Potentiostatic Polarization Method

Bruce etui, established a potentiostatic polarization method for solid polymer electrolytes [450], which is also used for diluted solutions because of its simphcity. For infinitely dilute electrolytes it was shown that this method is suitable for hquids as well [451]. Applying a small constant potential to a solution between nonblocking electrodes leads to decrease of the initial current value until a steady-state value is reached. The steady-state current is caused by the cations [450], so the cation transference number can be easily determined by dividing the cationic current by the initial current. Because electrode surfaces or rather passivating layers vary with time, this inaccurate description can be corrected by impedance measurements shortly before and after the potentiostatic polarization [452]. For small polarization potentials ( 10 mV), the steady-state current hs and initial current Iq are described as [450] 

This method is very popular for measuring transference numbers of lithium electrolytes (see references in Table 17.18) because of the very easy procedure and low time costs. But it must be taken into account that the measurement was developed for binary and ideal solid electrolytes, which is often not the case, especially for solid polymer electrolytes, where a large amount of ion-pairs is probable. 

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The potentiostatic polarization method has been used to study the inhibitive behavior of 5-nitrobenzothiazoles on the corrosion of pure aluminum in 0.1 M HC1... 

The number of precise methods to measure transference numbers in liquid aqueous electrolytes is quite acceptable [9-14]. Various methods are already used for more than a hundred years, such as the moving boundary, Hittorf s method, or the indirect determination of transference numbers by conductivity measurements. In contrast, accurate data for nonaqueous liquid electrolytes, especially with respect to hthium salts, are very rare. In hterature, the most often used methods are the potentiostatic polarization method and determination by NMR [21, 22]. Interestingly, the first was developed for solid electrolytes the latter is only valid for ideal solutions. To measure concentrated electrolyte... 

The behavior of the anion receptor TPEPB in a 1 M LiCl04 electrolyte in EC/DMC (1/2.5 v/v) was studied by Lee et al. [425]. The transference number of the lithium cation was enhanced by increasing the TPFPB content from 0.22 (0 M TPFPB) to 0.55 (0.1 M TPFPB), measured by the potentiostatic polarization method of Bruce and Vincent (see Section 17.4.6.4). The total ionic conductivity decreased due to the decreasing anionic conductivity, but the cationic conductivity increased. The interaction between TPFPB and Cl04, resulting in formation of a complex and advanced dissociation of LiCl04, was shown by FTIR analysis. 

Polarization probe.s. Polarization methods other than LPR are also of use in process control and corrosion analysis, but only a few systems are offered commercially. These systems use such polarization techniques as galvanodynamic or potentiodynamic, potentiostatic or galvanostatic, potentiostaircase or galvanostaircase, or cyclic polarization methods. Some systems involving these techniques are, in fact, used regularly in processing plants. These methods are used in situ or... 

Cyclic potentiodynamic polarization method. Electrochemical studies of pitting corrosion usually indicate that pitting occurs only within or above a critical potential or potential range. Therefore the susceptibility of passive metals to pitting corrosion is often investigated by electrochemical methods such as potentiodynamic or potentiostatic... 

Potentiostatic methods. Once the breakdown potential is determined by cyclic potentiodynamic polarization methods, polarizing individual samples at potentials above and below this value will indicate the validity of the chosen scan rate and give some kinetic data on the initiation and propagation of pits at different levels. Another possibility is to initiate pits above the pitting or breakdown potential and then shift to lower values above or below the protection potential. It is assumed that at imposed values below the protection potential, one should observe current decrease until complete repassivation. 

The test method ASTM F7464 covers the determination of the resistance to either pitting or crevice corrosion of passive metals and alloys from which surgical implants are produced. The resistance of surgical implants to localized corrosion is carried out in dilute sodium chloride solution under specific conditions of potentiodynamic test method. Typical transient decay curves under potentiostatic polarization should monitor susceptibility to localized corrosion. Alloys are ranked in terms of the critical potential for pitting, the higher (more noble) this potential, the more resistant is to passive film breakdown and to localized corrosion. (Sprowls)14... 

In a comparative study of potentiostatic anodic polarization methods, Greene and Leonard found that the anodic polarization characteristics were strongly controlled by the experimental procedure employed.They concluded that while potential sweep techniques yielded the most reproducible results, they were not necessarily the most accurate and that measurements should be conducted as slowly as possible, even though only qualitative data are required. 

Oxide Growth Kinetics and Mechanism. Formation of oxide films by potentiostatic polarization and their characterization by CV enables distinction of various oxide states as a function of the polarization conditions, here Ep, tp and T. This method allows precise determination of the thickness of oxide films with accuracy comparable to the most sensitive surface science techniques 4-7J1-20), CV may be considered the electrochemical analog of temperature programmed desorption, TPD, and one may refer to it as potential programmed desorption, PPD. Theoretical treatment of such determined oxide reduction charge densities by fitting of the data into oxide formation theories leads to derivation of important kinetic parameters of the process as a function of the polarization conditions. The kinetics of electro-oxidation of Rh at the ambient temperature were studied and some representative results are reported in ref 24. The present results are an extension of the previous experiments and they involve temperature dependence studies. 

The estimation method of Li transport number (ti i.) is considered here based on the results of the complex impedance and potentiostatic polarization measurements. 

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. 

The electrolyte dropping electrode [63] method, introduced in 1976, and subsequently used in conjunction with the four-electrode potentiostat [64], is a hydrodynamic technique, offering controlled convective transport. In essence, this approach is identical to the dropping mercury electrode [65] however, the drop consists of a flowing electrolyte liquid phase which forms a polarized ITIES with an immiscible continuous (receptor) phase. In... 

Although the CMT method was originally developed to measure the corrosion rate at the corrosion potential, it has been demonstrated that it can also be used, with some restrictions, to measure the dissolution rate of a polarized electrode. The device for polarization can be a galvanostat or a potentiostat, the operation of which must not interfere with the pH measurements. Most important, the counter electrode must be in the same cell compartment as the experimental electrode and its content well mixed. 

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... 

Standard Reference Method for Making Potentiostatic and Potentiodynamic Anodic Polarization Measurements", ASTM G5-72. "Evaluating Degree of Rusting on Painted Steel Surfaces", ASTM D610-68. 

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