Potentiostatic method

When the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarization of the sample at potentials below and above the breakdown potential gives information on the initiation and propagation of pits at different levels. Another method involves initiation of pits above the pitting or breakdown potential and then shifts to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential the current is expected to decrease until complete repassivation. 

The critical pitting potential ( cpp) lies between the breakdown potential and protection potential and may be determined by scratch repassivation method. The scratch repassivation method for localized corrosion involves scratching the alloy surface at a fixed potential. Then the change in current is monitored as a function of time, which will show the effect of potential on the induction and repassivation times. An informed choice of the level of potential between the breakdown potential and the critical pitting potential leads to the critical potential for a particular value in the given conditions (22). 

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Both the galvanostatic and potentiostatic method have their own particular spheres of application, and it is not always advantageous to reject the former in favour of the latter, although there is an increasing tendency to do so. Nevertheless, the potentiostatic method does have a distinct advantage in studies of passivity, since it is capable of defining more precisely the potential and current density at which the transition from the active (charge transfer controlled M to the passive state takes place this is fax... 

Pourbaix, M. and Vandervelden, F., Intentiostatic and Potentiostatic Methods, Their Use to Predetermine the Circumstances for Corrosion or Non-corrosion of Metals and Alloys , Corros. Sci., 5, 81 (1965)... 

The use of the potentiostatic method has helped to show that the process of self-passivation is practically identical to that which occurs when the metal is made anodically passive by the application of an external current" . The polarisation curve usually observed is shown schematically in Fig. 19.37a. Without the use of a potentiostat, the active portion of the curve AB would make a sudden transition to the curve DE, e.g. along curve AFE or AFD, and observation of the part of the curve BCDE during anodic polarisation was not common until the potentiostat was used. 

Potentiostatic methods, being capable of detecting differences in corrosion and passivation behaviour of various parts of a heterogeneous surface, have been applied to the electrochemical determination of grain boundary corrosion... 

Corradi and Gasperini claimed that the potentiostatic method was more effective and simpler than the Strauss test for determining intergranular corrosion of stainless steels, and suggested that the method may lend itself for use on finished equipment in service as a non-destructive test. 

S.C.C. has received a share of the potentiostatic approach to corrosion. Barnartt and van Rooyen reported that potentiostatically controlled corrosion in a potential range 50-100 mV above the corrosion potential provided an accelerated test for the s.c.c. of stainless steels. The elevation of the potential by means of a potentiostat eliminated the incubation period, and also increased the density of cracks. Booth and Tucker used potentiostatic methods in the s.c.c. of Al-Mg alloys. 

Steady-state measurements can be made under both galvanostatic and potentiostatic conditions. It is irrelevant for the results of the measurements whether the current or the potential was set first. But in certain cases in which the polarization (/ vs. E) curve is nonmonotonic and includes a falling section (BC in Fig. 12.4), the potentiostatic method has important advantages, since it allows the potential to be set to any point along the curve and the corresponding current measured. But when the galvanostatic method is used, an increase in current beyond point B causes a jump in potential to point D (i.e., the potential changes discontinuously from the value Eg to the value Eg,) and the entire intermediate part of the curve is inaccessible. 

Various types of controlled-potential pulsing are shown in Fig. 5.18. The simplest case is the single-pulse potentiostatic method (Fig. 5.18A). The current-time (I-t) curves obtained by this method have already been described in Section 5.4.1, Eq. (5.4.10) and (5.4.14). 

The double-pulse potentiostatic method (Fig. 5.18C) is suitable for studying the products or intermediates in electrode reactions, formed in the A pulse by means of the B pulse. For example, if an electroactive substance is reduced in pulse A and if pulse B is sufficiently more positive than pulse A, then the product can be reoxidized. The shape of the I-t curve in pulse B can indicate, for example, the degree to which the unstable product of the electrode reaction is changed in a subsequent chemical reaction. 

Fig. 5.18 Potentiostatic methods (A) single-pulse method, (B), (C) double-pulse methods (B for an electrocrystallization study and C for the study of products of electrolysis during the first pulse), (D) potential-sweep voltammetry, (E) triangular pulse voltammetry, (F) a series of pulses for electrode preparation, (G) cyclic voltammetry (the last pulse is recorded), (H) d.c. polarography (the electrode potential during the drop-time is considered constant this fact is expressed by the step function of time—actually the potential increases continuously), (I) a.c. polarography and (J) pulse polarography...

The simplest case occurs when a number of nuclei is formed at the beginning of the process and does not change with time instantaneous nucleation). This is the case when the electrolysis is carried out by the double-pulse potentiostatic method (Fig. 5.18B), where the crystallization nuclei are formed in the first high, short pulse and the electrode reaction then occurs only at these nuclei during the second, lower pulse. A second situation in which instantaneous nucleation can occur is when the nuclei occupy all the active sites on the electrode at the beginning of the electrolysis. 

To determine the potential-mdependent CPT by the potentiostatic method, it is necessary to select a potential placed between the pitting potential and the transpassive potential for the relevant stainless steel (Fig. 13). A suitable choice of potential is 700 mV SCE, and in order to obtain compatibility for a range of stainless steels, the polarization was always... 

