Free corrosion potential metal electrode

An important condition for potentiometry is high selectivity the electrode s potential shonld respond only to the snbstance being examined, not to other components in the solntion. This condition greatly restricts the possibilities of the version of potentiometry described here when metal electrodes are nsed as the indicator electrodes. The solntion shonld be free of ions of more electropositive metals and of the components of other redox systems (in particnlar, dissolved air). Only corrosion-resistant materials can be nsed as electrodes. It is not possible at all with this method to determine alkali or alkaline-earth metal ions in aqneons solntions. 

Counter electrodes should be made from a corrosion resistant metal, for example a noble metal, and their design should allow for uniform current (potential) distribution and free convection of aggressive agents to the working electrode. 

The standard electrode potentials , or the standard chemical potentials /X , may be used to calculate the free energy decrease —AG and the equilibrium constant /T of a corrosion reaction (see Appendix 20.2). Any corrosion reaction in aqueous solution must involve oxidation of the metal and reduction of a species in solution (an electron acceptor) with consequent electron transfer between the two reactants. Thus the corrosion of zinc ( In +zzn = —0-76 V) in a reducing acid of pH = 4 (a = 10 ) may be represented by the reaction ... 

Factors Involved in Galvanic Corrosion. Emf series and practical nobility of metals and metalloids. The emf. series is a list of half-cell potentials proportional to the free energy changes of the corresponding reversible half-cell reactions for standard state of unit activity with respect to the standard hydrogen electrode (SHE). This is also known as Nernst scale of solution potentials since it allows to classification of the metals in order of nobility according to the value of the equilibrium potential of their reaction of dissolution in the standard state (1 g ion/1). This thermodynamic nobility can differ from practical nobility due to the formation of a passive layer and electrochemical kinetics. 

If a polymer-covered metallic sample in contact with a halide-containing electrolyte is examined in a humid atmosphere free of oxygen, no corrosion reactions take place. Therefore, the electrode potential of the metal in contact with the electrolyte is almost equal to the thermodynamic Nernst potential. This value is significantly more negative than the electrode potential of the metal in contact with the polymer. 

When a chemically stable oxide (or salt) film is present on the surface of a metal (see the iron oxide stable regions of Figure 4), that metal may be free of subsequent corrosion. The conditions for this form of corrosion mitigation are that the underlying film is adherent, coherent and pore-free. In essence, these conditions merely stipulate that the film must be an effective barrier between the metal and the environment. This condition is called passivity and is characterized by measured electrode potentials in the regions where the film.is stable. Iron and its alloys have been shown to exhibit passive behavior (i6). ... 

When a metal (M) is immersed in a solution containing its ions (M ), several reactions may occur. The metal may lose an electron (corrosion) to form metal ions or the metal ions in solution gain electrons (reduction) and enter the solid metal state. The equilibrium across the metal-solution interface controls which reaction, if any, will occur at the metal-electrolyte interface. Because the equilibrium is determined by the equality of the partial Gibbs free-energy or chemical potentials (//) on either side of the electrode interface (i.e., Absolution=A dectrode). when any metal is immersed in the electrolyte, thermodynamics... 

Thermodynamically, dissolution should occur whenever the fiee energy of the solvated metal ion at a givai concentration is lower than the free energy of the atom in the metal, plus the thermodynamic potential of the electrons exchanged during the reaction via the standard hydrogen electrode. Such thermodynamic conditions arc summarized in the series of potential- H phase diagrams that have been extensively collated by Poutbaix [69]. Even in cases where a metal is coveted by a thin protective oxide, deleterious corrosion effects can... 

Fig. IV.1 shows schematically the energy relations in such a cell in the dark and under illumination. It also indicates the losses of free energy for the conversion of light with a quantum energy of the band gap whereby it is assumed that the redox reaction at the metallic counter electrode causes practically no energy loss. One sees, however, that the redox reaction at the semiconductor itself contributes very much to the loss in energy, if it has a redox potential which keeps the surface concentration of holes small enough in order to prevent corrosion.

Corrosion of steel is known by engineers as the result of electrochemical reaction when different potentials are developed by electrically connected metal parts in contact with a solution containing free ions. The so-called electrode potential is dependent on the particular metal and the nature of the solution. Comparative values of electrode potentials may be measured against a standard electrode-electrol)de system. For example, if hydrogen is considered of zero V electrode potential, then lead, iron, zinc and aluminum potentials are 0.13,0.44,0.75 and 1.66 V, resp>ectively. 

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