Cathodic control

Fig. 1.27 Evans diagrams illustrating (a) cathodic control, (b) anodic control, (c) mixed control, (d) resistance control, (e) how a reaction with a higher thermodynamic tendency ( r, ii) may result in a smaller corrosion rate than one with a lower thermodynamic tendency and (/) how gives no indication of the corrosion rate...

The primary function of a coating is to act as a barrier which isolates the underlying metal from the environment, and in certain circumstances such as an impervious continuous vitreous enamel on steel, this could be regarded as thermodynamic control. However, whereas a thick bituminous coating will act in the same way as n vitreous enamel, paint coatings are normally permeable to oxygen and water and in the case of an inhibitive primer (red lead, zinc chromate) anodic control will be significant, whilst the converse applies to a zinc-rich primer that will provide cathodic control to the substrate. 

Limiting Factors for Low-Temperature Operation. One controversial topic that has raised wide attention relates to the limiting factors of the low temperature of lithium ion cells. The researchers not only debated about whether the anode or cathode controls the overall low-temperature performance of a full lithium ion cell but also disagree upon the rate-determining steps that govern the low-temperature kinetics of lithium ion intercalation at the graphitic anode. 

Perry et al. [24] and Jaouen et al. [25] have provided useful diagnostic criteria. They concluded that cathodes controlled by either Tafel kinetics and oxygen diffusion in the agglomerate regions, or by Tafel kinetics and proton transport in the catalyst layer could result in double Tafel slopes. If the cathode was controlled by Tafel kinetics, oxygen diffusion, and proton transport all together, quadruple Tafel slopes would appear. 

A serious limitation of the use of anodic inhibitors is that they must be used in sufficiently high concentration to eliminate all the anodic sites, otherwise the anodic area that remains will carry the whole corrosion current, which is usually cathodically controlled. Intense local corrosion may then result, possibly leading to failure of the specimen. Cathodic inhibitors, on the contrary, are helpful in any concentrations for example, the blanketing of only half the cathodic surface will still roughly halve the corrosion rate. The presence of temporary hardness or magnesium ions can help reduce corrosion through deposition of CaCOs or Mg(OH)2, specifically on the cathodic surfaces where OH is produced in the oxygen absorption reaction ... 

Many other issues are involved in the application of cathodic protection. For example, consider the case of cathodic protection of underground structures in which the corrosivity of soil is likely to play a major role, as does the degree of aeration and the resistivity. Bacterial effects also can change the corrosion potential. AU these factors influence the corrosion process so that along a pipeline there can be varying cathodic control requirements that have to be estimated from potential measurements, experience, and so forth. 

Fig. 3.11 Effect of the polarization curve on the mixed potential of a corroding system (a) cathodically controlled, (b) anodically controlled.

The rate of the cathodic reaction is proportional to the CO2 partial pressure. Thus, the factor 0.67 in front of log (PCO2) indicates that the corrosion is not completely under cathodic control. (If it was, the factor would be = 1.) In the equation, mass transport limitation due to deposits of corrosion products has not been taken into consideration. Therefore it represents a so-called worst case . 

Conversely, for several aluminium alloys, pit initiation can be accepted under many circumstances. This is so because numerous pits are usually formed, and the oxide is insulating and has therefore low cathodic ability, so that the corrosion rate is under cathodic control. However, if the cathodic reaction can occur on a different metal because of a galvanic connection or for instance deposition of Cu on the aluminium surface, the pitting rate may be very high. Since we in other respects can accept pit initiation, the time dependence of pit growth and pit depths is important, and we shall consider this more quantitatively. 

Soils can be classified on the basis of particle size. Gravel contains the coarsest particles (> 2 mm) and clay the finest ones (< 0.002 mm), with sand and silt in between. Soils containing the finest particles, with ample distribution of small particle sizes are very dense and prevent supply of oxygen (but not of water), while gravel allows oxygen to be transported easily. Most metallic materials that are used in soils corrode under cathodic control, i.e. under control of oxygen transport. Thus, the density of the soil is important. The relationship is, however, somewhat... 

If the corrosion product on the metal surface is also an electronic conductor, the corrosion product does not hinder the flow of electrons, and the electrochemical reaction, instead of taking place at the metal/solution interface, occurs at the corrosion product/solution interface. In this case, if the process is under cathodic control, its total current may be also greatly increased by the presence of the corrosion product both for the possible greater catalytic activity of the corrosion product compared to that of the metal on the cathodic process (e.g., in the case of some iron sulfides with respect to hydrogen evolution) and for the much higher surface area of the corrosion product compared to that of the metal. On the other hand, if the corrosion product does not have the characteristics of an electronic conductor, the electron cannot flow across the corrosion product/solution interface, and the cathodic process occurs only on the limited free metal surface through the porosity of the corrosion product and with hindered diffusion. In the latter situation, the current density of the cathodic process has an upper limit, and it is drastically reduced. 

The overall balance of electric charges between the anodic and the cathodic processes must be equal to zero, expressed in terms of currents and not in terms of current densities. If we consider a hypothetical corrosion process, in which the kinetic corrosion resistances are those related to the cathodic process, and if we consider also that the cathodic surface is coupled with an anodic one of equal surface area, then both currents and the current densities will be equal on both areas. If the cathodic area is kept constant and the anodic surface area is varied, the total anodic current must be maintained constant by assuming cathodic control, it will also render constant the amount of dissolved metal. The anodic current density and, hence, the rate of penetration of the attack of the... 

The passive surface of the chromium layer greatly hinders the process of cathodic reduction of oxygen, while the anodic behavior of nickel is of an active type. The overall electrochemical process is therefore under cathodic control, and since the extent of the cathodic surface of chromium practically does not change by varying its detectivity, the total amount of oxygen is reduced, and consequently, the overall amount of dissolved nickel is almost independent of the number of defects in the layer of chromium. The rate of penetration in the layer of nickel is consequently decreases if the exposed surface is greater, that is, for higher surface density of defects in the layer of chromium. 

Considering the corrosion system as a whole, the sum of the cathode current must equal the sum of the anode currents for reason of electroneutrality. Thus, the cathode and anode reactions affect one another reciprocally, and the slowest, most inhibited one determines the overall reaction rate. In many cases the cathode reaction is the slowest, so the cathode controls corrosion. Lack of external current can only arise at a given mixed potential, which is the free corrosion potential. 

Increasing the electrical resistivity of the concrete Cathodic control creating conditions in which potentially cathodic areas of reinforcement are unable to drive an anodic reaction... 

When polarization occurs mostly at the cathode, the corrosion rate is said to be cathodically controlled. The corrosion potential is then near the thermodynamic anode potential. Examples are zinc corroding in sulfuric acid and iron exposed to natural waters. 

This reaction is rapid in most media, as shown by lack of pronounced polarization when iron is made anode employing an external current. When iron corrodes, the rate is usually controlled by the cathodic reaction, which, in general, is much slower (cathodic control). In deaerated solutions, the cathodic reaction is... 

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