Magnetite mechanism

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

In general there does not appear to be any direct correlation between the rate of the chemical dissolution of oxides and the rate of scale removal, although most work on oxide dissolution has concentrated on magnetite. For example, Gorichev and co-workers have studied the kinetics and mechanisms of dissolution of magnetite in acids and found that it is faster in phosphoric acid than in hydrochloric, whereas scale removal is slower. Also, ferrous ions accelerate the dissolution of magnetite in sulphuric, phosphoric and hydrochloric acid , whereas the scale removal rate is reduced by the addition of ferrous ions. These observations appear to emphasise the importance of reductive dissolution and undermining in scale removal, as opposed to direct chemical dissolution. 

Where caustic deposits occur, the resultant corrosion of steel by caustic gouging or stress corrosion cracking (SCC) mechanisms produces particulate iron oxides of hematite and magnetite. It is common to see white rings of deposited sodium hydroxide around the area of iron oxide formation. 

This view of the corrosion process is, however, more often than not too simplified an explanation. First, even when general corrosion take place (as in an idle or wet lay-up boiler), the reaction mechanisms tend to occur at many localized points on the boiler metal surface, typically where cracks and other imperfections in the magnetite film exist. Second, such processes almost always lead to derived forms of localized corrosion, which often result in severe metal wastage through the formation of deep pits. 

From Fig.2 (a), A solid phase transformation fiom hematite, Fc203 to magnetite, Fe304, is observed, indicating that the active sites of the catalj are related to Fc304. Suzuki et. al also found that Fe304 plays an important role in the formation of active centers by a redox mechanism [6]. It is also observed that the hematite itself relates to the formation of benzene at the initial periods, but no obvious iron carbide peaks are found on the tested Li-Fe/CNF, formation of which is considered as one of the itsisons for catalyst deactivation [3,6]. 

Li, Z Sun, Q. and Gao, M.Y. (2005) Preparation of water-soluble magnetite nanocrystals from hydrated ferric salts in 2-pyrrolidone mechanism leading to Fe304. Angewandte Chemie International Edition, 44 (1), 123-126. 

For a precipitated iron catalyst, several authors propose that the WGS reaction occurs on an iron oxide (magnetite) surface,1213 and there are also some reports that the FT reaction occurs on a carbide surface.14 There seems to be a general consensus that the FT and WGS reactions occur on different active sites,13 and some strong evidence indicates that iron carbide is active for the FT reaction and that an iron oxide is active for the WGS reaction,15 and this is the process we propose in this report. The most widely accepted mechanism for the FT reaction is surface polymerization on a carbide surface by CH2 insertion.16 The most widely accepted mechanism for the WGS reaction is the direct oxidation of CO with surface 0 (from water dissociation).17 Analysis done on a precipitated iron catalyst using bulk characterization techniques always shows iron oxides and iron carbides, and the question of whether there can be a sensible correlation made between the bulk composition and activity or selectivity is still a contentious issue.18... 

Blesa MA, Marinovich HA, Baumgartner EC, Maroto AJG. 1987. Mechanism of dissolution of magnetite by oxalic acid ferrous iron solutions. Inorganic Chemistry 26 3713-3717. 

Two different pathways of formation are possible (Stanjek, 2000). One route involves aerial oxidation of lithogenic magnetite as suggested for Brazilian Oxisols on basic igneous rocks. The mechanism of this topotactic reaction is described in Chapter 14. These maghemites are usually titaniferous as are the magnetites from which they are derived (see Chap. 15) and almost free from or very low in Al (Allan et al., 1989). Their unit cell size is a function of the residual Fe" and the Ti content. 

The magnetite is considered to form from a ferrihydrite precursor by interaction of this phase with dissolved Fe" ions (Kirschvink Lowenstam, 1979 Lowenstam, 1981 Nesson Lowenstam, 1985). The same mechanism operates for inorganic synthesis at around pH 7 (see chap. 13). Most probably the other iron oxides in the teeth form by a similar mechanism, but under conditions of slightly lower pH and/ or higher redox potential. The separation of these minerals in time and space suggests local variations in growth conditions. 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

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