Oxide film theory

There are various theories on how passive films are formed however, there are two commonly accepted theories. One theory is called the oxide film theory and states that the passive film is a diffusion-barrier layer of reaction products (i.e., metal oxides or other compounds). The barriers separate the metal from the hostile environment and thereby slow the rate of reaction. Another theory is the adsorption theory of passivity. This states that the film is simply adsorbed gas that forms a barrier to diffusion of metal ions from the substrata. 

Some of the objections to the oxide film theory were as follows ... 

There are two commonly expressed points of view regarding the composition and structure of the passive film. The first holds that the passive film (Definition 1 or 2) is always a diffusion-barrier layer of reaction products—for example, metal oxide or other compound that separates metal from its environment and that decreases the reaction rate. This theory is sometimes referred to as the oxide-film theory. 

The structure of the passive film on alloys, as with passive films in general, has been described both by the oxide-film theory and by the adsorption theory. It has been suggested that protective oxide films form above the critical alloy composition for passivity, but nonprotective oxide films form below the critical composition. The preferential oxidation of passive constituents (e.g., chromium) may form protective oxides (e.g., Cr203) above a specific alloy content, but not below. No quantitative predictions have been offered based on this point of view, and the fact that the passive film on stainless steels can be reduced cathodically, but not stoichiometric Cr203 itself, remains unexplained. 

Faraday, to whom Schonbein s first communication was sent as a letter, made further experiments. In his second paper he gives a history of earlier work previously unknown to him and Schonbein. As a result of experiments Faraday concluded that the peculiar condition of iron is due to the protection of the iron by a coat of oxide. .. so thin as not to be sensible , or, alternatively that the superficial particles of the metal are in such relation to the oxygen of the electrolyte as to be equivalent to an oxidation.. . having thus their affinity for oxygen satisfied , which would cover several theories proposed later. The oxide-film theory was experimentally confirmed by U. R. Evans. ... 

There are two theories regarding the formation of these films. The first theory states that the film formed is a metal oxide or other reaction compoimd. This is knovm as the oxide film theory. The second theory states that oxygen is adsorbed on the surface, forming a chemisorbed film. However, all chemisorbed films react over a period of time with the imderlying metal to form metal oxides. Oxide films are formed at room temperature. Metal oxides can be classified as network formers, intermediates, or modifiers. This division can be related to thin oxide films on metals. The metals that fall into network-forming or intermediate classes tend to grow protective oxides that support anion or mixed anion/cation movement. The network formers are noncrystalline, whereas the intermediates tend to be microcystalline at low temperatures. 

Films, anodic oxide Films, passivating Films, plastic Film theory Film wrappers Filter Filter aid Filter aids Filter fabrics Filtering centrifuges Filter media Filters... 

Several theories have appeared in the Hterature regarding the mechanism of protection by -PDA antiozonants. The scavenger theory states that the antiozonant diffuses to the surface and preferentially reacts with ozone, with the result that the mbber is not attacked until the antiozonant is exhausted (25,28,29). The protective film theory is similar, except that the ozone—antiozonant reaction products form a film on the surface that prevents attack (28). The relinking theory states that the antiozonant prevents scission of the ozonized mbber or recombines severed double bonds (14). A fourth theory states that the antiozonant reacts with the ozonized mbber or carbonyl oxide (3) in Pig. 1) to give a low molecular weight, inert self-healing film on the surface (3). 

Reflectance spectroscopy in the infrared and visible ultraviolet regions provides information on electronic states in the interphase. The external reflectance spectroscopy of the pure metal electrode at a variable potential (in the region of the minimal faradaic current) is also termed electroreflectance . Its importance at present is decreased by the fact that no satisfactory theory has so far been developed. The application of reflectance spectroscopy in the ultraviolet and visible regions is based on a study of the electronic spectra of adsorbed substances and oxide films on electrodes. 

In order to explain the changing optical properties of AIROFs several models were proposed. The UPS investigations of the valence band of the emersed film support band theory models by Gottesfeld [94] and by Mozota and Conway [79, 88]. The assumption of nonstoichiometry and electron hopping in the model proposed by Burke et al. [87] is not necessary. Recent electroreflectance measurements on anodic iridium oxide films performed by Gutierrez et al. [95] showed a shift of optical absorption bands to lower photon energies with increasing anodic electrode potentials, which is probably due to a shift of the Fermi level with respect to the t2g band [67]. 

