Iron oxides passive film layer

Figure 5 shows the relationship between the passive film thickness of an iron electrode and the electrode potential in an anodic phosphate solution and a neutral borate solution.6,9 A passive film on an iron electrode in acidic solution is made up of an oxide barrier layer that increases its thickness approximately linearly with increasing electrode potential, whereas in a neutral solution, there is a precipitated hydroxide layer with a constant thickness outside the oxide barrier layer. 

Re-immersion of the ordered oxide films into HC104 or HC1 solutions led to the disappearance of the LEED beams of Type 2. That is, the CrO phase was not stable in acid solutions. This is an indication that acidic electrolytes, particularly HC1, attacked the passive layer at the comparatively thin CrO regions, replacing or covering those regions with a thin, hydrated amorphous iron oxide layer. 

Corrosion and Passivity. The inhibition of dissolution is important m the corrosion of metals and building materials. Passivity is imparted to many metals by overlying oxides, the so-called passive films the inhibition of the resolution of these passive layers protects the underlying material. Figure 13 eves a schematic model of the hydrated passive film on iron. 

X 10 torr seconds. Auger electron spectroscopy and 16) has shown that a layer which can be represented as Fe O OH protects iron from further attack. Direct evidence of hydrogen being present in the passive film formed in solution was provided by mass spectrometry (30). Electron diffraction (2, 29) has shown tHat the protective surface layers wHTch form on iron which has been air oxidized or passivated in solution are surprisingly similar. The structures are based on the cubic close packed oxygen lattice of y Fe O OH, or Fe304 ... 

Moreover, gradients of stoichiometry limit the accuracy of thickness determinations that often only refer to parts of the passive film. Many oxide films show an increase of oxidation state from metal to the electrolyte, for example, on Cu [13]. In case of passive iron, many traditional techniques only evaluate the thickness of the outer Fe203-film, but not that of the inner Fc3 04-layer. While Vetter [28] described the film by a duplex model, Wagner [14] presented a model with continuous change of stoichiometry. 

Monel 400, a nickel alloy containing 66.5% nickel, 31.5% copper and 1.25% iron, has a marked tendency for the initiation of pitting in chloride-containing environments where the passive film can be disturbed. Under stagnant conditions chlorides penetrate the passive film at weak points and cause pitting attack. Sulfides can cause either a modification of the oxide layer, as described for copper, or breakdown of the oxide film of nickel alloys. Pit initiation and propagation depend on depth of exposure, temperature and presence of surface deposits. Little and coworkers [30] reported selective dealloying of nickel in Monel 400 in the presence of SRB from an estuarine environment. 

Other microorganisms promote corrosion of iron and its alloys through dissimilatory iron reduction reactions that lead to the dissolution of protective iron oxide/hy dr oxide films on the metal surface. Passive layers are either lost or replaced by less stable films that allow further corrosion. Obuekwe and coworkers [60] evaluated corrosion of mild steel under conditions of simultaneous production of ferrous and sulfide ions by an iron-reducing bacterium. They reported extensive pitting when both processes were active. When only sulfide was produced, initial corrosion... 

A point often neglected is the handling procedure between preparation of the surface and immersing the specimen in the test medium. For instance, oxide film formation on oxide-passive materials and or tarnishing layers on copper or iron alloys formed at this stage can influence the electrochenucal behaviour considerably these chemical changes on the surface depend on such factors as temperature, humidity and time [5]. 

The oxide layer that forms on the iron used in remediation applications, however, may differ from models developed to describe passive films because the iron used is an impure, recycled material that is manufactured primarily for use as a conditioner in building materials. Spectroscopic analyses of these materials show that they consist of a complex mixture of crystalline phases (25-27). Recently, Raman spectra obtained on iron particles from Master Builder Inc. (Cleveland, OH) and Peerless Metal Powder Abrasives (Detroit, MI) revealed maghemite, magnetite, and hematite (a-Fe203) on samples analyzed as received (27). 

Figure 10.15 Semiconductor model for the passive film of iron, with a band structure at the flat band potential (0.4 at pH = 0) and at 1.2 V jjg. The band gap is 1.6 V. With anodic polarization a potential gradient across the oxide layer A(p is buUt up, which is the origin of the transport of iron ions through the oxide.

The fields marked Fe203 and Fe304 are sometimes labeled passivation on the assumption that iron reacts in these regions to form protective oxide films. This is correct only insofar as passivity is accounted for by a diffusion-barrier oxide layer (Definition 2, Section 6.1). Actually, the Flade potential, above which passivity of iron is observed in media such as sulfuric or nitric acid, parallels line a and b, intersecting 0.6 V at pH = 0. For this reason, the passive film (Definition 1, Section 6.1) may not be any of the equilibrium stoichiometric iron oxides, as is further discussed in Chapter 6. 

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