Diffusion-controlled oxygen-reduction reaction

In the work of Alodan and Smyrl," the pH profiles over the particle were simulated by assuming a diffusion-controlled oxygen reduction reaction at the cathode and by setting a limiting diffusion current density of 0.1 itiA cm, calculated for a diffusion layer thickness of 20 pm and O2 concentration of 0.26 mol m. ... 

The production of hydroxide ions creates a localized high pH at the cathode, approximately 1—2 pH units above bulk water pH. Dissolved oxygen reaches the surface by diffusion, as indicated by the wavy lines in Figure 8. The oxygen reduction reaction controls the rate of corrosion in cooling systems the rate of oxygen diffusion is usually the limiting factor. 

It has been emphasised that the oxygen reduction reaction is diffusion controlled, and it might be thought that the nature of the metal surface is unimportant compared with the effect of concentration, velocity and temperature that all affect /Y and hence. However, in near-neutral solutions the surface of most metals will be coated (partially or completely) with either... 

Fig. 4.23 Schematic representation of polarization curves for the analysis of galvanic coupling when diffusion control of the oxygen reduction reaction is the dominant factor governing the corrosion rate...

At high cathodic potentials in Fig. 4.11, depending on the angular velocity, the oxygen reduction reaction is controlled by diffusion. Increasing the velocity from 25... 

A copper pipe and a pipe of unalloyed steel are joined as shown in Figure 3. Water with a resistivity of the order of 300 ohm cm is flowing through the pipeline. Assume that the oxygen reduction reaction is diffusion controlled on both materials, and that the dissolution of steel is activation controlled. Compare with a couple of examples in Seetion 7.3 (Figures 7.13 and 7.11). 

Sensor evaluations or fuel cell catalyst evaluations commonly use the oxygen reduction reaction and do not rec[uire the use of any external salt. The tip can use electrochemistry to detect products as they diffuse through porous membranes. Corrosion products may be able to undergo further electrochemical reactions, or could additionally benefit from using an ion-selective electrode as the tip. Many different applications can benefit from the ability to both control and monitor electrochemical reactions, with the added dimension of being able to provide spatial resolution thanks to the SECM. 

A striking example of the interaction of solution velocity and concentration is given by Zembura who found that for copper in aerated 0-1 N H2SO4, the controlling process was the oxygen reduction reaction and that up to 50°C, the slow step is the activation process for that reaction. At 75 C the process is now controlled by diffusion, and increasing solution velocity has a large effect on the corrosion rate (Fig. 2.S), but little effect at temperatures below 50°C. This study shows how unwise it is to separate these various... 

For diffusion controlled corrosion reactions e.g. dissolved oxygen reduction, and the effect of temperature which increases diffusion rates, then by substituting viscosity and the diffusion coefficients at appropriate temperatures into the Reynolds No. and Schmidt No., changes in corrosion rate can be calculated. 

Dissolved oxygen reduction process Corrosion processes governed by this cathode reaction might be expected to be wholly controlled by concentration polarisation because of the low solubility of oxygen, especially in concentrated salt solution. The effect of temperature increase is complex in that the diffusivity of oxygen molecules increases, but solubility decreases. Data are scarce for these effects but the net mass transport of oxygen should increase with temperature until a maximum is reached (estimated at about 80°C) when the concentration falls as the boiling point is approached. Thus the corrosion rate should attain a maximum at 80°C and then decrease with further increase in temperature. 

A complication that occurs on a low at.% Ru electrode is that, owing to the low Faradaic currents (low Ru content) and hence large Rt value, currents due to other trace redox reactions, e.g. oxygen reduction, become more detectable. This reveals itself in a phase-angle of 45° as co 0 as trace oxygen reduction would be diffusion-controlled. The impedance corresponding to this situation can be shown to be the same as that in Equation 5.3, with U(p) expressed by the relationship ... 

Generally, the reduction is achieved under deaerated conditions to avoid a competitive scavenging of Cjoiv and H atoms by oxygen. These atoms are as homogeneously distributed as the ions and the reducing species, and they are therefore produced at first as isolated entities. Similarly, multivalent ions are reduced by multistep reactions, including disproportionation of intermediate valencies. Such reduction reactions have been observed directly by pulse radiolysis for a variety of metal ions (Fig. 2), mostly in water [28], but also in other solvents where the ionic precursors are soluble. Most of their rate eonstants are known and the reactions are often diffusion controlled. 

In analyzing the polarization data, it can be seen that the cathodic reaction on the copper (oxygen reduction) quickly becomes diffusion controlled. However, at potentials below -0.4 V, hydrogen evolution begins to become the dominant reaction, as seen by the Tafel behavior at those potentials. At the higher anodic potentials applied to the steel specimen, the effect of uncompensated ohmic resistance (IRohmk) can be seen as a curving up of the anodic portion of the curve. 

Reaction (4.26) is controlled by concentration polarization (i.e., diffusion of O2 to the siuface limits the reaction rate) where the limiting current density for oxygen reduction is i, = 107 pA/cm ... 

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