Mass transport controlled oxygen reduction

Fig. 10.11 Log i - curves for a metal corroding via mass transport controlled oxygen reduction in a near neutral (pH 7) electrolyte showing the effect of dissolved oxygen level. An increased oxygen concentration results in an increased limiting current for oxygen reduction until a critical point is reached icoKKh when the cathodic reaction reverts to charge-transfer control. The cathodic reaction is oxygen reduction O2 + 4e- + 2H2O----------- 40H".

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. 

The objective of the mass transport lab is to explore the effect of controlled hydrodynamics on the rate at which a mass transport controlled electrochemical reaction occurs on a steel electrode in aqueous sodium chloride solution. The experimental results will be compared to those predicted from the Levich equation. The system chosen for this experiment is the cathodic reduction of oxygen at a steel electrode in neutral 0.6 M NaCl solution. The diffusion-limited cathodic current density will be calculated at various rotating disk electrode rotation rates and compared to the cathodic polarization curve generated at the same rotation rate. 

Ironically, once a crevice has initiated, the flow of solution across the fully exposed surface generally acts to increase the propagation rate. This effect results from the effect of increased flow on cathodic reactions on the fully exposed surface that are mass transport controlled, such as oxygen reduction. As the cathodic reaction rate increases, the polarization of the internal, crevice anode increases as well, leading to increased dissolution rates. This effect is mitigated to the extent that the... 

Many of the commercial probes are based on a Clark-type electrode (Fig. 12.15) which effectively responds to oxygen via its cathodic reduction under mass-transport control (section 12.4). The medical applications for dissolved oxygen include the following ... 

For oxygen reduction, a simple Randles equivalent circuit was also used because the reaction mechanism for oxygen reduction for the Pt electrode can be described by mass transport-controlled kinetics. The simulated curves are shown in Fig. 19.1l(A-C). The fits are reasonably good, with the charge-transfer resistances (based on geometric area) Rr values shown in Table 19.4. 

Therefore let us instead consider the more practical case of the tertiary current distribution. Based on the dependency of the Wagner number on polarization slope, we would predict that a pipe cathodically protected to a current density near its mass transport limited cathodic current density would have a more uniform current distribution than a pipe operating under charge transfer control. Of course the cathodic current density cannot exceed the mass transport limited value at any location on the pipe, as said in Chapter 4. Consider a tube that is cathodically protected at its entrance with a zinc anode in neutral seawater (4). Since the oxygen reduction reaction is mass transport limited, the Wagner number is large for the cathodically protected pipe (Fig. 12a), and a relatively uniform current distribution is predicted. However, if the solution conductivity is lowered, the current distribution will become less uniform. Finite element calculations and experimental confirmations (Fig. 12b) confirm the qualitative results of the Wagner number (4). 

Figure 16 shows the steady-state limiting current density, ilim, for the oxygen reduction reaction (ORR) on pure Al, pure Cu, and an intermetallic compound phase in Al alloy 2024-T3 whose stoichiometry is Al20Cu2(Mn,Fe)3 after exposure to a sulfate-chloride solution for 2 hours (43). The steady-state values for the Cu-bearing materials match the predictions of the Levich equation, while those for Al do not. Reactions that are controlled by mass transport in the solution phase should be independent of electrode material type. Clearly, this is not the case for Al, which suggests that some other process is rate controlling. 

An example is corrosion of steel in an aerated neutral solution in which the rate of oxygen reduction reaction is largely controlled by mass transport. An Evans diagram for this situation is given in Fig. 8. 

The rate of oxygen reduction on the oxide-free Pt surface is limited by the mass transport of dissolved oxygen to the electrode surface. The dependence of the oxygen reduction on the scan rate, v, was examined. Figure 31 shows the linear potential sweep voltammetric i-E curves for a Pt/diamond composite electrode in 02-saturated 0.1 M HCIO4 [125]. The peak current, ip, increases linearly with when the scan rate is varied from 50 to 400 v/s, indicative of a semi-infinite linear diffusion-controlled process. However, ip rather approaches a proportionality with v at scan rates higher than 400 v/s. This is expected as the reaction shifts from being mass transport limited to control by the surface adsorption process. 

Compared with oxygen reduction overpotential, both kinetic and mass-transport losses of the hydrogen electrode can be neglected. Therefore, the iR-free cell voltage, Ec(iR.jree), of a fuel cell operating on H2/O2 at low current densities (0.1 A/cnr) is controlled by voltage loss due to the O2 reduction kinetics, i.e., by the qact term in Equation 23.6. The cathode overpotential term, qact, for Pt catalysts at low current density (< 0.1 A/cm ) follows the Tafel equation ... 

Concentration polarization is the polarization component caused by concentration changes in the environment adjacent to the surface as illustrated in Fig. 5.4. When a chemical species participating in a corrosion process is in short supply, the mass transport of that species to the corroding surface can become rate controlling. A frequent case of concentration polarization occurs when the cathodic processes depend on the reduction of dissolved oxygen since it is usually in low concentration, that is, in parts per million (ppm) as shown in Table 5.2 [1]. 

Concentration polarization. When the cathodic reagent at the corroding surface is in short supply, the mass transport of this reagent could become rate controlling. A frequent case of this type of control occurs when the cathodic processes depend on the reduction of dissolved oxygen. 

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