Oxidation of the metal substrate

Electrodeposition was used to prepare a biaxially textured Gd2Zr207 (GZO) buffer layer on Ni-W substrates.129 Buffer layers provide chemically inert, continuous, and smooth bases for the growth of the superconductor oxide films. They also prevent both the diffusion of metal to the high-temperature superconductor (HTS) layer and the oxidation of the metal substrate when superconductor oxide films are processed at high temperature (-800 °C) in an oxygen atmosphere (100ppm or more). 

The continued oxidation of the metal substrate beneath the protective oxide layer must become a diffusion-controlled process for thick enough oxide films in which either metal atoms or oxygen atoms diffuse through the metal oxide layer to the appropriate interface where reaction proceeds. Let us assume a thick enough oxide layer on a plane metal surface where a steady state has been achieved. Then we can write for the rate of formation of metal oxide, MO, per unit area (assuming metal ion diffusion) ... 

The H and OH" ions are supplied by the simultaneous electrolysis of water during electrodeposition. Electrode reactions involving the resins are of little significance. However, oxidation of the metal substrate does play a role in anodic electrocoating. 

Due to ambient humidity, one or very few layers of absorbed water formed due to condensation results in the formation of water meniscus between the conductive tip and the substrate. Thus highly resistive electrolyte is formed by a thin water film in wet gas atmosphere which facilitates formation of nanostructures by oxidation of the metal substrate. The introduction of reference electrode in the electrochemical nanocell is not at all possible due to space constraint. A large potential drop in the electrolyte and at the counter electrode, i.e., AFM tip is encountered due to the absence of reference electrode. Several attempts have been made for nanostructure formation on metal substrates such as Si,... 

The corrosion resistance of some metals ultimately depends on the presence of a thin oxide film formed by the reaction of the metal with the environment. This is the case of titanium, tantalum, zirconium, molybdenum, aluminium, cobalt, chromium, etc. Table 9.23 gives the physicochemical properties of the oxides formed on some metals. A low oxide solubility is important to guarantee a low rate of corrosion, since any loss in oxide thickness, due to chemical dissolution, will tend to be balanced by oxidation of the metallic substrate. The oxides should also possess low ionic conductivity. 

The anodic reaction corresponds to the oxidation of zinc particles (loss of electrons) while the cathodic one usually involves oxygen reduction (gain of electrons) on the surface of iron or steel the "pressure" of electrons released by zinc prevents or controls the oxidation of the metal substrate. Theoretically, the protective mechanism is similar to a continuous layer of zinc applied by galvanizing with some differences because the coating film initially presents in general a considerable porosity (Jegannathan et al., 2006). 

There are several proposed mechanisms for the corrosion inhibiting behaviour observed for polypyrrole. One mechanism proposed in the literature is the release of a corrosion inhibiting counter ion from the polypyrrole as it is reduced from its oxidised state to its neutral state by the electrons produced by the oxidation of the metal substrate. This mechanism can be thought of as a smart or responsive coating... 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

There are several ways to prepare thin films for use as model catalyst supports.30-31 For the purposes of this review, we will point the reader toward other sources that discuss two of these methods direct oxidation of a parent metal and selective oxidation of one component of a binary alloy. 32 34 The remaining method consists of the deposition and oxidation of a metal on a refractory metal substrate. This method has been used extensively in our group323131 11 and by others33-52-68 and will be the focus of the discussion here. The choice of the metal substrate is important, as lattice mismatch between the film and the substrate will determine the level of crystallinity achieved during film growth. 

After this primer is applied vinyl chloride-vinyl acetate copolymer is added in a series of thin films. The total thickness is usually 5 mils. Pigments like iron oxide, lead, or zinc chromate prevent corrosion of the metal substrate in acid environments and may also be included in the coating. The final coated metal has good resistance to water and many chemicals with about a ten-year lifetime. 

Although the addition of free radicals to metal centers, leading to one-equivalent oxidation of the metal [see Eq. (288)] is an oxidative addition, we use the term here to describe those additions of substrates to metal centers that involve overall two-equivalent changes.381 386 The reactions of alkyl halides with cobalt(II) or with iridium(I) provide examples of one-equivalent and two-equivalent oxidations of the metal center, respectively ... 

Initiation factors that are believed to contribute to pit initiation are (1) imperfections or defects in the metal oxide, passive, or protective layer between the metal substrate and its environment and (2) exposure of the metal substrate to an aggressive medium. 

Electroless plating should not be confused neither with the electrochemical (galvanic) displacement deposition - process involving the oxidation (dissolution) of the metallic substrate and concomitant reduction of metallic ions in solution - nor with the homogeneous chemical reduction process - indiscriminate deposition over all objects in contact with the solution. 

The first step of the reaction is a single-electron oxidation of the aromatic substrate to form a cation-radical [193,194]. Oxidizing agents for this reaction include Bronsted acids, oxidative Lewis acids, halogens (i.e., Br2), metal salts (e.g., Ti(CF3C02)3), electron-donor-acceptor complexes, irra-... 

The adherence of the oxide to the metal substrate may be related to the epitaxial relationships of the oxide-metal system. Bardolle (53) showed that die adherence of the oxide diminished with the multiplication of the epitaxial possibilities. Thus on iron the oxide was less adherent on the Oil and 113 faces than on the 001 faces. The authors observed that when a copper single crystal sphere was heated in oxygen at a high temperature until a thick oxide scale formed on the high rate faces, the oxide cracked and flaked off easily on these faces but adhered tightly on the low rate faces. In this case the high rate faces show the greatest number of possible orientations, whereas the low rate faces show only one orientation. It should be noted that Bardolle s observations indicated that the faces with multiple orientations on iron were slow rate faces. 

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