Dissolved-oxygen reduction curve

Fig. 1.32 E-i curve for the simultaneous cathodic reduction of HjO (curve ImqpH) and dissolved oxygen (ABCG) which give the combined curve/4flC > E, Fis the anodic curve for...

Fig. 19.38 Schematic polarisation from potentiostatic polarisation. B shows the negative loop and represents the cathodic reduction of dissolved oxygen. The dashed curves in the diagram are cathodic currents and are frequently drawn on the left-hand side of the E axis...

In the presence of oxidizing species (such as dissolved oxygen), some metals and alloys spontaneously passivate and thus exhibit no active region in the polarization curve, as shown in Fig. 6. The oxidizer adds an additional cathodic reaction to the Evans diagram and causes the intersection of the total anodic and total cathodic lines to occur in the passive region (i.e., Ecmi is above Ew). The polarization curve shows none of the characteristics of an active-passive transition. The open circuit dissolution rate under these conditions is the passive current density, which is often on the order of 0.1 j.A/cm2 or less. The increased costs involved in using CRAs can be justified by their low dissolution rate under such oxidizing conditions. A comparison of dissolution rates for a material with the same anodic Tafel slope, E0, and i0 demonstrates a reduction in corrosion rate... 

Figure 9 Polarization curve of carbon steel in deaerated, pH 13.5 solution at 65°C. Sample was initially held potentiostatically at —1.2 V(SCE) for 30 min before initiation of the potentiodynamic scan in the anodic direction at 0.5 m V/s. The cathodic loop results from the fact that the passive current density is only 1 pA/cm2, which is less than the diffusion-limited current density for oxygen reduction for the 0.5 ppm of dissolved oxygen present. (From Ref. 8.)...

Figure 23-17 shows a residual current curve (obtained at high sensitivity) for a 0.1 M solution of HCl. This cuixent has two sources. The first is the reduction of trace impurities that are almost inevitably present in the blank solution. The contributors here include small amounts of dissolved oxygen, heavy metal ions from the distilled water, and impurities present in the salt used as the supporting electrolyte. 

For reasons stated previously, it is considerably more difficult to construct illustrative polarization curves for the reduction of dissolved oxygen. Reasonable estimates of the exchange current densities, Tafel slopes, and diffusion rates have been used to construct the curves of Fig. 3.17 (Ref 3, 6). These curves, identified by letters, are described as follows ... 

Fig. 5.42 Approximate polarization curves for iron, nickel, chromium, and titanium in 1 N H2S04. Approximate cathodic polarization curves for reduction of nitric acid, dissolved oxygen, and hydrogen ions. An explanation for predicting corrosion behavior based on intersection of anodic and cathodic curves can be found in the text.

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. 

Fig. 60. Stationary polarization curves at a platinum rotating disc electrode in diluted solution of H2SO4. Peaks and currents Pj reduction of the Pt-oxide, P2 oxidation of adsorbed hydrogen, P3 oxidation of Pt, I4 oxidation of H2O, I5 reduction of hydrogen ions dashed lines reduction of dissolved oxygen. Adapted according to [142].

Fig. 10.11 Log[f - curves for a metal corroding via mass transport oontrotled 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 (icoaa)s when the cathodic reaction reverts to charge-transfer control The cathodic reaction is oxygen reduction Oj + 4e- + 2HjO----------- 40H. ...

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