Electronegative potential

Except for A-tetrads, the electrostatic potential in the tetrad centre is negative with marked variations, however. The most electronegative potential is observed for G-, C- and U-tetrads. Cation binding is, however, not only affected by this potential but also by cavity size and, as will be discussed below, by hydration energy. 

Metal destrnction (dissolution) takes place in the material area with the more electronegative potential (anode) since this is where the metal ions from the surface enter the solution. The electrons released in the process migrate to the electropositive cathode and are consumed there at the phase boundary by redncible substances present in the solution (e.g., cations dissolved oxygen). 

Remarkable, if the electronegativity definition relation (4.33) is formally integrated by taking into account the nature of the electronegativity potential as minus chemical potential for the system in focus, the total energy would result as an integral that would correspond to the summing (4.24). 

Metals having an electronegative potential show a tendency to oxidise and thus to corrode in aqueous media, if the conditions allow. This tendency increases as the potential becomes more electronegative. It is well known that magnesium (standard electrode potential, —2380 mV) degrades much more under the effect of moisture than lead (standard electrode potential, — 126 mV). 

Zinc strongly decreases the potential. Alloy 7072 at 1% zinc is, therefore, used as a cladding of 3003 (see Section B.5.6). The alloys of the 7000 series have the most electronegative potentials. 

Copper alloys of the 2000 series have the least electronegative potentials. Alloy 2017A can, therefore, be protected by a cladding of 1050A (see Section B.5.6). 

The dissolution potential of aluminium in most aqueous media is in the order of - 500 mV with respect to a hydrogen electrode, while its standard electrode potential with respect to this same electrode amounts to — 1660 mV. Because of this highly electronegative potential, aluminium is one of the easiest metals to oxidise (Table B.1.1). However, aluminium behaves as a very stable metal, especially in oxidising media (air, water, etc.). 

Whenever two different types of metals are in contact, galvanic corrosion is possible. The metal with an electronegative potential (or the more electronegative metal, if both are... 

The comparison of the dissolution potentials of aluminium alloys may reach absurdity, for example, leading to a preference for alloys of the 2000 series, which have a dissolution potential far less negative, about — 650 mV, over those of the 5000 series, which have a more electronegative potential, on the order of - 800 mV (Table B.1.3). And yet the latter show excellent corrosion resistance, while alloys of the 2000 series are highly susceptible to pitting corrosion in natural environments. 

The intensity of cathodic corrosion of aluminium does not depend on the nature of the anions or on the concentration of cations [23]. According to the Pourbaix diagram (Figure B.1.10), cathodic protection of aluminium must be carried out at highly electronegative potentials, on the order of — 1750 mV SCE, which is well below the stability domain of water. At such a potential, there will always be cathodic corrosion. 

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