Cathode contamination chemical reaction

The metallic impurities present in an impure metal can be broadly divided into two groups those nobler (less electronegative) and those less noble or baser (more electronegative) as compared to the metal to be purified. Purification with respect to these two classes of impurities occurs due to the chemical and the electrochemical reactions that take place at the anode and at the cathode. At the anode, the impurities which are baser than the metal to be purified would go into solution by chemical displacement and by electrochemical reactions whereas the nobler impurities would remain behind as sludges. At the cathode, the baser impurities would not get electrolytically deposited because of the unfavorable electrode potential and the concentration of these impurities would build up in the electrolyte. If, however, the baser impurities enter the cell via the electrolyte or from the construction materials of the cell, there would be no accumulation or build up because these would readily co-deposit at the cathode and contaminate the metal. It is for this reason that it is extremely important to select the electrolyte and the construction materials of the cell carefully. In actual practice, some of the baser impurities do get transferred to the cathode due to chemical reactions. As an example, let the case of the electrorefining of vanadium in a molten electrolyte composed of sodium chloride-potassium chloride-vanadium dichloride be considered. Aluminum and iron are typically considered as baser and nobler impurities in the metal. When the impure metal is brought into contact with the molten electrolyte, the following reaction occurs... 

Continuous and semicontinuous electrochemical reactors are normally employed for effluent metal ion remediation, where the anode reaction is usually oxygen evolution from water [compare with Equation (26.4)]. After the metal contaminant is captured on the cathode, the cathode can be discarded, the collected metal can be resold, or the deposited metal can be chemically or elecfro-chemically etched into a small volume of a suitable leaching liquor (e.g., water) so as to increase its concentration substantially. 

Chemical contamination - anything that inhibits or promotes the anodic or cathodic reactions on the steel surface can distort interpretation of potential measurements. This may include surface applied corrosion inhibitors. 

The first step in the chemical degradation mechanism is the production of hydrogen peroxide, which may be produced as a by-product of the oxygen reduction reaction on the cathode, or may be produced chemically by crossover of either hydrogen or oxygen to the opposite electrode. The hydrogen peroxide reacts with metal ion contaminants (M"+) acting as Fenton s catalysts to produce very reactive hydroperoxy and peroxy radicals, as described by equation (1.35) and equation (1.36). 

Evidence for the formation of some type of Solid Permeable Interface (SPI) has been obtained in all cases smdied. It can be stated generally that the organic species formed on the different cathode electrodes are more or less the same varying more in degree than in their precise chemical nature layer thickness also vary from material to material they also tend to increase significantly with temperature. However, the inorganic species found are more dependent on electrode material type. Reactions with the lithium-salt anion used are also material dependent. It is especially important to reduce the impact of the PF anion and its related contaminants (HF and PF,) on electrode surface chemistry through the implementation of more stable salts. Such a development is currently underway. 

The acid gas contaminants (carbon dioxide and in particular, hydrogen sulfide) increase environmental embrittlement tendencies. Their effect is to increase the volume of hydrogen entering the steel by causing corrosion that supplies hydrogen ions and by interfering with cathodic reactions. Chemical treatments can be utilized to overcome some of these effects. 

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