Corrosion reduction process

Dissolved oxygen reduction process Corrosion processes governed by this cathode reaction might be expected to be wholly controlled by concentration polarisation because of the low solubility of oxygen, especially in concentrated salt solution. The effect of temperature increase is complex in that the diffusivity of oxygen molecules increases, but solubility decreases. Data are scarce for these effects but the net mass transport of oxygen should increase with temperature until a maximum is reached (estimated at about 80°C) when the concentration falls as the boiling point is approached. Thus the corrosion rate should attain a maximum at 80°C and then decrease with further increase in temperature. 

Later work on aluminium alloys has also focused more closely upon the role of hydrogen which had not previously been widely considered as an embrittling species in the stress-corrosion cracking process for these alloys. The idea was not new, however. Reports of intergranular failure under cathodic charging conditions had been made at a much earlier time . A reduction in stress-corrosion life and alloy ductility in a high purity Al-5Zn-3Mg alloy had been found in specimens pre-exposed to a 2% NaCI solution" , an effect that was accentuated if specimens were stressed". ... 

The main metallic impurities that contaminate the primary powder, due to chemical corrosion of the retort and other metal parts of the reactor, are Fe, Ni and Cr. From this point of view, reactors that are equipped with larger retorts usually provide better purity due to a relatively low ratio between the internal surface of the wet metal parts of the reactor and the total volume of the melt. Recent investigations on the decreasing of Fe, Ni and Cr impurities during the sodium reduction process were performed by Li [591]. It was shown that one of the most effective ways to reduce contamination of the product is to reduce the duration of time K2TaF7 is present in the reactor. 

Three general reaction types compare the activation-control reduction processes. In Fig. 25-12, in Case I, the single reversible corrosion potential (anode/cathode intersection) is in the active region. A wide range of corrosion rates is possible. In Case 2, the cathodic curve intersects the anodic curve at three potentials, one active and two passive. If the middle active/passive intersection is not stable, the lower and upper... 

The chemical reactivity of the material to be processed for size reduction can pose a great problem. For example, the plant construction itself may be exposed to the threat of corrosion. The size reduction process generally raises the temperature of the material in question and this effect may alter the material in some undesirable way. 

Reduction of nitrate (and nitrite) occurs by interaction with the metal, for example, by metallic iron that has been widely studied for elaborating a technology to remove nitrate from water [66-70]. The very principle of this procedure that the anodic transformation (dissolution) corrosion of metallic iron Fe(0) leading to the formation of Fe(II) and Fe(III) species is coupled with the reduction process. The effect of pFi and the iron-to-nitrate ratio, temperature, and mass transport were studied under various conditions in order to elaborate a suitable technology. 

The process of corrosion and electtochemical reactions are very visible and useful illustrations of the oxidation-reduction process. As metals are exposed to oxygen in air and water, the constituent metal atoms become oxidized, forming metal ions/metal oxides. Put simply, the oxidation-reduction process results in the metal msting and becoming severely corroded and weakened. In the case of enviromnental exposure of iron, the metal forms iron(III) oxide, known as rust (see Figure 4.2). 

Also, statues with copper skins are particularly prone to the effects of oxidation-reduction processes, and good examples are the Statue of Liberty or roofs of old buildings. While these would have had a characteristic rich copper-brown coloration, enviromnental exposure caused oxidation of the copper and other constituent metals, changing their appearance to a light-green coloration with additional corrosion. 

The selection of materials must also consider oxidation/reduction processes that occur in the absence of an aqueous electrolyte. Examples include sulfidation, destructive oxidation of alloys in air or steam at high temperatures, carburization, nitriding, fuel ash corrosion, and high-temperature hydrogen attack. 

In this method, the artefact is immersed in a tank of electrolyte in which the metal will not corrode, i.e. it remains passive. An electrolytic cell is formed with the artefact being the cathode with an inert anode. A small dc current is passed between the two electrodes and the corrosion products on the surface undergo a reduction process to a different compound. In the case of iron, the following takes place on the cathode surface. 

Oxidation-reduction processes are responsible for many t)q es of chemical change. Corrosion, the operation of a battery, and biochemical energy-harvesting reactions are a few examples. In this section we explore the basic concepts underlying this class of chemical reactions. 

The deterioration of metals caused by an oxidation-reduction process is termed corrosion. Metal atoms are converted to metal ions the structure, hence the properties, changes dramatically, and usually for the worse (Figure 9.8). 

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