The mechanism of anodic dissolution of silver in cyanide solutions has been studied by Bek and coworkers [378-380]. For example, using [379] the rotating disc electrode and pulse potentiostatic method, it has been found that the limiting step involved the formation, at the electrode surface, of the adsorbed complex with two... 

For the reduction of />-nitrophenol, where the irreversible step (35a) is replaced by a reversible two-electron process (36), another equation has been derived (91, 92) for the calculation of the rate constant k of reaction (35 b). The numerical values obtained for the rate constants are claimed (92) to be in agreement with the values obtained by chrono-potentiometric and potentiostatic methods. Reaction (35 b) has also been studied as a consecutive reaction to the reverse reaction of (35c), i.e. oxidation of -aminophenol. 

Sn(II) and Nb(V) species changed their coordinations with the liquid composition. Nb-Sn alloy samples were prepared by the potentiostatic method and analyzed. The results showed that the Nb content in the alloy could be increased by increasing the bath temperature to 160 °C and increasing the NbCh content in the bath. However, increasing the NbCls mole fraction in the bath also increased the viscosity of the bath. Pulse electrolysis was found to be effective in increasing the Nb content in the alloy. The maximum Nb content in the alloy was 60.8 wt% from constant potential electrolysis and 69.1 wt% from pulse electrolysis. XRD diffraction patterns showed that the electrodeposits contained crystalline Sn and NbsSn which is a superconductor material. 

Rather, electrochemical cells containing a second electrode will be considered in the following. These are the electrochemical systems of practical interest, as shown in Figure 3.1.1. Still, one should keep in mind that the behavior of a two-electrode cell is the combination of the two individual electrodes. And indeed, experimental methods have been developed, in particular potentiostatic methods, by which the effects of the second electrode are virtually eliminated so that one electrode of interest may be studied [6],... 

The chemical reaction mechanism of electropolymerization can be described as follows. The first step in course of the oxidative electropolymerization is the formation of cation radicals. The further fate of this highly reactive species depends on the experimental conditions (composition of the solution, temperature, potential or the rate of the potential change, galvanostatic current density, material of the electrode, state of the electrode surface, etc.). In favorable case the next step is a dimerization reaction, and then stepwise chain growth proceeds via association of radical ions (RR-route) or that of cation radical with a neutral monomer (RS-route). There might even be parallel dimerization reactions leading to different products or to the polymer of a disordered structure. The inactive ions present in the solution may play a pivotal role in the stabilization of the radical ions. Potential cycling is usually more efficient than the potentiostatic method, i.e., at least a partial reduction... 

Conductive polymer films based on polypyrrole and hyaluronic acid were synthesized using both a galvanostatic method—by applying current at a constant intensity ranging between 0.5 and 10 mA for periods varying between 60 and 150 minutes—and a potentiostatic method with the constant potentials ranging between 0.3 and 0.75 V... 

P. LACOMBE I have no personal experience with the potentiostatic method. But it can be said from a thermodynamic point of view that the potentiostatic method gives far more in-... 

Soc., 87, 521 (1945)], Bartlett and Stephenson [Jf. Eleetrochem. Soc., 99, 504 (1952)3, Frank [Thesis, G tingen 1954] and Oliver f International Committee for Electrochemical Thermodynamics and Kinetics, p. 314, Butterworths, 1955]. The greatest interest in the potentiostatic method as compared to the amperostatic method is apparent by the increasing number of researches carried out in England by Edeleanu, in Belgium by Pourbaix, in France by Jacquet and his co-workers. 

There are several techniques to evaluate reactivation activity of structurally different AChE reactivators. Probably the most frequent are Eltmann s method (Ellman et al, 1961 Bajgar, 2005) and the potentiostatic method (Kuca and Kassa, 2003). Each one has its advantages and disadvantages. For example, Ellmann s method does not use real substrate ACh, while the potentiostatic method is very time consuming. 

Although the results obtained by both methods are proportionally comparable, to discuss the influence of structural moieties on final reactivator activity only one method should be used. Because of our experiences with the second potentiostatic method, the stracture-activity relationship mentioned in this chapter is based on these results. 

The validity of the SECV method was first tested for a stage II PEVD sample at 550°C. Thus, the results from SECV and the steady-state potentiostatic method could be compared. 

R.V. Bucur and V. Mecca, The duffusivity and solubility of hydrogen in metallized polymer membranes measured by the non-equilibrium stripping potentiostatic method. Surf, Coat, TechnoL 28 3S7 (1986). 

Yet when applied to current reversal techniques, such as double-step chronampero-metry of cyclic voltammetry, these methods require that an appreciable current be observed during the backward perturbation, that is, for t > 0, in potentiostatic methods or after the potential scan inversion in cyclic voltammetry. This requires that the characteristic time 0 of the method is adjusted to match the half-life ti/2 of the electrogenerated intermediate. Today, owing to the recent development of ultramicroelectrodes, 0 can be routinely varied from a few seconds to a few nanoseconds [102]. Yet with basic standard electrochemical equipment, 0 is usually restricted from the second to the low millisecond range. Thus for experimental situations involving faster chemical reactions, current rever-... 

O ) polarography uses a dropping mercury cathode and the current is measured 00 the rotating-disc method uses a spinning platinum disc as the cathode and the current in the steady state situation at a series of rotation speeds is measured Hi) the potentiostatic method uses fixed electrodes and the fall of current with time is measured. 

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