Sulfuric acid anodic polarization

The Tafel equation also describes the evolution of oxygen at a platinum anode. Bockris and Huq found that, with solutions carefully purified by preelectrolysis, the oxygen electrode exhibits reversible behavior (E = 1.24 V, compared with the theoretical 1.23 V). The exchange current density, however, is only of the order of 10" to 10" °A/cm in dilute sulfuric acid so polarization occurs readily, and relatively large overpotentials are observed at moderate current densities. In solutions of ordinary chemical purity the Nemst relation fails for the oxygen electrode because of mixed-potential behavior. Criddle, using platinum electrodes in highly purified 1 M KOH, obtained a rest potential of 1.59 V. The potential is reduced by peroxide, which may be formed with impurities such as metals, protein, or carbon. 

In the polarization curve for anodic dissolution of iron in a phosphoric acid solution without CP ions, as shown in Fig. 3, we can see three different states of metal dissolution. The first is the active state at the potential region of the less noble metal where the metal dissolves actively, and the second is the passive state at the more noble region where metal dissolution barely proceeds. In the passive state, an extremely thin oxide film called a passive film is formed on the metal surface, so that metal dissolution is restricted. In the active state, on the contrary, the absence of the passive film leads to the dissolution from the bare metal surface. The difference of the dissolution current between the active and passive states is quite large for a system of an iron electrode in 1 mol m"3 sulfuric acid, the latter value is about 1/10,000 of the former value.6... 

Figure 9—4 shows the polarization curves observed for the transfer reaction of cadmium ions (Cd Cd ) at a metallic cadmium electrode in a sulfuric acid solution. It has been proposed in the literature that the transfer of cadmium ions is a single elemental step involving divalent cadmium ions [Conway-Bockris, 1968]. The Tafel constant, a, obtained from the observed polarization curves in Fig. 9-4 agrees well with that derived for a single transfer step of divalent ions the Tafel constant is = (1- P) 1 in the anodic transfer and is a = z p = 1 in the cathodic transfer. 

Fig. 11-10. Anodic polarization curves observed for metallic iron, nickel, and chromium electrodes in a sulfuric acid solution (0.5 M H 2SO 4) at 25°C solid curve = anodic metal dissolution current dot-dash curve s anodic oxygen evolution current [Sato-Okamoto, 1981.]...

The above CVs (Figs. 24 and 25) display well-formed reduction peaks independent of the blank solution and the type of active carbon materials. The combined shape of the cathodic peaks indicates that surface species participate in electrochemical processes in different local environments, or with various structures but convergent peak potentials. The effect of anodic polarization is more readily observed in a basic environment than in an acid solution. Similarly, a positive shift of cathodic peak potential with a decrease in anodic sweep potential limit takes place. Similar results were obtained for studies of electrochemical oxidation of graphite [17] and glass-like carbon [222] electrodes. There was considerable enlargement of both anodic and cathodic peaks after anodic polarization in 20% sulfuric acid (Fig. 26) [17]. 

Some tricks have been reported to avoid multiplication of nuclei, such as a low temperature electrolysis, a low concentration of the solutions, aspects of the nature and preparation of the electrode surface. The fewer defects on the electrode surface, the fewer are the nucleation sites.26,27 As a first step, the electrodes are mechanically polished using abrasive paper. In a second step, the surface is electrochemically polished by successively generating hydrogen and oxygen on the electrode while immersed in a sulfuric acid bath. Electrodes intended for use as anodes are cathodically polarized in a final step. They are then washed and dried before use. The more commonly used electrodes are Pt wires (typically 1 cm long and 1 mm diameter). Other types have been studied.28... 

FIGURE 22.7 Polarization curves forthe anodic dissolution and the passivation of metallic iron in 0.5 kmolm 3 sulfuric acid solution at 25°C [9,10] fpe = anodic iron dissolution current, io7 — oxygen evolution current, p = passivation potential, and Etp — trans-passivation potential. 

As suggested earlier, the n-type GaAs does not corrode in the dark but does corrode under photoexcitation. Figure 22.21 shows semi-schematically the polarization curves for corrosion of an n-type GaAs electrode in sulfuric acid solution under the dark and photoexcited conditions [5-6]. It is seen that in acid solution the rc-type GaAs electrode does not corrode in the dark but does corrode under photoexcitation. The anodic dissolution occurring at the photoexcited rc-type electrode is essentially the same as that which will occur at the p-type electrode, except that the potential region... 

