Magnetite porous form

Caustic gouging corrosion caused by localized high concentrations of caustic developing within and under porous surface deposits dissolving the protective magnetite film forming ferrite and hypoferrite ions. 

Coarse magnetic corrosion Fe3°4 NOTE Passivation is a form of corrosion, albeit the resulting magnetite is desirable. Thick, porous, bulk water or boiler surfaces deposit. Formed under both high and low pH and other adverse BW conditions. Of no practical passivation benefit. 

Still following the macro-structural hypothesis which we favored at that time, we abondoned the idea of a specific favorable influence of flux promoters and assumed instead that the cause for the success of the magnetite experiment was the compact porous structure of the iron sponge which was formed in the test oven by the reduction of the Swedish ore. An apparent support of this idea was that contrary to the favorable action of the dense iron sponge obtained from magnetite, catalysts of a looser structure such as, e.g., iron asbestos preparations had always been particularly ineffective. 

Maghemite formed by oxidation of magnetite is non porous, whereas that obtained by dehydration of lepidocrocite is meso porous. 

The protective oxide on ferritic steel consists of two layers, which are porous to carbon dioxide. The inner layer consists of crystallites of Cr and Si and the outer layer Fe304 (magnetite) formed by the reaction of metallic iron and C02 and the product CO giving elemental carbon. 

The reduction of oxidic catalyst is generally effected with synthesis gas. The magnetite is converted into a highly porous, high surface area, highly catalytically active form of a-iron. The promoters, with the exception of cobalt, are not reduced [33]. 

The subsequent reduction of the magnetite is of crucial importance to the quality of the catalyst. It is normally carried out with synthesis gas in the pressure reactor of the ammonia plant at not too high pressures (70 to 300 bar, depending on the plant type) and at temperatures between 350 and 400°C, whereupon highly porous a-iron is formed ... 

When steel or iron is exposed to an atmospheric environment, a thin layer of magnetite, Fe304, is formed, covered by a layer of FeOOH. Atmospheric oxygen then penetrates though the almost water-free, porous outer layer of FeOOH and oxidizes the magnetite to hydrated ferric oxide, Fe203, or FeOOH. The presence of Fe " in the electrolyte initiates the precipitation of various corrosion products. The electrochemical mechanism of atmospheric corrosion of iron suggested by Evans is briefly summarized in this chapter [8]. 

Figure 4.4.41. Disaetized form of the transmission line model. e and e, are the potentials in the magnetite and solution phases, respectively. Here i and i, are the currents in the magnetite and solution phases, respectively I and / are the total current and the current flowing across the metal-solution interface and base of the pore, respectively RE and M designate the reference electrode and metal (working electrode) locations, respectively. (Reprinted with permission from J. R. Park and D. D. Macdonald, Impedance Studies of the Growth of Porous Magnetite Films on Carbon Steel in High Temperature Aqueous Systems, Corros. Sci. 23, 295 [1983]. Copyright 1983, Pergamon Journals Ltd.)...

For the reduction process of magnetite a core-and-shell mechanism was proposed and tested in detail (8). Thereafter, a catalyst particle is continuously converted into a porous shell of metallic iron topotactically formed on a nonre-duced core by a propagating reaction front. It was, however, found that this simple concept has to be modified and that the reduction process is, in fact, rather complex in the presence of the promoters (9). In particular, it was concluded that the resulting ammonia-iron consists of Fe particles with a preferred (111) texture determined by the preferential migration of iron ions separated from each other by particles of the structural promoter. 

XRD results for the geopolymer samples are shown in Fig. 2. For the GP50-7 sample (Fig. 2(a)), the relative proportions of magnetite and hematite remained similar to that present in the initial precursor powder. This indicated that the crystalline phases were stable under the alkali conditions after being cured at S0°C for 24 h. However, a broad peak near 28° 20 was also formed and is similar to what has been observed in aluminosilicate systems. Because this sample was completely water soluble, it is believed that this broad peak was due to the silica gel phase formed. Heating of sample GP50-7 to > 100°C resulted in the further growth of crystalline phases, and the formation of a porous gel. 

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