Anodic process curves

Fig. 3.7 Anodic SWV curves of flavones (1 and 2, reversible processes), an anthraquinone dye (3, a quasireversible process) and a pyran dye (4, a totally irreversible process). Scans from open-circuit potential toward positive potentials (reprinted from [186] with permission)...

Steady-State Kinetics, There are two electrochemical methods for determination of the steady-state rate of an electrochemical reaction at the mixed potential. In the first method (the intercept method) the rate is determined as the current coordinate of the intersection of the high overpotential polarization curves for the partial cathodic and anodic processes, measured from the rest potential. In the second method (the low-overpotential method) the rate is determined from the low-overpotential polarization data for partial cathodic and anodic processes, measured from the mixed potential. The first method was illustrated in Figures 8.3 and 8.4. The second method is discussed briefly here. Typical current—potential curves in the vicinity of the mixed potential for the electroless copper deposition (average of six trials) are shown in Figure 8.13. The rate of deposition may be calculated from these curves using the Le Roy equation (29,30) ... 

We now repeat the measuring procedure in the same manner, but with an organic substrate added to the SSE. The polarogram in a standard case now looks as curve B, Fig. 2. It is evident that a new anodic process involving the substrate takes place in the system, since the current starts to rise at a much lower anode potential. If the concentration of substrate is not too high, we observe that the current reaches a plateau value, the diffusion-controlled current. At the plateau,... 

From the slope of the polarization curve and its variation with time (exposure time of the iron electrode), information on the kind of inhibition can be gained. An inhibition of anodic processes decreases the ia versus E current density and increases the corrosion potential correspondingly, an increase in cathodic inhibition causes a decrease in the i. and lowers the corrosion potential. 

This work further showed that there are two types of polarisation curves of minerals, one for cathodic processes and another for anodic processes, with two electrochemical reactions at potentials (pi and cpj (Fig. 2-37, curve I) and one electrochemical reaction at the potential cp, (Fig. 2-37, curve II). When recording the polarisation curves of minerals. 

For anodic processes there are also two wave steps on the polarisation curves at potentials cpi and 92 corresponding to two electrochemical reactions. The first anodic electrochemical reaction for sulphide at the potential (pi corresponds to its anodic decomposition (transition of ions of metals in solution),... 

Marshakov et al. studied the effect of an ultrasonic field on the anodic dissolution of a variety of metals [120]. Anodic polarization curves were measured in 0.1—0.5 N HC1, NaCl, H2S04, and NajSC solutions in an ultrasonic field of 20 kHz frequency. The effect of ultrasound was different for various metals due to differential effects on the rate-controlling process. Anodic dissolution of Fe was actually slowed down in the presence of ultrasound because the rate-controlling adsorption of anions at the iron surface was inhibited. At a cadmium anode, the energy of metal atoms in a lattice was increased in the presence of ultrasound and, therefore, the ionization of cadmium was accelerated. Anodic dissolution of copper and silver... 

The effect of ultrasound on the process of tellurium anodic dissolution in alkaline solutions was studied by the method of plotting polarization and galvanostatic curves [148]. Tests were made in NaOH solutions (concentrations of 0—20 g/L), subjected to the action of ultrasound at a frequency 17.5 kHz and using Te electrodeposited under ultrasound. The anodic polarization curves plotted without ultrasound and in its presence shifted with increased NaOH concentration towards negative values as a result of the increasing rate of Te anodic dissolution. The presence of ultrasound inhibited the process of Te anodic dissolution, probably due to the desorption of OFT anions from the anode surface. This sonoelectrodeposited Te thus showed greater corrosion resistance in alkaline solution than that deposited... 

Curves 1 and 2 of Fig. 11 have been taken on passive electrodes in hexamethylphosphotriamide solutions of different composition (sodium and lithium salts) and curve 6 — on a nonpassivated electrode (tetramethyl-, tetraethyl- and tetrabutylammonium salts). The rate of the process is equal for sodium and lithium, and is independent of the alkyl radical chain lenght. The generation rates in tetraalkylammonium salt solution were close to those in hexamethylphosphotriamide solutions of lithium salts on a copper electrode specially activated by anodic treatment (curve 3 in Fig. 11). The practical equality of the currents in these systems opposes the idea that intermediate adatoms are participating. 

Chronoamperograms and chronopotenti-ograms More information with respect to the processes that are responsible for the low-current potential regime of the partial anodic polarization curves may be derived from current-time transients. As shown by Fig. 13, there is a continuous decay of the dissolution rate of Cu from a Cu—20 at.% Pd alloy in acidified 1 N Na2S04 (Ec 0.72 V) as long as < Ec-Similar transients have been reported in the hterature for other aUoy/electrolyte systems. Usually, they follow a power law of the form /(/) where m... 

When one is conducting an experiment more or less data are collected usually in the form of numbers. These raw data are only a sequence of numbers without any value, which require a proper meaning to become information. A series of potential and current density values is raw data, but can be processed to give polarization resistance values or anodic polarization curves which provide valuable information. en the same type of analysis is used for samples of similar nature, the evaluation and interpretation of a measurement can be totally automated. This situation occurs mostly in corrosion monitoring. In research laboratories a changing variety of samples requires flexible evaluation procedures and a more active role of the human operator. 

Once the corrosion (mixed) potential is known, the estimation of the cathodic protection current is relatively simple the cathodic Tafel line is extended until the ordinate reaches the anode equifibrium value. The current corresponding to that ordinate value is the minimum value of the external current that must be suppfied to stop the corrosion process. For processes in which there are multiple species undergoing cathodic or anodic reactions, the resultant cathodic and anodic Tafel curves are calculated by adding the individual polarization curves within the respective potential range. 

The anodic polarization curve for a specimen with an active crevice will be in principle as shown in Figure 7.17. In this case a very small free external surface is assumed, and any internal hydrogen reduction is disregarded. is the potential as measured with the reference electrode positioned outside the crevice. As explained above, the real potential in the crevice, Ei , is more negative. The lower limit for corrosion in an active crevice is the protection (or repassivation) potential Epr. However, the critical potential that must be exceeded for initiation of the ereviee corrosion process, the crevice corrosion initiation potential, is higher than the protection potential. 

The rate of nanotube growth can be increased by bigb-intensity ultrasonication during the anodization process [8]. Under ultrasonication, the transient curve (current density plotted vs. anodization time) is modified probably due to more rapid dissolution of the titanium foil. 

In aerated solutions, the cathodic reaction is the O2 reduction reaction (Eq. 22). In such media, the anodic polarization curve will be the same as in acidic solutions (Fig. 14.6) in the absence of O2. However, the cathodic polarization curve (Fig. 14.7) has a different shape, because the reduction of O2 is a diffusion-controlled process. 

---