3O4 forms rapidly in dry air at room temperature. An air pressure of 10—3 mm for a few minutes gave a measurable effect. Exposure to air at atmospheric pressure for periods longer than a few minutes does not greatly strengthen the oxide pattern. The factors affecting the sensitivity of this method of examining thin films on metals are discussed.