Oxidation of iron

Thus an oxidising agent is identified as an electron acceptor and the oxidation of iron(II) by chlorine can be written as two "half equations, viz. 

Consider also the oxidation of iron(II) ions by dichromate(Vl)... 

The best known oxoanion of iron is the ferrate(VI) prepared by oxidizing a suspension of hydrous iron(III) oxide in concentrated alkah with potassium hypochlorite or by anodic oxidation of iron in concentrated alkah. Crystals of potassium ferrate [13718-66-6], K FeO, are deep purple, orthorhombic, and contain discrete tetrahedral [FeOJ anions. Barium ferrate [13773-23A] can be precipitated from solutions of soluble ferrate salts. 

The treatment units used for color removal are the same as those used for turbidity removal. However, the pH must be increased prior to filtration so that the metal hydroxides are removed by the filters. At low pH values, metal ions or their soluble complexes readily pass through the filters and form insoluble species in storage tanks and in the distribution system. For iron salts, it is important that the pH be greater than 6 as the oxidation of iron(II) to iron(III) occurs rapidly above this pH in the presence of dissolved oxygen or other strong oxidants (18). 

Purification actually starts with the precipitation of the hydrous oxides of iron, alumina, siUca, and tin which carry along arsenic, antimony, and, to some extent, germanium. Lead and silver sulfates coprecipitate but lead is reintroduced into the electrolyte by anode corrosion, as is aluminum from the cathodes and copper by bus-bar corrosion. 

I. G. Wright, Oxidation of Iron-, Nickel-, and Cobalt-Base Alloys, MCIC Report 72-07, BatteUe-Columbus Laboratories, BatteUe Memorial Institute, 1972. 

Impurities ate elirninated in fire refining in the foUowing sequence slag, that is, oxides of iron, magnesium, aluminum, and sihcon fluxing, that is, arsenic and antimony and vapors, that is, sulfur (as SO2), cadmium, and zinc. 

Naturally occurring colored minerals that contain oxides of iron are known by such names as ochre [1309-37-1], umber [12713-03-0], sienna [1309-37-1], etc. These show greater variation in color and tinting power than the synthetic equivalents, and the nature and amount of impurities in the national products is also variable. Most of the pigments identified in Table 9 are, therefore, manufactured synthetically. They are primarily used in skin-makeup products and in eye-area colorants. 

The difference in stability between FeO and NiO is not as large as that between iron and copper oxides, and so the preferential oxidation of iron is not so marked in pentlandite. Furthermore, the nickel and iron monoxides form a continuous series of solid solutions, and so a small amount of nickel is always removed into die oxide phase (Table 9.2). 

The result is that the oxidation of iron in aerated water (rusting) goes on at a rate which is millions of times faster than that in dry air. Because of the importance of (c), wet oxidation is a particular problem with metals. 

Formation of rust inside a elosed steel tank (oxygen is removed from the atmosphere by the oxidation of iron). 

At present, chlorine dioxide is primarily used as a bleaching chemical in the pulp and paper industry. It is also used in large amounts by the textile industry, as well as for the aching of flour, fats, oils, and waxes. In treating drinking water, chlorine dioxide is used in this country for taste and odor control, decolorization, disinfection, provision of residual disinfectant in water distribution systems, and oxidation of iron, manganese, and organics. The principal use of chlorine dioxide in the United States is for the removal of taste and odor caused by phenolic compounds in raw water supplies. 

Several authors " have suggested that in some systems voids, far from acting as diffusion barriers, may actually assist transport by permitting a dissociation-recombination mechanism. The presence of elements which could give rise to carrier molecules, e.g. carbon or hydrogen , and thus to the behaviour illustrated in Fig. 1.87, would particularly favour this mechanism. The oxidant side of the pore functions as a sink for vacancies diffusing from the oxide/gas interface by a reaction which yields gas of sufficiently high chemical potential to oxidise the metal side of the pore. The vacancies created by this reaction then travel to the metal/oxide interface where they are accommodated by plastic flow, or they may form additional voids by the mechanisms already discussed. The reaction sequence at the various interfaces (Fig. 1.87b) for the oxidation of iron (prior to the formation of Fe Oj) would be... 

It has already been shown that bulk lattice diffusion is not generally considered to be the rate-controlling process for the oxidation of iron in most real situations. Hence the classical Wagner treatment, whereby the valency of the alloying element increases or decreases the number of lattice defects. 

Effects of Specific Alloying Elements on the Oxidation of Iron... 

Over the years, Pourbaix and his co-workers in the CEBELCOR Institute, founded under his direction, extended these diagrams by including lines for metastable compounds. Figure 7.66 illustrates such a presentation for the Fe-O system over the temperature range 830-2200 K. Pourbaix used these diagrams as a basis for a discussion of the stability of metallic iron (solid, liquid and vapour phases), the oxides of iron as a function of oxygen pressure and temperature from which he explained the protection of iron at high temperature by immunity and passivation. He also pointed out the... 

Some pigments promote corrosion owing to their content of soluble salts, their reactivity, or their electrochemical action, and thus should be avoided. Rust of the spotted type can be the consequence of their presence in a paint, especially the hrst coat, e.g. of graphite (noble to steel), some red oxides of iron, gypsum, ochre or lamp black. 

The current-potential relationship ABCDE, as obtained potentiosta-tically, has allowed a study of the passive phenomena in greater detail and the operational definition of the passive state with greater preciseness. Bonhoeffer, Vetter and many others have made extensive potentiostatic studies of iron which indicate that the metal has a thin film, composed of one or more oxides of iron, on its surface when in the passive state . Similar studies have been made with stainless steel, nickel, chromium and other metals... 

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