Nonelectrochemical methods

A very important characteristic of surface constitution for any metal is the position of its PZC. Table 10.1 reports values for the PZC for a number of metals. We can see that these values vary within rather wide limits. An important difference between platinum group metals and most other metals is the ability of the latter upon anodic polarization to form relatively thick superhcial oxide or salt layers. Owing to their great practical value, these layers are considered in more detail in Section 16.3. For investigations of the structure and of properties of platinum and other electrodes, many nonelectrochemical methods are also widely applied, which is discussed in more detail in Chapter 27. 

As was cited in the case of immersion testing, most SCC test work is accomplished using mechanical, nonelectrochemical methods. It has been estimated that 90 percent of all SCC testing is handled by one of the following methods (1) constant strain, (2) constant load, or (3) precracked specimens. Prestressed samples, such as are shown in Fig. 25-17, have been used for laboratory and field SCC testing. The variable observed is time to failure or visible cracking. Unfortunately, such tests do not provide acceleration of failure. 

Lithography With the STM Nonelectrochemical Methods. The prospect of atomic density information storage has spurred applications of the STM as a surface modification tool. In this application, the anisotropic current density distribution generated by an STM tip is exploited to "write" on a substrate surface. Features with critical dimensions < 5 nm have been written in UHV, in air, and under liquids. 

Even refined electrochemical methods cannot alone provide full information about the molecular structure of the metal/ solution interface. Hence, many nonelectrochemical techniques have been developed in the past few decades to study the double layer. They include spectroscopic, microscopic, radiochemical, microgravimetric, and other methods. A combination of electrochemical (chronovoltammetry, chronocoulometry, impedance spectroscopy, etc.) and nonelectrochemical methods is often used in studying mechanisms of the electrode process. 

A facile synthesis of cyclic alkylsilanes consisting in the electrochemical reduction of aliphatic dibromides in the presence of polychlorosilanes of the formula R SiCl4 ( = 0, 2) affords heterocyclic compounds in good yields <1995JOM213>. According to the procedure described in Equation (8), the compound 26 was obtained in 57% yield. In contrast to nonelectrochemical methods, which are based on the ring closure of terminal unsaturated compounds, the electrochemical route is claimed to be more efficient and selective. 

Chronocoulometry and photon polarization modulation infrared reflec-tion/absorption spectroscopy have been employed [311] to study the fusion of dimyristoylphosphatidylcholine vesicles onto an Au(lll) electrode. The fusion was controlled either by the electrode potential, or charge. Film characteristics was also potential dependent. After removing the film from the electrode surface (negative potential), phospholipid molecules remained in its close proximity, in the ad-vesicle state. Several electrochemical and nonelectrochemical methods have been apphed [312, 313] to investigate the spreading of small unilamellar vesicles onto Au(lll) electrode. Vesicles fused onto the surface at > —0.5 V (versus SSCE), to form defected bilayers in contact with the metal surface. At more negative potentials, the film was removed from the electrode surface, but it still remained in its close proximity. 

One may believe that the current intensive efforts will eventually be crowned with the development of simple, economically reasonable, photoelectrochemical processes capable of competing with other, nonelectrochemical methods of utilizing solar energy. 

Goals of Testing and Classification of Test Methods Nonelectrochemical Methods Electrochemical and Electrical Methods Barrier Characteristics of Coatings Adhesion of Organic Coatings Transport Properties of Coatings Other Film Properties Corrosion Resistance of Painted Metals... 

Nonelectrochemical Methods. Nonelectrochemical methods of studying corrosion include exposure tests of performance and primary film property measurements. Standard exposure tests include salt water immersion (3-5% aq. NaCl, usually at room temperature, sometimes oxygen saturated) cyclic immersion (e.g., salt water immersion alternated with drying periods) salt fog or spray (5% aq. NaCl fog,... 

