Oxygen depolarization

When water pH is between about 4 and 10 near room temperature, iron corrosion rates are nearly constant (Fig. 5.5). Below a pH of 4, protective corrosion products are dissolved. A bare iron surface contacts water, and acid can react directly with steel. Hydrogen evolution (Reaction 5.3) becomes pronounced below a pH of 4. In conjunction with oxygen depolarization, the corrosion rate increases sharply (Fig. 5.5). 

Guild AF, Gancs L, Allen RJ, Mukerjee S (2007) Carbon-supported low-loading rhodium sulfide electrocatalysts for oxygen depolarized cathode applications. Appl Catal A 326 227-235... 

The effects of impurities are less important for oxygen-depolarized than for hydrogen-depolarized corrosion, since the values of polarization for oxygen reduction found at different metafs differ fess strongfy than those for hydrogen evofution. 

Meyers, J. P. Villwock, R. D. Darling, R. M. Newman, J. In Advances In Mathematical Modeling and Simulation of Electrochemical Processes and Oxygen Depolarized Cathodes and Activated Cathodes for Chlor-Alkall Processes, Van Zee, J. W., Puller, T. P., Poller, P. C., Hine, P., Eds. The Electrcohemical Society Proceedings Series Pennington, NJ, 1998 Vol. PV 98-10. 

Dioxygen is also evolved at the anode. An alternative cathodic reaction is the reduction of dioxygen using oxygen depolarized cathodes. Here, the anodic reaction is the same as above, while the cathodic reaction is ... 

Fig. 14 Schematic representation of the chlor-alkali electrolysis using an oxygen depolarized cathode.

Oxygen depolarized cathodes can be used in hydrochloric acid electrolysis, too. 

Fig. 17 Scheme of producing chlorine from hydrochloric acid using an Oxygen Depolarized Cathode (ODC). The ODC/catalyst/membrane unit is displayed in more detail in the coming Figure 23. 

The effect of ultrasonic field on the polarization curves of Cu-Pb, and some brasses has been studied in chloride and sulfate solutions in the presence and absence of the respective metal ions [108]. The main effect of the ultrasound at low current densities is the acceleration of diffusion. The passivation current density in solutions free of the respective metal ions is considerably increased when ultrasound is applied. Stable passivity cannot be attained because of the periodic destruction of the salt film. The hydrogen evolution reaction is accelerated because of the destruction of the solvation shell. The oxygen depolarization reaction is also enhanced due to the increased diffusion. The rate of metal deposition is likewise increased by ultrasound. The steady-state potentials of reactions with anodic control are shifted in the negative direction when ultrasound is applied. 

Practical application of the oxygen depolarized electrode has been limited to fuel cells with pure oxygen for special... 

Figure 3. Energy expenditure for 1 t chlorine production I- Current density effect (SPE with oxygen depolarization concentration of HCl =8.3 mole/1). II- Hydrochloric acid concentration effect (i = 4-3 kA/m ). Ill- Current density effect in industrial condition [2].

The proximate goal of the research is to invent an oxygen depolarized air cathode which, if fitted to a membrane cell for the production of pure caustic, could save the U.S. chloralkall industry billions of kilowatt hours of electricity annually. 

Iron corrodes in a sea water solution of pH 7. Assuming corrosion proceeds by oxygen depolarization, calculate the minimum current required for complete cathodic protection. Exchange current density, °, for oxygen reduction is... 

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

Within the acid region, pH < 4, the ferrous oxide film is dissolved, the surface pH falls, and iron is more or less in direct contact with the aqueous environment. The increased rate of reaction is then the sum of both an appreciable rate of hydrogen evolution and oxygen depolarization. 

The increased corrosion rate of iron as pH decreases is not caused by increased hydrogen evolution alone in fact, greater accessibility of oxygen to the metal surface on dissolution of the surface oxide favors oxygen depolarization, which is often the more important reason. The sensitivity of the corrosion rate of iron or steel in nonoxidizng acids to dissolved oxygen concentration is shown... 

Acid salts, which are salts that hydrolyze to form acid solutions, cause corrosion with combined hydrogen evolution and oxygen depolarization at a rate... 

Although carbon content of a steel has no effect on the corrosion rate in fresh waters, a slight increase in rate (maximum 20%) has been observed in seawater as the carbon content is raised from 0.1 to 0.8% [41]. The cause of this increase is probably related to greater importance of the hydrogen evolution reaction in chloride solution (with complexing of Fe " by Cl ) supplementary to oxygen depolarization as the cathodic surface of cementite (FesC) increases. 

Iron corrodes in seawater at a rate of 2.5 gmd. Assuming that all corrosion is by oxygen depolarization, calculate the minimum initial current density (A/m ) necessary for complete cathodic protection. 

Figure 29.2. Polarization diagram for metal corroding under control by oxygen depolarization.

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... 

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