Anodic process phenomena

The dissolution of zinc in a mineral acid is much faster when the zinc contains an admixture of copper. This is because the surface of the metal contains copper crystallites at which hydrogen evolution occurs with a much lower overpotential than at zinc (see Fig. 5.54C). The mixed potential is shifted to a more positive value, E mix, and the corrosion current increases. In this case the cathodic and anodic processes occur on separate surfaces. This phenomenon is termed corrosion of a chemically heterogeneous surface. In the solution an electric current flows between the cathodic and anodic domains which represent short-circuited electrodes of a galvanic cell. A. de la Rive assumed this to be the only kind of corrosion, calling these systems local cells. 

The idea of excess potential is useful in applying the process of separating a certain kind of ions at unattackable cathodes. For reduction it has up to the present only been proved for hydrogen it is nevertheless possible that the separation potential of every ion changes with the nature of the electrode, since the opportunity for the reaction of discharged ions being catalytically influenced to form stable molecules is always present. E. Muller 1 and Coehn 2 have shown that the excess potential phenomenon also occurs in anodic processes. 

The generally used expression anodic protection concerns anticorrosion protection methods for some metals and alloys in aggressive chemical environments by anodic polarization with an external dc power source. Maintaining the metal surface in the passive state practically stops corrosion processes. However, it should be mentioned that the anodic passivity phenomenon can occur in the case of contact with other metals characterized by more noble potentials. Cathodic coatings causing anodic passivity are an example. The same role is played by platinum, palladium, or copper additives introduced into steel alloys. This method of protection is sometimes called galvanic anodic protection. 

In the aluminum electrowinning process a phenomenon called the anode effect is normally encountered when the alumina content in the electrolyte falls below 2%. The anode gets partially covered with a gas blanket and as a consequence, sparking occurs and the cell voltage fluctuates considerably due to frequent breaking and reestablishment of local contact between the anode and the electrolyte. A heavy current passes through the anode area... 

M. Faraday was the first to observe an electrocatalytic process, in 1834, when he discovered that a new compound, ethane, is formed in the electrolysis of alkali metal acetates (this is probably the first example of electrochemical synthesis). This process was later named the Kolbe reaction, as Kolbe discovered in 1849 that this is a general phenomenon for fatty acids (except for formic acid) and their salts at higher concentrations. If these electrolytes are electrolysed with a platinum or irridium anode, oxygen evolution ceases in the potential interval between +2.1 and +2.2 V and a hydrocarbon is formed according to the equation... 

However, despite those positive reports, the authors would not recommend precalcination of the starting NiO-YSZ powder mixture as a necessary step in the processing of solid oxide fuel cells for the following reasons. First, the mechanism for the enhanced electrochemical performance for anodes when the NiO-YSZ precalcinated together has not been explained clearly. The phenomenon is actually counterintuitive because it has been shown that coarsening of NiO or YSZ alone leads to lower anode... 

Oscillations have been observed in chemical as well as electrochemical systems [Frl, Fi3, Wol]. Such oscillatory phenomena usually originate from a multivariable system with extremely nonlinear kinetic relationships and complicated coupling mechanisms [Fr4], Current oscillations at silicon electrodes under potentio-static conditions in HF were already reported in one of the first electrochemical studies of silicon electrodes [Tul] and ascribed to the presence of a thin anodic silicon oxide film. In contrast to the case of anodic oxidation in HF-free electrolytes where the oscillations become damped after a few periods, the oscillations in aqueous HF can be stable over hours. Several groups have studied this phenomenon since this early work, and a common understanding of its basic origin has emerged, but details of the oscillation process are still controversial. 

The presence of water does not only create conditions for the existence of an electrolyte, but it acts as a solvent for the dissolution of contaminants [10], Oxygen plays an important role as oxidant element in the atmospheric corrosion process. The thickness of the water layer determines the oxygen diffusion toward the metallic surface and also the diffusion of the reaction products to the outside interface limited by the atmosphere. Another aspect of ISO definition is that a metallic surface is covered by adsorptive and/or liquid films of electrolyte . According to new results, the presence of adsorptive or liquid films of electrolyte perhaps could be not in the entire metallic surface, but in places where there is formed a central anodic drop due to the existence of hygroscopic particles or substances surrounded by microdrops where the cathodic process takes place. This phenomenon is particularly possible in indoor conditions [15-18],... 

The process is characterised by the electrofluorination of volatile organic substrates within the matrix of pores of a carbon anode immersed in molten KF 2HF as electrolyte (as in a mid-temperature fluorine generator cell), and depends on the phenomenon that the anodically charged porous carbon is not wetted by the electrolyte. The fluorination probably takes place at the three phase interface of organic vapour, solid carbon, and liquid electrolyte in close proximity to, or at the sites where fluorine is being evolved. 

Another interesting phenomenon observed so far is that after purge the cell HFR gradually decreases in a time scale of hours, which is called HFR relaxation after purge. Typical results of FIFR relaxation are shown in Fig. 20. When the 60-s purge is completed, the valves at the inlet and outlet of the cell for both anode and cathode gas lines are closed and the cell temperatures are maintained constant at the purge cell temperature during the whole relaxation process. 

For the above study the usual value of the transfer coefficient a = 0.5 has been considered. With small a values, DDPV peaks are found to show a special shape under certain conditions. As can be seen in Fig. 4.17a, fora < 0.3 the DDPV curves corresponding to quasireversible processes with k° 10-3 cm s-1 present a striking splitting of the peak, with a sharper peak appearing at more anodic potentials. This phenomenon is promoted by small transfer constants and is more obvious for positive pulse heights (AE > 0, reverse mode, where the anodic peak is even greater than the cathodic one) and at planar electrodes, since it becomes less apparent as the electrode size is reduced (see Fig. 4.17b). The description of this phenomenon is of great interest since this could lead to erroneous interpretation of... 

Analogously to the processes at the cathode, oxidation at the anode takes place in the course of reversible processes in which the phenomenon of limiting current can be observed. Ions of bivalent iron can be anodically oxidized with a current efficiency of 100 p. c., if the density of the current used does not exceed the value of the limiting current above this value also oxygen is liberated from the solution. 

It has already been mentioned that the excess potential phenomenon occurs also with the oxidation phenomena. Thus it is possible to conyert p-nitrotoluene into p-nitrobenzoic acid at lead-peroxide anodes, while at platinum anodes only the alcohol is formed. It still seems inexplicable how this peculiar action of the anode material takes place. The simplest yet sufficient. explanation is to assume that the anode is capable of influencing catalytically the oxidation process as well as the formation of molecular oxygen. If the first process is accelerated and the second retarded, we obtain the excess potential by which the evolution of oxygen occurs only at a higher potential. Inversely, the oxygen and ozone formation cart be made reversible, and the oxidizing action decreased. 

---