FIGURE 22.21 Polarization curves for the corrosion of a photoexcited n-type GaAs electrode in 0.5 kmol m-3 sulfuric acid solution [15] curves are exaggerated around Ecolr solid curve = photoexcited, dashed curve = dark, and H2 = cathodic hydrogen reaction coupled with anodic GaAs dissolution. 

FIGURE 22.24 Anodic polarization curves for passivation and transpassivation of metallic iron and nickel in 0.5 kmol m-3 sulfuric acid solution with inserted sketches for electronic energy diagrams of passive films [32] /ip = passivation potential, TP = transpassivation potential, fb = flat band potential, /Fe = anodic dissolution current of metallic iron, Nl = anodic dissolution current of metallic nickel, and io2 — anodic oxygen evolution current. 

Limited information is available on the anodic polarization of the four metals in Fig. 5.42 in nitric acid. As an approximation, the behavior in sulfuric acid is assumed to apply in nitric acid. The overall reaction for the reduction of nitric acid is ... 

Corrosion behavior was screened by a 1 week immersion in pH 2 sulfuric acid at 80°C in order to optimize alloy composition and nitridation conditions. Anodic polarization testing was then conducted on the most promising nitrided alloys in aerated pH 3 sulfuric acid at 80°C to quantify... 

Kaesche and Hackerman (13) have investigated the inhibition of several aliphatic and aromatic amines on pure iron corroding in IN hydrochloric acid. These authors observed in thirteen out of fourteen cases that the inhibition was both anodic and cathodic, albeit predominantly anodic. The exception was methylamine which acted only cathodically. In the case of the corrosion inhibition on pure iron by B-naphthoquinoline in sodium sulfate/sulfuric acid solution (13). one observes a simple parallel shift of the anodic and cathodic Tafel lines towards smaller values of current density. Here the effect is almost symetrical, indicating that this inhibitor acts to the same extent upon anodic and cathodic reaction rates. Therefore, the effect of B-naphthoquinoline can be explained on the basis that its adsorption blocks a fraction 0 of the metal surface for all electrode reactions. If equation 9 describes the external polarization behavior in terms of a function of the partial current potential relationship for the anodic and cathodic reactions in the usual terms ... 

A case in point is a study made by Ross (J ) on the dissolution of iron in 0.5 molar sulfuric acid in the presence of thiourea at 40° C. The results of this study, which was conducted as a function of the flow rate, are shown in Fig.9. It appears that the uninhibited dissolution of iron follows expected mass transfer behavior both in the laminar and turbulent regions. However, at two inhibitor concentrations marked deviations from the expected mass transfer behavior are observed. Ross attempted to explain these results on the basis that different inhibitor concentrations affect the anodic and cathodic polarization in different ways, taking also into consideration that at small... 

In sulfuric acid, iron could he passivated hy anodic polarization, which leads to low metal dissolution rates without the polarization, active iron corrosion appears. Alloying elements influence the corrosion rate. Copper has a positive effect and normally reduces the corrosion rate, whereas sulfur and phosphorus increase the corrosion rate. In low-suUur-containirig iron, copper can also increase the corrosion rate [16]. 

In sulfuric acid, chromic acid, or mixtures of the two, the thickness of the oxide layer can be increased by anodic polarization. Homogeneous dissolution of aluminum is promoted and oxides are formed at the surface. The rate of dissolution of the oxides is lower than... 

Fig. 4.14 Anodic polarization curves for Fe-10.5%C and SS (304 L) in 1 N sulfuric acid [47]. Reproduced...

As shown in Fig. 4.15, the anodic polarization curves obtained for steels with chromium content between 3.54% and 19.20% indicates that increasing the chromium content enhances the Fe-Cr-Ni alloy passivation ability by decreasing the critical current density of the alloys from 10 to 10 pA/cm [49]. The passive current also decreases from 50 pA/cm for 3.54% Cr alloy in sulfuric acid to 10 pA/cm for the alloy with 19.2% Cr. 

When polarization occurs mostly at the cathode, the corrosion rate is said to be cathodically controlled. The corrosion potential is then near the thermodynamic anode potential. Examples are zinc corroding in sulfuric acid and iron exposed to natural waters. 

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