Techniques for Studying Passivity

As described in the chapter on passivity (see Chapter 3.2, Passive Metals ) passive films play a critical role in the field 

Many different techniques have been used to depassivate or bare small areas of metal to follow the repassivation kinetics, including, scratching, breaking, guillotining, impinging particles, and incident laser irradiation. Burstein and coworkers have developed and extensively used the scratching and guOlotining approaches [42-44, 

To study the earliest stages of repassivation where the current densities are the highest, it is necessary to create the fresh area as quickly as possible. A very small area of fresh metal can be created extremely quickly (on the order of microseconds) by the thin-film-brealdrig experiment [48]. In this approach, a thin film deposited onto a brittle substrate such as glass or Si is suspended into the solution. Breaking of the thin-film electrode results in the creation of a fresh metal area of size equal to the cross-section of the thin film. Current densities on the order of 1000 A cm were measured using this technique on A1 thin films [48]. 

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I. Epelboin and M. Keddam, Electrochemical techniques for studying passivity and its breakdown, Passivity of Metals (R. R Frankenthal and J. Kroger, eds.), Electrochemical Society, Pennington, NJ, 1978, p. 184. 

Froment and co-workers " have employed REFLEXAFS (vide supra) for studying passive films on iron and nickel. Their early studies were concerned with demonstrating the applicability of the REFLEXAFS technique to electrochemical systems. Most recently, they have used this technique to study the structure of passive films on Ni and on Ni-Mo alloy electrodes. For the Ni electrodes, they performed studies after reduction at — 700 mV (vs. saturated mercurous sulphate electrode) as well as in the passive (-l-3(X)mV) and transpassive (-1-800 mV) regions. The Fourier transforms for the films in the passive region have a Ni—O peak at a distance that corresponds closely to that in bulk nickel oxide. However, no Ni-Ni interactions were observed. These investigators interpreted these results as consistent with a model that postulates an amorphous hydrated polymeric oxide. ... 

A particularly useful technique for studying such systems involves labelling one individual component with a dye. Depending upon the dye one can study the quenching of its fluorescence or phosphorescence by a mobile probe added passively to the system. From the rates and efficiency of the quenchin, one can learn about local morphology (with resolution of 10 A to 100 A) one can measure the extent of interphase formation and one can study molecular relaxation processes in individual phases of the material. 

Use and Uimitations of Electrochemical Techniques A major caution must be noted as to the general, indiscriminate use of all electrochemical tests, especially the use of AC and EIS test techniques, for the study of corrosion systems. AC and EIS techniques are apphcable for the evaluation of very thin films or deposits that are uniform, constant, and stable—for example, thin-film protective coatings. Sometimes, researchers do not recognize the dynamic nature of some passive films, corrosion produc ts, or deposits from other sources nor do they even consider the possibility of a change in the surface conditions during the course of their experiment. As an example, it is note-... 

Much of the microscopic information that has been obtained about defect complexes that include hydrogen has come from IR absorption and Raman techniques. For example, simply assigning a vibrational feature for a hydrogen-shallow impurity complex shows directly that the passivation of the impurity is due to complex formation and not compensation alone, either by a level associated with a possibly isolated H atom or by lattice damage introduced by the hydrogenation process. The vibrational band provides a fingerprint for an H-related complex, which allows its chemical reactions or thermal stability to be studied. Further, the vibrational characteristics provide a benchmark for theory many groups now routinely calculate vibrational frequencies for the structures they have determined. 

The advantages of the in situ techniques include an intact blood supply multiple samples may be taken, thus enabling kinetic studies to be performed. A fundamental point regarding the in situ intestinal perfusion method is that the rat model has been demonstrated to correlate with in vivo human data [46 19], Amidon et al. [36] have demonstrated that it can be used to predict absorption for both passive and carrier-mediated substrates. However, the intestinal luminal concentrations used in rat experiments should reflect adequately scaled and clinically relevant concentrations to ensure appropriate permeability determinations [50], There are limitations of the in situ rat perfusion models. The assumption involved in derivation of these models that all drug passes into portal vein, that is drug disappearance reflects drug absorption, may not be valid in some circumstances as discussed below. 

Several techniques for miniaturization of simple chemical and medical analysis systems are described. Miniaturization of total analysis systems realizes a small sample volume, a fast response and reduction of reagents. These features are useful in chemical and medical analysis. During the last decade many micro flow control devices, as well as the micro chemical sensors fabricated by three dimensional microfabrication technologies based on photofabrication, termed micromachining, have been developed. Miniaturized total analysis systems (pTAS) have been studied and some prototypes developed. In microfabricated systems, microfluidics , which represent the behavior of fluids in small sized channels, are considered and are very important in the design of micro elements used in pTAS. In this chapter microfluidics applied flow devices, micro flow control devices of active and passive microvalves, mechanical and non-mechanical micropumps and micro flow sensors fabricated by micromachining are reviewed. 

An improved technique for entrapment of plant cells has been recently developed with Capsiaum frutesaene cell suspensions. Cells were shown to passively invade the pores of a reticulated polyurethane matrix, inside which viability and metabolic activity were better preserved than in other immobilizing matrices (23). This sytem was used in a deteuled study of the synthesis of capsaicin (an ester of vanillylamine), which is the pungent principle of chili pepper fruits. In the absence of specific precursors to capsaicin, the entrapped cells produced 2-3 orders of magnitude higher yields than c ll suspensions over a 5-10 day culture period (4-5 mg capsaicin g dry weight 1 versus 30 yg... 

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