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Corrosion 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... 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...
For situations controlled by anodic dissolution of a film P = 1/density of metal, but if the corrosion is controlled by the cathodic reaction P = 1/density of metal x nc Ma/na Me where n and M are the number of electrons and the molecular masses of anodic and cathodic reactants. [Pg.296]

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 ... [Pg.350]

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. [Pg.415]

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 . [Pg.80]

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. [Pg.127]

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. [Pg.318]

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... [Pg.318]

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. [Pg.539]

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. [Pg.68]

If corrosion is controlled by concentration polarization at the cathode, as when oxygen depolarization is controlling. Equation (5.8) simplifies to... [Pg.72]

When the corrosion rate is cathodically controlled and the corrosion potential approaches the open-circuit anode potential, the required current density is... [Pg.257]

Equation (29.10) is the Stern-Geary equation. Should the cathodic reaction be controlled by concentration polarization, as occurs in corrosion reactions controlled by oxygen depolarization, the corrosion current equals the limiting diffusion current (Fig. 29.2). This situation is equivalent to a large or infinite value of Pc in (29.10). Under these conditions, (29.10) becomes... [Pg.458]

Corrosion reactions consist of at least one anodic partial reaction and one cathodic partial reaction, each of which involves several steps (Figure 4.6). The overall reaction rate is limited by the rate of the slowest partial reaction. We can thus make a distinction between corrosion reactions under anodic or cathodic control. [Pg.124]

Cathodic control protection protects the substrate by coating with a less noble metal, for which the slopes of the cathodic polarization curves are steep. The cathodic overpotential of the surface is increased by the coating therefore, the corrosion potential becomes more negative than that of the substrate. Coating materials used for this purpose are zinc, aluminum, manganese, cadmium, and their alloys. The electrode potential of these metals are more negative than those of iron and steel. When exposed to the environment, these coatings act as sacrificial anodes for the iron and steel substrates. [Pg.275]


See other pages where Corrosion cathodic control is mentioned: [Pg.1263]    [Pg.1292]    [Pg.59]    [Pg.92]    [Pg.96]    [Pg.226]    [Pg.234]    [Pg.307]    [Pg.331]    [Pg.810]    [Pg.348]    [Pg.350]    [Pg.71]    [Pg.321]    [Pg.352]    [Pg.348]    [Pg.552]    [Pg.559]    [Pg.15]    [Pg.174]    [Pg.248]    [Pg.133]    [Pg.13]    [Pg.9]    [Pg.290]    [Pg.781]    [Pg.6]    [Pg.285]    [Pg.104]    [Pg.319]    [Pg.839]    [Pg.1478]    [Pg.127]    [Pg.143]    [Pg.384]   
See also in sourсe #XX -- [ Pg.1292 ]

See also in sourсe #XX -- [ Pg.329 ]

See also in sourсe #XX -- [ Pg.329 ]




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