It is clear that the problem of anodic oxide films is increasingly of multidisciplinary interest, shared by specialists in different areas of physics and chemistry. It is felt, from reviewing the literature, that these researchers often tend to overlook some aspects which are somewhat removed from their immediate field. Thus, an otherwise excellent experiment or theory may lose significance because of some neglected or ill-defined detail. Hence, it is considered useful at the outset of this article to try and give an overview of the system as a whole, summarizing all the factors which contribute to its extreme complexity. Though some of the points in this overview may be considered only too well known by some scientists, it is hoped that they will arouse a new awareness in others. 

A lively subsection in applications of quantum theory to transitions at electrodes concerns the tunneling of electrons through oxide films. This work has been led by Schmickler (1980, 1996), who has used a quantum mechanical approach known as resonance tunneling to explain the unexpected curvature of Tafel lines for electron transfer through oxide-covered electrodes (Fig. 9.21). 

W. Schmickler, J. Electroanal. Theory 100 533 (1979). Theory of electrodic currents through coatings (and oxide films) in terms of resonance tunneling. Tafel lines curve. 

In a typical spectroelectrochemical measurement, an optically transparent electrode (OTE) is used and the UV/vis absorption spectrum (or absorbance) of the substance participating in the reaction is measured. Various types of OTE exist, for example (i) a plate (glass, quartz or plastic) coated either with an optically transparent vapor-deposited metal (Pt or Au) film or with an optically transparent conductive tin oxide film (Fig. 5.26), and (ii) a fine micromesh (40-800 wires/cm) of electrically conductive material (Pt or Au). The electrochemical cell may be either a thin-layer cell with a solution-layer thickness of less than 0.2 mm (Fig. 9.2(a)) or a cell with a solution layer of conventional thickness ( 1 cm, Fig. 9.2(b)). The advantage of the thin-layer cell is that the electrolysis is complete within a short time ( 30 s). On the other hand, the cell with conventional solution thickness has the advantage that mass transport in the solution near the electrode surface can be treated mathematically by the theory of semi-infinite linear diffusion. 

The differences in antiwear properties of disulfides are related to their ability to be physisorbed about 100 to 1000 times faster than monosulfide on metal surfaces. The differences can be explained in terms of the lower energy needed for the formation of the same number of RS" ions from disulfides (Kajdas,1994). The exposed metal surface is extremely reactive to lubricant components, especially antiwear and extreme-pressure additives resulting the formation of a film on the contact surface. The reaction of emitted electrons of low energy (1 to 4 eV) with molecules of oil additives adsorbed on the friction surface may lead to formation of negative ions and negative ion radicals. The investigator (Kajdas, 1994 and 1985) pointed out the indispensability of the metal oxide film on the rubbing surface from the viewpoint of the theory of sulfide film formation. 

The vibrationally excited precursor AB/s/(fs) can decay not only via energy transfer to the bulk but also via a chemical transformation (desorption of B and reaction with the formation of D and C/s/). These chemical processes can be characterized by the chemical lifetime Tch, which can be estimated in the framework of the statistical RRKM theory (see, e.g., Refs. [50, 51]) using the reaction parameters of reagents B and A/s/, precursor AB/s/, and transition complexes determined based on the results of quantum-chemical calculations. Such estimates were performed for many reactions of interest for the growth of metal oxide films [20]. It appeared that in the wide temperature range... 

Civilization depends on the protection of metals, for most of them are unstable in normal environments unless they are protected by some kind of oxide film. The basic idea about the theory of corrosion is that the metal gets involved in a kind of local fuel cell in which it consumes itself. The partner to most of this self-dissolution is the deposition of hydrogen (favored in acid solutions) or the reduction of oxygen (favored in alkaline). Corrosion is measured in many ways, but the quick way in the laboratory is to move the potential a little bit away ( 5 mV) from the corrosion potential in both anodic and cathodic directions and measure the corresponding current. A simple equation takes the data from this type of measurement and produces the corrosion rate. 

As one example of a coupled-currents theory, the growth of very thick oxide films under local space-charge neutrality conditions will be considered in detail in the following section. 

Refs. [i] Strehblow HH (2003) Passivity of metals. In Alkire RC, Kolb DM (eds) Advances in electrochemical science and engineering. Wiley-VCH, Weinheim, pp 271-374 [ii] Vetter KJ, Gorn F (1973) Electrochim Acta 18 321 [Hi] Strehblow HH (2002) Mechanisms of pitting corrosion. In Marcus P (ed) Corrosion mechanisms in theory and practice. Marcel Dekker, New York, pp 243-285 [iv] Strehblow HH (2003) Pitting corrosion. In Bard AJ, Stratmann M, Frankel GS (eds) Corrosion and oxide films. Encyclopedia of electrochemistry, vol. 4. Wiley, Weinheim, 337... 

---