Hydrogen depolarization

In the above mechanism, both hydrogen ion and molecule are utilized by SRB to convert SO to H,S. The consumption of hydrogen depolarizes the cathode and leads to an increased rate of corrosion. 

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. 

But the conventional membrane setup described is only one possibility of a sodium sulfate electrolysis. Other proposals include the use of an ODC or even an HDA (Hydrogen Depolarized Anode) for cell voltage reduction or the use of anion exchange membranes instead of the cation variety [7]. Last but not least electrodialysis may be used in a three compartment unit the salt is converted into free acid and the free base [40]. 

Neutral Salt Splitting with the Use of Hydrogen Depolarized Anodes (HydrinaTechnology, Example from... 

Hydrogen Depolarized Anodes (Hydrina-Technology, Example from De Nora Permelec)... 

Energy consumption may be substantially reduced by replacing the oxygen-evolving anode with a hydrogen depolarized anode [230], [232]. The modified schemes are shown in Figures 104 and 105. 

A conventional structure used for the hydrogen depolarized anode consists of an electrically conductive sheet with controlled porosity and hydrophobicity, coated with a catalytic layer. Under optimum operating conditions the electrolyte partially fills the pores, with the gas-liquid meniscus in the area of the sheet containing the catalyst. 

The three relationships apply to current densities up to 3000 A/m. The DAP assembly comprises a hydrogen depolarized anode and a Nafion 117 membrane the electrolytic compartments are limited by Nafion 324 and Selemion AAV ionexchange membranes. The relationships further apply to operating temperatures of 60 - 70 °C and to 13-18% caustic soda, 200-300 sodium sulfate, and 10% sulfuric acid. 

A second Hydrina process involves recovery and recycle of caustic soda and a mixed solution of Na2S04 and H2SO4. This is similar to the two-compartment cell of Fig. 15.18, but it uses the hydrogen-depolarized anode instead of the conventional type. A third version uses a three-compartment cell with the hydrogen anode [106,107]. Saturated Na2S04 enters the central compartment, and water is added to the electrode compartments. The products are sulfuric acid and caustic soda of high pmity. 

Oswin, H. G., Chodosh, S. M. (1965). Nonporous hydrogen depolarized anode for fuel cells. In R. Gould (Ed.), Advances in chemistry series vol. 47. Fuel cell systems (pp. 61—72). 

In connection with the above statements a next step has been made in the investigations, namely studying the influence of the Ce and Ce + ions as components of the corrosion medium (O.IN H2SO4) on the anodic behavior of stainless steel. These investigations were provoked by the observed occurrence of cathodic depolarization reaction of Ce + (CeCb) reduction, as a result of which the surface concentration of cerium is decreasing and theoretically it should approach zero value (Stoyanova et al., 2010). For this purpose an inverse experiment was carried out at different concentrations of Ce + ions in the corrosion medium we monitored the changes in the stationary corrosion potential of the thermally treated steel by the chronopotentiometric method. The aim of this experiment was to prove the occurrence of a reversible reaction of reduction of Ce + Ce +- e <-> Ce , (instead of the reaction of hydrogen depolarization), which in its turn creates also the option to form a film (chemically insoluble) of cerium hydroxides/oxides on the active sections of the steel surface. 

The corrosion rate is controlled mainly hy cathodic reaction rates. Cathodic Reactions 5.2 and 5.3 are usually much slower than anodic Reaction 5.1. The slower reaction controls the corrosion rate. If water pH is depressed. Reaction 5.3 is favored, speeding attack. If oxygen concentration is high. Reaction 5.2 is aided, also increasing wastage hy a process called depolarization. Depolarization is simply hydrogen-ion removal from solution near the cathode. 

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). 

Thus, hydrogen gas is generated in most acid solutions. If dissolved oxygen is present, the cathodic Reaction 7.2 can be accelerated by depolarization as in Reaction 7.3 ... 

Depolarization of the cathode by SRB. SRB contain an enzyme called hydrog-enase, which allows the utilization of hydrogen to reduce sulfate to sulfide ... 

Oxygen dissolved in aqueous solutions, even in very low concentrations, is a leading cause of corrosion problems (i.e., pitting) in drilling. Its presence also accelerates the corrosion rate of other corrodents such as hydrogen sulfide and carbon dioxide. Oxygen plays a dual role both as a cathodic depolarizer and an anodic polarizer or passivator. Within a certain range of concentration the... 

An increase in circulatory flow rate (which can sweep away the hydrogen film and depolarize the cathode)... 

The minimum rate of boiler steel corrosion (i.e., the maximum development of dense, adherent, and protective magnetite) occurs at pH of 11 to 12. However, at significantly lower pH values (acid conditions) hydrogen ions are discharged (depolarization), the magnetite layer becomes porous, and corrosion rates increase. 

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