Hydrogen Ions reduction

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... 

The hydrogen and oxygen evolution processes on Pb are affected by other metal additions to Pb, for example, addition of bismuth enhances both the hydrogen and oxygen evolution [127], while addition of silver inhibits hydrogen ions reduction [128]. 

ACIDS, CARBOXYLIC (see also HYDROGEN-ION REDUCTION) Al jphatic Aromatic... 

CARBAZONES, see CARBAZIDES CARBOHYDRATES and SUGARS CARBOXYLIC ACIDS, see ACIDS, CARBOXYLIC CATALYTIC HYDROGEN-ION REDUCTION DEUTERATED COMPOUNDS, see ISOTOPES D IAZ0 COMPOUNDS DIAZONIUM SALTS ESTERS, CARBOXYLIC... 

Figure 1. A correlation between the exchange current of hydrogen ion reduction and a bond energy (Q Me-H).

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. 

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. 

Fig. 3.19 Cathodic polarization curves for 100 and 10,000 ppm Fe3+ (as FeCI3) on platinum in nitrogen-deaerated solution. The increase in current density at 400 mV (SHE) is due to a velocity effect in introducing nitrogen sparging into the solution. The limiting current density is increased by a factor of about 100 on increasing the concentration from 100 to 10,000 ppm. The increase in current density near-100 mV (SHE) is due to hydrogen ion reduction resulting from a decrease in pH dueto Fe3+ hydrolysis.

Curves for hydrogen-ion reduction are based on experimental values of the polarization parameters governing the polarization curves. The anodic polarization curves for iron show a dependence on pH due to the influence of hydrogen ion concentration on the kinetic steps in the iron oxidation. Based on Ref 7 and 8... 

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. 

All of the curves in Fig. 5.6 start in the active dissolution potential range and hence do not show the complete polarization curve for the iron extending to the equilibrium half-cell potential as was done in Fig. 5. 4. This extension was shown as dashed lines and the equilibrium potential was taken as -620 mV for Fe2+ = 10 6. Qualitatively, the basis for estimating how the active regions of the curves in Fig. 5.6 would be extrapolated to the equilibrium potential can be seen by reference to Fig. 4.16. There, the corrosion potential is represented as the intersection of the anodic Tafel curve and the cathodic polarization curve for hydrogen-ion reduction at several pH values. It is pointed out that careful measurements have shown that the anodic Tafel line shifts with pH (Ref 6), this shift being attributed to an effect of the hydrogen ion on the intermediate steps of the iron dissolution. 

Fig. 5.17 Net polarization curves, N, resulting from the metal anodic curve, M, and the sum cathodic curve, SC, for the oxygen-reduction and hydrogen-ion-reduction curves. Curves M and SC are from Fig. 5.16. pH = 1. PQ = 0.05 atm...

Fig. 5.19 Potentiostatic polarization curve for pure chromium in hydrogen-saturated (deaerated) 1 N H2S04at25 °C. Dashed section is a cathodic "peak" where the hydrogen-ion reduction dominates over the passive chromium oxidation. Redrawn from Ref 9...

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-... 

Fig. 6.4 Schematic experimental polarization curves (solid curves) assuming active-passive behavior for the individual metal-oxidation curve and Tafel behavior plus limiting diffusion for the individual dissolved-ox-ygen and hydrogen-ion reduction curves (dashed curves)...

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