Oxygen limiting current

The corrosion current of the uncoupled cathode on the aerated side, fcorr,Fe(02)> is controlled by oxygen limiting current, Fl2, O2 > /corr,Fe(02) = O2 The uncoupled cor-... 

The optimal feed composition (4.91), therefore, provides equality of water- and oxygen-limiting currents at the channel inlet. This means that everywhere along z the current is not limited by membrane drying. In other words, = /fr is the optimal water fraction preventing membrane dehydration. 

Consider cell operation at the oxygen-limiting current density. Formally, operation at the limiting current is equivalent to infinite voltage loss E. The expression under the last logarithm on the left side of (4.207) should... 

Figure 5.31b shows that the carbon corrosion current density in the R-domain is equal to the oxygen-limiting current density (2 A cm in this simulation. Table 5.10). Thus, during the start-stop cycle, carbon corrosion runs very fast and even short transients can severely damage the catalyst. The solution to this problem is in lowering the cell potential during the transient (Takeuchi and Fuller, 2(X)8). 

Eigure 5.32 shows the same as in Eigure 5.31 curves for the hundred times smaller oxygen-limiting current density on the anode side (yjj = 0.02 A cm ). This mimics the situation when in the R-domain oxygen penetrates to the anode side through the membrane (crossover). 

Oxygen-limiting current density on the anode side (A cm ) Oxygen-limiting current density on the cathode side (A cm ) Butler-Volmer ORR current density (A cm ). Equation 5.212 Reference (characteristic) current density (A cm ). Equation 4.52 Reference current density (A cm ). Equation 5.199 Vapor flux density (A cm )... 

In oxygen-free seawater, the J(U) curves, together with the Tafel straight lines for hydrogen evolution, correspond to Eq. (2-19) (see Fig. 2-2lb). A limiting current density occurs with COj flushing for which the reaction ... 

On the other hand, it can be assumed for the oxygen corrosion of steel in aqueous solutions and soils that there is a constant minimum protection current density, 4, in the protective range, U limiting current density for oxygen reduction according to Eq. (4-5) (see Section 2.2.3.2). Then it follows, with V = +1,1 = 2nr, S = 27crs and d = dU from Eq. (24-54), instead of Eq. (24-58) [12-14] ... 

It follows from this that the limiting current density / l is the most significant parameter in a corrosion reaction in which oxygen is the cathodic reactant, and that any factor that increases / l will increase the corrosion rate, since at E ... 

In Section 1.4 see Fig. 1.31h) it has been shown that when a corrosion reaction is controlled by the rate of oxygen diffusion, both the rate of corrosion and the corrosion potential increase with / l. the limiting current density, i.e. 

Turning now to the acidic situation, a report on the electrochemical behaviour of platinum exposed to 0-1m sodium bicarbonate containing oxygen up to 3970 kPa and at temperatures of 162 and 238°C is available. Anodic and cathodic polarisation curves and Tafel slopes are presented whilst limiting current densities, exchange current densities and reversible electrode potentials are tabulated. In weak acid and neutral solutions containing chloride ions, the passivity of platinum is always associated with the presence of adsorbed oxygen or oxide layer on the surface In concentrated hydrochloric acid solutions, the possible retardation of dissolution is more likely because of an adsorbed layer of atomic chlorine ... 

Figure 10.5 demonstrates that, even when semi-logarithmic cathodic kinetics are not observed, partial or total cathodic protection is possible. Indeed, Fig. 10.5 shows that the corrosion rate approximates to the limiting current for oxygen reduction (/,ij and the current required for protection is substantially lower than when semi-logarithmic cathodic behaviour prevails. 

It was indicated earlier that the cathodic current was a poor indicator of adequate protection. Whilst, to a first approximation the protection potential is a function of the metal, the current required for protection is a function of the environment and, more particularly, of the cathodic kinetics it entails. From Fig. 10.4 it is apparent that any circumstance that causes the cathodic kinetics to increase will cause both the corrosion rate and the current required for full (/") or partial (1/ — /, ) protection to rise. For example, an increase in the limiting current in Fig. 10.5 produced by an increase in environmental oxygen concentration or in fluid flow rate will increase the corrosion rate and the cathodic protection current. Similarly, if the environment is made more acid the hydrogen evolution reaction is more likely to be involved in the corrosion reaction and it also becomes easier and faster this too produces an increased corrosion rate and cathodic current demand. 

FIG. 24 Steady-state diffusion-limited current for the reduction of oxygen in water at an UME approaching a water-DCE (O) and a water-NB (A) interface. The solid lines are the characteristics predicted theoretically for no interfacial kinetic barrier to transfer and for y = 1.2, Aj = 5.5 (top solid curve) or y = 0.58, = 3.8 (bottom solid curve). The lower and upper dashed lines denote the... 

FIG. 28 Normalized steady-state diffusion-limited current vs. UME-interface separation for the reduction of oxygen at an UME approaching an air-water interface with 1-octadecanol monolayer coverage (O)- From top to bottom, the curves correspond to an uncompressed monolayer and surface pressures of 5, 10, 20, 30, 40, and 50 mN m . The solid lines represent the theoretical behavior for reversible transfer in an aerated atmosphere, with zero-order rate constants for oxygen transfer from air to water, h / Q mol cm s of 6.7, 3.7, 3.3, 2.5, 1.8, 1.7, and 1.3. (Reprinted from Ref. 19. Copyright 1998 American Chemical Society.)... 

The conceptual development of limiting-current measurement was advanced substantially by Agar and Bowden (A2), who investigated the current-overpotential relationship for oxygen evolution at nickel electrodes in fused sodium hydroxide. Here water transport is the limiting step ... 

Oxidation of ferrocyanide, although used occasionally, offers no advantages relative to reduction of ferricyanide. Because the potential for oxygen liberation in alkaline solutions is close to the oxidation potential of the ferrocyanide couple, the limiting-current plateaus obtained in this case are quite narrow (El). 

Changes in bulk reactant concentration during the limiting current measurement, due, for example, to variations in gas pressure (oxygen reduction) or to the presence of other species susceptible to reduction at the... 

---