Cathodic water polarization

Fig. 5.8 Schematic representation of relative positions of anodic metal, cathodic hydrogen, and cathodic water polarization curves, pH = 1. Curve M, anodic polarization for metal (e.g., Fe-18% Cr) curve H, cathodic polarization for H+ curve W, cathodic polarization for H20 curve SC, sum of H+ and H20 polarization...

Fig. 5.12 Sum (SC) of cathodic oxygen, hydrogen, and water polarization curves of Fig. 5.11. Oxygen curve dominates above -300 mV (SHE) and hydrogen curve below-300 mV (SHE). Water reduction makes negligible contribution to the current density. pH = 1.Po2 =0.2 atm...

Fig. 7 Polarization curve of cathodic oxygen reduction with superimposed cathodic water dissociation at 18-8-CrNi steel in air-saturated, stirred NaOH/0.5 M NaCi solution, pH 11,25°C ( ) as well as in practically 02-free solution (o) potential referred to saturated calomel electrode [2].

When cathodic polarization is a result of negative total current densities 7., the potential becomes more negative and the corrosion rate lower. Finally, at the equilibrium potential it becomes zero. In neutral water equilibrium potentials are undefined or not attainable. Instead, protective potentials are quoted at which the corrosion rate is negligibly low. This is the case when = 1 flA cm (w = lOjUm a ) which is described by the following criteria for cathodic protection ... 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

Cathodic protection can be used to protect steel in concrete (see Chapter 19). There is no fear of damage by H2 evolution due to porosity of the mortar. Local corrosion attack can be observed under extreme conditions due to porosity (water/ cement ratio = 1) and polarization (f/jq = -0.98 V) with portland cement but not with blast furnace cement, corresponding to field IV in Fig. 2-2 [53]. However, such conditions do not occur in practice. 

The polarization cell must be inspected at regular intervals (e.g., twice a year) to check water loss caused by electrolysis. If necessary, the correct level must be restored with deionized water. In addition, the electrolyte should be renewed every 4 years. It is recommended that the dc decoupling device be designed so that the maximum expected failure current flows through the smallest possible polarization cell in order to load the cathodic protection as little as possible. 

As the measurements show, the small heater without an electrical separation (from the boiler) is not detrimental to cathodic protection. However, with the uninsulated built-in Cu heat exchanger without an electrical separation, cathodic protection was not achieved. As expected, the polarization increased with increasing conductivity of the water. It should be pointed out that the Cu tube was tinned and that the tin could act as a weak cathodic component. Apart from the unknown long-term stability of such a coating, the apparent raising of the cathodic polarization resistance of tin is not sufficient to provide cathodic protection with such a large fixture. This applies also to other metal coatings (e.g., nickel). 

The existence of an electrical potential causes not only cation and anion movement but also migration of moisture toward the cathode. This movement of water (electroendosmosis) is due to the asymmetrical nature of the polar groups of the water molecule. In arid regions water leaving the anode area may cause the soil surrounding the anodes to become so dry that proper current densities cannot be maintained along the line. To alleviate this, some pipe-line companies have had to transport water into desert areas to re-moisten anode beds. 

A process involving water electrolysis is the production of heavy water. During cathodic polarization the relative rates of deuterium discharge and evolution are lower than those of the normal hydrogen isotope. Hence, during electrolysis the solution is enriched in heavy water. When the process is performed repeatedly, water with a D2O content of up to 99.7% can be produced. Electrochemical methods are also used widely in the manufacture of a variety of other inorganic and organic substances. 

Fig. 6. Polarization of the oxygen separator. Air cathode area = 10 cm2 water temperature = 40 °C air feed = 4 dm3/min.

The total output photovoltage must exceed the thermodynamic potential difference for water splitting (1.229 V at 25°C), the energy level mismatches for the anodic and cathodic processes, and the polarization loss or overvoltages due to kinetic, diffusion, and IR potential losses in the bulk of electrolyte. 

Even though the effect of moisture on the anode kinetics is well known, interpretation of experimental results on the effect of moisture can be tricky. As Nakagawa et al. [52] pointed out, the measurement of the total cell impedance under the OCV condition is not convincing since the reduction of polarization could as well be due to the availability of H20 for the cathodic reaction. In addition, the measurement of cell performance under the constant voltage or constant current conditions may also lead to wrong conclusions about the effect of water, because the addition of H20 will... 

The silver deposition experiments of Sonnenfeld and Schardt (94) provide a representative example. After the deposition of silver on HOPG, the freshly plated surface was imaged in the presence of aqueous 0.05 M AgClO (Fig. 7)(94). Assuming a positive tip polarity is used, the STM tip will function as an anode and its potential will be that necessary to oxidize water, Ea jj2q/02 - -+0.95 V (pH - 7). The substrate cathode will drive the reduction of silver ion at the silver plated substrate at a formal potential of Ec Ag+/Ag V. Thus, an imaging window of AEp - 230 mV is... 

Fig. 10-28. Polarization curves for cell reactions of photoelectrolytic decomposition of water at a photoezcited n-type anode and at a metal cathode solid curve M = cathodic polarization curve of hydrogen evolution at metal cathode solid curve n-SC = anodic polarization curve of oxygen evolution at photoezcited n-type anode (Fermi level versus current curve) dashed curve p-SC = quasi-Fermi level of interfadal holes as a ftmction of anodic reaction current at photoezcited n-type anode (anodic polarization curve r re-sented by interfacial hole level) = electrode potential of two operating electrodes in a photoelectrolytic cell p. sc = inverse overvoltage of generation and transport ofphotoezcited holes in an n-type anode.

The cathodic and anodic polarization potentials, Ec and E, in the stationary state of the cell for photoelectrolytic decomposition of water, in which the metallic cathode and the n-type semiconductor anode are short-circuited, are given, respectively, in Eqns. 10-55 and 10-56 ... 

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