Galvanic microelement

When metal parts rub in an electrolyte, it is possible to form short-circuit galvanic microelements (Fig. 1.7). Potentials 1E3 and 2E3 appear at the metal-electrolyte interface and contact potential difference 1E2 in the contact sites of the parts. The electromotive force of these elements promotes electrode processes on the friction surfaces. The processes appear even though lEs = 2E3. because of the galvanic elements resulting from crevice corrosion in the friction zone. 

In contrast to the processes commonly considered in electrochemistry, electrochemical processes occur during friction under conditions of moving and deforming discrete contacts of individual microasperities. The participation of electrolytes as a liquid layer in the friction pair leads to potential leaps of 9 3 and 933 the metal-solution interface and to the contact potential difference 9 2 ill metal contacts (see Fig. 1.7) [22]. As a result, a short-circuited galvanic microelement appears with a probability of redox reactions on its electrodes. 

Two atoms exchanging electrons in a solution are a separate galvanic microelement. The atoms serve as electrodes and water serves a conductor. In the absence of equihbrium between these electrodes arises potential difference AE, which forces electrons to pass from one atom to the other, thereby generating electric current. The value of the maximum useful work of such current is proportionate to the charge carried by it. This charge is equal to the number of electrons, i.e., to the stoichiometric coefficient of the electron in semi-reactions multiplied by the Faraday number... 

This mechanism has been proposed by Hung et al. [14]. These authors have investigated oxidation of Pb powder in an ampoule with humid air in the absence of Ch- Despite the lack of oxygen, lead is oxidized and H2 is formed in the ampoule. Galvanic microelements form on the lead particle surface where the following reactions proceed ... 

The conjugated electrochemical processes that take place in galvanic microelements on the Pb surface are influenced by pH of the solution that covers the surface of Pb particles. With increase of pH of this solution, the overvoltage of hydrogen evolution (reaction (8.13)) increases and hence these reactions are slowed down. Thus, formation of a layer of Pb(OH)2 and PbO on the surface of Pb particles suppresses oxidation of lead through the galvanic microelement electrochemical mechanism. 

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