Of oxide films

Studies have been made on the rate of growth of oxide films on different crystal faces of a metal using ellipsometric methods. The rate was indeed different for (100), (101), (110), and (311) faces of copper [162] moreover, the film on a (311) surface was anisotropic in that its apparent thickness varied with the angle of rotation about the film normal. 

C2.18.4.2 DEPOSITION OF OXIDE FILMS BY ATOMIC LAYER PROCESSING... 

Sometimes the formation of oxide films on the metal surface binders efficient ECM, and leads to poor surface finish. Eor example, the ECM of titanium is rendered difficult in chloride and nitrate electrolytes because the oxide film formed is so passive. Even when higher (eg, ca 50 V) voltage is apphed, to break the oxide film, its dismption is so nonuniform that deep grain boundary attack of the metal surface occurs. 

Probably the most important powder property governing the formation of atomic bonds is the surface condition of the particles, especially with respect to the presence of oxide films. If heavy oxide layers are present, they must be penetrated by projections on the particles. This results in only local rather than widespread bonding. A ductile metal such as iron which has a heavy oxide layer may not form as strong or as many bonds as a less ductile metal. 

Fig. 21.5 Breakdown of oxide films, leading to linear oxidation behaviour.

Biocorrosion of stainless steel is caused by exopolymer-producing bacteria. It can be shown that Fe is accumulated in the biofilm [2.62]. The effect of bacteria on the corrosion behavior of the Mo metal surface has also been investigated by XPS [2.63]. These last two investigations indicate a new field of research in which XPS can be employed successfully. XPS has also been used to study the corrosion of glasses [2.64], of polymer coatings on steel [2.65], of tooth-filling materials [2.66], and to investigate the role of surface hydroxyls of oxide films on metal [2.67] or other passive films. 

Compared with XPS and AES, the higher surface specificity of SSIMS (1-2 mono-layers compared with 2-8 monolayers) can be useful for more precise determination of the chemistry of an outer surface. Although from details of the 01s spectrum, XPS could give the information that OH and oxide were present on a surface, and from the Cls spectrum that hydrocarbons and carbides were present, only SSIMS could be used to identify the particular hydroxide or hydrocarbons. In the growth of oxide films for different purposes (e.g. passivation or anodization), such information is valuable, because it provides a guide to the quality of the film and the nature of the growth process. 

When active, as in a pit or a crevice or when depassivated by mechanical damage of oxide film or chemical removal in nonoxidizing acid. 

Formation conditions Nature of oxide film Thickness (nm)... 

Fig. 1.45 Breakdown of oxide film leading to a pit and crack when a high-strength steel is subjected to a tensile stress in a chloride solution (after Brown )...

Bianchi, G., Corguetti, A., Mazza, F. and Torchio, S., Electronic Properties of Oxide Films and Pitting Susceptibility of Type 304 Stainless Steel , Corros. Sci., 12, 495 (1972) Szkiarska-Smialowska, Z. and Mankowski, J., Effect of Temperature on the Kinetics of... 

The existence of oxide films of varying resistance on the surface of a cathode results in a corresponding variation of galvanic current flow. This... 

Pick-up of stray current (a.c. or d.c.) (Section 10.5). Decreased use of d.c. in many areas has led to less possibilities of pick-up of direct current from utilities, mines, etc. The importance of grounded a.c. systems has been discounted, but Waters has shown that alternating currents can accelerate corrosion. Furthermore the rectifying effects of oxide films, clay minerals and other soil factors are not understood. 

Anodising the formation of oxide films on metals by anodic oxidation of the metal in an electrolyte solution. The term can be used for thin dielectric films but is more particularly applied to thicker films formed on some metals such as aluminium at higher anodic voltages. 

These facts are different demonstrations of the same event degradation reactions occur simultaneously with electropolymerization.49-59 These reactions had also been called overoxidation in the literature. The concept is well established in polymer science and consists of those reactions between the pristine polymer and the ambient that promote a deterioration of the original polymeric properties. The electrochemical consequence of a strong degradation is a passivation of the film through a decrease in the electrical conductivity that allows a lower current flow at the same potential than the pristine and nondegraded polymer film did. Passivation is also a well-established concept in the electrochemistry of oxide films or electropainting. 

As a rule, different types of oxide film will form simultaneously on metal electrodes for instance, porous phase layers on top of adsorbed layers. Often, aging processes occur in the oxide layers, which produce time-dependent changes in the properties or even transitions between different forms. 

Dickinson T, Povey AP, Sherwood PMA. 1975. X-ray photoelectron spectroscopic studies of oxide-films on platinum and gold electrodes. J Chem Soc Faraday Trans 171 298-311. 

Tremiliosi-Eilho G, Jerkiewicz G, Conway BE. 1992. Characterization and significance of the sequence of stages of oxide film formation at platinum generated by strong anodic polarization. Langmuir 8 658-667. 

Damjanovic A, Hudson PG. 1988. On the kinetics and mechanism of O2 reduction at oxide film covered Pt electrodes. I. Effect of oxide film thickness on kinetics. J Electrochem Soc 135 2269-2273. 

Obtaining of oxide film throu sedimentation or by electrophoretic depositing from water, alcohol, or benzene solution also makes it possible to produce a fairly thin film with sufficient sensitivity to adsorption of above particles. However, poor mechanical features of the films obtained are their major drawbacks. 

The most reliable and easy to control method of oxide films application is provided by either sputtering of an oxide layer (e. g., by a laser beiun), or evaporation of a thin metal zinc film (its resistance is usually 5-10 Ohm) from tantalum vessel onto a thoroughly prepared clean quartz substrate in a vacuum of Torr (containing no oil and... 

Over large periods of time, the conductivity of oxide film grows linearly with time during chemisorbtion of atoms of oxygen and nitrogen at ultra low concentrations on the zinc oxide film over wide time interval at moderate and elevated temperatures, i. e. the following simple kinetic equation is valid ... 

Our comments on adsorption of oxygen and nitrogen atoms lead to conclusion that practically under all conditions the initial rate of variation of conductivity of zinc oxide film due to adsorption of acceptor particles discussed in this section is proportional to the concentration of particles in the space adjacent to the surface of oxide film. This is similar to the case of donor particles. This means that the following equation is applicable ... 

Provided that the value d is small enough, there is a simple linear relation between the initial rate of variation in conductivity of oxide film (e.g., ZnO) and concentration of free radicals in the space adjacent to the film surface. When adsorbing the simplest alkyl radicals as well... 

It was shown in a number of works [29] that impurity conductivity of thin zinc oxide films are extremely sensitive to adsorption of atoms of various metals (see Chapters 2 and 3). Using this feature of oxide films, we first employed the sensor method to study evaporation of superstechiometric atoms of metals from metal oxide surfaces, zinc oxide in particular [30]. 

From the positive results of these experiments we concluded that the behaviour of atomic and molecular particles of silver with respect to their influence on electrophysical properties of oxide films is similar to that of atoms and molecules of nonmetals, with the only difference that metal-atom interstitials behave similar to hydrogen-like donors of electrons, independent of the kind of a metal. As to metal molecules, at low temperatures of a semiconductor film, when their surface dissociation does not occur, they do not reveal considerable activity with respect to electrophysical properties of the film. 

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