Surface oxide electrochemical phase

In tenns of an electrochemical treatment, passivation of a surface represents a significant deviation from ideal electrode behaviour. As mentioned above, for a metal immersed in an electrolyte, the conditions can be such as predicted by the Pourbaix diagram that fonnation of a second-phase film—usually an insoluble surface oxide film—is favoured compared with dissolution (solvation) of the oxidized anion. Depending on the quality of the oxide film, the fonnation of a surface layer can retard further dissolution and virtually stop it after some time. Such surface layers are called passive films. This type of film provides the comparably high chemical stability of many important constmction materials such as aluminium or stainless steels. 

These differences in the chemical structure of the surface depend not only on the process of manufacture but also on additional treatments or processing conditions. In oxidized carbon fibers, the concentration of carbonyl and, more particularly carboxyl groups, is substantially increased at the expense of hydroxyl groups. In the treatment of carbon fibers, several methods of oxidation are used. Liquid phase oxidation is carried out by the electrochemical and chemical methods whereas gaseous oxidation is carried out in air, oxygen or in the presence of catalysts. Plasma treatment is also used for the surface oxidation of formed fibers. Different methods of oxidation produce different surface characteristics. For example, interlaminar strength is improved by a factor of 10 by electrolytic oxidation over crude oxidation in air. 

Mercury electrodeposition is a model system for experimental studies of electrochemical phase formation. On the one hand, the product obtained is a liquid drop, corresponding very well with the liquid drop model of classical nucleation theory. Besides, electron transfer is fast [61] and therefore the growth of nuclei is controlled by mass transport to the electrode surface [44]. On the other hand, the properties of the mercuryjaqueous solution interface have been the object of study for over a century and hence are fairly well understood. The high overpotential for proton reduction onto both mercury and vitreous carbon favor the study of the process over a wide range of overpotentials. In spite of the complications introduced by the equilibrium between the Hg +, Hg2 " ", and Hg species, this system offers an excellent opportunity to verily the fundamental postulates of the electrochemical nucleation theory. In fact, the dependence of the nucleation rate on the oxidation state of the electrodepositing species is fiiUy consistent with theory critical nuclei appear with similar sizes and onto similar number densities of active sites... 

Well-dispersed ternary PtRuSn catalysts of various atomic ratios (60 30 10, 60 20 20, and 60 10 30) were deposited onto carbon using a modified alcohol-reduction process by Wu et al. [303]. The alloy phase structure and surface morphology for each variation of the PtRuSn/C catalysts were determined by XRD and HR-TEM. In order to evaluate the contributions of Ru and Sn in the different stages of ethanol oxidation, electrochemical oxidations of adsorbed CO, ethanol, acetaldehyde, and acetic acid were performed on each PtRuSn/C catalyst. The results indicated that the Ru-rich PtRuSn/C catalyst (60 30 10) exhibited the lowest... 

DL Luscombe, AM Bond. Determination of tocopherols by reverse-phase liquid chromatography and electrochemical detection at a surface-oxide modified platinum microelectrode, without added electrolyte. Talanta 38 65-72, 1991. 

Because these alloys have been commercially developed, the full compositional, heat treatment and corrosion testing details are not available. It is difficult to identify the particular effects of R additions on the corrosion resistance of these alloys from the data shown, because of the absence of a systematic variation in the particular R addition or concentration. There is however, sufficient evidence in table 5 to suggest that R additions play a role in improving the corrosion resistance of Mg casting alloys, but it is difficult to draw conclusions regarding the nature of that role. As with other alloys, the corrosion resistance of Mg alloys is generally associated with the stability, solubility and defect structure of the naturally occurring oxide film, and the tendency for the various phases in the microstructure to act as local anodes and cathodes. Unfortunately there is no evidence in the literature to indicate whether or not R additions modify the properties of the surface oxide film on the cast Mg alloys or alter the electrochemical balance of the microstructure. 

Carbon fibers, when used without surface treatments, produce composites with low mechanical properties. This has been attributed to weak adhesion and poor bonding between the fiber and matrix. Therefore, the carbon fibers are given surface treatments, the exact nature of which is a trade secret. This surface treatment increases the surface active sites which results in the improvements of the bonding between the fibers and the resin matrix. This tends to increase the wettability of the carbon fibers and enhances the mechanical properties [4-7]. Surface treatments may be classified into oxidative and nonoxidative treatments. An oxidative treatment involves gaseous oxidation, liquid-phase oxidation carried out chemically or electrochemically and catalytic oxidation. The nonoxidative surface treatments involves deposition of more active forms of carbon or metals such as whiskerization, pyrolytic coating, the grafting of the polymers, and metal deposition on the carbon fiber surfaces [8-11]. 

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