Hydrogen-ion-reduction reaction

Creation of adsorbed hydrogen atoms due to the local hydrogen ion reduction reaction, followed by absorption. The kinetics of these reactions are believed to be accelerated both by the necessity to balance the enhanced oxidation reactions and by the presence of the sulfur species in the crack tip environment. 

Zembura has made specific use of the rotating disc for investigation of the effect of flow on corrosion reactions. This work has shown that it is possible to determine the type of control (activation or concentration polarisation) of zinc dissolving in 0.1 N Na2S04 (de-aerated), which followed closely the predicted increase in hydrogen ion reduction as the flow rate increased, and proved that in this example... 

Dissimilar metals. Galvanic corrosion occurs when two metals with different electrochemical potentials are in contact in the same solution [Figures 6.7 and 6.8]. In both cases, the corrosion of iron (steel) is exothermic and the cathodic reaction is controlling the corrosion rate. The more noble metal, copper increases the corrosion through cathodic reaction of hydrogen ion reduction and hydrogen evolution A passive oxide film on stainless steel for example can accelerate hydrogen reduction reaction. 

For ordinary redox couples in aqueous solution, the most probable acceptor level, ox, is higher by about 3 eV than the most probable donor level, ,.ed. The standard Fermi level, p(rcdox), of a redox couple is set just at the middle between the most probable acceptor level, ox, of the oxidant and the most probable donor level, red, of the reductant. The hydrogen-hydrogen-ion redox reaction is one of the basic reactions in electrochemistry and is called hydrogen electrode reaction ... 

Fie. 3.18 Illustration of the effect of exchange current density on the polarization curve for oxygen reduction in aerated environments of pH = 0.56 and Pq2 = °-2 atm- Curves converge to the same diffusion limit and are identical when the hydrogen ion reduction is the dominant reaction. 

Two cases are considered, one with hydrogen-ion reduction supporting the corrosion (Case III) and the other representative of aerated conditions in which the reduction of oxygen is the governing cathodic reaction (Case IV). The first example, Case III, is shown in Fig. 4.22 in... 

Cathodic protection also can be accomplished by lowering the electrode potential to E M, the equilibrium potential for the metal to be protected, by an external power source. The circuit used to accomplish this is the same as shown in Fig. 2.12. With slight modification, it is again shown in Fig. 4.25 in which the metal to be protected is iron and the cathodic reaction supporting corrosion is either hydrogen-ion reduction, oxygen reduction, or both. 

In deaerated 1 N H2SO4 (pH = 0.56), hydrogen-ion reduction is the cathodic reaction with the cathodic polarization curve intersecting the iron, nickel, and chromium curves in the active potential region. Hence, active corrosion occurs with hydrogen evolution, and the corrosion rates would be estimated by the intersections of the curves. The curves predict that the titanium will be passivated. However, the position ofthe cathodic hydrogen curve relative to the anodic curves for titanium and chromium indicates that if the exchange current density for the hydro-... 

Activation Polarization. Activation polarization is caused by a slow electrode reaction. The reaction at the electrode requires an activation energy in order to proceed. The most important example is that of hydrogen ion reduction at a cathode, H + - H2. For this reaction, the polarization is called hydrogen... 

In practice, electrode reactions other than hydrogen ion reduction are used to construct practical reference systems. Potentials determined using these halfcells can be related back to the hydrogen reference or absolute potential scale if... 

Hydrogen ion reduction, or hydrogen evolution, has already been discussed. This is the cathodic reaction that occurs during... 

Ru or Os, the resulting products are methane, ethylene, and hydrogen. These reduction reactions of carbon dioxide proceed via an electron transfer pathway involving hydrogen ions and metal-activated carbon dioxide rather than by a hydrogenation route from the hydrogen that may be also formed in the photoreduction (Refs. 128 and 129) ... 

The overall electrochemical reaction cannot explain its corrosion rate in frequent conditions. The electrochemical dissolution of magnesium can be carried out through two successive steps the electrochemical formation of Mg " and its oxidation to a divalent ion through hydrogen ion reduction (chemical reaction) or the electrochemical formation of divalent ion directly or both. The mechanism of dissolution can depend also on the stability of magnesium dihydride as a functiou of pH at the interface. 

An isolated half-cell reaction cannot occur by itself, so we need to choose a reference half-cell reaction to complete the reference cell. Thermochemical data for electrochemical half-reactions are commonly published as the standard half-cell potential for reduction reactions, E°. Representative values are reported in Table 9.1 however, these are but a few of the extensive sets of values that are available. This potential is measured with reference to a hydrogen—hydrogen ion oxidation reaction, whose potential is defined as zero ... 

The reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

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