While electrochemical methods provide powerful and sensitive ways of studying modified electrodes to provide information about electron-transfer kinetics and film porosity, they cannot provide information about structure or elemental composition. Thus complete characterization requires application of many of the nonelectrochemical methods described in Chapter 17. These encompass microscopy, high vacuum surface analysis, Raman and IR spectroscopy, and methods based on scanning probes, the quartz crystal microbalance, and measurements of contact angles. 

A wide variety of the experimental technique is available for the study of sorption phenomena and for the characterization of surface structure and state via sorption phenomena. Although the classical electrochemical methods—galvanostatic, potentiostatic, potentiodynamic (voltammetric, cyclicvoltammetric) and transient—are widely used, new methods were coming into foreground during the last two decades. The main cheir-acteristic of the new experimental methods is the simultaneous use (coupling) of electrochemical techniques with other nonelectrochemical methods. 

A nonelectrochemical method also sensitive to the presence of water is infrared spectroscopy with multiple internal reflection, applied by Nguyen and coworkers to study the swelKng of an organic coating at the metal-polymer interface in situ [58]. The integration of the stretching vibration of water was used as a measure of the water uptake at the interface. Stratmarm and coworkers measured the increase of the water concentration at the Fe-alkyd-polymer interface by means of single attenuated total infrared reflection spectroscopy (ATIRS) [59]. An ultrathin Fe film was evaporated onto a ZnSe crystal and coated with an alkyd primer. The authors concluded that no separate water film is... 

Yeh and Kuwana " were the first to report on the electrochemistry of cytochrome c at doped metal oxide semiconductor electrodes. A nearly reversible electrode reaction was indicated by the cyclic voltammetry and differential pulse voltammetry of cytochrome c at tin-doped indium oxide electrodes. Except for the calculated diffusion coefficient, all of the characteristics of the electrochemistry of cytochrome c at this electrode indicated that the electrode reaction was well-behaved. A value of 0.5 x 10" cmVs was determined for the diffusion coefficient which, like previously determined values at mercury, is lower than the value obtained by nonelectrochemical methods (i.e., 1.1 X 10 cm /s " " ). The electrochemical response of cytochrome c at tin oxide semiconductor electrodes was reported to be quasi-reversible, although no details were given. " ... 

The direct in situ study of the specific adsorption of ions and organic species was dictated by the desire to obtain more and more direct information on the processes occurring on the surface of the electrode. This was one of the most important factors leading to the elaboration of nonelectrochemical methods for the study of specific adsorption. For instance, radio-tracer method, ellipsometry, and various spectroscopic methods (such as Raman spectroscopy) should be mentioned here. 

Despite the development and apphcation of new mostly nonelectrochemical methods, classical electrochemical methods remained in use for solid electrodes. 

Although on the basis of current-potential relationships important conclusions can be drawn regarding the mechanism of the electrode processes - especially if the experimental parameters are varied over a wide range - the use of combined electrochemical and nonelectrochemical methods is inevitable to elucidate the mechanism of the complex electrode processes. As we will see later in this volume, a great variety of advanced electrochemical and in situ probes are available, which give different types of information and therefore provide a better insight into the nature of... 

Another in situ nonelectrochemical method is the measurement of the electric conductivity of thin Pt films deposited on isolated substrate, usually glass.The conductivity of such films changes in a characteristic way with potential as a result of changes in the electronic properties of the metal electrode surface induced by the electric field across the metal-electrolyte... 

For over a decade, the use of poly aniline for corrosion protection of metals, particularly stainless steel, has been investigated. In the first study [105] by DeBerry, polyaniline was electrochemically deposited on ferritic stainless steels and found to provide anodic protection that significantly reduced corrosion rates in acid solutions. Numerous studies since then have confirmed the corrosion-preventing properties of polyanilines [102-104,106-111]. In addition, nonelectrochemical methods of applying polyaniline have been demonstrated [102-104,106-1111. In one recent study (I09J, dispersions based on doped polyaniline. Versicon, ... 

Electrochemical methods for the determination of diffusion coefficients are not as popular as nonelectrochemical methods. Compared to the number of references wi th N M R data, the number of diffusion coefficients determined by electrochemical methods is insignificant... 

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