Films, oxide passive, oxidizing properties

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

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. 

Initially the growth rate is lower than on crystalline silicon. It is known that hydrogen has the ability to passivate the crystalline silicon surface and so its presence on the a-Si H film is probably the reason for the slower rate. The different oxidation properties of PVD and CVD films are a dramatic illustration of the different a-Si H structures that can be produced. 

A New Model. The results of the studies on anodic oxide films (see section 5.9 and chapter 3 on passive film and anodic oxides) show that anodic oxide properties (oxidation state, degree of hydration, 0/Si ratio, degree of crystallinity, electronic and ionic conductivities, and etch rate) are a function of the formation field (the applied potential). Also, they vary from the surface to the oxide/silicon interface, which means that they change with time as the layer of oxide near the oxide/silicon interface moves to the surface during the formation and dissolution process. The oxide near the silicon/oxide interface is more disordered in composition and structure than that in the bulk of the oxide film. Also, the degree of disorder depends on the formation field which is a function of thickness and potential. The range of disorder in the oxide stmcture is thus responsible for the variation in the etch rate of the oxide formed at different times during a period of the oscillation. The etch rate of silicon oxides is very sensitive to the stmcture and composition (see Chapter 4). 

Passivators are inorganic substances possessing oxidative properties whose reaction products with metals form a passive film on the substrate surface, which shifts the corrosion potential of the substrate to the positive side by a few tens of volts. Like a depolarizer, the passivator generates a current on the anodic areas of the substrate of 1 > i density, where i is the critical density of the passivation current. This means that the chemical composition of the passivating film on a metal substrate is the same whether the substrate is passivated by anodic polarization in an acid or is treated with solutions of chromates (CrO ), nitrates (NO ), molybdates (MoO ), tungstates (WO ), ferrates (FeO ) or pertechnates (TcOj). 

As a final comment, the self-passivation response is not only advantageous for enhancing durability in service, but also provides unique opportunities for post-processing modification of nanocomposites. For example, pretreatment of the nanocomposite in oxygen plasma may enhance the materials resistance to thermal-oxidative or UV-electron environments. Alternatively, self-generation of the inorganic surface on commercial nanocomposites films may enhanced barrier properties before introduction to service. 

At the transpassivation potential (Figure 6.3), the properties of the passive film change and one observes a renewed increase in the rate of dissolution. This behavior is referred to as anodic depassivation. It may be the result of film oxidation at high anodic potentials or of film breakdown favored by the presence of certain anions. Generally speaking, in the transpassive potential region, one observes three types of metal dissolution behavior ... 

The efficiency of calcium and sodium nitrite as inhibitors in concrete has been reported by several authors (Gaidis and Rosenberg, 1987 El-Jazairi and Berke, 1990) since the early 1970s. The investigations, conducted using different experimental techniques in solutions, mortar and concrete, revealed a critical inhibitor (nitrite) to chloride concentration ratio of about 0.6. This implies fairly high nitrite concentrations in the pore water of concrete. Nitrite acts as a passivator due to its oxidizing properties and stabilizes the passive film accord-... 

The microstructure influences not only the composition but also the initial growth of the passive film, which determines the compact properties. The initial oxide layer formation is accompanied by the gradual growth of the oxide film. Oxide growth by cation diffusion over vacancies requires the annihilation of the vacancies by the increase of either misfit or misorientation interfacial dislocations. ... 

Dielectric Film Deposition. Dielectric films are found in all VLSI circuits to provide insulation between conducting layers, as diffusion and ion implantation (qv) masks, for diffusion from doped oxides, to cap doped films to prevent outdiffusion, and for passivating devices as a measure of protection against external contamination, moisture, and scratches. Properties that define the nature and function of dielectric films are the dielectric constant, the process temperature, and specific fabrication characteristics such as step coverage, gap-filling capabihties, density stress, contamination, thickness uniformity, deposition rate, and moisture resistance (2). Several processes are used to deposit dielectric films including atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), or plasma-enhanced CVD (PECVD) (see Plasma technology). 

Niobium is used as a substrate for platinum in impressed-current cathodic protection anodes because of its high anodic breakdown potential (100 V in seawater), good mechanical properties, good electrical conductivity, and the formation of an adherent passive oxide film when it is anodized. Other uses for niobium metal are in vacuum tubes, high pressure sodium vapor lamps, and in the manufacture of catalysts. 

More effort has probably been devoted to study of the corrosion and passivation properties of Fe-Cr-Ni alloys, e.g. stainless steel and other transition-metal alloys, than to any other metallic system [2.42, 2.44, 2.55, 2.56]. The type of spectral information obtainable from an Fe-Cr alloy of technical origin, carrying an oxide and contaminant film after corrosion, is shown schematically in Fig. 2.13 [2.57]. 

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